# Router sled to flatten slab - how best to place the wood?



## LiveEdge (Dec 18, 2013)

I've watched a number of videos on using a router sled and am comfortable with the concept, but one thing that never seems to get mentioned is how to decide how to lay the slab down before you get going. After looking at my slab and doing some surveying with a straight piece of MDF it appears I have some twist. I suppose I could just put the slab down as it lies and start chewing away, but wondered if this is the best way to go about it. Would it be better to initially shim one high corner so as to "split the difference" with how much wood comes of each end? or does that not work because I'll wind up taking off just as much when I do the other side?

Is it possible that one side has "twist" while the other has cupping? It may actually be a product of exactly how the slab was cut, but it seems like one side has two opposite high corners while the other seems to have more high edges. If this is the case, is there any advantage to flattening one particular side before the other?


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## Mahdeew (Jul 24, 2013)

Think of your router sled as a joiner, it flattens one side so the other side can be of the same thickness once planed. For me, I would rather placing the slab on its cup side up, brace it in place with boards and shimmy it so that the two high sides measure the same compared to the board it lays on. You may want to screw in the shims so they don't move from all the movements and vibrations.


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## LiveEdge (Dec 18, 2013)

Excellent. I was hoping you'd chime in jinx.  You seem to be a man in the know…

That makes sense to me.


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## Mahdeew (Jul 24, 2013)

Thanks.. I have been doing a lot of thinking and research about this as well since I have about 46 walnut and cherry slabs most about 26" wide. Here is the sled on my wishlist.


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## LiveEdge (Dec 18, 2013)

waitaminute…you've only thought and read about this?


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## LiveEdge (Dec 18, 2013)

BTW, I've decided to basically do this on my garage floor which is as flat (read: flatter) as any table I have. That also allows me to use a sled without the "parallel tracks" that I see in videos like the one done by the Wood Whisperer. The flat floor will be my parallel track and the sled will ride along that.


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## Mahdeew (Jul 24, 2013)

LOL, Yes.. So far, my hand plane has been doing the job just fine with small projects 22"wide by 48" long. Not sure how big is yours. I plan to make a conference table 52" wide 8' long. I would need a sled for that.


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## Mahdeew (Jul 24, 2013)

Maybe caulk the shimmies to the floor or screw them to the backside of the slab.


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## LiveEdge (Dec 18, 2013)

The desk is going to be about 36×70 so not quite as big as your table, but I don't have the arms to take that down a few thousands per stroke…


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## OSU55 (Dec 14, 2012)

Never tried the router sled using the garage floor, but it should work fine for rough flattening. I know my floor is not quite level enough for final passes. I agree with thinking of the set up as a face joiner, with the floor as the table. I used 1 x something side rails shimmed out from the piece being planed and screwed into the side of the piece. I could then measure down from the cross slide and move each corner of the cross sled plane. Shims on the floor do the same thing. A hot glue gun might be the ticket for sticking the shims.

I gave up on the router sled approach. It created a lot of tear out and required a lot of work to get the tear out and cut lines out. However, I was planning glued up panels for table tops. I switched to hand planes for that work. For rough flattening a large, rough slab piece like you have, I think I would go with the sled route again, and use hand planes to finish it out.


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## CharlesA (Jun 24, 2013)

I've had great luck with router sled on slabs. If you have to choose between cup up or down, I'd say cup up. The slab is more stable that way. You get the first side flat (usually after multiple passes-haven't had problems with tear out). Once that side is flat (or at least a large enough section is flat-I like leaving some parts on the edges a little uneven), then you can flip it over do the other side.

First slab I did I later wished I had fixed the shims in-they kept slipping. Last slab was concave on one side so it was stable on my table to flatten the other side.


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## ADHDan (Aug 17, 2012)

I've also had good results flattening oddly shaped or wide slabs with a router carriage. I secured the slabs a few different ways, depending on what I was eventually going to do with them.

If you don't mind tiny holes the slab (either on one face or both sides), you can secure it by screwing up through bottom of the base into the bottom face of the slab, or through the sides of the base (or some other anchoring point) into the sides of the slab. The number of screws you'll need depends on the size/shape of the slab, but I don't think you need to punch more than 1/16" or 1/8" into the workpiece to anchor it pretty solidly.

If you don't want screw holes in your slab, shims work just fine and double-sided carpet tape can help keep the slab, shims, and base all attached to each other to prevent sliding. It also helps if your base and shims aren't made of slippery-smooth material (maybe veneer the base with high-grit sandpaper?).

Another way of looking at this is to check out what people have done for face-jointing planer sleds. There are lots of different designs, and they all accomplish the same thing you need for using a router to face-joint: provide a stable base that prevents the workpiece from shifting or rocking, so that the overhead cutter creates a perfectly flat surface.


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## gfadvm (Jan 13, 2011)

Jinx, Thanks for the link to that sled! I really like the design and I have all the metal needed.


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## Nomad62 (Apr 20, 2010)

I would recommend doing just what you are asking about, to average out the imperfect surface rather than to just cut it however it sits… if that will get you the thickest piece. I cut odd pieces of log and stump, I often find the imperfection is in one corner but sometimes the whole thing is twisted or cupped, and either way requires a different set-up to get the thickest piece when you are done.


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## LiveEdge (Dec 18, 2013)

I'm hoping to do this over the weekend. I'll probably have to have a beer first to get the courage to hit the power switch on the router.


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## Mahdeew (Jul 24, 2013)

gfadvm, welcome.. It doesn't look all that difficult to make. I am thinking about using square "pipe" instead of round ones. Also, plan on welding a nut to a 2" over-sized pipe with a hole in it that is welded to the runners so I can adjust both the support beams and the top part by tightening the bolts to the legs. Still have to figure out how to secure shims to it.

LiveEdge, if you screw up, you can always blame it on the beer… Good thinking!


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## LiveEdge (Dec 18, 2013)

Well, I put in about six hours today. but half of that was going to get supplies, building the sled, setting up the tracks (I decided to do it on a table after all). I took about four passes (two on each side) and it is definitely much flatter than when it started. The thinnest corner is still 13/4 inch but there are parts closer to two inches. I think there are a few issues that I may be able to resolve by tomorrow.

1) It's still hard to figure out what the best plane is to set the tracks on. I'm good at making the tracks parallel to each other, but I'm have difficulty deciding exactly which parallel plane to use.
2) because either my router doesn't plunge very far or the shank on my bit isn't long enough I have to periodically lower the tracks. This compounds the possibility of having the planes out of parallel; especially the planes on each side of the slab. I noticed at the end of the day that one end was thicker than the other so I will need to take this into account tomorrow.

All in all it could be going worse. . I may not get things perfect, but I think I'll get them close enough. I'll post some pictures in a few seconds.


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## LiveEdge (Dec 18, 2013)




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## Mahdeew (Jul 24, 2013)

May have to do the rest by hand plane.


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## LiveEdge (Dec 18, 2013)

Coming along. The top is pretty flat and I took some 80 grit on the belt sander to it. There are still a few marks that will need more 80 grit including an "oh crap"'moment where one of the screws holding one of the tracks somehow worked its way loose and the sled came crashing down allowing the router to make a nice divot. Doh!

The underside isn't going to be perfect. On of the corners falls away, but I decided to leave it rather than keep taking width off the slab. I'll just have to take it into account when mounting on the desk posts.


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## Mahdeew (Jul 24, 2013)

That is one beautiful piece of wood. Glad it worked out for you.


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