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Curved Edge on Long Panel Cut, Using Table Saw, Help?

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Forum topic by clin posted 06-18-2016 12:12 AM 933 views 0 times favorited 12 replies Add to Favorites Watch
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clin

510 posts in 458 days


06-18-2016 12:12 AM

I just cut a 13” x 72” piece of 1/2” MDF in half, lengthwise. I was trying to get a very straight cut, so I used my clamp-on straight edge to ride against the fence.

The edge on the piece between the blade and fence came out perfectly straight, as measured against that same clamp-on straight edge. But the other side curved slightly. By slightly, I mean 0.010” gap in the middle when the ends are touching the straight edge.

So this other edge has a slight curve in it. I’m wonder if anyone has any idea why? My guess is that the offcut piece turned toward the blade as I was ripping it, and that the back of the blade cut into the offcut very slightly.

Again, this is MDF which I believe has next to zero internal stresses.

Also, I did some other checks, using both sides of the clamp-on straight edge, to make sure it wasn’t the straightedge with a slight curve. And also using a shorter 4’ straight edge that is supposedly accurate to 0.0005”.

A few more notes, the MDF cuts like it wasn’t even there. I had to apply almost no pressure to feed it through. But I kept the feed rate normal, focusing on keeping things tight to the fence.

Any idea why, and is there a technique to avoid this?

-- Clin


12 replies so far

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JeffP

573 posts in 854 days


#1 posted 06-18-2016 10:56 AM

Do you have a riving knife or splitter in place?

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

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simmo

58 posts in 2934 days


#2 posted 06-18-2016 11:06 AM

Is the curved edge uniform in width, if so the internal stress may cause the bowing, if not the blade has removed the material and the feed may have not been perfect, not many things are truly inert.
sept me when drunk,
Chris

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Lazyman

690 posts in 849 days


#3 posted 06-18-2016 01:13 PM

Even MDF may bow al little after cutting. The piece away from the fence is probably just rubbing against the back side of the blade as it passes and closes in after the cut. A riving knife or splitter should help that. Also, the riving knife or splitter needs to be as close to the same thickness as the blade as possible (but never thicker). Also make sure that it is properly aligned with the blade. If it is not straight it can deflect or bind a little. Be careful not to push against the away piece behind the blade with your push stick and if using a feather board, it should be in front of the blade.

One last thought…MDF usually has a very straight factory edge so if it wasn’t straight to begin with that could possible indicate some deformities in the MDF?

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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Nubsnstubs

826 posts in 1192 days


#4 posted 06-18-2016 02:13 PM


I just cut a 13” x 72” piece of 1/2” MDF in half, lengthwise. I was trying to get a very straight cut, but the other side curved slightly. By slightly, I mean 0.010” gap in the middle when the ends are touching the straight edge.

Also, I did some other checks, using a shorter 4 straight edge that is supposedly accurate to 0.0005”.

Any idea why, and is there a technique to avoid this?
- clin

My thoughts, and you can ignore them if you feel offended…...

If all you got was .010” bow on a 72 inch piece of wood, any wood, man made or natural, should be thought of as a blessing instead of a detriment.

Using a straight edge with a tolerance of half a thousandth for wood? Wood reacts when cut, whereas metal, plastic or glass doesn’t. It’s wood and despite the fact the tree doesn’t have it’s toes in the soil doesn’t mean it’s completely dead. It’s alive and will keep moving until you turn it into ashes or dust, then the wind will finally disperse it back into the soil. So, sell your straight edge to a machinist, if you haven’t dropped it yet, and get yourself a 1×1” piece of square metal tubing to use for checking straightness. It’s just as effective, and if you drop it, it’ll still be as accurate as before unlike that square with the .0005” tolerance that loses it’s accuracy after the first drop….......

I sure hope this help you in your current predicament. ................. Jerry (in Tucson).

-- Jerry (in Tucson)

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johnstoneb

2143 posts in 1635 days


#5 posted 06-18-2016 02:33 PM

x1 nubs

-- Bruce, Boise, ID

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clin

510 posts in 458 days


#6 posted 06-18-2016 05:35 PM

Of course I realize that a 10 mil error over 72” (actually the error is really 10 mil over 36”), is pretty small. It’s just that this was only on the offcut. So it seems unlikely that it was the material relaxing.

There was a splitter. But again being such a small error, and the splitter has to be a hair thinner than the blade, I don’t think there is anyway for a splitter to help this. Unless it was in fact exactly the width of the blade.


Be careful not to push against the away piece behind the blade with your push stick and if using a feather board, it should be in front of the blade.

One last thought…MDF usually has a very straight factory edge so if it wasn t straight to begin with that could possible indicate some deformities in the MDF?

- Lazyman

When you say behind the blade, in my mind that means on the outfeed side. Though I get confused because, for example, when talking about crosscut sleds, people call the front fence the farthest away, and the back or rear fence the one nearest my belt buckle (that always seems backwards to me). Front is what is towards me back is what is away from me.

I certainly did not touch the offcut or anything else on the outfeed side (what I would call behind the blade). I was gripping both sides of the assembly out in front of the table saw. Remember it was a 13” wide piece with a clamp-on straight edge on top of it. Ripping it in half.

The MDF cuts like it isn’t even there, so it was very easy to guide it through. As I neared the end of the cut, I don’t know exactly what I did. The cut was about 7” wide, so no push sticks involved. I’m sure I pushed the piece between the blade and fence through with my hand at the trailing edge. Mostly pushing on the end of the clamp.

I probably pushed the offcut as well. And I do make a point to not push it off center. But at this point 95% of the cut is already done anyway. So that wouldn’t account for much of this.

Again, this was a pretty gentle sweep or curve off of straight throughout the entire cut.

You’d think the offcut would drag a little and if anything try to swing away from the blade.

Maybe this is as simple as some slight irregularity in the outfeed table pulling the offcut to the side.

At this point I don’t even know if this is repeatable. But I’m getting ready to rip a bunch of strips for a torsion box build. I’d prefer to be able to just rip finished strips off the longer panels and not have to run them through again to straighten an edge using the clamp-on straightedge. And then of course run it again, to get the other side parallel.

-- Clin

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Lazyman

690 posts in 849 days


#7 posted 06-18-2016 06:58 PM

I think about front and back of the saw and blade the same way you do. The front of the saw and blade is the part closest to where I stand but suppose that the front of the board (or a sled) might be the part that goes through the blade first.

Anyway, after seeing Jerry’s post, I just reread your OP and realized that you said 1/100th of an inch. When I responded, in my mind I was thinking 1/10th. .010” is about the same as 3 sheets of 20 lb printer paper and is not going to normally be a problem when working with wood, especially when you spread that over half of a 72” board. In fact, if you got only 1/100th of an inch over 36”, that pretty darn precise. Wood working tools almost never have that sort of precision. Even for a torsion box, because of its design, this should not be a problem. If you need that sort of precision, wood is probably not the right material. My suggestion is to not worry about it unless it actually causes a problem.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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clin

510 posts in 458 days


#8 posted 06-18-2016 07:54 PM



I think about front and back of the saw and blade the same way you do. The front of the saw and blade is the part closest to where I stand but suppose that the front of the board (or a sled) might be the part that goes through the blade first.

Anyway, after seeing Jerry s post, I just reread your OP and realized that you said 1/100th of an inch. When I responded, in my mind I was thinking 1/10th. .010” is about the same as 3 sheets of 20 lb printer paper and is not going to normally be a problem when working with wood, especially when you spread that over half of a 72” board. In fact, if you got only 1/100th of an inch over 36”, that pretty darn precise. Wood working tools almost never have that sort of precision. Even for a torsion box, because of its design, this should not be a problem. If you need that sort of precision, wood is probably not the right material. My suggestion is to not worry about it unless it actually causes a problem.

- Lazyman

I’m just trying to make the table as flat as I can. I don’t have a specific requirement. And since I saw this on one rip cut, I thought I’d ask if there might be some common reason for this and some better technique I need to learn.

As it is now, I have another long piece to rip in half, and will do that and see if this repeats. Since I will be using the ripped edge up against the fence, for the next cut, there is potential for accumulated error.

-- Clin

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Lazyman

690 posts in 849 days


#9 posted 06-18-2016 10:43 PM

Again, I don’t think it will matter. While plenty of guys spend a lot of time getting their fences aligned to within .001” of their miter gauges, most rulers are only marked to 16ths. You occasionally see a 6” or 12” rulers marked at 32nds (3 times your error) and 64ths (1.5 times) but almost anyone over the age of 42 needs readers to see much less mark and saw to those markings. If you use a 5 mm lead mechanical pencil, the narrowest line you can draw is 0.19” wide—19 times wider than the slight deviation you are worried about.

Remember that you are working with a medium that flexes, expands and contracts during every phase of the process and for years after you complete whatever projects you build with it. You’ve actually achieved a level of precision that most of us don’t even shoot for when running a large piece of wood by hand across a spinning blade that can easily flex more than your margin of error. Heck, I’ll bet my 3’ steel ruler has more deviation over its length than your cut.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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Loren

8301 posts in 3110 days


#10 posted 06-18-2016 11:14 PM

Sheet goods have internal stresses. MDF does this
banana thing often in my experience. If the warp
cannot be clamped out in assembly due to how
the part is used, I like to rip it a bit oversized and
give it a night to stabilize.

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TheFridge

5765 posts in 948 days


#11 posted 06-19-2016 01:20 AM

.010 is a noticeable gap in most situations.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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clin

510 posts in 458 days


#12 posted 06-19-2016 01:44 AM

I completed the other cuts, and didn’t have as much of an issue. I think my worst case is less than 0.005” after trimming that first edge. And most of the pieces have no measurable gap between the MDF pieces and my straightedge. Which is actually pretty cool. I’ve got a good saw, and since the material cut like it wasn’t there, it made it easy to hold it tight the the fence.

Perhaps the first cut issues was due to internal stress, or some other issue with how I moved the offcut. But it doesn’t appear to be “built in” to my methods.


.010 is a noticeable gap in most situations.

- TheFridge

I agree, if finished work had a 0.010” gap, it is quite visible. But others are correct that this was the maximum deflection over a 72” length. So in that sense it is splitting hairs. And in this application it isn’t a question of something being seen, but of how truly flat the finished surface is.

I think things are on track for this to come out really well. It will be interesting to see just how flat this torsion box is when I’m done. This application isn’t that important, but it’s certainly good exercise trying to cut pieces accurately.

-- Clin

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