Best Plastic for a Bench Block?

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?


Doug White

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Doug White
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PVC is all over the map, depending on the mix, in terms of hardness (ie, raincoats through conduit are all "PVC") - however, I note that PVC at McMaster is harder than UMHW-PE at McMaster. But - impact strength will matter, since you are pounding on things - and UMHW-PE will take some pounding, deform a bit, and spring back (barring massive deformation), where the harder plastics may just crack and be done with it.

ABS at McMaster is similar in hardness (different scale, same general hardness) to PVC but has better impact. I think it's more prone to permanent deformation, though it's also another product where Canoe ABS and Helmet ABS are very different hardness, so they behave differently, and it's hard to generalize about the plastic as a class.

Another option (which might allow not [or less] machining your custom blocks, if you fiddle about with parts and clay) that has great impact and can be got a bit harder than UMHW-PE, though not as hard as ABS, is castable urethane - not uncommon as a "rubber mallet" face among other things. Mind, McMaster does not comment on the impact behavior, but other sources do (and McMaster does stock it.)

Still, if you have the time and the plastic, you could try PVC and see how (or if) it fails, and use that as a guide to what next. Or, try hardwood, paying attention to the anisotropic nature of wood, and perhaps using a sheet of plywood on the bottom.

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My gut reaction is that if you're at MIT why are you asking _us_ for advice? You should have better resources on campus.

Reply to
J. Clarke

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.

Reply to
Ned Simmons

How about casting the parts from aluminum, then attaching wear blocks made up UHMW to the surfaces that wear. That way you could just use sheet stock that would be a simple cut and stick operation instead of needing to machine the entire piece every time.

OR machine blocks of billet to the base shape and attach to that.

Reply to
Steve W.

Ned Simmons wrote in news:

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

Reply to
Doug White

Why not find some rock maple, rout your block out of it to try out your ideas and carry on looking for polymer chunks while testing things out?

My general purpose bench block was made out of a hunk of Micarta, my dad brought home some healthy chunks about 2' square and 4" thick out of some UHV switchgear, were shipping blocks for the terminals. Made one up about the same pattern as the metal ones they used to sell only larger, complete with a V across the diameter. Have been pounding on it for 25 years, hardly shows any wear.


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Kiln-dried White Maple or Beech

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Tom Gardner

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