Precision Puzzlemaking Primer -- Volume 1

Step 4 Taking measurements

Once you've figured out what saw and blade you'll be using, it's time to design a crosscutting sled for it. Start by taking lots of careful measurements. Some important dimensions to consider are:

• the width and depth of the miter slots
• the distance between the miter slots, and where the saw blade sits between them
• the thickness of the blade itself (this is called "kerf"), and also its maximum depth of cut
• the thickness and diameter of any blade dampeners or stiffeners that you plan to use
• how far back the blade sits from the front of the saw
• how far the saw is front to back, and how much travel the sled will have

You also need to think about the thickness of the plywood that you plan to use. I recommend using the highest quality 1/2 inch "cabinet grade" plywood that you can find. The higher the ply-count, the better. Even if both sides of your sheet of plywood look good, you should pick one side to be "bad" and mark it as such by scribbling light pencil marks across the surface. When you are assembling the layers of the sled plates and cross supports, make sure that the bad sides are always facing each other (good sides facing out) to ensure that any bow in the plywood is balanced out. You want the finished sled to sit as flat on the saw as possible.

Making accurate measurements is an art form in and of itself. The thousandth of an inch (0.001") is a popular dimensional increment, and there are a wide range of readily accessible (and occasionally reasonably priced) tools which will take certain measurements accurate down to one thousandth. By far, the most versatile of these tools are four-way dial calipers. This picture shows my drawer of inspection (i.e. measuring) tools -- I've got three sets of calipers in there.

I strongly recommend that you get yourself a set of calipers. I will make several more tool recommendation throughout the course of this project, but this one is by far the most important of them. Although you won't actually need calipers to build this sled, you will need them to calibrate your saw and jigs, and to measure for error in the resulting cuts. Your cuts can only be as accurate as your ability to measure.