The AK receiver flat bending jig



After about a year of experimentation and no small number of messed up parts, we finally came up with a working jig for bending the unfinished (meaning no FFL) receiver flats that have been sold by a number of vendors including Tannery Shop, FAC, and others.   It is most likely that these flats were all made by the same manufacturer - Hesse Arms.   While many people have criticisms about Hesse quality, many will concede that the "quality" of the guns is largely a factor of the attention that goes into the final assembly, since the AKM receiver design is fairly simple and straightforward.    The flats have, however, not proven to be very popular among the do-it-yourself crowd because of a fairly high failure rate among people trying to make the main U channel bend who end up with deformed magwell feed lips.   What usually happens is that the bend will not stay in a straight line and the receiver gets deformed.    That is a common ailment on a lot of the sheet metal based open source firearm projects.    The main thing about such weapons from a national level manufacturing standpoint (and what the Soviets looked at) is that such weapons could be made cheaply after considerable development in design and tooling.   Once the design and tooling are set up, just about any uneducated peasant could actually construct the guns.   Well, in theory anyway.    The problem for the hobbyist is that the tooling has been unavailable, or cost prohibitive.   Most of the manufacturers have, until now, kept their tooling designs a secret, and in the case of nationalized manufacture, the tooling designs were considered state secrets.  

I had concentrated development of the jig on the most difficult aspect of the AKM receiver bending operations, which was the main "U" channel bend.   The bends on the two upper lips which then become the top rails of the receiver can be successfully done a number of ways, with the most simple and straightforward being to clamp the receiver up against a steel bar as a straight edge and simply hammer the lip down into place.   This is a fairly simple high school level metal shop operation.   The hard part has always been that main bend, and it is probably the reason most successful AK makers and firearm regulators chuckle at the many aborted attempts by home gunsmiths at constructing their own AKs.   

On a cost analysis factor, this jig system worked out to be a micro-economic version of exactly what the Soviets intended.   A fairly high outlay in effort on tooling to make what is, on economy of scale, a relatively low cost weapon.   Given the prices of parts kits hovering in the $150 price range, receiver flats (with rails) in the range of $50 and the rest of the 922R compliant parts in the range of $40 to $70 depending on the maker, and investment in a $500 jig.   That makes the jig less than worthwhile for the construction of a single rifle when you compare the whole operation to the purchase of a Romanian or Chinese semi-auto AK at around $300 to $500.    On the other hand, club, small group, or avid collector who wants to make four or five guns, then sell the durable tooling off at a nominal discount off of the purchase price, the deal starts to make good sense in a hurry.    It would, in theory, make sense for a rental deal, but experiences with others who had AR jig rentals on very high quality jigs were not so good.   The primary reasons the rentals did not work out were the required shipping costs and time constraints placed on people who participated in the deal.   The other problem was that the jigs cost a lot to produce which made the necessity of a high security deposit.   The high security deposits were necessary to prevent loss, but it still made the whole proposition pretty costly.    It also presents the privacy threatening situation of a database needing to be maintained on who is renting the jigs and making the deposits, as opposed to a situation where the jig can be bought outright and no tracking needed after that.   Since it is fairly easy for us to recognize our work, we are in a position to offer warranty repairs on the jigs regardless of who owns them down the line.    The way this jig is designed, it could be used for hundreds of bending operations with no need for repair.   Consider the price of a good barrel press and the other tooling you will likely need, and an AK factory for your garage is probably going to end up in the $1000  range not including the rivet gun and air compressor if you are hell bent on duplicating Soviet manufacture.  

Now for the parts which might be beyond the ability of your average drunken Siberian peasant.   We did not develop barrel pressing or riveting tools for use with the tooling package yet.   I found that welding is a good enough substitute for rivets on every significant part of AK construction that uses rivets.   In places where there is a hole for a rivet, you can also just stick a rivet or small screw in place, weld the opposite end of it down to a blob and grind the head to shape with a dremel.  I found that the lighter duty wire feed welders are great for this. 

Most of the parts kits come with the barrel already installed in the trunion, and welding the trunion in place is not something which looks so incredibly difficult that it would require removal of the barrel to facilitate.    Furthermore, even a cheap flux core wire feed welder is up to the task and a savvy shopper can get one of them in a basic welder's starter kit for around $250 from any one of a number of tool suppliers.   We did make the jig in a way that you can avoid needing to use the big vice in the picture, and it definitely did not need a hydraulic press as anticipated (a $160 waste of my money I might ad) .   The thing about the big vice is that it is fast and illustrative of the versatility of the jig.   We also found that placing a block of wood up on the steel and hammering the pieces together with a big hammer is crude, but effective.    

Another way of dealing with the trunion to barrel interface issue is to use a MAPP gas or Oxy-Acetylene torch to heat the trunion and cross pin to remove the barrel before installation, then install the trunion into the receiver (rivets, welds, whatever) while keeping the barrel and the barrel pin in the freezer.   When you are ready to re-install the barrel, have the receiver and trunion securely held in a vice, heat up the trunion to where it expands a bit so that you can just wiggle the cold barrel and pin in the place it was before.   99.9% of the time, your headspace will turn out right.    Do not "quench" the trunion in any part of this operation.   The trunion is heat treated and can take a lot of heat without losing its strength, but quenching might make it brittle.   If you must cool it down, use oil, not water.    Note, there are some experts who hotly disagree with this method and will only recommend the use of a barrel press because they consider any heating of the trunion to likely cause irreparable damage to the heat treat done at the factory.   It is a judgment call on the part of the builder as to how they want to approach this issue.  

Of the guns we did, we also were able to cut the "homebrew" receiver to match the stub of the receivers that were in the parts kits that already had barrels installed.   We then simply (but carefully) welded them together.   This worked on both a milled receiver parts kit and a more conventional sheet metal receiver parts kit.   The sheet metal one was thin and for a measure of safety and longevity, I put some additional metal on the outside over the welds to beef it up a little more.   

Note that there are going to be differences in methods of improvisation when it comes to building or restoring AKs, but the common denominator to successfully building them is an investment in tooling which will hopefully pay off in the long run on an economy of scale.  IE, ten guns or more.   This bending jig takes care of what has so far been the most frustrating and difficult part of the puzzle - e.g. the problem of getting the main U channel bend done in a way that does not mangle the lips along the magazine well. 

Setting up the Jig

The first step is to drill and or widen the small holes that run along the center of the AK receiver flat.  This is necessary for  centering it on the bottom plate of the tool.   Most of the holes are already there and just need to be reamed out a little.   If you are making an "AK channel" out of plain 1mm steel, you may may not need to drill use the bottom plate, but it can help.   The main reason we need to use this arrangement is to keep the thin lips on the magwell from deforming when the metal is bent.   Grease the parts of the flat which will be pressing up on the plates of the bending "shoe".  This takes friction off of the plates and helps to ensure the bend goes at the edge of the "foot" where the plates are clamped tight against the receiver flat. 

It will also help to put some grease on the lower part of the jig in order to keep the metal guiding through freely instead of tearing. 


Preparing to bend the flat


The big pins on the jig will line it up with the sleeves tight so there is not much room for wiggle or error (if any).   Now it is time to force the "foot" into the "shoe".   We opted for the bench vice in this illustration, but other methods work form using the threaded pins and large nuts, to a hydraulic press to a large hammer.     We prefer the vice because it is fast and gives an even pressure across the length of the bars.   

Bending and final steps




Almost there.  It is hard to see in this picture, but the lower is trying to bow out, forcing it all they way down into the jig makes sure it stays straight and the bend is straight along the desired line.   

The jig will seem "stuck" together at this point, and we now see the reason for the grease.  Without the  grease, we would risk tearing the sheet metal when we hammer the jig about to get the newly bent receiver out.   A few careful whacks and the jig pops apart. 
To the right you see the removal of the jig and "foot" from the lower fixture.  
The receiver will seem stuck in the fixture, but the  sheet metal is  resilient enough that some spreader pliers will not damage it in removing it from the jig.  

The critical part of the final product, a bent AK receiver with the lips of the magazine well having a straight bend that matches the rest of the receiver.    Here is another project to eventually take to the range out in the backwoods and shoot at our steel targets which are made from the failed jig designs.   Still needed will be welding in the rails, and attaching all of the other parts to make a gun.    I will likely leave this one for after the 2004 sunset of the AW ban. 


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