Mechanical puzzles made of wood require safe, accurate cuts of small pieces. This cutoff box design satisfies that requirement. Dimensions are somewhat arbitrary and the design lends itself to modification.
Start with a 12-in. by 24-in. piece of 3/8-in.-thick acrylic. Drill four, ¼ -in. holes for dowels to serve for bearing axles. Dowels should be a press fit in both the bearing inner diameter and the hole drilled in the acrylic. The outside diameter of the bearings should be less that the width of the table saw's miter-gauge slots. The four bearings are positioned so that two bearings will be in each miter-gauge slot. The bearings do not need to be precisely located at this stage.
With two bearings installed for one of the miter-gauge slots, saw the acrylic base in half while holding the bearings against the miter-gauge slot-edge, which is next to the saw blade. Set the half of the base with the bearings aside, install the two remaining bearings in the other half, and saw this half while again holding the bearings against the miter-gauge slot-edge, which is next to the saw blade, using the other miter-gauge slot. You now have two pieces of acrylic separated by a sawkerf when the bearings are against the inboard edges of the miter-gauge slots.
Attach the front fence to one of the acrylic base halves using flat-head bolts. The bolts should be long enough to extend above the top of the fence and be held in place with a nut and washer. The acrylic base can be counter sunk to accommodate the bolt head. Use a square to position the fence at right angles to the sawkerf-edge of the base. Attach the back fence in the same manner to the same base-half. Both fences can now be bolted to the other base-half. Use clamps to pre-load the bearings against the inboard edges. By carefully positioning the fences on that first half with a square, the fixture is now ready to be used.
However, the table saw is not ready for the fixture to be used. A stop must be provided to prevent the bearings from moving out of the miter-gauge slots; especially so at the back of the saw. A board clamped judiciously will work as a stop. Or, at the back of one of the miter-gauge slots, a hole can be drilled and tapped for a ¼-in., socket-head screw. This screw is a very good stop, and will less likely be forgotten.
Additional Suggestions: 1) If it can't be made precise, make it adjustable. 2) To make precise adjustments, know where you are before the adjustment and then know how much adjustment is made.
For example, a fixed stop attached to one side of the cutoff box will serve as a reference-stop. To cut a desired length, place something that is the desired length against the reference-stop. A piece is then cut which will fill the gap between the desired-length object and the saw blade. Replace the desired-length object with the newly cut piece. The space between the end of the cut-stop and the saw blade should equal the desired length. If a measurement indicates that the length is not quite correct, the cut-stop length can readily be corrected. Feeler gauges, accurate to 0.001-in., can be used to either shorten or lengthen the space between the stop and the saw blade. To shorten the space, add the suitable feeler gauge between the reference-stop and the cut-stop. To lengthen the space, shorten the cut-stop by placing the feeler gauge between the desired-length piece and the cut-stop and saw the cut-stop shorter.
*This cutoff box was described in "Methods of Work", Fine Woodworking , number 117, April 1996, page 16.