Monday, 3 June 2013

Crosscut Sledding!

During our brief but frequent rain showers on the weekend, I took some time to build a crosscut sled for the table saw.

Most woodworkers have at least one crosscut sled around, and some have more. A large one for cutting big stuff, a small one for cutting - well, you probably already know - small stuff. I decided to try building a small sled first.

A crosscut sled allows you to cut stock square - right at 90° on the first try, and with greater accuracy than sliding a piece across the table of the saw. The science is pretty simple. On the bed of the saw, there are 2 slots. These slots act as a guide for the sled, and if you can find a way to hold your hunk of wood that is consistently 90° to the blade, then you have a sled that is super-fantastic. But wait - that's not all!

Knowing that he board is at 90° when held properly, you can insert shims to move the board off square to any angle you want quickly and easily - even easier than using a mitre gauge, and without having to fiddle with the mitre gauge settings every time.

The sled is actually pretty simple, with only 6 components and no moving parts. There are tonnes of crosscut sled plans on line, and a few tutorials on getting them square, but my favourite tutorial is this (incredibly boring) video that lays out the process pretty well:

Now, my sled is not as nice as his, and not as high tech either, but the five cut method was great for getting it accurate. But I'm getting ahead of myself. Lets start with making parts.

Since this is a small sled, it only takes small parts.

The work surface - 12" by 24" piece of 1/2" plywood
The runners - 2 pieces of hardwood measuring 3/4" X 12" X 1/2"
The fence and push rail - 2 pieces of hardwood measuring 2-1/2" X 24" X 1-1/4"
The guard - a piece of Plexiglas 1/4" thick by 4" wide by 12"

Start by cutting your plywood as square as possible. You can cheat and have the dude at home depot cut it on the panel saw if you want. Also, ask about pre-cut sheets of plywood at the lumber store. Mine has pieces 24" square, so I just had to cut a piece in half. Be sure your plywood is good on one side so you have a smooth surface for the top side.

Put a sheet of waxed paper on the saw, and groove it to sit down into the T-slots. Set your runners in the slots. Now put the board on top with the centre of the board aligned with the blade. It doesn't need to be exact. Push down hard so the runners bottom out in the slots.

Bring the fence over, so the end of the plywood is against the fence. This will make the plywood roughly square to the blade and slots. Now put glue on the runners, and place the top in position. Put a bunch of bricks or other heavy things on there to hold it in place while the glue dries. I also pre-drilled and set a couple screws.

Go to bed, and come back in the morning.

Attach the back push rail. This one doesn't have to be perfectly square, but of course its better if it's at least close. Now its time for the tricky part, the fence.

The fence needs to be 100% perfectly square to the blade, but this is tricky to accomplish. Here is how I did it using the 5 cut method above for measuring, and playing card shims to adjust.

  1. Set the saw to a height to just clear the plywood and cut into the sled about 4"
  2. Attach the fence using one screw in the far left corner. Measure the distance from the screw to the cut and write it down someplace.
  3. Make a small wooden block
  4. Using a square, set the fence to "as square as possible" to the cut you just made, and draw a line at the far right corner where you think the fence needs to be.
  5. Hold a playing card or two against the side of the block, and set it in place so that the cards are in line with where the fence needs to go. Screw the block in place solidly. At this point you have a floppy fence, and a solid block. The block will be your adjustment surface, and the playing cards will be your shims to adjust the fence's position.
  6. Using a C-clamp. Hold the fence in place against the block with no playing card shims. You should be out of square bythe thickness of the cards divided by two. At this point you want to be out of square, so you have a reference to build from.
  7. Make a 5-sided test cut in some scrap material like in the video above. If things are to plan, your fence will be out of square by the half the thickness of the playing card shims at the cut line. Check it by measuring with the most accurate tool you have available (Vernier callipers are awesome!) Remember to divide the error by four like the video says.
  8. Add playing card shims between the block and fence to reduce the out-of-squareness until everything is lined up. A normal playing card is about five thousandths of an inch thick. Printer paper is about three thousandths of an inch. Shim away until things are perfect.
  9. Once everything is lined up, flip the sled over and screw the fence in place using a billion screws. You really don't want it to move when you undo the clamp.
  10. Remove the clamp and block, and put on your Plexiglas guard
    Yippee! you built yourself a crosscut sled!

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