Sunday, May 18, 2008

Small Crosscut Sled

A crosscut sled is an improvement to the design of the typical table saw. It allows the "table" to slide over the spinning blade, taking the workpiece with it, rather than sliding the workpiece itself over the blade. It also provides a convenient place to mount stop blocks, T-track, and other gadgets, if so desired.

This allows for a safer and more accurate cut, especially on small pieces. No more needing to hold your hands close to the sharp blade, trying to control kickback, or needing a Rube Goldberg-esque configuration of blocks, clamps, featherboards, and fences.

First I cut a scrap of 3/4" birch cabinet ply from the BORG. It is only 5 ply, but works fine for many uses. I didn't measure the base of the sled, but it is around 30" x 18". For a sled the exact measurements aren't as important as squaring it to the blade is.

I next cut some 1/2"x4" reclaimed oak boards to make the front and rear fences. I like to use reclaimed wood whenever possible. Partly for cost and partly to preserve the good wood on the market for a little longer. These boards had some embedded nails that could not be removed. So I cut them out on the table saw, leaving several shorter boards. Two were more than long enough so I cut them to match the base.

I jointed them on my 4" Rockwell jointer, making sure the unfinished face and the bottom were exactly 90*. The bottom will be glued to the base, and the face will control the workpieces. I used a homemade pushblock for the face jointing. It is just a scrap of plywood with some rubber shelf liner stuff attached with spray adhesive. No fancy handles. Just enough to keep control of the workpiece.

Once the oak boards were jointed, I checked for square on the sled. My digital angle gauge is perfectly square, and makes for a convenient square when I'm too lazy to walk across the garage to get a "real" square. If the fence on the jointer is square, the workpiece will be too. And this jointer is perfectly square.

I cut some thin strips of oak 3/4" wide to use as rails. I placed these in the slots on the table saw and used shims to raise them just proud of the table saw surface. I then set my fence to fit the sled base so the blade is centered. A square table saw fence will also ensure the front and rear edges of the base are also square to the blade. This is critical because those edges will set the alignment of the fences!

I applied glue to the rails and clamped the base down to the table saw. I was unable to get a clamp on the front left corner so I piled a bunch of weight on that corner. Surprisingly it worked well!

After the glue dried I trimmed the rails to fit and tested the glide. As is normal, the glide is too tight. I used a cabinet scraper to pare down the rails until they would slide in the slots on the table saw with just a medium amount of force. I want the fit tight because once the rails are waxed it's perfect.

I had debated about using mechanical fasteners to hold the fences on, as I have for my previous sleds. I decided against them because they would add complexity without much strength. For a sled this size there is no need for screws.

I set the boards on the sled base again and drew lines so I could easily tell how wide to make the glue on the base. I then applied a nice even layer of glue to both the sled base and the rail, one rail at a time. Several clamps hold it together to dry.

Check the square several times throughout the process. The clamps will often draw the fence one way or another (a good argument for "parallel clamps" that don't do this) and require adjusting. Small position adjustments will usually resolve any angle issues.

Once both rails have been glued and allowed to fully dry, use a card scraper to clean up any glue. Especially on the inside of the sled. Bumps of glue squeeze-out will throw off the perfect square you are going for!

Now is the time to apply wax to the rails. If you haven't waxed your table saw lately, this would be a good excuse to get that done too.

Smear the wax on liberally and allow to dry 5 minutes. Then buff with a clean cloth. I prefer microfiber cloths for both tasks, but any soft cloth will work just fine.

Check the glide again on the table saw. There should be zero side to side play, yet the sled should move smoothly the entire length of the saw surface with just a push from your pinkie finger. Feel free to scrape the rails a bit more and re-wax if needed.

Check your saw blade is a good one for plywood or laminate so you don't get any tearout when cutting the initial kerf. I forgot to check when building this sled, so I got some tearout from using a 24 tooth ripping blade. It may not be critical, depending on the location of the tearout. Luckily for me the tearout was in the middle of the sled, not at the fence.

Sneak up on the cut, moving the blade up just 1/8" of an inch at a time until the kerf is exposed in the sled bottom. Don't worry about making kerf cuts up into the fences. You'll do that automatically when you start cutting workpieces with the sled. Right now you just want a clean kerf to begin with.

Take your time, moving the sled over the blade slowly. There are no awards for speed, and you will just risk tearout of the plywood or fences.

Check that the rear fence is square to the blade. Use your square (a real one this time) and hold the edge to the face of the blade, not the cutting tips. If it is not square you'll have to cut it apart and start over. But if you followed these instructions, you should have a rear fence that is perfectly square to the blade. Do some test cuts to confirm.

At this point you can either be done and call it good, or apply a finish. For small "throwaway" sleds like this it really doesn't matter. the bed of the sled will be chewed up before the wood surface shows any signs of wear. So a finish will be just for appearances. My reclaimed oak boards had a poly finish on one side, I didn't apply one. And I saw no need to remove it either. On some of my sleds I apply two coats of shellac, especially if they're made from MDF to provide some moisture protection. Like, in case I spill coffee/tea/kool-aid on it.

But I'm perfect and would NEVER do THAT!

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