Using Timber for Zero Clearance Inserts

I’ve been discovering why timber is not used as a zero clearance insert on tablesaws.

The first problem discovered was with the anti-kickback pawls that on my table are attached to the riving knife, so they rise and fall with the blade height (which is a good thing), but the dig into the soft timber surface as you drop the blade height (pretty much doing what they were designed to do – stopping the timber moving backwards relative to the pawls), and that stops the blade height from being able to be dropped.

The second problem I discovered yesterday is that I made the inserts on a warmer day than it was yesterday, so when I went to raise the blade, the insert was binding one the blade as the whole insert was being squeezed by the now smaller hole in the tabletop, and the insert broke, being so tightly held in place.

Not even sure how a different material would react in the same circumstances (perhaps I can find one with a similar thermal coefficient of expansion to that of the tabletop). I might also look to see if I can source some of those grub screws that have the spring loaded bearing at the end and include those in the next insert.

Does go to show one other thing – despite nothing obvious having changed between one workshop session and the next, it is worth checking that the blade is spinning freely before use. It also proved useful that I leave the saw with the blade wound down and parked below the table surface, so the first job I had to do was raise the blade to the required height. If I had just left the blade exposed, and had not checked if it was spinning freely then in this case I probably would have stalled the motor, and blown a circuit breaker at best, or at worst, ripped the insert out of the table and thrown it across the room (in my general direction).