Receiver Fabrication — Riveting and Heat Treatment
We have already bent and milled the top rails of the Kalashnikov receiver, the Ствольная коробка, or stvol'naya korobka, the "barrel box" as the Russians call it, producing what you see below. Now it's time to spot-weld the internal rails into place and rivet the front and rear trunnions and the trigger guard into place. Then we will heat-treat the areas around where a few of the cross-receiver pins will be mounted.
This is one in a series of pages about reconstructing an AK-47, Автомат Калашникова 47, Avtomat Kalashnikova 47 or Kalashnikov Automatic Rifle 1947 Model.
We must very thoroughly clean the receiver and internal rails using acetone.
You must remove all the oil before spot-welding those rails into place! Acetone will dissolve and lift off all the oils, including skin oil, and also lift off any water moisture. Wipe it down with acetone moistened clean paper towels. The discoloration you see in these towels is from the oil that was cleaned off.
Now we have spot-welded the internal rails into place.
The welds need to be cleaned up with a small sanding wheel to remove the small flecks of metal.
A customized fixture provides a way to install the rivets.
You also need a simple but customized striking tool to install the rivets holding the trigger guard in place down in the bottom of the receiver, and especially for the interior ends of the rivets holding the front trunnion in place.
Most of the slots are left over from this being scrap in another project. The key dimensions here are the overall width and the two notches near the bottom.
We are ready to rivet a rear trunnion into place. The rear trunnion uses two long rivets. The first one is already in place, the second one is ready to be hammered.
Start by tapping the rivet with a ball peen hammer. It is important to use the right amount of force. You don't want to try to mash it all the way down with a few extra-hard blows!
Wrong: BOOM BOOM BOOM!
Right: tap tap tap tap tap tap tap tap tap tap tap.
Once the head starts to mushroom, use a rivet driving tool. Its driving face encourages the formation of a proper mushroom head.
The head mushrooms further out. Continue to strike it with the driving tool with the cup at its end. This rivet is finished!
Three rivets on each side hold the front trunnion in place. They are short rivets, passing only through the receiver and the outer face of the trunnion.
The single rivet above the front end of the trigger guard passes all the way through the receiver, passing through a steel tube.
Two rivets hold the rear trunnion in place. They are two long rivets, passing all the way through the trunnion and both sides of the receiver.
Coming through the bottom of the receiver, one rivet holds the rear of the trigger guard in place.
Two rivets on either side hold the front of the trigger guard and the magazine catch in place.
You can see the single rear rivet and two of the front rivets on the trigger guard and magazine catch.
Looking through the magazine well, you can see part of one of the internal rails.
Looking through the magazine well, we can see the central rivet passing through the tube across the receiver.
We can also see some of the internal rails. The bolt was assembled into place and tested for fit.
There was not enough clearance for the bolt to translate forward and back. A comparison measurement was made to a commercial receiver, and metal was very gradually removed from the tab on that rail with a file. A small amount of metal would be removed, and then the bolt was tested for clearance again.
We can see the internal ends of the rivets holding the trigger guard and magazine catch in place.
These internal rails were already heat treated. If they hadn't been, that would have been required before spot-welding them into place.
Here you can see the internal ends of some of the rivets holding the front trunnion in place.
These are the most difficult rivets to install. A customized striking tool is definitely needed for this!
Here we are looking into the front trunnion, where the chamber end of the barrel will be inserted. A pin pressed through the hole across the top of the trunnion, seen on top at right here, will lock the barrel and receiver together.
This is the other side of the front of the receiver, with a flash rather than ambient light as seen above.
The real Bubba method here is to use something like 10-32 machine screws! You can probably imagine how quickly they would work loose when the gun was fired.
The remaining holes need to be heat treated. The goal is to make the steel harder, but not brittle.
The steel was relatively soft, allowing us to bend the flat into place and allowing dimples to be stamped into the original flat. For the most part, the steel is of appropriate hardness. However, we need to harden the steel around the three holes that mount the pins holding the moving parts of the action.
The trick is to take the steel up to a certain temperature and then quench it by dropping it into the pan of oil. That makes it hard, but leaves it brittle.
Then, take it back up to a slightly lower temperature and hold it there for a few seconds, allowing the newly formed grains to release their stress. Then dunk it back into the oil.
There's a real art to the heat treatment. With experience, the temperature can be judged by the color.
The result looks a bit ugly for the time being. I have removed the scale from the burned oil with a small high-speed sanding disk.
The three heat-treated holes from front to back, or from left to right in this picture, are for the hammer pin, the trigger pin, and the safety level pin.
The next step for the receiver will be bead-blasting and parkerizing, which will form a protective outer layer on the metal and clean up the appearance.
Next step: Restoring the wood stock