Receiver Fabrication — Bending
The receiver, the ствольная коробка, or stvol'naya korobka, the "barrel box" as the Russians call it, is the part considered to define the firearm. It is the one part completely destroyed in a parts kit. And, as mentioned on the previous page, the BATFE has recently ruled that the barrel must be destroyed in order to provide more business for U.S.-based manufacturers of replacement Kalashnikov barrels. We need to learn how to bend a commonly available receiver flat into a new receiver.
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 have already see about its history, and we have looked at what's included in a parts kit and how to do some initial work to fully disassemble the receiver fragments destroyed as per BATFE regulations. Now it's time to start building a replacement receiver.
Here you can see why this part of the project is required.
This receiver has been cut into three main pieces with a saw and a torch and the central part is missing. You see the receiver cover at top, the rear and front trunions at left and right in the middle, and the receiver fragments below.
Here is the starting point: a "flat" showing both sides and a pair of rails.
The flat is just a specifically shaped piece of sheet steel. It is slightly tapered and has a number of notches and holes. Many of the holes are small and round for rivets, three are roughly rectangular, and one is shaped like a keyhole.
The largest rectangular cutout is the magazine well, the next largest is for the trigger, and the small square one is for mounting the pistol grip.
The flat will be folded along its long axis into three main faces, with two more narrow folds for the top rims.
The first picture shows the face that will become the exterior, the second shows what will be the interior.
Notice that despite its name, the "flat" is not really flat. There are oval dimples that will be on the sides of the box-like receiver, above and to either side of the magazine well. There is also a slight ramp at the forward end of the magazine well.
Apparently there are people whose have the job of deciding just precisely how flat is "flat enough", the precise point at which a pre-drilled, specifically shaped, and slightly bent sheet of steel becomes close enough that it's more of a receiver (and therefore a gun) than it is a sheet of steel.
The rules are complex and vary by specific model.
For the Avtomat Kalashnikova, "flat enough" means almost completely flat. However, a "flat" for an MP5/HK94/G3 receiver has almost no planar surfaces, ask Google to show you pictures.
You can find lots of videos on line showing someone pinching a steel sheet in a vise at the end of the workbench and going at it with a hammer.
The ultimate page is the DIY: Shovel AK project, which starts by converting a shovel handle into a buttstock similar to a folding paratrooper model, and progresses to build the receiver from the shovel blade!
Our goal here is to be significantly more precise.
We will assemble the flat between a pair of steel blocks. A rectangular die will be sprayed with white lithium grease. We will then form the bends by forcing the block through the shaping die with a hydraulic press.
The flat has been placed on the bottom block. It has been precisely located on pins that fit through the existing holes that will be used in later assembly steps.
The top plate has been mounted onto those pins and screwed tightly into place.
This is the forming die. The assembled block with the protruding "wings" of the flat, seen at left rear, will be forced down through this opening to form clean bends.
Notice that the edges of the tool steel die are slightly rounded. This ensures that the flat is bent, and not cut or pinched.
The lower block has been sprayed with white lithium grease.
In the second picture you can see where we have placed it below the ram of the hydraulic press.
Here is the lower block sitting in the press below the hydraulic ram.
The assembled upper block and flat have been placed in position.
Notice the pockets in the side of the block. They have been milled out to accept the dimples on the flat which is not quite flat.
Some pieces of steel bar stock have been placed on top, and the press is starting to force the upper block down into the lower one.
Notice that the "wings" of the flat are starting to bend up. You can see where the white lithium grease has sort of bubbled underneath it as it lifted away.
Do this slowly. The bends should form evenly and symmetrically. Release the pressure and move the assembly to the left or right to direct the force toward one end or the other as needed to keep the bend smooth and even.
Some time later the upper block has been forced entirely into the lower one, and the major bends (which will be the lower edges of the finished receiver) are complete.
The greatest pressure is needed to get the bends started. But you want to apply that pressure very smoothly to get smooth, straight bends.
It gets very easy once the main bends get to about 30°. However, take it very gently and smoothly.
After the block was fully seated down within the larger block, the remaining protruding edges of the flat have been bent down to form the rails along the upper edge. This was done with a Delrin rod driven by a light hammer and many light taps.
Everything has been disassembled and the lithium grease cleaned up with acetone.
There's the bent receiver, lying right-side up on the block. Let's have a closer look at it.
More work needs to be done. The rails need to be cut down in a milling machine. The flat is intentionally over sized, so the rails are wider than they should be. But it would be almost impossible to get a clean bend if they were pre-cut to leave nothing but the proper narrower width.
These pictures show the result of those oval dimples after bending.
The block used for forming the bends has to have pockets machined out where these dimples have to go during the bend.
This bottom view shows the magazine well ramp.
Next step: Milling the top rails to the proper dimensions.