Torsion Box Planer Sled Issue-Please Help!

[edwarmr]

So I just made a torsion box planer sled like the one pictured below. I shimmed my work surface (MFT/3) to make it flat. When I glued up the base on the MFT I checked the base for flat using a precision straight edge and it seemed flat. After letting the glue dry and rechecking everything it is definitely bowed (lower in the middle by ~1/16”). I realize this isn’t a huge amount but the purpose of the sled is getting a perfectly flat face on my lumber so I’m hoping to correct it without starting over.

Here’s my question: If I glue another layer of MDF to the base and make sure my surface is dead flat when I clamp it will it stay that way when I unclamp it or will it warp back to the original shape?

Any help is greatly appreciated. Thanks guys!

[attachimg=1]

 

[edwarmr]

Anybody? 

 

[Michael Kellough]

Post a picture of what you’ve actually made. Also give the dimensions.

 

[edwarmr]

Post a picture of what you’ve actually made. Also give the dimensions.

Will do. I work this weekend so I won’t be able to get a picture until Monday. The dimensions are approximately 48”L x 12”W x 1-1/4”H. I’m sure part of the problem is that a torsion box that is only    1-1/4” thick probably doesn’t give a ton of rigidity. The bottom skin in 1/4” MDF; the webbing is 3/4” MDF; and the top skin is 1/4” MDF. It’s based off of plans from Woodsmith.

 

[Michael Kellough]

It’s not easy to build something truly flat.
Did you check the flatness of the MFT after adding the weight of materials and clamps?

Maybe you can use tall straight cauls to span the structural parts of the MFT to support the sled material.

With the flat support structure sorted out I’d crosscut the existing sled skins a bunch of times top and bottom then laminate additional layers of mdf.

 

[Svar]

Planer rollers will likely flatten out a 1/16" bow. Moreover, torsion box is unnecessary for a planer sled. In this application it offers no advantages over a simple strip of MDF.
I doubt gluing another layer will fix anything, as it may warp again unpredictably. I'd try to plane it flat with whatever means available.

 

[edwarmr]

It’s not easy to build something truly flat.
Did you check the flatness of the MFT after adding the weight of materials and clamps?

Maybe you can use tall straight cauls to span the structural parts of the MFT to support the sled material.

With the flat support structure sorted out I’d crosscut the existing sled skins a bunch of times top and bottom then laminate additional layers of mdf.

You can say that again  [laughing] It's super tough to keep something dead flat. I checked for flat after adding everything but I may have made a mistake (maybe my lighting wasn't right and I didn't see a gap under the straigtedge). I also only have a 38" straightedge so a longer one would help. I just ordered a 50" one so that will definitely help me in the future.

And good idea about crosscutting, thanks! That would probably take some of the stress out and make it more likely to hold if I glue another layer.

I appreciate the advice

 

[edwarmr]

Planer rollers will likely flatten out a 1/16" bow. Moreover, torsion box is unnecessary for a planer sled. In this application it offers no advantages over a simple strip of MDF.
I doubt gluing another layer will fix anything, as it may warp again unpredictably. I'd try to plane it flat with whatever means available.

[member=15585]Svar[/member]
Thanks for the input! I'm trying to picture what you're saying. The planer rollers will definitely flatten the bow and that was actually my concern. I believe that would also mean the board that the planer sled is helping flatten will be forced down by 1/16" of an inch and will return to it's original somewhat warped shape after planing. If I'm understanding this wrong please correct me.

And you could very well be right about laminating another board to the bottom. The only tough thing about planing the skins of the torsion box flat is that they are only 1/4" thick. I suppose I could glue another piece of MDF to the bottom just to give me more to work with when planing it down.

 

[Birdhunter]

I’m totally at a loss as to why this device is needed. It looks unlikely to successfully resist the planer roller pressure or the torque a warped board would exert.

What are you trying to accomplish with the sled.

Sorry for being so dense, but I truly don’t understand.

 

[ChuckM]

I’m totally at a loss as to why this device is needed. It looks unlikely to successfully resist the planer roller pressure or the torque a warped board would exert.

What are you trying to accomplish with the sled.

Sorry for being so dense, but I truly don’t understand.

It came from the Woodsmith site which I believe to be a variant of this:https://www.finewoodworking.com/2005/10/25/a-planer-sled-for-milling-lumber

WS:https://www.woodsmithplans.com/plan/double-duty-planer-sled/

I myself use this sweat-free method (no need to build or use a heavy, bulky jig) - image shows some 80 bft planed in a session (4" - 8" wide & 5' - 6' long):
https://m.youtube.com/watch?feature=youtu.be&v=8UONmuQt_98

 

[edwarmr]

I’m totally at a loss as to why this device is needed. It looks unlikely to successfully resist the planer roller pressure or the torque a warped board would exert.

What are you trying to accomplish with the sled.

Sorry for being so dense, but I truly don’t understand.

The idea is to get a dead flat planer sled. Once you do that you can place warped boards on top of the sled and using adjustable leveling bars you can stabilize your warped lumber so the feed rollers won’t compress it. This is critical to getting one flat face (basically an alternative to having a jointer). Once you have one flat face you can take your board off the sled and run it through the planer flat face down to get your second face flat and parallel to the first.

[attachimg=1]

 

[ChuckM]

It’s not easy to build something truly flat.
Snip.

Many woodworkers spend lots of money on accessories, trying to get things "truly flat", "truly square", "truly straight" or "truly out of wind", with some only to find it either truly unachievable or truly not mattering as once the build is subject to use or environmental changes (humidity, for example), "truly whatever" is not really that critical in a lot of cases.

 

[Birdhunter]

I just glued plywood “runners” to each side of a warped board the runners were placed on top of my table saw bed so they were perfectly level. The runners left about 1/8” of the board proud. I then ran the board, runners down through the planer thus a perfectly flat top. I cut the runners off on the table saw and ran the board back through the planer flattened side down. I then had a totally flat non-warped board.

 

[Michael Kellough]

The purpose of such a sled is to joint boards with a thickness planer.

It does this by providing a place to set shims to support the stock.
The thinner the stock the more shims are required to prevent the stock bending under the rollers and cutter head.

The ideal sled is dead flat, rigid, lightweight, and slippery on the bottom.

In theory a sheet of laminate would work. If the shims are adjusted when the laminate is supported by a truly flat surface (a good workbench) that’s all you need to properly support the stock as it passes under the rollers and cutter head. But in practice, since it has no rigidity, you’d need something else just to support the plane of shims to get it from the bench to the planer. A single piece of 3/4” mdf might be enough. It just needs to be even in thickness and stiff enough to maintain the shim to stock relationship.

The Shopnotes sled seems unnecessarily elaborate and it uses a lot of the thickness capacity of the planer. The method ChuckM uses is efficient.

You don’t need a torsion box to make a planer sled but I do see the utility if you want to have movable shims ala the Shopnotes design. A planer sled does not need to be stiff along it’s length like a surfboard. It just needs to be stiff enough that the stock won’t get dislocated relative to the shims when the whole package is moved from the bench to the planer (that’s why the laminate alone is not a good sled).

 

[Michael Kellough]

I just glued plywood “runners” to each side of a warped board the runners were placed on top of my table saw bed so they were perfectly level. The runners left about 1/8” of the board proud. I then ran the board, runners down through the planer thus a perfectly flat top. I cut the runners off on the table saw and ran the board back through the planer flattened side down. I then had a totally flat non-warped board.

That’s really clever but it’s a one-off solution and you have to wait for the glue to dry. If you have a bunch to stuff to joint you want a reusable jig that is quickly adjustable to suit the different boards.

 

[Birdhunter]

That’s true about The runners not being a “production” method. The advantage is that there isn’t any truing that has to be done.

I guess if I was doing a number of boards, I’d use hot glue to attach the runners. To me, that would be preferable to using a jig that requires a set up for each board.

That’s what makes this forum so great, you get exposed to so many ways to solve a problem.

 

[ChuckM]

Sniop.

I guess if I was doing a number of boards, I’d use hot glue to attach the runners. To me, that would be preferable to using a jig that requires a set up for each board.

That’s what makes this forum so great, you get exposed to so many ways to solve a problem.

Hot glue sometimes fail for me (I use it to glue shims on the sled). I saw somewhere people nail their runners.

 

[Michael Kellough]

Back to Star’s comment (which pretty much says it all) if you can easily flatten your existing sled enough to set the shims then it’s good enough already.

 

[Alanbach]

Can’t you just shim the bases of the adjuster assemblies so that they are all dead flat relative to each other?

 

[Svar]

Looking at Shop Notes picture I'd be tempted to use aluminum t-slot extrusions for the frame with hardboard for skins. All secured with bolts. T-slots on the sides will be handy.
If you want to keep what you have, just surface it flat somehow adding a layer of hardboard if needed.
You can even add two thin runners on the bottom instead (wood or UHMW) and plane those straight. It will slide easier.