# Flattening problem using planer sled



## TravisN (Jan 15, 2019)

Hi all,

I wasn't sure if I should post this here or the jig forum. It's a follow-up from my previous post here.

I decided to go with the planer sled for a variety of reasons. I used an 8' piece of 3/4" melamine and tried to support the sled all the way through and out. I tried it out with a 5' board of 8/4 ash. The board had a little twist and was cupped. I placed it with the bow up (ends down) to try and minimize any rocking effect as it went through the planer. I made sure to shim it with the sled on the flattest surface I had (my kitchen counter top) because I read in other posts that that was perhaps the most important step. It seemed to work fine, but when I came back to test the newly flattened surface on my counter top I noticed it now had an opposite cup.

In other words, it seems to narrow in the middle of the board. Laying the rough side down (as originally placed) and the board rests on its ends. Flip it over and lay the supposedly flat side down and it also rests on its ends. The gap is smaller than the rough side, perhaps about 1/16th of an inch.

I'm just trying to figure out what would cause this. I don't want to keep running this board through the planer if there is a problem because I'm trying to preserve as much thickness as possible. I thought the ends were secure when I ran it through on the sled, but I can't think of another explanation for the thinner middle. I'm assuming the rollers pushed the ends down a fraction and then they "spring back up" when free. Are there any other reasons I could be observing this? I suppose my counter top might have a valley, but I don't have a long enough straight edge to test that with.

Pictures below are the "flat" side down.


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## bilyo (May 20, 2015)

When I first tried to use a sled, I did exactly as you did and got similar results. It was only when I made my sled into a torsion box, stiff enough to stay flat, that I was able to get good results. Your melamine board (sled) is quite flexible and there is no way you are going to keep it perfectly flat as to goes through the planer. Another way you can try (I haven't done it) is to place another piece of melamine through the planer, make sure it is flat and well supported and anchor it in position. Then let your sled slide on it. That way it will stay flat all the way through.

I have had this discussion before here and others have said that they have no problem doing it the way you did. That's fine. I haven't been able to succeed that way.


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## Unknowncraftsman (Jun 23, 2013)

Now don't you feel silly spending all that time and some money on a silly fixture. 
A planer is not a jointer.

Good Luck


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## MrUnix (May 18, 2012)

You just didn't shim/secure it properly… somewhere, it had some give. Not the sled, but the stock you were trying to flatten. Placing it with the bow up (ends down) probably made shimming it properly more difficult as well. You also don't want to try and take much off on each pass, so go lightly.

Cheers,
Brad


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## Andybb (Sep 30, 2016)

> When I first tried to use a sled, I did exactly as you did and got similar results. It was only when I made my sled into a torsion box, stiff enough to stay flat, that I was able to get good results.
> - bilyo


+1
I posted this in your other thread. This is a torsion box sled and it works great.


> Planer sled.
> 
> Nothing unique or special about it as many have made ljs these and  plans are readily available but I ve been meaning to make one of these for a few years now and finally got around to it. Just thrilled that it worked so well. Boards came out dead flat. It ain t pretty but it works great for flattening warped boards too wide or twisted for my 6 jointer. Spent about $15 on various hardware at The Borg.
> 
> ...


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## ChefHDAN (Aug 7, 2010)

Travis, you're clearly diving into this hobby, I've seen your many posts, and applaud your vigor. What is you time worth and where do you see yourself going in this endeavor? How do you feel about that? .... What is your time worth? I found these;

Craftsman Jointer

Delta Jointer

Jet Jointer

Time is a valuable commodity, if you figure what your hourly rate is, the amount of time dealing with sleds etc. between construction adjustment and use. I really believe that you'll be very pleased owning a jointer, the best work starts with square stock and some stock you can only get for a reasonable price in the rough. If I were in your position I'd be standing in front of the Craftsman Jointer with 2 $100 bills and 2 $50's, Offer $200 and see if he'll take $250. That is of course unless the Delta is less, but I'll bet the seller wants +$400.

Not trying to bust your chops, but I see you're "Into It" and you'll be so happy after you run your first board over it, and I'm pretty confident it will not go to waste in your shop as you're diving in and building your skills… eventually you're going to NEED a jointer, I say do it now!


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## controlfreak (Jun 29, 2019)

I don't think the problem is with th MDF flexing as it should be a uniform thickness with full bottom support. I think the slight movement in the adjustable support under the board combined with the board movement itself combine to create what you show in your pictures. When I look at the downward pressure that the planner rollers exert I don't see how there will be no deflection in the board being flattened as it rides on the sled without saturating it with contact points. The same is true over a jointer without the subtle adjustments in pressure by the operator. Even with those adjustments it will require multiple passes. I know this may not help you Travis but I am not being critical here, more like thinking out loud.


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## RichT (Oct 14, 2016)

An eight foot long piece of melamine that won't flex? Someone will have to tell me where I can buy that!

I'm with billyo.


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## TravisN (Jan 15, 2019)

> When I first tried to use a sled, I did exactly as you did and got similar results. It was only when I made my sled into a torsion box, stiff enough to stay flat, that I was able to get good results. Your melamine board (sled) is quite flexible and there is no way you are going to keep it perfectly flat as to goes through the planer. Another way you can try (I haven t done it) is to place another piece of melamine through the planer, make sure it is flat and well supported and anchor it in position. Then let your sled slide on it. That way it will stay flat all the way through.
> 
> I have had this discussion before here and others have said that they have no problem doing it the way you did. That s fine. I haven t been able to succeed that way.
> 
> - bilyo


I looked at similar discussions. I decided to try the simplest solution first. Hopefully I can get this to work, otherwise I'll look into a torsion box design.


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## TravisN (Jan 15, 2019)

> Now don't you feel silly spending all that time and some money on a silly fixture.
> A planer is not a jointer.
> 
> Good Luck
> ...


Um, thanks?


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## Lazyman (Aug 8, 2014)

I don't think that you said how wide the board is. If you have room, you might try putting an already straight board on either side of the board you are flattening as you run it through the planer. This may help reduce any flex. Take extremely light passes. Did you use hot glue to lock the shims and the board down? Not sure how well the hot glue will stick to the melamine but it can't hurt.

You may also be able to use a straightened board to see if your countertop is actually flat. Turn it at different angles and test it across the ends of the countertop to verify that the theoretically straight board is actually straight. If you make 2 straight boards, you can use them against each other to verify that they are both straight.


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## TravisN (Jan 15, 2019)

> You just didn t shim/secure it properly… somewhere, it had some give. Not the sled, but the stock you were trying to flatten. Placing it with the bow up (ends down) probably made shimming it properly more difficult as well. You also don t want to try and take much off on each pass, so go lightly.
> 
> Cheers,
> Brad
> ...


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## TravisN (Jan 15, 2019)

> I posted this in your other thread. This is a torsion box sled and it works great.
> 
> Planer sled.
> 
> - Andybb


I appreciate that walk-through. If I were doing this repeatedly I would invest the time to make a deluxe sled like that. I was looking for an easy one-off type solution.


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## TravisN (Jan 15, 2019)

> Travis, you re clearly diving into this hobby, I ve seen your many posts, and applaud your vigor. What is you time worth and where do you see yourself going in this endeavor? How do you feel about that? .... What is your time worth? I found these;
> 
> Craftsman Jointer
> 
> ...


Thanks for the links ChefHDAN! I have a benchtop jointer that works great for most of my small needs. I am building a kitchen table ATM, not something I will do frequently. While I love woodworking and making things, my "shop" is also a functioning garage and I can't justify a full-size jointer. I would love it, and I would be willing to buy a used model like you linked. I just don't have space for it.

I could probably get pretty decent results with my benchtop model on these 5' boards, it's got 5' of combined in/outfeed. But, I know I will not get good results on the tabletop planks, which is why I am testing this method on the 5' boards.


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## TravisN (Jan 15, 2019)

> An eight foot long piece of melamine that won t flex? Someone will have to tell me where I can buy that!
> 
> I m with billyo.
> 
> - Rich


It's not that the melamine won't flex. It's that theoretically it doesn't have to be that rigid. Since the planer references off the sled and the sled is flat/supported through the planer, and the board is shimmed so it won't flex and the twists/cups are leveled out, you should still be able to get a flat surface. It's not the same as just running the plank unsupported through the planer.


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## TravisN (Jan 15, 2019)

> I don t think the problem is with th MDF flexing as it should be a uniform thickness with full bottom support. I think the slight movement in the adjustable support under the board combined with the board movement itself combine to create what you show in your pictures. When I look at the downward pressure that the planner rollers exert I don t see how there will be no deflection in the board being flattened as it rides on the sled without saturating it with contact points. The same is true over a jointer without the subtle adjustments in pressure by the operator. Even with those adjustments it will require multiple passes. I know this may not help you Travis but I am not being critical here, more like thinking out loud.
> 
> - controlfreak


Thanks controlfreak. I was probably going easy on the shims. Since it is a 8/4 piece of ash, I assumed it would be more resistant to the rollers. I'll try again with more shims


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## AlaskaGuy (Jan 29, 2012)

Are you sure your reference (counter top) is truly flat? What did you use to check that?

I agree with Aj2,


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## RichT (Oct 14, 2016)

> It s not that the melamine won t flex. It s that theoretically it doesn t have to be that rigid. Since the planer references off the sled and the sled is flat/supported through the planer, and the board is shimmed so it won t flex and the twists/cups are leveled out, you should still be able to get a flat surface. It s not the same as just running the plank unsupported through the planer.
> 
> - Travis


So, you're telling me that a piece of melamine that's sagging at each end is still going to be perfectly flat in the section going through the planer?


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## Andybb (Sep 30, 2016)

> I posted this in your other thread. This is a torsion box sled and it works great.
> 
> Planer sled.
> 
> ...


I understand, but investing a day to build a jig that will get the job done is to me well worth the time investment vs the time and frustration in using something that doesn't work. People have explained that the melamine is probably flexing and have given you a solution that is proven to work. JMHO and 2 cents worth.


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## TravisN (Jan 15, 2019)

> I don t think that you said how wide the board is. If you have room, you might try putting an already straight board on either side of the board you are flattening as you run it through the planer. This may help reduce any flex. Take extremely light passes. Did you use hot glue to lock the shims and the board down? Not sure how well the hot glue will stick to the melamine but it can t hurt.
> 
> You may also be able to use a straightened board to see if your countertop is actually flat. Turn it at different angles and test it across the ends of the countertop to verify that the theoretically straight board is actually straight. If you make 2 straight boards, you can use them against each other to verify that they are both straight.
> 
> - Lazyman


I'm not sure the countertop is absolutely flat. That's one of the variables here. I don't have an 8' straightedge to confirm it with. I'm obviously questioning my ability to make an 8' straight board. So…. the only way I could really tell if it is flat is to purchase an 8' straight edge. I did run another 5' board through the planer on the sled and placed the two supposedly flat faces on top of each other. They didn't naturally rest flush but I was able to squeeze them together with hand pressure. So I think they're pretty close.


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## TravisN (Jan 15, 2019)

> It s not that the melamine won t flex. It s that theoretically it doesn t have to be that rigid. Since the planer references off the sled and the sled is flat/supported through the planer, and the board is shimmed so it won t flex and the twists/cups are leveled out, you should still be able to get a flat surface. It s not the same as just running the plank unsupported through the planer.
> 
> - Travis
> 
> ...


Yes, I am assuming that the rollers which are apparently strong enough to flex 8/4 ash are able to keep the melamine flat through the cutting section. I am also using additional infeed and outfeed support, so it's not as if the melamine is just hanging except for the couple feet of planer bed.

That said, I could be wrong, and the issue could be that the melamine is sagging through the cutters.


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## RichT (Oct 14, 2016)

> That said, I could be wrong, and the issue could be that the melamine is sagging through the cutters.
> 
> - Travis


Bingo.


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## tmasondarnell (Jul 2, 2013)

Travis,

Can feel your pain. This is what I did for several years.

Did you shim in the middle of the board at the high spot-hot glue the shims in place? It really looks like either the sled is sagging or the board is being compressed as it goes through the planer.


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## Andybb (Sep 30, 2016)

> They didn t naturally rest flush but I was able to squeeze them together with hand pressure. So I think they re pretty close.
> - Travis


"Pretty close" doesn't work.

An 8' slice of MDF along the factory edge will work as an 8" straight edge.


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## controlfreak (Jun 29, 2019)

> It s not that the melamine won t flex. It s that theoretically it doesn t have to be that rigid. Since the planer references off the sled and the sled is flat/supported through the planer, and the board is shimmed so it won t flex and the twists/cups are leveled out, you should still be able to get a flat surface. It s not the same as just running the plank unsupported through the planer.
> 
> - Travis
> 
> ...


Good point, if the infeed and outfeed are not supported it could cause problems but I doubt most people would allow a newly made sled to flop around like a wet noodle.


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## Lazyman (Aug 8, 2014)

You may be able to use a shorter straight edge. Just see if you can can slide a feeler gauge under it as you try it in several spots.


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## TravisN (Jan 15, 2019)

> That said, I could be wrong, and the issue could be that the melamine is sagging through the cutters.
> 
> - Travis
> 
> ...


Wow, I walked right into that one.



> An 8 slice of MDF along the factory edge will work as an 8" straight edge.
> 
> - Andybb


Of course, thanks!

I did find such an edge. I didn't test the counter top, just one of my finished faces. It does have a dip in the center, maybe 1/16th of an inch at the most.



> Travis,
> 
> Can feel your pain. This is what I did for several years.
> 
> ...


I did hot glue shims anywhere I could see a gap, spaced probably about 4" apart. I couldn't feel any flex after shimming, but I also didn't really force it. I don't know how strong the rollers are. I'll watch a few more videos to see how others shim, maybe I'm not supporting it enough.

If it is the sled itself that is flexing, I would expect a different pattern. That is, assuming the flexing board creates the cup in the middle (because the two ends are sagging down, lifting the center and cutting away more), then you would expect the sled is flexing the most at the center of the plank being cut, which is approximately 2.5' in to the 8' sled. But those first few feet are also the most supported: extended infeed, the planer bed, and extended outfeed, plus I'm personally supporting the end of the sled that is hanging off. The tail end may sag after I go to support the front, but that would affect the end of the board more than any other part, and not result in a center cup. Am I understanding that correctly?

I appreciate all your comments, I know it's hard to troubleshoot without seeing in person, and this is not the ideal way to flatten a board.


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## Unknowncraftsman (Jun 23, 2013)

Your time would be better spend learning how to use a Bench plane. It's something that will take your work to a level of satisfaction and build your confidence.
Plus when your done it stores away in a small drawer.
My experience with plywood fixtures is they don't always stay flat for the long haul. Esp the bigger ones.
This is not a jab at Travis.
This is my experience strength and hope for the new woodworkers passing through. The smart ones that want to learn. I know your out there.
Good Luck


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## Andybb (Sep 30, 2016)

You could have built that torsion box sled by now!


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## TravisN (Jan 15, 2019)

OK, I'm pretty sure I figured it out. I took a straight edge (thanks Andybb) to the counter top and it has the exact same cup (depressed middle) that I'm seeing in my finished boards. I assumed when I was setting up my shims that the counter was flat. The jig has to be set up on a flat surface, and mine wasn't. When the sag in the middle of the counter top (transferred into the jig via shims) is forced flat by the planer, extra gets chopped off the top.

So I can build a torsion box with its own flatness like others have suggested, or find a way to temporarily flatten the counter top when I set up my shims.


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## clin (Sep 3, 2015)

You certainly can make a flat torsion box as a sled, but all that is needed is a flat surface to place the sled on while shimming. If you don't already have a flat assembly table, I'd put your torsion box efforts into making a flat work surface that you can use for many other things. Of course if you plan to do a lot of this, have the sled be the flat shimming surface (a torsion box) can simplify things.

Remember when you make a torsion box you must assemble it on a flat reference surface. This creates a sort of chicken and egg problem. Creating a reference surface to assemble a torsion box is actually the biggest part of making one. Once you have that, the torsion box goes together quite fast. While discussing torsion boxes, keep in mind that joints between webs DO NOT matter one wit. The webs don't even need to touch each other. What does matter is the skins have good attachment to the webs.

I've had great success with using a Melamine shelf board while flattening boards in the range of 5-6 feet long. Sometimes the sled would actually partially fall off while carrying things around. But as long as everything ends up in the same position it was in when shimmed on the flat surface, it will come out fine. In the end, the planer is a thickness planer and all that matters is the thickness under the cutting head at any given moment. So the sled and stock don't need to be kept absolutely flat while feed through the planer. Just normal support so the whole thing feeds through properly.

The sled is just a flat reference surface temporarily attached to the piece of work. So if shimmed on a flat surface, the reference will be established and you will get a flat result. Of course supporting every part of the board is not possible and the planer can push the stock down. I think this is where shallower cuts can help and of course err on using more shims than needed.

Also, it's generally better to have any cup in your stock facing up so the gaps are at the edges, between the stock and sled, where you can fit shims. If the cup is down, the gap is in the middle of the board and you have no way to get shims under the board. The result is the planer can push some of the cup out of the board and the cup won't get removed. I'm not saying that was the OP's issue, because that's not what happened, just pointing it out for anyone else reading through the topic.


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## TravisN (Jan 15, 2019)

> You certainly can make a flat torsion box as a sled, but all that is needed is a flat surface to place the sled on while shimming. If you don t already have a flat assembly table, I d put your torsion box efforts into making a flat work surface that you can use for many other things. Of course if you plan to do a lot of this, have the sled be the flat shimming surface (a torsion box) can simplify things.
> 
> Remember when you make a torsion box you must assemble it on a flat reference surface. This creates a sort of chicken and egg problem. Creating a reference surface to assemble a torsion box is actually the biggest part of making one. Once you have that, the torsion box goes together quite fast. While discussing torsion boxes, keep in mind that joints between webs DO NOT matter one wit. The webs don t even need to touch each other. What does matter is the skins have good attachment to the webs.
> 
> ...


Thank you Clin, that is so helpful. Exactly the information I needed. Now I just have to decide what to do about it.


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## bilyo (May 20, 2015)

I'll chime in one more time briefly as this thread is getting long. I favor the torsion box sled because I have had good success with it. I just wanted to tell you that it doesn't need to be as sophisticated as the one Andybob made. That is a special piece of work very nicely done. Mine is a basic torsion box with a flat surface top and bottom covered with plastic laminate. I lay the board I want to plane on it and, using little wedges, stabilize it by placing the wedges where ever there are gaps that need support. A dab of hot melt glue holds the wedges in place. Because the sled is stiff, I don't worry about how level the surface is that it is resting on. Again, because it is stiff, I know it will stay flat as it goes through the planer. I do use roller stands on both infeed and outfeed sides to support it (I'm working alone). Once made, the torsion box will save you time and a lot of futzing around trying to get everything straight and true. I hope this helps you make up your mind.


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## TravisN (Jan 15, 2019)

> I ll chime in one more time briefly as this thread is getting long. I favor the torsion box sled because I have had good success with it. I just wanted to tell you that it doesn t need to be as sophisticated as the one Andybob made. That is a special piece of work very nicely done. Mine is a basic torsion box with a flat surface top and bottom covered with plastic laminate. I lay the board I want to plane on it and, using little wedges, stabilize it by placing the wedges where ever there are gaps that need support. A dab of hot melt glue holds the wedges in place. Because the sled is stiff, I don t worry about how level the surface is that it is resting on. Again, because it is stiff, I know it will stay flat as it goes through the planer. I do use roller stands on both infeed and outfeed sides to support it (I m working alone). Once made, the torsion box will save you time and a lot of futzing around trying to get everything straight and true. I hope this helps you make up your mind.
> 
> - bilyo


Thanks for staying with it all the way bilyo!

I think I will make a simple torsion box. I have a workbench that is not completely flat, but I am not ready to remake that and it's not long enough anyway, so I think I will need a dedicated box for this project anyway. I had put off doing this because it did seem intimidating, especially finding a flat surface to make the torsion box on. But, this is my reality. I want these planks flat for my kitchen table top, and the only way I'm going to get that is with a torsion box. I could try and shim my current sled flat on my counter top, but I would run the risk of inconsistent results since I would have to repeatedly reset it with every new plank.

So….off to YouTube I go to try and find the simplest torsion box design I can.

Thanks for everyone's contributions!


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## Lazyman (Aug 8, 2014)

BTW, I think that on The Wood Whisper's YouTube channel, he shows how to make a torsion box when you don't have a flat bench to build it on. I think it was the outfeed and assembly table episode. He uses saw horses and takes you through the steps to level it. Norm Abrams may have also shown that technique on the New Yankee Workshop while building the original version that TWW's table was based upon.


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## Andybb (Sep 30, 2016)

The Wood Whisper's YouTube channel, he shows how to make a torsion box when you don't have a flat bench to build it on.


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## bilyo (May 20, 2015)

I watched the first few minutes of the video and noted that he was using fairly heavy duty components. This is, of course, a good thing for a table. I probably don't need to say this but, I will: remember that your sled needs to be as light as you can make it in keeping with making it stiff. You will be carrying it back and forth a lot. I made mine out of cedar which is light and cut the grid members about 1/4" thick and used 1/4" ply top and bottom. Completed thickness is about 1 1/2". 
Good luck.


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## Andybb (Sep 30, 2016)

Mine is about the same. I used scrap 1/4" hard board for the top and bottom skins and some kind of mystery light weight scrap hardwood for the interior stringers. Mine is only 4' long so my cast iron table saw top was the perfect flat surface to build it on.


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## TravisN (Jan 15, 2019)

> BTW, I think that on The Wood Whisper s YouTube channel, he shows how to make a torsion box when you don t have a flat bench to build it on. I think it was the outfeed and assembly table episode. He uses saw horses and takes you through the steps to level it. Norm Abrams may have also shown that technique on the New Yankee Workshop while building the original version that TWW s table was based upon.
> 
> - Lazyman





> I watched the first few minutes of the video and noted that he was using fairly heavy duty components. This is, of course, a good thing for a table. I probably don t need to say this but, I will: remember that your sled needs to be as light as you can make it in keeping with making it stiff. You will be carrying it back and forth a lot. I made mine out of cedar which is light and cut the grid members about 1/4" thick and used 1/4" ply top and bottom. Completed thickness is about 1 1/2".
> Good luck.
> 
> - bilyo





> Mine is about the same. I used scrap 1/4" hard board for the top and bottom skins and some kind of mystery light weight scrap hardwood for the interior stringers. Mine is only 4 long so my cast iron table saw top was the perfect flat surface to build it on.
> 
> - Andybb


I appreciate the follow-up and recommendations. I will dive into those thoroughly. It seems most people either use thin plywood or mdf for the skins, and a mix of either mdf or scrap hardwoods for the web. I was looking at Keith Rust's design from FWW and he used dried 2×4s for the web, which surprised me because I thought those were notoriously unstable.

My biggest question in proceeding is what to use for components. I get the importance of building on a flat reference. I plan to mill some scrap 2×4's that I can shim to level on my workbench. Since the sled will only be about 12" wide, I can easily get some 13-14" pieces flat and square with my benchtop jointer and planer. I can set a skin on those and build up from there. But, the sled will be 8' long. There is no sheet product (MDF or plywood, especially in the thinner stocks) I can buy that will be flat over the 8' span. What good is building on a flat reference if the components I'm building with are not flat? Perhaps the inner components (i.e., web) keep the sled flat. But then again, I need inner pieces that are flat for 8'. It's another chicken-egg problem, isn't it?


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## controlfreak (Jun 29, 2019)

Why am I getting the feeling that you need to build a torsion box to build a torsion box. It reminds me of a video to build a router table and you guessed it, they were using a router table to finish some of the pieces.


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## TravisN (Jan 15, 2019)

> Why am I getting the feeling that you need to build a torsion box to build a torsion box. It reminds me of a video to build a router table and you guessed it, they were using a router table to finish some of the pieces.
> 
> - controlfreak


My sentiments exactly!


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## Lazyman (Aug 8, 2014)

This is the video I was thinking of. At about 7 minutes in it shows his approach to making one without already having a flat table to make it.


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## TravisN (Jan 15, 2019)

> This is the video I was thinking of. At about 7 minutes in it shows his approach to making one without already having a flat table to make it.
> 
> - Lazyman


I will watch that video when I get home. Conceptually, I think I understand how to shim a platform. But, if the materials for the torsion box are not flat (e.g., the skins, the inner supports), how do you make a flat torsion box? I ask because I can't think of any 8' material I could use for my box that starts out flat.


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## TravisN (Jan 15, 2019)

Hello again,

I thought it would be more polite to try and resurrect this thread than start another. I have one more question as I construct this torsion box planer sled.

I am using Keith Rust's basic idea, extended to 8 feet. 1/2" BB plywood skins, with three 1×1 poplar runners extending almost the full length and two 10" end runners spanning the width.

When I bought the poplar I had the store mill it flat for me so the resulting box would be flat. Well, I ran out of time and wasn't able to get back to the project for a week. So, either the poplar has moved and/or the lumber yard just planed both faces rather than flattening one side and planing the other. Even though I specifically asked them to flatten it, I suspect they just planed it.

Regardless, I ripped my poplar strips tonight and they have a definite cup in them. I imagine this is a problem. Surely the 1/2" BB skins will not force the 1" poplar flat, will they? What are my options? I can run them through my existing planer sled which does a pretty good job (leaves a cup of about 1/16") and will reduce overall thickness. I thought about flipping them so they sit on their edges instead of their faces. My rips are not 100% straight but they are less off than the faces. I could alternate the direction of the cup in the three runners (e.g., two down and one up), but I worry that will just create more problems.

Any recommendations? I assume there must be some tolerances for inaccuracies in torsion boxes because all the ones I've seen are made of wood and all wood moves. I just don't want to build this thing and have it be no better than my plain melamine sled that I started with.


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## davezedlee (Feb 22, 2016)

might be too late, but i use a sheet of plate glass as a flat surface for assembling, and it seems to work well

its one of the ikea ones that used to have "love" in a million languages (as if that matters), but is 10mm thick and 60×32"

lay it on a carpeted floor or just a mattress fully supporting it, and you're good to go

even better, it was free and glue scrapes off with a razor


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## dbw (Dec 2, 2013)

I would have to agree building a torsion box is the answer. This is on my "to do" list. I will tell you investing in a real straight edge was, for me, priceless. I don't mean a long ruler from a big box store. I mean a real straight edge. Wood River has a 38" straight edge for +/- $40. It is guaranteed flat +/- .003" along the entire length. Woodpeckers makes one as well except it is pricey. Using a real straight edge will eliminate one piece of guess work.


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## TravisN (Jan 15, 2019)

Thank you for the feedback. I have a solution for the flat assembly surface.

My problem is that the components for the torsion box are not flat. Once I attach the cupped poplar to the BB plywood skin, won't the skin now be cupped? I'm asking about the best way to get a flat torsion box without perfectly flat innards.

I assume the torsion box works best when the runners are continuous strips. If that doesn't matter, *I can break them down into shorter segments that I flatten on my bench top jointer. *That will allow me to preserve thickness and get them very flat. Would that work, or should they be continuous planks?


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## Lazyman (Aug 8, 2014)

If you had a chicken instead of an egg, you could run them through your planner with your jointer sled. Sorry, I couldn't resist. 

I think that I would take them back to the guys you paid to mill them for you and ask them to try again.


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## Bill_Steele (Aug 29, 2013)

My guess is if the wood you are using for the core of the sled is not flat, then the warp will translate to the bottom and top skins. You are using 1/2" plywood for the skins, so that may help some to reduce the bow.

I just finished building one of the sleds like Andybb shows above. I used Poplar as the core for mine and 1/4" MDF for the top and bottom skins. The wood was dry (6%-8%) and I made sure to joint and plane all the boards so that they were flat. I glued up the sled on the top of my workbench, which I made sure is flat. I now have a tiny bit of bow in my sled. If I push down on the top of the sled it will flatten out. I have not tested the sled yet to see how well it performs or if the bow is translated to the stock being surfaced.

I wish I would have used MDF as the core of the sled rather than a hardwood. I'm not a very experienced woodworker, so it's entirely possible I made a mistake along the way that introduced this warp to my sled. My thought is that MDF (or BB plywood) would be more dimensionally stable than Poplar and maybe that would give me a better chance of getting a truely flat sled.

I need to test the sled to see what the end result is. If the stock ends up with a little bow, then I'll probably make another sled and use MDF in the core.


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## bilyo (May 20, 2015)

> I assume the torsion box works best when the runners are continuous strips. If that doesn t matter, *I can break them down into shorter segments that I flatten on my bench top jointer. *That will allow me to preserve thickness and get them very flat. Would that work, or should they be continuous planks?
> 
> - Travis


No. They don't have to be continuous. They are only spacers to keep the two skins equally spaced from each other. The stiffness of the completed torsion box comes from the two skins that are held in place by the interior ribs. The interior grid can be made up of many short pieces glued to the two skins. You can cut the interior ribs straight and to consistent width on your table saw.

You can think of the torsion box as a lamination. If you lay 4-5 thin strips together dry and then bend them you will notice that they slide on one another (the ends no longer line up). But, if you glue them together, they become a rigid unit. Same thing with the torsion box. Only, you are making the interior "lamination" a light weight grid of thin pieces.


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## bilyo (May 20, 2015)

Sorry. I didn't notice your post #43 until after I finished my above comment #49. I think your plan is going to result in a very heavy torsion box. Although, not wrong. You can make it stiff, but still light, by using 1/4" ply skins with 1/4" grid pieces glued on edge. I say "grid" because that is what it should be; a grid of 1/4" spacers about 6" O.C. both ways. The height of the grid can be about 3/4" to 1" and, with the 1/4" skins, the overall torsion box thickness will be from 1 1/4" to 1 1/2". If the skins are completely glued to the interior grid, it should be plenty stiff.


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## clin (Sep 3, 2015)

Don't use ANY solid wood in the torsion box. As we all know wood moves, so even if the box is flat when you build it it will change shape with changes in humidity. Since you are already using 1/2" ply for the skins, use it for the webs as well. Or it sounds like you are just planning three long pieces (two sides and one down the middle).

Rip the plywood into strips on the table saw. If there is a straight factory edge on the plywood, that will be easy to do. If not, then you will need a temporary edge. For example, buy a melamine shelf board, as these tend to have nice straight edges. Screw this to your plywood so that this shelf edge rides against the table saw fence. Once you do this, you will have a nice straight edge on you plywood for ripping as many strips as you want.

Or of course if you have a track saw use that to get a straight edge.

MDF works well for torsion box webs as well. While even plywood and MDF aren't 100% free from wood movement, they will move much less than solid wood.

Also, I would still put some cross pieces in the torsion box. But again, these don't need to even touch the long "runners' as you call them. They just need to have a good bond to the skins. This will ensure the torsion box stays flat side to side.

I know many don't understand why the webs don't need to be joined. The mistake is thinking the webs are a frame holding the skins in place. What the webs do is hold the skins the same distance apart in all places. The geometry works such that if the two skins are held a fixed distance apart, they MUST stay parallel. And if the skins are bonded to the webs the skins cannot slide across the webs and therefore the skins cannot curve relative to each other. Or more specifically cannot change their curve relative to each other.


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## TravisN (Jan 15, 2019)

> If you had a chicken instead of an egg, you could run them through your planner with your jointer sled. Sorry, I couldn t resist.
> 
> - Lazyman


LOL!


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## TravisN (Jan 15, 2019)

> Don t use ANY solid wood in the torsion box. As we all know wood moves, so even if the box is flat when you build it it will change shape with changes in humidity. Since you are already using 1/2" ply for the skins, use it for the webs as well. Or it sounds like you are just planning three long pieces (two sides and one down the middle).


I had thought this too. The plan the Keith published used "dried 2×4 studs," which I believe are fairly unstable. So I assumed there was some dark secret that made it work for this application.



> Also, I would still put some cross pieces in the torsion box. But again, these don t need to even touch the long "runners as you call them. They just need to have a good bond to the skins. This will ensure the torsion box stays flat side to side.


Excellent. I was going to put one on each end, but now I think I should probably put a few more along the way since it is an 8' sled.



> I know many don t understand why the webs don t need to be joined. The mistake is thinking the webs are a frame holding the skins in place. What the webs do is hold the skins the same distance apart in all places. The geometry works such that if the two skins are held a fixed distance apart, they MUST stay parallel. And if the skins are bonded to the webs the skins cannot slide across the webs and therefore the skins cannot curve relative to each other. Or more specifically cannot change their curve relative to each other.


This sounds really important here, and I'm trying to digest it all. I was totally thinking of it like a frame holding skins in place. I'm not sure I understand your alternative explanation, though. I get that the web keeps the skins equidistant, i.e., parallel. But if that's the sole purpose, how does this differ from a single sheet of uniform thickness? Similarly, what does it matter then if my runners are cupped, so long as the runners are uniformly thick? I'm missing something. I'll keep thinking on it.


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## bilyo (May 20, 2015)

> But if that s the sole purpose, how does this differ from a single sheet of uniform thickness? Similarly, what does it matter then if my runners are cupped, so long as the runners are uniformly thick? I m missing something. I ll keep thinking on it.
> 
> - Travis


There is no functional difference. When making a torsion box, you are essentially making a plank with a hollow center to make it lighter but just as stiff. Have you ever torn into a hollow core door? Same principle. Years ago, I had to cut one open for some reason and found that the ribs inside were made of corrugated cardboard. The door, of course, was plenty stiff for it's purpose.


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## clin (Sep 3, 2015)

> This sounds really important here, and I m trying to digest it all. I was totally thinking of it like a frame holding skins in place. I m not sure I understand your alternative explanation, though. I get that the web keeps the skins equidistant, i.e., parallel. But if that s the sole purpose, how does this differ from a single sheet of uniform thickness? Similarly, what does it matter then if my runners are cupped, so long as the runners are uniformly thick? I m missing something. I ll keep thinking on it.
> 
> - Travis





> There is no functional difference. When making a torsion box, you are essentially making a plank with a hollow center to make it lighter but just as stiff. Have you ever torn into a hollow core door? Same principle. Years ago, I had to cut one open for some reason and found that the ribs inside were made of corrugated cardboard. The door, of course, was plenty stiff for it s purpose.
> 
> - bilyo


Bilyo has it spot on. A torsion box does NOT bring any magical stiffness qualities over a solid slab made from the same material. A torsion box is just a structure with most of the material that contributes very little to the stiffness and strength removed. It is therefore lighter.

Having said that, the reason woodworkers look at torsion boxes is becasue it is a way to make a structure very flat by using pieces that only need to be cut straight. For example, start with a big thick slab of solid wood, ignoring it isn't stable, how would you make it flat without a giant jointer or giant planer and sled. Yet if you can cut straight piece of sheet material AND have a flat surface to assembly it on, you can get the job done. Being lighter, especially for a planer sled, is helpful too.

Aside from the flatness of your sled, there are other considerations like the planer pushing down on the stock and you do need the skins to support the local pressure where shims and such are put. This is another reason to put in cross pieces. I'd do them every 6" or so.

Concerning visualizing how a torsion box works and why the webs do not form a frame that keeps in from bending, think of the skins like two sheets of paper. Do this experiment. Take a thick stack of paper, like 1" stack of printer paper. Get the edges all lined up, now bend the stack. Noticed how the edges of the stack slide around. How the side of the stack on the inside of the curve, smaller radius, necessarily has to slide out relative to the outside.

Now image if you had webs that didn't allow the paper to slide around, this would be similar to the sheets of paper being glued together. The shape cannot change.

An I-beam is like a one-dimensional torsion box. The outside flanges take the load. With a typical floor I-beam, the top flange is under compression and the bottom flange is under tension. The web of the I-beam keeps the two flanges parallel and prevents them from 'sliding" relative to each other.

If for some odd reason, you put a third, thin flange in the middle, it would do almost nothing to add strength. It would not be in compression or tension. It would just add weight. This relates to the concept of moment of inertia.

This is easy to imagine when you think of a beam under load and bending. The outsides edges (like torsion box skins) have to compress or stretch more than the inside. So for any structure, the further away a structural member is from the center the more of the load it will carry, all other things being equal. And to be clear, in a torsion box it is the skins that carry the load.

Of course all materials have some ability to stretch and compress, so just as a solid slab can bend under heavy load, a torsion box can also. It's just a torsion box is much lighter than the corresponding solid structure and is almost as stiff and strong.

Anyway, I hope the wall of words helps you understand it a bit.

As long as I'm on a roll here. Another thing is if you add a cleat at the end of the sled to keep your stock from sliding off the sled, this cleat goes on the end of the sled you feed into the planer first. I see many videos of people having the cleat on the trailing end of the sled. The fact that this works indicates a cleat isn't even needed. With typical planers, the stock is pulled through by rollers on top. So when the wood is on a sled, the wood is pulled through by the rollers and the wood is pulling the sled through. The sled is not pushing the stock through.

So if there is going to be a tendency for the wood to slip of the sled, it's going to try to slip off the front of the sled, not the back. The reality is the friction between the wood and sled is high. Maybe dabs of hot-melt glue on shims, or just sandpaper. While the bottom of the sled and planer bed are typically both waxed and therefore relatively low friction.


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## TravisN (Jan 15, 2019)

I'm not going to quote clin's response here for brevity sake, but let me thank you for the wonderful response. I imagine that took some time to write up, and it is really helpful. I appreciate the detailed information. It's easy to see that I'm an overthinking analytical guy. I really like to understand why I'm doing what I'm doing instead of just following a "recipe," and that's exactly the kind of information that helps me.

One final question: why is it so important to assemble on a super flat surface? If the web pieces are cut straight and uniform, won't the skins conform to the web pieces? I can see in extreme cases how a sagging/cupped or twisted assembly surface could transfer that shape into the torsion box (e.g., the web doesn't make seamless contact with the skin). But it seems like proper web pieces would overcome some small imperfections in the assembly surface, right? Many of the web tutorials show people really agonizing over this, busting out levels and winding sticks, etc. But a dead flat assembly table won't overcome twisted components, and I assume that flat and straight components will have more impact on final shape than the assembly table. This is important to me so I know how careful to be in my assembly preparation. My workbench top is a 6' MDF (3/4") panel. It may have been flat once, but now it is only flattish. I was going to square some 2×4 blocks and shim those on the workbench until a panel lays flat on those. I could probably spend a long time in getting them flat and level. How good is "good enough?"


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## FreshCutFarmhouse (Feb 29, 2020)

I have not done this personally, but I have seen people run a board wider than the jointer through the jointer and only flattened a portion of it. Then they take a flat piece of stock and place under the jointed section. Run the non jointed side up through a planer until its completely flat on top. Then flip it over, ditch the flat board and run it through the planer until its flat. Obviouosly, this process may cause more loss than you want though.

Or you could rip the Ash down to boards you jointer can candle to flatten, then glue them all back up.

Another option, is to fix the option after the fact. After gluing up the table, build a box for a router with a surfacing bit and run it across until you have a completely flat surface.


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## RichT (Oct 14, 2016)

> I have not done this personally, but I have seen people run a board wider than the jointer through the jointer and only flattened a portion of it. [...]
> 
> - FreshCutFarmhouse


I do that regularly. With my 6" jointer, I can easily mill boards up to 10" wide, and have done 11" successfully. There are some tricks to getting it right, but anyone can figure it out.


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## clin (Sep 3, 2015)

> One final question: why is it so important to assemble on a super flat surface? If the web pieces are cut straight and uniform, won t the skins conform to the web pieces? I can see in extreme cases how a sagging/cupped or twisted assembly surface could transfer that shape into the torsion box (e.g., the web doesn t make seamless contact with the skin). But it seems like proper web pieces would overcome some small imperfections in the assembly surface, right? Many of the web tutorials show people really agonizing over this, busting out levels and winding sticks, etc. But a dead flat assembly table won t overcome twisted components, and I assume that flat and straight components will have more impact on final shape than the assembly table. This is important to me so I know how careful to be in my assembly preparation. My workbench top is a 6 MDF (3/4") panel. It may have been flat once, but now it is only flattish. I was going to square some 2×4 blocks and shim those on the workbench until a panel lays flat on those. I could probably spend a long time in getting them flat and level. How good is "good enough?"
> 
> - Travis


Certainly straight-edged webs will create straight sections of skin. And these would certainly pull out any small scale curves in the skins. However, remember that unless the webs were unusually thick, they can twist and curve in ways that would create undulations of their straight edge.

Just as to some degree the skins themselves will span imperfections in the build surface. I.E., you could use a bed of nails without concern that the skin would droop between the nails. But of course you're looking for it being flat over relatively long spans of several feet.

But even a straight cut board over a longish distance can be forced out of straight with relatively minimal effort. Just think how much even a 2×4 in the flat orientation can bend under it's own weight. It's not a huge amount, but much more than you want in a flat reference surface.

It's simply easier to start flat build surface . Also, the normal technique is this first skin is just laying on the reference supports to create a temporary flat work surface. You wouldn't normally attach webs to this.

Most people do assemble the webs into a grid, getting them nice and flat on the bottom skin, but NOT attached to it. Then they glue and tack down the top skin on the web frame. Then flip this all over and attach what was the bottom skin to the web frame.

When I built my torsion box assembly table, I used MDF for the webs (and skins) and I cut the webs into long strips that were full length and full width. I then slotted the webs strips so they interlocked like cardboard separators in a case of beer. Again, the webs don't need to be joined to work in the torsion box, but that doesn't mean you might not want to tack them together just to hold them in place until you attached the skins. Sometimes a single web may want to stand proud of the other for some reason. You can push it into place and tack it with a brad or CA glue or whatever you like.

While I have been preaching that the webs don't even need to be joined, you could of course attempt to create perfectly square interlocking webs that would self align to be flat. But has ANY woodworking project had joinery that accurate and tight that it would self align nearly perfectly to form a reference surface. I don't think so. I don't even think if the frame could be assembled flat that it would stay flat under it's own weight. That's what the skins are for.

I'm pretty sure the "Wood Whisperer's" torsion box build has been linked in this thread, if not you won't have any trouble finding it. Some version of his approach is a good way to go. I wasn't kidding earlier when I said making the reference surface to build on is the larger effort. This is also why I originally recommended building a large flat torsion box assembly table. Once you have that, you're good to build all manner of flat things on it.

Concerning good enough, that of course is up to you and how good you want this to work as a planer sled which is ultimately functioning as a jointer. You said you want an 8 ft sled. Obviously there's no need to have it flat over 8 feet to an error 0.010' of an inch when a few ounces of pressure over 8 feet would easily bend any board into the desired position. But flatter is better. In may case, I have a 4 foot aluminum level that I think claimed to be straight to something like .001" and using this and a set of thickness gauges I think I had mine flat to less than .005" at any point over any 4 foot length.

In your case, you won't have trouble keeping it flat across the relatively narrow width of the sled, but you have to be carefully not to build a twist into it over the length of the sled. It would be much easier to start with a flat build surface, than to try to shim and adjust every web to be flat before attaching the first skin and praying something didn't shift right after you applied the glue and the clock started ticking.


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## Unknowncraftsman (Jun 23, 2013)

With a flat reference surface you will end up with two parallel curved,cupped,twisted or bowed surfaces.
If you don't have a jointer learn the ways of hand planing. 
Those planer sled are a waste of time plus they don't stay flat and are cumbersome to store.
Building a hand tool cabinet is a good use of shop time. Always


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## clin (Sep 3, 2015)

> With a flat reference surface you will end up with two parallel curved,cupped,twisted or bowed surfaces.
> If you don't have a jointer learn the ways of hand planing.
> Those planer sled are a waste of time plus they don't stay flat and are cumbersome to store.
> Building a hand tool cabinet is a good use of shop time. Always
> ...


A planer sled takes up a lot less room than a jointer. My sled is just a shelf board I hang on the wall when not in use. That's hardly cumbersome. I shim my stock on this while laying on a flat assembly table. A table saw table can be used if it has been adjusted flat. Or as the OP is looking to do, making a torsion box sled so he doesn't need to setup the planer sled on a flat surface. I well built torsion box will stay plenty flat.

It works fine for what it is and that is a perfectly acceptable alternative to a jointer. Just as a hand plane is another alternative. It is not as convenient as a jointer, but it has it's place for those of us who don't have room for a jointer or don't use one often enough to want to make the investment in dollars or space.


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## Unknowncraftsman (Jun 23, 2013)

I just don't agree.
It's almost impossible to describe how much more a jointer can bring to your work flow.
Nothing makes a flat surface to a square edge as fast and accurate a decent jointer.
I guess if your passion is working with plywood i could why there wouldn't be space for a jointer.

Good Luck everyone.


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## TravisN (Jan 15, 2019)

> With a flat reference surface you will end up with two parallel curved,cupped,twisted or bowed surfaces.
> If you don't have a jointer learn the ways of hand planing.
> Those planer sled are a waste of time plus they don't stay flat and are cumbersome to store.
> Building a hand tool cabinet is a good use of shop time. Always
> ...


Thank you again Clin! I'm "off to the races" so to speak. Ready to get this done!


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