Machine vibrations sometimes cause threaded adjusters to wiggle out of alignment.

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Machine vibrations sometimes cause threaded adjusters to wiggle out of alignment. For jigs that require threaded adjusters (like the router-table fence adjustment jig shown above), try high-density polyethylene (HDPE) instead—available in sheets and blocks from plastics manufacturers.

The trick to getting an HDPE block to hold tight against movement caused by machine vibration: When tapping the threads for the adjuster, stop just short of completing the threads through the block; instead, let the screw itself cut the last few threads.
—Doug Spencer, West Jordan, Utah