Reverse Engineering and machining the MP-44 trunnion

Here at Gun Lab we do a fair amount of reverse engineering, most of what we like to make have no drawings. However when there are drawings or solid models available we will use them. With this said I have found that most of what is available on the internet or in books is just not correct.

A case in point is the MP-44 trunnion. I have all the drawings that I have been able to find on this part, a number of different sets are out there, and when compared with the actual part have found them to be lacking. Some are just wrong and in some cases I don’t think the person has actually looked at a part.

Over at weapons man he had a write up about reverse engineering a while back.

http://weaponsman.com/?p=21915

It was a very interesting write up and I recommend reading it.

Now back to the MP-44 trunnion. We were contracted a while back with making a limited number of new trunnions  for the MP-44. He sent us a very good original one and we had a poor copy of one at the shop. Using these two pieces we started the project of reverse engineering it. The easiest thing to do was look for engineer drawings off the web. These are the ones that I found.

The measurements have been removed from these copies, however you can find them on the internet. I did use the basic drawing as a starting point. The sheets were cleaned and measurements were taken using a cmm, micrometers and  pin gauges. Tolerances were set using not only the trunnion but also matching parts. When there was a doubt other parts were located to increase the measurement standards. This allowed us to come up with a reasonable solid model that we felt was accurate enough to start programing.

Even with this solid model and prior to making the parts we still caught a few errors that have been corrected.

Now comes the  portion of the project were we start working on the machining of the part. A master cam  programed was written to run the trunnion on the cnc machining center. It was determined that 7 set ups would have to be done to machine this part. This is a video of the master cam operation to complete operations 1 through 4.

With the program checked out we then ordered the proper tooling and set up all the off sets on the machine. Test material was brought in cut and machined to size to do a test run. This is the progress so far.The material was saw cut to size.

Then machined to the proper size and squared.

The first 4 ops have been completed. Op 1 and 2 cut the groves on both sides and drilled the ejector holes.

The next phase was op 3 and 4.

The parts are set up in the vises ready to machineThe barrel hole drilled and machined.

The hole for the bolt carrier is drilled, machined and then bored to a very nice finish.

The gas tube support is machined in.

The first six test pieces.

Even now we are still not working on the actual parts. These are 6 test pieces that will be completed and set to the buyer to test on his kits to verify proper fit. In this case because of tolerances we went with a smaller bore diameter for the barrel fit, it is easier to remove metal then to add it.