Spotwelding a trunion for a HK91/G3 clone

[slyguy2000]

I have a new 91 receiver coming in from Ohio Rapid Fire, and I was wondering what people were using to spot weld the trunions in? I have a 220 spotwelder that I used for my rails on my Ak build projects, and wanted to know if that would work, or what I would need to do to make it work. Also any assistance on things I would need to know about alignment, and spacing issues would be greatly appreciated too.

Thanks!

[paul123]

i dont think a spot welder will work, personally i have only ever used a tig welder i have heard of people useing a mig welder but with mixed results

[cieneguita]

A standard auto body technique is "plug welding" with a MIG where holes through the outer sheet are filled with the inner sheet as a backer simulating a spot weld. In this application I would think a TIG would be the better option to control heat on the trunion.

[slyguy2000]

ahhh... so I should drill several holes in the receiver and tig weld them closed to the trunion. On the german receiver stub that I have it looks like it was spot welded (it's very flat and uniformed ~.25" circles, and appears to be spot welded) on my bobcat BW89SP, it does look like it was drilled and filled in with a tig weld.

[slyguy2000]

are there any alignment snafu's I need to worry about? I was going to lock the trunion onto the bolt and carrier thru the receiver to hold the trunion in place and keep it aligned with the b/bc. Is there any other trick I can use to do this?

[paul123]

i think the germans use a robot :)

what i do is drill 4 3/16 holes around the front of the receiver and tig it, i start in the center and get a good puddle and start doing little circles to bring the reeiver into the puddle while adding filler, it doesent look like the german ones but it doesent need any grinding when im done. you just have to be very carefull not to put too much heat into the trunnion it can make it brittle and it could crack.

i do it at around 75 to 85 amps it only takes a few seconds to me the hardest weld is at the back of the trunnion where it meets the rails, on the corners of the little slot you have to keep it around 30 amp or it will burn through but in the middle you have to go back up to about 65 to 75 to fuse the rec. to the trunnion.

as far as alignment thats what i useually do is lock the bolt in and do the front welds and then remove it to do the back ones, just incase i do burn through a glob of molten metal wont fall into my bolt

[slyguy2000]

many many thanks for your words of wisdom. I'll give it a shot and see if I can do my first build succesfully.

I've been looking forward to building this puppy for a while now, and can't wait for the receiver to get here...

[paul123]

another thing you might want to look into is a piece of alluminum rod that you slide into the cocking tube where it meets the receiver to act as a heatsink it helps ALOT! it keeps the metal from burning through

[slyguy2000]

LOL you read my mind... I just got two from my machine shop here at work for that, one to fit into a barrel'd trunion that I plan on using to to check alignment of the barrel to the receiver before any thing gets welded, and one to fit into the cocking tube for the same (and now as you mentioned, for heatsync purposes also.)

[jfk]

another thing you might want to look into is a piece of alluminum rod that you slide into the cocking tube where it meets the receiver to act as a heatsink it helps ALOT! it keeps the metal from burning through

Sounds like good advice....