Pistol Frame Material

Danimal300 · May 7, 2016

I have been designing some pistol frames in CAD/CAM. They are basically copies of STI 2011 and SW Bodyguard 380, but with some small changes here and there, mostly to accommodate custom grips and features. The dimension of the rails remain exactly the same as the stock pistols so that I can use stock components in the builds. I want the STI to be a 9mm, so low pressure, lower recoil than 45. These are just for me, and just for fun, so they just need to be safe.

I am looking at what material to machine them out of. The STI site states 4140 heat treated to 28-32 Rockwell for their frames. That seems pretty low hardness, but I understand that they probably need to be higher strength, lower hardness to minimize the chance for cyclic fracture. The SW specifies stainless steel. I am a hobby machinist, and have not had the best luck with stainless steel or that would be my choice.

Looking at the cost of 4140 and if I am only shooting for a 9mm/380, what would be a good cheap alternative? This is a test of ability for me, and there is a good chance that they will end up in the scrap bin. But if by chance they make it through it would be nice to have something that will last a while and is safe to shoot.

paracongo308 · May 7, 2016

The 28-32 Rc hardness level is more then enough, those frames could also be made out of aluminum then hard coat anodized.

Danimal300 · May 7, 2016

paracongo308 said:
The 28-32 Rc hardness level is more then enough, those frames could also be made out of aluminum then hard coat anodized.

I have built a 2011 out of aluminum and type II anodized it to around .0007 which was as thick as I could get it. I have had no problems with it, but for some reason I still just prefer the ones that I have built from steel. I purchased 80% frames, and got really comfortable with the whole build process.

would 1018 be a bad choice? For the 380 there are some cuts that I need to use a 1/16th end mill and make some kinda tight little turns.

ulav8r · May 8, 2016

Danimal300 said:
would 1018 be a bad choice? For the 380 there are some cuts that I need to use a 1/16th end mill and make some kinda tight little turns.

1018 does not machine well but would be OK for low use(maybe 40-50 rounds)if it had a nitride finish to give it some wear resistance. It would probably become un-useable due to peening deformation. 4140 pre-hardened is at 28-32 Rc when received and will machine better than 1018. It will cost no more than 1018 with nitride finish, probably less since no heat treatment or high wear finish would be required.

steve_p · May 8, 2016

It's not like we are talking a huge block of material for a mold here...
The difference between 1018 and 4140ph, even from
mcmaster, will be minimal in a 2011 size frame.
Dont cheap out & do it right the first time.

Sent from my SM-G900P using Tapatalk

Danimal300 · May 8, 2016

Thanks for the help guys, I can read all day about material properties, but the experience of working with different materials is lacking up to this point. I can machine aluminum all day long, but steel from stock is my new challenge. I will go with the 4140 and give that a shot.

Any specific do's and dont's for 4140 on CNC? I am using a 3/16 .020 radius long carbide end-mill and a 1/16 stub length carbide end-mill for the detail work. So far my method for steel is to set the SFM ~100 and adjust the feed to get the chip-load right. The chip load on those tiny end-mills is like .0005 - .001 for <32 Rockwell and .0003 - .0005 for >32. I plan on setting feed per tooth to ~.0005 to start and adjusting up if there is any chattering.

Here is a screen shot of the 380, it is only .7" tall, just over 3" long so the tiny little details in there should be some fun.

gwilson · May 8, 2016

I have a Kel-Tec .380 pistol with a THIN barrel made of 4140. It has a steel slide. I don't know which steel. The FRAME is plastic. It is not the most ideal frame material,and it can wear out. But,this pistol is for self defense shooting at man sized targets not very far away. A belly gun,essentially. But,it shoots hot loads and will not blow up. It is not meant to be shot a lot at target ranges. It is a concealed carry pistol,made as light weight as possible. I don't even LIKE the gun,but it is light enough to carry,is very small,and will hold 7 rounds although the grip is only about 2" long. Very LIVELY when shot,but very effective,too,with hot loads of large hollow points. They will make BIG holes in flesh. I developed a much larger respect for .380's after I fired the gun into a dry wood stump that my anvil sits on.

But,back to your original question,since my frame is probably glass fiber filled polymer,it points out that you can make your frame out of most any metal,and be better off than my pistol is with its plastic frame.

I'll recommemd pre hard 4140 too. I have used it,and it seemed no harder to machine and drill holes in than mild steel.

steve_p · May 8, 2016

Danimal300 said:
Thanks for the help guys, I can read all day about material properties, but the experience of working with different materials is lacking up to this point. I can machine aluminum all day long, but steel from stock is my new challenge. I will go with the 4140 and give that a shot.

Any specific do's and dont's for 4140 on CNC? I am using a 3/16 .020 radius long carbide end-mill and a 1/16 stub length carbide end-mill for the detail work. So far my method for steel is to set the SFM ~100 and adjust the feed to get the chip-load right. The chip load on those tiny end-mills is like .0005 - .001 for <32 Rockwell and .0003 - .0005 for >32. I plan on setting feed per tooth to ~.0005 to start and adjusting up if there is any chattering.

Here is a screen shot of the 380, it is only .7" tall, just over 3" long so the tiny little details in there should be some fun.
View attachment 170860

Are you using cam, or programming by hand?
If using cam, HSM paths are your best bet for roughing out with small tools.

Did you build your aluminum frame from your cad model? Any problems?
Would you mind sharing the model?

Thanks,
Steve

Sent from my SM-G900P using Tapatalk

wesg · May 8, 2016

Danimal300 said:
Here is a screen shot of the 380, it is only .7" tall, just over 3" long so the tiny little details in there should be some fun.
View attachment 170860

That looks like it could be machined from a short piece of barrel blank. Shouldn't be too hard to scare up. Either CM or SS would be fine.

Chris_1969 · May 9, 2016

I would build one from aluminium first, to ensure dimensions and tool path etc are all good, if it all works fine, use 4140 for the next one, I like the way it machines.

For the 2011 STI frame, start with a bit of 2" dia round 4140 and slab it down.

Do you have a full print/model for the 2011 or are you using the cncguns model?

I have a solidworks model of an SVI Infinity 2011 that is very good, but no cnc.

Danimal300 · May 10, 2016

Chris_1969 said:
I would build one from aluminium first, to ensure dimensions and tool path etc are all good, if it all works fine, use 4140 for the next one, I like the way it machines.

For the 2011 STI frame, start with a bit of 2" dia round 4140 and slab it down.

Do you have a full print/model for the 2011 or are you using the cncguns model?

I have a solidworks model of an SVI Infinity 2011 that is very good, but no cnc.

I am using a model that I made, and haven't tested it yet. I plan on making a 7075 version first to check dimensions. I added some features to it that I have designed from competition pistols. I want to design an aluminum SV style grip, but do not have the tooling yet.

Danimal300 · May 10, 2016

steve_p said:
Are you using cam, or programming by hand?
If using cam, HSM paths are your best bet for roughing out with small tools.

Did you build your aluminum frame from your cad model? Any problems?
Would you mind sharing the model?

Thanks,
Steve

Sent from my SM-G900P using Tapatalk

I am thinking about sharing my models on grab cad, but I want to make it clear that they are not exact because I change some details to make them fit my uses. For example my STI frame has a small thumb rest built in and a more beveled edge on the trigger relief that needs to be blended into the grip. It allows for higher grip, and is really a competition preference. It is still a work in progress.

jabezkin · May 29, 2016

Danimal300 said:
I have been designing some pistol frames in CAD/CAM. They are basically copies of STI 2011 and SW Bodyguard 380, but with some small changes here and there, mostly to accommodate custom grips and features. The dimension of the rails remain exactly the same as the stock pistols so that I can use stock components in the builds. I want the STI to be a 9mm, so low pressure, lower recoil than 45. These are just for me, and just for fun, so they just need to be safe.

I am looking at what material to machine them out of. The STI site states 4140 heat treated to 28-32 Rockwell for their frames. That seems pretty low hardness, but I understand that they probably need to be higher strength, lower hardness to minimize the chance for cyclic fracture. The SW specifies stainless steel. I am a hobby machinist, and have not had the best luck with stainless steel or that would be my choice.

Looking at the cost of 4140 and if I am only shooting for a 9mm/380, what would be a good cheap alternative? This is a test of ability for me, and there is a good chance that they will end up in the scrap bin. But if by chance they make it through it would be nice to have something that will last a while and is safe to shoot.

Go with 416 SS. Machines like butter.......

partsproduction · May 29, 2016

I sure agree with the 416 advice. The 4140 at 28-32 Rc is what is commonly sold prehardened 4140.
A while ago the local BLM wanted me to make a gravel spike to test the road gravel thickness that contractors lay on their roads. It was made from 4140 prehardened 1 1/2" round about 13" long, tapered most of the way. There is a channel milled about 5" long on one side so they can see where the gravel ends and the dirt starts. This project came to me without recommendations as to material or heat treatment, well, I guessed that the 4140 was at least as hard as the sample they brought so I made it out of that. (The sample was almost dead soft)
It came back the other day with 9 splits or cracks equidistant around the crown where they had been hitting it with a sledge hammer. I was amazed! The guy said they only used it one day. I then heat treated the crown to 48 Rc after tapering the crown area to act as a belt, we'll see if it comes back.
The point is that I really thought I knew the material I'd been using every day for over thirty years. So the strength may be directional.

Monarchist · May 30, 2016

partsproduction said:
I sure agree with the 416 advice. The 4140 at 28-32 Rc is what is commonly sold prehardened 4140.
A while ago the local BLM wanted me to make a gravel spike to test the road gravel thickness that contractors lay on their roads. It was made from 4140 prehardened 1 1/2" round about 13" long, tapered most of the way. There is a channel milled about 5" long on one side so they can see where the gravel ends and the dirt starts. This project came to me without recommendations as to material or heat treatment, well, I guessed that the 4140 was at least as hard as the sample they brought so I made it out of that. (The sample was almost dead soft)
It came back the other day with 9 splits or cracks equidistant around the crown where they had been hitting it with a sledge hammer. I was amazed! The guy said they only used it one day. I then heat treated the crown to 48 Rc after tapering the crown area to act as a belt, we'll see if it comes back.
The point is that I really thought I knew the material I'd been using every day for over thirty years. So the strength may be directional.

I've had Chinese-made chisels (in China) that had a softer 'ring' shrunk on to a rebated diameter at the striking end. Works a treat.

And yes to 'directional'. Back in the '50's US Steel sent movies around to shop classes.

One of them was of the making of railway car wheels. Started with a bar with the proportions of half a soup-can, writ large.

Went under a monster forging press at heat, came out upset to a much greater diameter as well as shorter. Then they showed the Metallurgists etched cross-sections.

Radial 'spokes' - differences in the grain of the steel - had been made to flow outward like petals of a flower. Which was what exactly was intended for that application.

Bill

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