The Lap Hold-down Screw that came with my Omni Faceting Machine is a small knurled screw with a right-hand thread. The standard washer that comes with it is brass. The problem with this is that it would undo itself as the machine was running, possibly due to the vibration caused by a poorly-sized Spindle that the laps are centered by. This meant the laps would slowly loosen and then start sliding on the Platen. And the Hold-down Screw itself would come off and bounce around the lap and room like a crazy grasshopper.
The other problem is that the threaded hole in the Spindle that the Hold-down Screw fits into isn’t deep enough. When fully-tightened, my Hold-down Screw has about 7/16″ of space between it and the top of the Spindle.
I have some old steel Maja Manufacturing laps – about 5/16″ thick – which the Hold-down Screw could not be tightened down to reach.
My solution for this was to purchase some 11/16″ diameter plastic tap washers pictured below (note that this is before I had the Platen size reduced). The right package shows the thick plastic washers I found – just had to thread one onto the screw and use it instead of the brass washer. This now allows me to tighten the screw sufficiently without forcing anything and the plastic has enough grip to prevent the screw from coming undone by itself.
For my Maja laps I use 2 of these plastic washers stacked together.