Doug White wrote:
"Lloyd E. Sponenburgh"lloydspinsidemindspring.com wrote in
. 3.70:


Can anyone tell me how I can avoid this?

When I mounted splash shields on the table of my R2E4 (cast acrylic,
and VERY prone to cracking and "spidering" from holes), I drilled the
holes in the plastic to accept a grommet. IIRC, the holes were 7/16".
I don't even want to have special bits around for plastics, so I
drilled them way undersized, and used a tapered reamer to enlarge them
without cracking the plastic.

The grommets were made for whatever hole size I drilled, and
themselves had a bore of 1/4". The 1/4"-20 screws with a washer under
the head fit just loosely in the grommets. When you squish 'em a
little, the hole swells shut, and it holds firmly to both bolt and
plastic, without any tendency for the plastic to be stressed at the
union. Unless you use a shoulder bolt, you'll have to put a bit of
threadlocker in the screw hole, since you cannot tighten the bolt too
far without undoing the whole purpose of the exercise.

Just make sure you keep the threadlocker AWAY from the plastic. I'm not
sure about acrylic, but I know from sad experience what happens to Lexan
when you get Loctite on it. We had a rush job that was just about
finished when an overzealous technician decided to applied threadlocker
to some 3/8" Lexan parts it took several days to machine. They swelled
up& cracked like mad. Had to start all over again, with even less time.

Doug White

Reminds me of a misadventure I had with acrylic and an acetone
spill once.

Thanks for the reminder.

--Winston