Welcome to our saw dust pile, Ron (if that's actually your name). Please take the time to update your profile (upper right, under the Welcome ________ on this screen). We already have too many people named N/a on this forum.
Don't hesitate to ask us questions, because we love to help by providing answers, especially to new members.
The grommet is just a cushioned stop for the router bit when you insert ite into the router collet. You should never drop a router bit shank all the way into the collet and then tighten the collet without lifting the bit slightly above the stop. Tightening the collet pulls the bit in tighter against the stop and this can jam the collet, making the collet and bit nearly impossible to remove.
If you do this it's quite possible that the bit will become stuck and not come back out easily when you loosen the collet to remove the bit. You should always lift the bit up off the stop about 1/8" before tightening the collet. Using the rubber grommet in the bottom of the router collet will provide a repeatable, yet flexible stop for the router bit, that will also prevent the bit from becoming stuck when the collet is tightened.
Marc Sommerfeld pioneered this grommet idea and now sells router bits with their shaft lengths standardized so that when using one of his bit pairs like the tongue and groove bits, that the cutting portion of the bits will match if you bottom out the bit shafts against the grommet before tightening the collet. He now supplies these grommets when you buy bits from him, but you can also buy them from electronic suppliers. The grommet eliminates the need to change the router height adjustment when changing the bit if the bit is part of one of his bit pair sets. Some other bit sources are beginning to adopt this as well, but you will need to check this yourself if using router bits from any other source. I now have a grommet or an O-Ring (alternate method) in the bottom of the collet of each router that I own (12 total). It's a perfect solution for keeping the bit from becoming stuck, as well as helping to reduce the need to adjust the bit height when switching bit pairs.
Central North Carolina
Last edited by CharleyL; 04-20-2017 at 08:33 AM.