Steel for Action?

[oldred]

IMHO you are definitely making the right choice using bar/flat stock rather than starting with a casting, getting that 4140 stock to the point of where the casting would be at the start of the project is the easy part. After that I honestly believe that working with your raw stock will be much easier than working with the casting because setup will be a LOT easier! Those castings would probably be easier and quicker if a person had the production setup jigs but without them setup would be the major portion of the work and besides any serious mistake (mistakes do sometimes happen unfortunately) can quickly turn a fairly expensive receiver casting into a paperweight! Having to start over with raw stock might be disappointing but it would be much less expensive, of course to be fair finishing the exterior contours of the casting would be simpler and would require a LOT less shaping and file/stoning work. That exterior finishing however is the most fun part for me and I don't consider that work at all, seeing it take shape under the files is very satisfying and goes a lot faster than most folks might imagine, in fact I was very surprised at how quickly the first one I did took on it's final shape.

However you decide on each step please keep us posted, for a long time I have wanted to see other builders get involved and I think it would be a lot of fun if we could get several folks together to share info and to discuss problems that might arise or help with the inevitable "head scratching" episodes that are surely going to arise. I have built three of these from scratch and currently working on a fourth but each time some new puzzle presents itself and has to be solved so if I can be of any help at all please feel free to PM me or just post it here.

[john.k]

Ive found its easiest to start the machining with the block mortice....once this is square or whatever it needs to be,its much easier to match the block size,and its also much easier to machine the externals square with the internals. ...Obviously if I was an accurate worker this wouldnt matter,but I aint.,so I just go with the flow.......not so much a gun as a work of art.

[oldred]

Ive found its easiest to start the machining with the block mortice....once this is square or whatever it needs to be,its much easier to match the block size,and its also much easier to machine the externals square with the internals. ...Obviously if I was an accurate worker this wouldnt matter,but I aint.,so I just go with the flow.......not so much a gun as a work of art.

Couldn't agree more about that block and mortise as I too cut the mortise first but before doing anything I first mill the receiver blank on one side, the top and what is to be the front all flat and squared to each other. These surfaces are then ALWAYS used as reference and setup surfaces, I then establish a center-line (referenced from the flat squared sides) both top and bottom along the length and vertically on the front end 90 deg surface. As long as a person works from the same side and maintains those reference lines getting lost will not be a problem, trying to just start at one random machined point and then locate everything else relative to that makes it too easy to lose reference. By machining not only the breech block mortise first but also the lower tang area, etc it's much easier to get a really good fit. I know that theoretically starting with the small parts first shouldn't be a problem with fit but tiny errors can occur especially for those of us who can't lay claim to being pros and fitting the attaching parts to the finished receiver is a lot easier than the other way around!

[Willbird]

One point of order, while 4140HT is pretty strong, it may not have the same properties as an action heat treated to the ideal hardness. I have tested a few rem 700 actions I had when I had access to a rockwell hardness tester and 36-38 sounds like about what I got. So if you duplicated a rem 700 from 4140HT which is 28-32 rockwell C it would not have the same properties as the action you copied, it might still be plenty strong though.

Received the results of the Rockwell test and the action was in the 36-37 HRc range and the bolt was in the 41-42 HRc. So I will not find out if the remainder of the bolt will separate or not?

Note the difference in hardness between the two parts of the action, that is on purpose too, running the same metal at the same hardness together will wear more than if one part is harder than the other.

I have also seen a lot of 4140HT parts that were scrapped due to cracks in the bar stock, some of them BIG parts 24" in diameter and 24" long....cracks found after about 30 hours of machine work . Gunsmith friend made up some 1911 compensators from 4140HT and after he had them finished and blued he saw the crack that probably ran through the whole bar of material.

Just a few things to be wary of....those things are not material specific....for something you are going to put a LOT of time into, buying an aerospace grade of material with certifications might be worth some extra cost.

Bill

[oldred]

One point of order, while 4140HT is pretty strong, it may not have the same properties as an action heat treated to the ideal hardness. I have tested a few rem 700 actions I had when I had access to a rockwell hardness tester and 36-38 sounds like about what I got. So if you duplicated a rem 700 from 4140HT which is 28-32 rockwell C it would not have the same properties as the action you copied, it might still be plenty strong though.



Note the difference in hardness between the two parts of the action, that is on purpose too, running the same metal at the same hardness together will wear more than if one part is harder than the other.

I have also seen a lot of 4140HT parts that were scrapped due to cracks in the bar stock, some of them BIG parts 24" in diameter and 24" long....cracks found after about 30 hours of machine work . Gunsmith friend made up some 1911 compensators from 4140HT and after he had them finished and blued he saw the crack that probably ran through the whole bar of material.

Just a few things to be wary of....those things are not material specific....for something you are going to put a LOT of time into, buying an aerospace grade of material with certifications might be worth some extra cost.

Bill

All very good points and buying certified material certainly offers peace of mind if it's worth the added cost to the builder. A finished receiver made from 4140 will be fairly easy to check for any cracking however voids or inclusions are a bit more of a problem, I think checking for cracks after machining is complete and a rather hefty proof testing step should render a home built firearm about as safe as we are going to get them. However I still want to err on the side of safety and while I pressure tested my rifles (except for the WMR 22) at approximately 50,000+ PSI as indicated by loading data I still limit my loads to around 25,000 to 28,000 PSI. If I were building a modern design rifle for high pressure cartridges I would want to use steel that has been certified but obtaining such materials in small quantities can be a bit pricey for what we are doing, still if a high pressure cartridge is wanted/needed then maybe it should be considered regardless.

[Willbird]

All very good points and buying certified material certainly offers peace of mind if it's worth the added cost to the builder. A finished receiver made from 4140 will be fairly easy to check for any cracking however voids or inclusions are a bit more of a problem, I think checking for cracks after machining is complete and a rather hefty proof testing step should render a home built firearm about as safe as we are going to get them. However I still want to err on the side of safety and while I pressure tested my rifles (except for the WMR 22) at approximately 50,000+ PSI as indicated by loading data I still limit my loads to around 25,000 to 28,000 PSI. If I were building a modern design rifle for high pressure cartridges I would want to use steel that has been certified but obtaining such materials in small quantities can be a bit pricey for what we are doing, still if a high pressure cartridge is wanted/needed then maybe it should be considered regardless.

Really tough stuff to machine without cnc and flood coolant, but one prior employer made some parts out of Inconel 100 (we actually used aeromet 100...same stuff more or less) because it was as strong as machined as 6150 that was machined and heat treated post machining. Might be OK home shop machining for simple parts. The SFM for an HSS drill was something like 18-20 SFM...watching a 3/4" drill loaf along at that speed was amusing .

The stuff is still pretty much as strong at 1000F as it is at room temperature too .

S7 might be an ideal steel if you had a way to have it heat treated in a controlled atmosphere furnace.

Kinda wonder if boiling a part like happens in bluing, or warming it to that temp then cold bluing it might make cracks visible ?? I have used the spray bomb crack detection stuff before, it works fairly well.

Bill

[fast ronnie]

I am kind of late to this thread, but here is my two cents worth.
4130 or 4140 will warp during heat treat. A2 and D2 are both thru hardening steels that are designed for cutting. When heat-treated they will have the same hardness all the way through, and as such will have no warning of failure. They will not tolerate shock very well. S7 might be a reasonable material to work with, as it is designed for heavy shock loads. There is a better grade of steel for this purpose, however. 4340 is somewhat similar to to 4140, is easy to machine, is very strong and shock resistant. It warps very little during heat-treat compared to similar grades of steel (4130 and 4140) and and when heat-treated to approximately 40 rockwell, has a great strength along with just enough "give".
It is used in making high-performance crankshafts. Those things take an awful lot of torsional flexing. It is one of my "go to" steels for anything requiring high strength and has some really good properties. It is not hard to find, as you can order it from Mcmaster Carr or several other suppliers. It is only very slightly more expensive than 4140. There is not a single reason that I can think of not to use it.

[oldred]

I have about 250 lbs of Inconel 718 in 4"x8"x18" blocks and I think it will come in really useful someday for an anchor if I ever get around to buying a boat, Lol!


Joking aside I have the stuff and simply don't know what to do with it, I know a few things I would like to make with it but about the only thing I have actually made so far using my manual machines is a big pile of dulled and/or broken cutters. I mean that's some tough stuff! I can get started cutting Ok it seems but then all of a sudden it will work harden and after that even carbide can't handle it but then so far all I have done is just play around with it to see what it was like, not much to like I suppose.

[oldred]

4130 or 4140 will warp during heat treat.

That's what's so nice about 4140PH it machines quite well as is, for a rifle receiver it is very strong and needs no further heat treating. I am sure there are other alloy steels out there that fill the bill just as well but 4140 seems perfect for this, a good combination of hardness and incredible strength plus good machining characteristics also, maybe not the best of any alloy but still very well suited for the home rifle builder. Also as noted earlier by several others it is the alloy of choice for Shiloh's rifle receivers and unless I am mistaken even the Ruger no 1.

[John Taylor]

Fun stuff to work with is Titanium. I have rifled about a dozen barrel made of this stuff and it is the only time I have had smoke coming out of the bore of a barrel in the rifling machine.

[Willbird]

I have about 250 lbs of Inconel 718 in 4"x8"x18" blocks and I think it will come in really useful someday for an anchor if I ever get around to buying a boat, Lol!


Joking aside I have the stuff and simply don't know what to do with it, I know a few things I would like to make with it but about the only thing I have actually made so far using my manual machines is a big pile of dulled and/or broken cutters. I mean that's some tough stuff! I can get started cutting Ok it seems but then all of a sudden it will work harden and after that even carbide can't handle it but then so far all I have done is just play around with it to see what it was like, not much to like I suppose.

If you use the Machinery Handbook speeds and feeds, and use flood coolant I have run the same 3/4 HSS steel twist drill for days running the machine 24 hours a day (two 12 hour shifts)....use a power feed of about .01" per revolution.

As far as S7 goes, where it excels is the Jominy Notch test.....which tests how a steel does under impact when a deep cut stress riser is present. Lots of gun designs include stress risers in their design.

The strongest attribute for 4140 is that it blues really nice . ...it is not bad by any means, but it has nothing on S7 heat treated then hit with some nice ion nitride .

[oldred]

Lots of gun designs include stress risers in their design.

Just a note about that and any falling block design such as the High wall and Low wall designs, when cutting the Breech block mortise be aware that the inside corners need to have a small radius to eliminate stress risers at that point in the receiver to prevent receiver failure in this highly stressed area. Then there is that extractor notch in the chamber that has lead to disastrous failures when cut using sharp tools and no radius is left in the corners of the cut, that one can be critical especially with higher pressure rounds but even moderate pressures may cause failures.

[kywoodwrkr]

"be aware that the inside corners need to have a small radius to eliminate stress risers at that point in the receiver to prevent receiver failure in this highly stressed area"
oldred,
What radius is ideal, or a range for radius?
I'm going to try and use an EDM for machining and wire in it is .010" dia., compensation wise that is considered .006 with electrical gap.
Therefore my smallest radius would be .006, larger would just be drawn in as fillet.
Cutting outside of breech block not a problem, 0.000 to, reasonable.
Thanks.

[oldred]

What radius is ideal, or a range for radius?

The print calls for a .015 radius of the inside corners of mortise.

I started to reply with this info last night but rather than just take it from memory I decided I would wait and post a shot of the actual print his morning, wouldn't you know there was an oil stain on the large version in that area and I couldn't get a satisfactory shot of the original small print. Both are clearly readable when looking at them but when transferred to the computer neither was clear enough, at least to me but I can try again if you want. In any case it is .015 for certain.