www.MG42.us

I just got my reciever welded up and here is how I did it,along with some things I learned.
Alignment-
I wanted to do a first class job, so I put in the effort to make a proper alignment mandrel to hold the reciever pieces in place for tack welding.
My alignment mandrel started out as a 2 1/2 inch diameter steel round bar, 40 inches long. I then machined it down to fit inside the reciever.
I set the overall length,using bipod mount and other holes,about 1/8 inch long to allow for weld shrinkage. This dimension worked out exactly right. When finished, the booster could be screwed on all the way and the barrel went fully forward into battery.
Welding-
I tack welded the pieces with a stick welder, at home, then spent 4 1/2 hours TIG welding at work. I also used a MIG welder a little bit on the inside of the muzzle end, because of limited access. I used copper weld backup. Brass weld backup is not very good because the zinc boils out and causes poor quality welds. In some places, where internal clearance allowed, I used 1/16 inch thick steel backup that remained in place as permanent weld reinforcement.
The most difficult part of this sort of welding is building up edges. I used run off tabs. These allow the weld bead to continue past the edge of the part. They are ground off later to leave a clean, sharp edge, without undercuts or craters.
Straightening-
After welding I heated the welded areas red hot and bent the reciever so it was perfectly straight and centered on my alignment mandrel, front and rear. The straightness changed when the metal cooled off, so I had to do it several times to get it right.
Weld cleanup-
I think it is important not to grind the welds completely flush. The weld bead buildup is needed for strength. I can`t tell you how many times,over the years, seen a machinist or welder use inadequate weld preparation chamfers and then grind the weld almost completely off !!, causing failure.
A nice weld bead is a thing of beauty and need not be hidden.
For internal grinding, I had to buy a better dremel tool. There was one difficult spot where the recuperator goes. I bought extra long carbide burrs for my air die grinder, and also welded a handle on to the side of a 1/2 inch wide, half round file, to reach that area.
Riveting-
I made an alignment mandrel to line up the rails with cam piece , at the front, and to center them vertically at the rear.
I had a lot of trouble riveting the cam piece. I tried using 6mm drill rod for a rivet, but the tool steel material would just not mushroom properly , and kept buckleing in the middle. I had to do it four times, next time I will buy a bolt.

All my effort was worth it. When I put the bolt in, it just fell right in and locked up with the barrel perfectly, with no binding or resistance.

john s

PLEASE!!

A properly done weld can be ground flush with no loss of strength. the reason some
welds break after grinding is there is not enough penetration. A weld with 100%
penetration will not weaken when groound flush with the surface.
When welding a part like the receiver you need to compensate for shrinkage and
warping and plan the welds to minimize it. by properly tacking the parts in place and filling
the gap from both sides a small section at a time it will minimize the shrinkage and
warping, and eliminate the heating and beating.

Cast weld filler metal is ALWAYS less ductile and more prone to cracking than the wrought original material.

In an ideal situation , one could weld the outside, then back gouge the inside until sound metal is reached, then weld the inside and grind both sides flush. This method would most likely require several cycles of welding, then grinding, more welding, more grinding...... with each cycle introducing more warping and distortion.
On a simple part that is easily accessible from both sides, this method would be practical and would provide a weld joint that is ALMOST as good as the original.

In my many years as a machinist, one of the most common welding mistakes I see is attempting to hide a weld repair by grinding it flush. In trying to remove all signs of welding, the weld reinforcement is removed and the metal stock usually ends up being thinner than it was before.
100% weld penetration is an excellent goal , but it requires extraordinary effort, and rarely happens in the real world.

Because of the shape of the mg42 reciever, it is almost impossible to get a perfect weld on the inside, with no undercuts or defects of any kind, and then grind it perfectly plane and true, without reducing the thickness of the metal to a dangerous degree.

I chose not to try to hide the fact that my gun has been welded.

Properly tacking parts and welding a gap from both sides, are excellent methods to reduce warpage and shrinkage.
Unfortunately, even the best laid plans go wrong.
I heated the welded areas red hot to relieve stresses that are introduced by welding. Only a little gentle bending was needed to bring it into perfect alignment.

Sounds like you really went to a lot of trouble - certainly your receiver is probabaly better that mine.

I ground off all the slag. Then I made a copy of Pirates jig with a few mods. I set my length using the top cover, bipod, and barrel/booster, then checked to make sure my OAL was close. Next I made up some backer plates of copper and got to mig'ing. I ground all the welds down flush, didnt like some due to thinest so I re-migged them and reground them. I xrayed my receiver and found no ugly voids. Fixed a few craters, ground , sand blasted and KG coated it.

Over the course of its new life, Ive had 3-4 out of battery explosions at least. Ive had to bend the barrel door back in at least twice since it was blown out half an inch on 2 occasions. My welds have been re-x-rayed and show no signs of cracking. Ive got around 1800 rounds thru it of 308 and 8mm so far.

My point is while your weld up is probably superior to mine, does it matter? If I take bench rest quality reloads, will it matter in a mini-14?

Remember the 42 is really a design that will simply quit if the receiver fails - its not like your rewelding a chamber to contain the round as it fires. Having talked with a few guys that have rewelded post sample FA guns they agree.

Im still impressed with your patience and skills though -

All of mine is MIG welded. First tacked, checked for alignment, cleaned up the welds, tacked some more, cleaned up, etc. Its straight, w/o the full jig work. Cleaned(ground and filed) all the welds outside, cleaned as needed inside. Did not want to fully remove the evidence of it being destroyed.
Kept welding to very small areas prevented warping

Impressive work on your part. It ought to be perfect

hey the other the I neglected to mention was in relation to grinding welds - when I took my test to become certified in plate welding (taking the pipe test in December) a few weeks ago.

Ther test consists of welding 2 pieces of 1/2 plate end to end, grinding the weld flush, cutting out a 2 inch section and then it has to survive a U-bend without cracking. The 'coupon' is bent so as to put maximal strain on the weld. When you take the pipe cert test the cut 4 sections from the 2 rounds you welded together and they all have to pass without cracking. Again the weld gets ground flush before it gets bent...sounds kinky.