My 3D printed PCP hammer

[Scotchmo]

Over the last two days, I did a little research on this problem. I wanted to compare plastic properties that might be used in an airgun hammer. I looked at MDS and Nylon, POM (Delrin) and PLA. I don't think the MDS is available as a 3D printer filament but I looked at it for comparison since it has been used to good effect by some tuners.

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Delrin had the lowest coefficient of friction. MDS is close:
Delrin ~0.25
MDS ~0.28
PLA ~0.4

More thoughts on friction later.

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PLA has the lowest coefficient of thermal expansion:
PLA ~2.3x10^-5 in/in per degree F
MDS ~3.7x10^-5 in/in per degree F
Delrin ~4.6x10^-5 in/in per degree F

Those might be average values which can vary across the temperature spectrum.

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Delrin has the least affinity for water absorption. The rate of absorption is low for all three, though Nylon and MDS will absorb quite a bit (7%) given time. I've never measured anything that severe.

Maximum water absorbtion:
Delrin ~0.9% (0.4% in 24hrs)
PLA ~1.5% (.15% in 24hrs)
MDS ~7% (1.2% in 24hrs)

Unless subject to constant high humidity, I don't see this to be as big a problem as thermal expansion (though I could be wrong).

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I really wanted to 3D print a hammer out of POM (Acetal). But after trying many different printer settings and bed adhesion trys, all unsuccessful, I decided to go with PLA for now. I might try again later as I learn more. PLA is about the easiest to use 3D printer filament.

After some testing, I don't think that a low friction material is that important. There is very little side load to cause drag. I decided to run the PLA dry since any lube seemed to create viscous drag on the hammer. When there is sufficient clearance, no lube seems best.

Delrin machines and taps nicely. Not so much for PLA. After some intermittent success with tapping holes in PLA. I decided to go with inserted nuts to retain the hardware.

Left to right - Final PLA design, PLA without inserts, PLA with Teflon rings, fluted and threaded PLA, machined Acetal, and two factory Marauder hammers.


Current design. I can probably improve on the finish in the future by adjusting the printer settings:


The factory hammer is 79grams w/hardware.
The current PLA hammer is 26grams w/hardware.

I tried several different spring spacings with the PLA hammer. Giving different velocities from 870fps up to 920fps max. It took about 200 shots over the chronograph and several disassembly/assembly sessions to get the tune that I settled on.

Factory hammer, AA 10.3gr@905fps, 40-45 shots before coming of the reg (1900psi).
PLA hammer, AA 10.3gr@900fps, 60 shots before coming of the reg (1900psi). And the maybe 3 or 4 more good shots.

With the factory hammer, I kept the hammer spring at the weakest setting. The PLA hammer is 1/3 the mass and required another 0.3" of preload.

0.24" additional preload - 870fps
0.3" additional preload - 900fps ~60 shots
0.325" additional preload - 905fps ~55 shots
0.4" additional preload - 915fps, ~44 shots
0.5" additional preload - 918fps

I also made a PLA baffle. The baffle and slightly better efficiency gives less noise.

It's been a good shooting gun for years. So I was somewhat reluctant to mess with it. This is the first time in almost 5 years that it has been disassembled. I'll shoot some targets soon to see how it does now.

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PLA - Polylactic acid
a bioplastic usually made from corn starch or sugar cane. Circa 2004 I think. It was unknown when I was designing things commercially. It seems to be the most common plastic used in 3D printers.

POM - Polyoxymethylene
Patented as Delrin in 1956, also known as Acetal

PA - Polyamide
Patented as Nylon in 1938

MDS - Molybdenum Disulfide filled Nylon

[Rob M]

Very interesting , was it 100 % infill ? I have never printed anything aside from PLA personally , and really haven't had the need.. I guess POM and nylon would be interesting but for most applications , pla fits the bill.A quick tip for tapping PLA , if you are making blind holes in the design , or any holes you plan on tapping after the print , triple your " shell thickness " in cura.. THis will assure a thick wall around those holes for the threads to cut into , even if infill is lower than 50 %

[Scotchmo]

Very interesting , was it 100 % infill ? I have never printed anything aside from PLA personally , and really haven't had the need.. I guess POM and nylon would be interesting but for most applications , pla fits the bill.A quick tip for tapping PLA , if you are making blind holes in the design , or any holes you plan on tapping after the print , triple your " shell thickness " in cura.. THis will assure a thick wall around those holes for the threads to cut into , even if infill is lower than 50 %

I use 1.6mm for the shell thichkness. And I usually use 20% or 25% infill. The final hammer was 100% infill as I needed the extra mass.

The hand tap often tears out the first few threads in the PLA. I think either a larger diameter hole or a tapered entry for the tap would help.

I'm liking the inserts, as it eliminates some secondary operations, and provides a stronger thread, and nuts are cheap enough. Providing threaded inserts is often too much work, or not possible, when machining plastic, but easy enough when modeling for a 3D print.

PLA came about just in time as it seems to be the key to the success of simple 3D printing.

[Rob M]

yes, those first few turns seem to be an utter waste.. Ive drawn the threads in cad into several prints, they were slightly better , but getting the specs exact was not easy , usually they would need to be chased afterwards .. Inserts are good idea, as well as recesses to hold nuts.aand yes. 1.6mm is more than enough to get the best thread results ..maybe a chamfer in the design of the hole would help , I have never tried it

[Gipper]

Curious as to how well this hammer is wearing where it is engaged by the sear?  PLA is often knocked for not being very strong. I like the idea of being able to do this with some of my Crosmans if wear does not end up being an issue.

Brian

[Scotchmo]

Curious as to how well this hammer is wearing where it is engaged by the sear?  PLA is often knocked for not being very strong. I like the idea of being able to to this with some of my Cosmans if wear does not end up being an issue.

Brian

Your comments made me think about it.

The Acetal hammer I use in my Marauder pistol did not show any noticeable wear after much use. I doubt that the PLA will be as good as POM (Acetal/Delrin) with respect to wear. Though I don't know if it will be an issue in this case. Some wear in that area may not affect the feel of the trigger much, at least with the fall-away linkage used in the Marauder rifle and high end Crosman pistol triggers. The pull-away linkage used in the lower end pistols and Disco/Maximus might be affected to a greater degree as the sear can drag heavily along the hammer on a slow trigger pull.

In either case, your questions prompted me to add a small glop of Super-lube along the bottom of the hammer. In a narrow strip that only contacts the sear.

[Scotchmo]

yes, those first few turns seem to be an utter waste.. Ive drawn the threads in cad into several prints, they were slightly better , but getting the specs exact was not easy , usually they would need to be chased afterwards .. Inserts are good idea, as well as recesses to hold nuts.aand yes. 1.6mm is more than enough to get the best thread results ..maybe a chamfer in the design of the hole would help , I have never tried it

By "inserts", I also meant recesses to hold nuts. Left view shows hammer, right view is a cutaway showing "inserts":


I use a very old program for modeling (AutoCAD 2000), which uses constructive solids. The solids are somewhat cumbersome to edit but it can output STL files that work perfectly in Cura. It does not have an easy way to make helical solids, otherwise I would have tried to print the threads before tapping.

Stuff we just dreamed of 30 years ago. Even solid modeling was state of the art and available to a select  few.  The average person could not afford the stereo-lithography machines of 20 years ago. And then now - going from a 3D model to usable part, on your desk, using inexpensive equipment - I'm still amazed:

[Rob M]

awesome , yes it is still mind bending .. My dad visited and saw the 3d printer, then saw a 3d printed object and said " but where did this come from ?" 

[WhatUPSbox?]

Great stuff......this does not help my patience on the 3d printer front

Have you had success with the knurled type threaded inserts for 3d prints?

[Motorhead]

Scott,
Last year while visiting my local interstate plastics ( Right about there inventory time ) they put out in there retail shop salvage bins a bunch of medical / implant grade Acetal.
Yellow, Red, Blue, Green, Orange and few others.  Dang near purchased it all for making some really fun and cool parts with.

I have to date made a couple M-rod / WarP hammers with it and MAN IS IT SLIPPERY STUFF !!
It is soft like black acetal being no where near as abrasion resistant as the MDS, but given a clean polished tube and no sharp edges on sear and tube where it slides ... DANG !!!

have no clue what the blend is, but was told when inquiring about it that it is make for artificial limb joints and such "In Body" points of motion and claimed it had VERY HIGH self lubricity.

Just a shout out to at least some acetal being Hammer worthy ... tho not a fan of Black being it scars and gouges far to easily 

[Gipper]

Curious as to how well this hammer is wearing where it is engaged by the sear?  PLA is often knocked for not being very strong. I like the idea of being able to to this with some of my Cosmans if wear does not end up being an issue.

Brian

Your comments made me think about it.

The Acetal hammer I use in my Marauder pistol did not show any noticeable wear after much use. I doubt that the PLA will be as good as POM (Acetal/Delrin) with respect to wear. Though I don't know if it will be an issue in this case. Some wear in that area may not affect the feel of the trigger much, at least with the fall-away linkage used in the Marauder rifle and high end Crosman pistol triggers. The pull-away linkage used in the lower end pistols and Disco/Maximus might be affected to a greater degree as the sear can drag heavily along the hammer on a slow trigger pull.

In either case, your questions prompted me to add a small glop of Super-lube along the bottom of the hammer. In a narrow strip that only contacts the sear.

Almost all my Crosmans are outfitted with the PROD trigger group so I am anxious to give this a shot when I get some time.  My hammers will be far less complicated to print as well.

Brian

[Scotchmo]

Scott,
Last year while visiting my local interstate plastics ( Right about there inventory time ) they put out in there retail shop salvage bins a bunch of medical / implant grade Acetal.
Yellow, Red, Blue, Green, Orange and few others.  Dang near purchased it all for making some really fun and cool parts with.

I have to date made a couple M-rod / WarP hammers with it and MAN IS IT SLIPPERY STUFF !!
It is soft like black acetal being no where near as abrasion resistant as the MDS, but given a clean polished tube and no sharp edges on sear and tube where it slides ... DANG !!!

have no clue what the blend is, but was told when inquiring about it that it is make for artificial limb joints and such "In Body" points of motion and claimed it had VERY HIGH self lubricity.

Just a shout out to at least some acetal being Hammer worthy ... tho not a fan of Black being it scars and gouges far to easily 

The black acetal seems just as durable as other colors, but scratches can be much more visible against the black. The white gets scratches too, just harder to see them.

Until it gets black grease/dirt embedded in the scratches.  Now the scratches are more obvious on the white.

If it's just black dyed acetal, it should not matter much. For unknown samples, it's possible it has some other additive (carbon, PTFE, graphite, etc.).