Ned Simmons wrote in
:

On Mon, 10 Oct 2011 13:47:34 GMT, Doug White
wrote:

I help coach the MIT collegiate pistol team, but end up spending half
my time doing gunsmithing. We have a whole raft of match CO2 pistols
that need occasional O-ring replacements, and I'd like to make a
custom bench block to hold some assemblies while I work on them. I
have a couple of general purpose commercial blocks, and I _think_ they
are made out of Nylon.

I'd like something easy to machine, but tough enough to put up with
some pounding. A couple operations require pressing very large snap
rings on/off or pounding out cross pins with the piece supported
mostly on a coarse sharp threaded section. I don't want the block
getting chewed up too quickly from those operations. I have thick
blocks of PVC, but I think that is too soft.

Likely suspects would be Nylon (several flavors), Delrin, or UHMW
polyethylene. Extra points for something I can get easily from
McMaster Carr in ~ 1.5" x 3" cross-section. They indicate that
polyethylene has better impact strength than Nylon or Delrin, but I'm
not sure that is the best metric.

Comments? Suggestions?

Thanks!

Doug White

As long as you don't need to do much more than cut it up into blocks,
I'd start with high durometer urethane. See McMaster p.3518. It's not
cheap, but will be more durable than the alternatives. Good plastic
dead blow hammers are made of urethane. Machinability is fair for the
hardest grades.

It sounds like it would work well, but I need more than just a
rectangular block. The part I'm working on is cylindrical, with several
stepped diameters to support. I'm going to bore some holes in a block,
to match the diameters of the part. Then cut the block in half to split
the holes, and mill slots to provide clearance for the E-clip & the like.
Never having machined polyurethane, I'm not sure how well that might
work. The hard polyurethane is also almost twice as expensive as most of
the other plastics.

The general concensus (thanks to all those that replied!) seems to be
that polyethylene is a good candidate. It's certainly rugged enough not
to get chewed up too quickly. I'll end up with two blocks, in case we
actually manage to wear one out. By then, I suspect the pistols will be
so worn out as to be unrepairable.

Doug White