Weld Questions On My Receiver

[Gorbachov]

As you can see by the picture, I have my four-cut German receiver set up on a jig for TIG welding and I’ve got a couple questions before I take it any further.

1. Is there any reason one would extend the grip stick slot and weld in the bolt block stud prior welding up the receiver pieces, or can these procedures be done after the receiver is welded together?

2. To prevent weld filler from protruding inside the receiver, do the all three cuts at the trunnion and behind it require copper backing around the entire inside circumference of the cut to back up the weld, or is it just the sides that need backing?

3. I’ve bolted the rails in to align and hold the two receiver sections in their correct position and am planning to tack the sections together with the rails in, then remove them and finish the welds. Is this going to be a problem?

4. Lastly, I haven’t attached a line down the side or the top of the receiver yet to align it top to bottom or side to side and want to know if the holes for the rivets on the rails are centered on the receiver top to bottom? If so, I will use the rear bolt that holds the rail to attached the line and then center it down the side of the receiver.

For anyone that’s interested, I’m using highway sign post for the base of my jig, which is 1.75” square with 3/32” walls made out of galvanized steel, with 7/16” holes 1” OC on all four sides. It’s very ridged and works extremely well and I got it from my local Fastenal store.

[Pirate]

DO NOT do any welding untill you do the semi mods!! you will be building an illeagle mg if you do.

the copper backers serve a dual purpose, the y keep air from the back of the weld and also prevent excess buildup in the receiver.

to align the rear sections use a peice of 1 1/4" x 1/8" flat stock set it on the bottom edge of the receiver & drill it for the rivet holes and screw it in.

good luck with the project, looks like you spent a lot of time prepping the parts, it will pay off in the end.

[Gorbachov]

Thanks for the response Pirate! I don’t know what I was thinking…Of course it’d be an illegal machine gun and I’m definitely not going down that path!! By the way, I’ve read your “Build Pictorial” multiple times and am using it as my guide to build my gun.

I’m still unclear though on the copper backing. Do all three cuts at the trunnion and behind it require copper backing around the entire inside circumference of the cut to back up the weld, or is it just the sides that need backing?

I was planning on using 1 1/4" X 1/8" copper flat stock on the tail piece at the very end and am curious what the risks are of using the rails for this alignment on the two sections just in front of it, as it positions them exactly as they were prior to being cut.

[TbcSEOD]

Good lord, that is the finest prep job I have seen to date (at least that I can recall seeing) thing looks like it should be in a museum!

As for the copper backing it would be best to use it all the way around if you can - the o2 getting into the weld will contaminate it and in theory/practice make it a weaker weld. (at least thats my understanding)

~T

[Pirate]

I just use copper sheet, you can also get copper couplings and cut and flatten them out. you only need the backer where you are welding

getting the first rear section is critical as it affects the alignment of the buffer, if it is crooked your recoil spring will bind.

[Gorbachov]

I truly appreciate the input guys! I had all the parts bead blasted and probably went a little overboard. The place that did it even blasted the inside of the feed cover, which I’m hoping isn’t going to affect its ability to cycle. I want to blue the gun as it was originally and if I do I’m going to have to polish the entire receiver back to a nice shinny smooth finish first.

I’m clear now on the copper backing, but am still curious what the risks are of using the rails for tacking purposes to align the two sections just behind the trunnion. Pirate, you say “getting the first rear section is critical as it affects the alignment of the buffer, if it is crooked your recoil spring will bind”. Are you speaking of the small section at the very end?

I’ve been looking through the tutorials trying to find a measurement for this and the closest thing I can come up with is the CAD drawings by “Flemgunner” that show the very end of the gun at 3” from DC of the rear rivet hole on the rails. Another reference in another thread says the measurement from the receiver rear to the rear of the cam window is 382 mm.

I’m planning on starting the welding process this next week and am still a bit unclear as to some of the dimensions. There are some great photos showing some of the measurements in metrics, but I haven’t found any that show the entire length of the gun. Can someone please help point me to the right place in the forum to find this?

[TbcSEOD]

I truly appreciate the input guys! I had all the parts bead blasted and probably went a little overboard. The place that did it even blasted the inside of the feed cover, which I’m hoping isn’t going to affect its ability to cycle. I want to blue the gun as it was originally and if I do I’m going to have to polish the entire receiver back to a nice shinny smooth finish first.

I’m clear now on the copper backing, but am still curious what the risks are of using the rails for tacking purposes to align the two sections just behind the trunnion. Pirate, you say “getting the first rear section is critical as it affects the alignment of the buffer, if it is crooked your recoil spring will bind”. Are you speaking of the small section at the very end?

I’ve been looking through the tutorials trying to find a measurement for this and the closest thing I can come up with is the CAD drawings by “Flemgunner” that show the very end of the gun at 3” from DC of the rear rivet hole on the rails. Another reference in another thread says the measurement from the receiver rear to the rear of the cam window is 382 mm.

I’m planning on starting the welding process this next week and am still a bit unclear as to some of the dimensions. There are some great photos showing some of the measurements in metrics, but I haven’t found any that show the entire length of the gun. Can someone please help point me to the right place in the forum to find this?

Rewelds can never be too clean - it looks assume!

As for OAL - 37 11/16 as ref: viewtopic.php?f=10&t=6606&p=50195#p50195 and http://www.salt6.com/pirates%20build.htm (about 1/2 way down)

my .02 on using the rails is - careful and watch the heat, other than that I don't see why you couldn't use them for alignment - just don't damage them. The only thing I can think of would be that the welds might shrink a little (go slow and little sections that bounce from side to side and cut to cut to distribute the heat and min warping and twisting) in the process of shrinking the rivet holes might come out a little askew - but I mean were talking hundredths of an inch so I don't think that would really come into play - just a matter of reaming them back to round and riveting as usual.

Hope that helps

~T

[Bil]

I used a flat piece of steel,like Pirate said.I used the rails to determine the holes,and bolted and clamped it.I found it easier to clamp and align with the flat surface,also to hold the copper sheet I used as backer.Also I was afraid of damaging the rails.It worked good this way,but I suppose using the rails would work too. ---bil

[Pirate]

I’m clear now on the copper backing, but am still curious what the risks are of using the rails for tacking purposes to align the two sections just behind the trunnion. Pirate, you say “getting the first rear section is critical as it affects the alignment of the buffer, if it is crooked your recoil spring will bind”. Are you speaking of the small section at the very end?

Yes, the very end section, it needs to be straight so that the buffer is centered straight in the receiver.

I’ve been looking through the tutorials trying to find a measurement for this and the closest thing I can come up with is the CAD drawings by “Flemgunner” that show the very end of the gun at 3” from DC of the rear rivet hole on the rails. Another reference in another thread says the measurement from the receiver rear to the rear of the cam window is 382 mm.

the best way to set the rear dimention is to use the feed cover. when all the parts fit the receiver sections wil be in the right place.

as far as using the rails they will not align or hold the parts as well as a peice of flat bar stock.

[cz2]

Concerning use of the rails as a guide in the re-welding,do not do it. As a first time sa42 builder I felt the rails would be the best way to align the sections also.After 3 welds and 2 re-cuts , I can say the rails are not strong enough to hold the sections straight! As a long time welder I thought welding up the sections would be a snap.
The experienced builders here have it down,measure,tack,cool,check and repeat.
I think a lot of stress is in the sections from original forming,use,and the cutting de-mil process.The sections I have move a lot during welding.
Still have 4 of 5 welds to go,maybe some day? Good luck on your build.
cz2.

[88comm]

From what I have read in other posts DO NOT plan on doing all the welding in one sitting. It involves tacking and checking, a little more welding and more checking, then some more. You want to minimize heat to the receiver, so don't do a whole line of weld bead anywhere - almost garanteed to warp.

Copper as a backer helps also in cutting down on inside clean-up of the weld. the copper water pipe fittings are a good idea. Even the pipe will work. 3/4" would probably be a pretty good size. Cut it down the side and hammer it flat. A note aboput copper. it work hardens (getting stiffer as you bend it). To anneal/soften it, heat it red hot with a propane torch then IMMEDIATELY quench in water. It is now soft and will bend to shape much more easily.

As you tack it together, assemble the various parts to it to check alignment and fit. A short tack can be easily cut and rewelded. The longer ones obviously take more work.

[Gorbachov]

I sincerely appreciate all your help guys…There’s now way I could do this without your help! I spent the entire weekend running around getting supplies like copper, 1-1/4” flat stock, etc., so I’m ready to tackle the re-alignment on my jig as you all have suggested.

One other question I haven’t gotten answered. I need to pull a string down the center on the side of the receiver and want to know if the holes for the rivets on the rails are centered top and bottom on the receiver to line up horizontally with the bore of the barrel?

[88comm]

Gorby -

I would say that theoretically this would be the case. Others may state that a little stronger. I found that when looking by eye down the length of the tacked together receiver, there are few lines that are easliy identified as straight. By looking at it this way, and rotating it (like a chicken on a spit) you can get a better feel of how straight it is. The truely important alignment is between the bolt and the barrel - when they lock-up together. IMHO the receiver was designed with a looseness that allowed many (some?) misalignments and still worked as designed. The M1 Carbine and the FAL and the AK (so I understand) all have some slack as part of the design. Those designs allow for dirt in the action and continued operation. The M16, on the other hand, is relatively intolerant of dirt in the action.
To get back to your question - I did not make special efforts to use a string to check my receiver. I did use one, but I checked "by eye" much more than I used any string. I am completely happy with my results (and as a college professor I expected my students to hold a plus or minus tolerance of .005", I could do better)
Logically, if the bolt slide surfaces are parallel to the bolt centerline, then the rails would/should run parallel to the barrel bore. I think the rails are interchangeable left and right, so their holes should also be parallel to the bolt sliding surfaces (on the rails).
As noted when starting this discussion - theoretically all these lines should be parallel to the barrel bore. What the design tolerances for misalignment were back in '42 would be interesting to know...

[Bil]

Note-not all rails are interchangable left to right!!In the 42 the left side rail has a thinner lower flange for clearance of the recuperator.I think later that was changed.Check in the stickies section here and on the top of the page.It would not made a differance for your center line,but would cause trouble later. ---bil

[Pirate]

I have done mine by eye, with all the tapers etc it is tough to get measurements. if you fit all the parts as you go and eyeball it it will be straight.

[MGMike]

The lower left corner of the receiver is the only square part that's easy to work off of. I've notched a piece of hardware store aluminum angle to clear the bumps that are there, and after I learn to use my Harbor Freight TIG welder, that will be part of my line up for welding. Thanks, Pirate for "TIG 101"

That said, the important line up is the barrel, the front bearing, and the rails. If that stuff lines up, it should work

Have fun.

MikeD

[88comm]

Thanks Bil - now that you mention it I seem to remember that there were lefts and rights on the rails. My mistake. I agree with Pirate 100% -

It will work, and the nice thing about working metal - if it is wrong you can cut it apart and reweld it (can't do that with a wood project )

[drooling idiot]

Took these quotes out because they sum up how I've done 7 receivers so far.

there are few lines that are easily identified as straight. By looking at it this way, and rotating it (like a chicken on a spit) you can get a better feel of how straight it is.

The lower left corner of the receiver is the only square part that's easy to work off of.
MikeD

mount the receiver upside down on a uni-strut jig. align by eye with a string line and tack. Remove from jig and rotate the receiver so your eye can see where its off. The thing about the 42 receiver is its very "organic" in appearance with many curves & few strait lines.

[Gorbachov]

I’ve spent the entire day (Sat) re-aligning my receiver sections with 1-1/4” X 1/8” steel flat stock and am finding that the width of this piece of steel is a little bit too much and it doesn’t lay really flat inside the receiver but rides up in the radiuses. The result is the sections aren’t aligning real well down the side.

It appears that a 1-1/18” wide piece of steel would correct this and I’m going to oversize the holes so the screws going through the rivet holes allow the pieces to be moved and adjusted after the whole thing is screwed back together. Anybody here experience this problem and if so, how did you correct it?

[Gorbachov]

It appears from the alignment and rigidity I’ve gained on the three rear sections behind the trunnion after installing the 1-1/8” steel that I don’t really need to set them up on the jig in order to weld them together.

All three pieces making up this rear tail section are aligned perfectly and I’m thinking I’ll tack and weld these up completely and then weld that completed section to the front section surrounding the barrel in the jig.

Any thoughts on this?