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I've built a used the router sled for flattening slabs made famous by Nick Offerman a few years back. Multiple YouTube videos now on how to build them correctly, including one by Marc.  Im having an issue with mine that I've not seen addressed in any of the videos that I've found.  I have a 4 foot wide slab in the shop, which means my sled is about 5 feet long.  Im having issues with the sled sagging in the middle from the weight of the router, causing a slight dip in the slab in the middle.  Im not pressing down with any more or less force in the middle vs the ends, so I think I need to make a change to the structure of the sled (all made with 3/4" ply). Anyone have a similar issue or know what I might do to fix this? Thanks guys!

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Just gonna throw my two cents at this while we wait for other, likely more intelligent, contributions.  

It sounds like you have 3/4 inch plywood spanning a 4 foot distance.  And the plywood sled is just not stiff enough.

You could increase the stiffness by adding steel angle extrusions along the front and back edges of the sled.  These are generally straight (check against a level) and can be had in various thicknesses (i.e. stiffnesses).

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Either option above should have you set fine. The goal just being to stabilize the whole bottom from sagging. 

I personally would not want to have 5" sides on the sled as I alredy get a lot of material sticking up there during large flatterings but it would work. A few bucks of angle iron spanning to support would also be fine.

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16 minutes ago, Brendon_t said:

Either option above should have you set fine. The goal just being to stabilize the whole bottom from sagging. 

I personally would not want to have 5" sides on the sled as I alredy get a lot of material sticking up there during large flatterings but it would work. A few bucks of angle iron spanning to support would also be fine.

Need to make sure the angle iron is cold rolled. Hot rolled isn't going to be strait. May look strait but it could wave enough to leave marks on the slab that would lead to more work later.

5" is drastic the jig shouldn't sag at 3.5 - 4" from the weight of the router, if it does you need to buy a lighter router. Other option if you don't want to have more material sticking up is to tag it on to the sides with glue and brad nails. doubling the sides will also benefit but again 3" tall.

This guy is always helpful

http://www.woodbin.com/calcs/sagulator/

Using sagulator with 35lb router (is this realistic?) 6" wide base with 3.5" tall sides sags .02" if you double the sides the sag is eliminated. If you increase the sides to 4.5" the sage is eliminated. If you double up the bottom the sag goes to .01"

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48 minutes ago, drzaius said:

Post a pic of your sled & then we can provide more informed suggestions.

Nothing special obviously, but here is what I put together.  Thanks for all the input.  If possible I'd like to avoid having to add any angle iron to it, but not against it completely.  Router is a basic Skil combo base. Around 15 lbs.  Current height of my walls are 3". Glued and screwed the bottom up into the walls. IMG_1933.thumb.JPG.8745883f72c716fe19ac7c144bfceacb.JPG

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I can confirm what's already been said; I also built the Offerman sled; stiffness came from the walls.  I also tried angle iron - more sag than the wood version. 

One tip, in case it helps:  a medium size plunge router is probably better than a large fixed base: less sag and more cutting tool extension. 

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53 minutes ago, Pondhockey said:

 

One tip, in case it helps:  a medium size plunge router is probably better than a large fixed base: less sag and more cutting tool extension. 

Plunge yes if at all possible but not going to the lighter side. My slab flattener is about 3" across. I want the router it's in to have as much ass as possible.  I know I can build the sled to not flex, I also want as much weight in the tool spinning the bit for stability.

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I have a small, 24” x 36” slap I need to flatten. After building the jig and attaching it to my work table, do I secure the slab to anything in any way to prevent movement. I’m assuming that I need to shim on the bottom to prevent anykind of rocking. 

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I shim with playing cards. I tape equal amounts under the opposite  high corners. You can run a sled through a drum sander with 36  grit or use a  router sled to flatten one side. 

I flattened & sanded both sides  of a 20" x 38" x 3" slab of 200 year old cypress the other night in about 90 minutes. 36 grit then 60, 80, 120, 150, and dressed it w 220.  Big twist on a longer slab and I would definitely use a router sled first. Maybe 65 or 70% of the surface flat and you can switch to a finer grit.

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1 hour ago, wdwerker said:

I shim with playing cards. I tape equal amounts under the opposite  high corners. You can run a sled through a drum sander with 36  grit or use a  router sled to flatten one side. 

I flattened & sanded both sides  of a 20" x 38" x 3" slab of 200 year old cypress the other night in about 90 minutes. 36 grit then 60, 80, 120, 150, and dressed it w 220.  Big twist on a longer slab and I would definitely use a router sled first. Maybe 65 or 70% of the surface flat and you can switch to a finer grit.

I hate changing the paper on my Jet sander. 

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You check your sled with a straight edge while it doesn't have anything just to make sure it's flat to start with?

If it really is sagging (check with straight edge after putting weight in middle) then just get some more plywood strips to reinforce/extend the edges as mentioned above. Just screw them into your current sides.

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23 hours ago, wtnhighlander said:

Coop, you might use some carpet tape to hold your slab to the bench, so it won't slide around. 

Good idea. Or, I could take it to Atlanta:D

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I just wish the jig was the worse part of the leveling. After an hour of flatening, my right hand is in the shape of the Bosch handle. 2.25” to 3/4”, steady as she goes mate! 

76A725F1-01F6-4FDA-8E5F-D49B52214679.thumb.jpeg.deec123c6ef32d20399aa8fa86165b8e.jpeg

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I would think 1 7/8 - 2" should  be achievable.  If you take that slab down to 3/4 on purpose or out of ineptitude I will be ashamed of you. And the likelihood of it moving considerably will be substantial. 

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