MDF or Ply?

[nblong]

I'm getting ready to build a cross-cut sled (Shop Notes #99). The plans call for a ply base. I don't have any ply but I've got quite a lot of MDF. Is there any reason I shouldn't use the MDF for all of it (base, fences, etc.)? Thanks for the help.

 

[DaveO]

No, other than screw holding capacity of MDF vs Ply. You will want to use screws to attach your main fence so you can adjust it for square to the blade. I would also coat the MDF well with shellac to prevent any moisture related swelling, before squaring everything up.
My router table top and fence are made of MDF, with shellac, and they are as straight and flat as the day I made them.

Dave

 

[fergy]

I've got one that is melamine-coated 3/4" MDF. The only other thing to remember is that it's going to be heavier than plywood.

 

[nblong]

Thanks for the tips guys. I've got some laminate, fergy so I might try that. I would not have thought to use shellac DaveO, thanks.

 

[Dudelive]

Thanks for the tips guys. I've got some laminate, fergy so I might try that. I would not have thought to use shellac DaveO, thanks.

You probably already know, but be sure to use it on ALL sides and not just top or bottom. It is easy to forget and get in a hurry and not do the bottom or the sides when you get the top looking real nice.

 

[skysharks]

I have to agrees with DaveO, watch the screws holding strenght. Other than that you should be fine.
Add some Johnson's Paste wax to it and watch it slide.

 

[Adirondax]

A bit off topic but has anybody else noticed that the quality of MDF available at the box stores (at least HD and Lowe's) has gone down? It's lighter in color, flexes more and doesn't hold an edge the way the stuff I bought last year did. I use MDF for templates but now I'm thinking I need to use something tempered.

 

[DavidF]

I used 1/2" MDF for my sled, with poplar fences. The only place I have had to screw into the mdf is the slide. The fence is held by screwing through the MDF into the fence. Tip: don't put the fence right on the back edge (nearest you) move it in an inch or so and put a single pivot screw on the left end and a small adjustment block on the right. This has a hex head screw in it that adjusts the fence angle to set it square. When it's all done screw through the same block into the fence to keep it tight against the hex head. When its all done butt some brackets behind the fence near the center. This is much easier than trying to make adjustments when the fence is screwed from below.

I have not coated ANY surface of my sled and it still seems ok. probably better than doing one side only.

Use the "5 cut" method to square up. Mine ended up about 0.0005" over a ft, if it moves a bit I can live with that!!