I’ve been working on the lathe for a couple days at school, and been doing turning and drilling (no threading yet). I finished the sample piece my instructor wanted (a cylinder with several different diameters and grooves, which we will be using for threading instruction tomorrow), and I was looking for something to practice these operations on today. After flipping through some old copies of Home Shop Machinist, I realized that the perfect item to make would be a gas piston from the Walther G41(W) that I was looking at last week over at Forgotten Weapons. It’s a very simple piece, which slides over the barrel and inside a thin cover. It is pushed by gas briefly trapped by the muzzle cone of the rifle, and acts on a connecting rod to move the bolt. The upshot is that it’s a simple cylinder with three gas seal grooves cut on the outside surface and two more on the inside surface.
So, I broke out the gas piston from our G41(W), and began by doing a basic reverse engineering of it. I drew out the part and used series of tools to measure all the important dimensions. A basic dial micrometer for the outside measurements, a micrometer with pointed contacts for the groove diameters, and a digital caliper for the inside diameter (I don’t have a good tool for precise measurement of inside diameters, as it’s larger than the set of plug gauges we have).
Once the part was drawn out, I pulled a piece of 1″ diameter stainless stock from the steel room (the original is made of stainless, since it is exposed to hot, high pressure gasses on every shot), and chucked it up in the lathe. I first faced it off, and then turned down the OD with a series of cuts until I had the proper 0.904″ diameter. Next I used a cutoff tool to cut the exterior grooves, followed by a series of drilling operations to cut the ID. We don’t have a reamer of the specific 0.650″ diameter necessary for this part, so I instead used a boring tool to cut the final inside diameter. Not nearly as fine a finish as a reamer, but it will work. A second boring cutter allowed me to cut the interior gas relief grooves, and then back to the cutoff tool to, well, cut off the part from the stock.
This took me about 4 hours from start to finish, and it does have several problems. My speeds and feed rates on the first two grooves were not ideal (and I was using an HSS cutting tool on a stainless part, which is not recommended), and that led to a lot of chatter. The inside diameter should be reamed for a smoother finish, and the inside gas relief grooves are not deep enough (we would be best served to make a specific tool for that operation if we were to make a bunch of these parts).
Once I had the final piece in hand, I did a little buffing to the outside to clean up some burred edges, and tested it out on the rifle. It actually fits! My first actual gun part, and it fits both over the rifle barrel and inside the cover with the same feel as the original part.
I expect we will be revisiting this piece down the road when I start running the CNC lathe – it could be done in a single program on the CNC, and will be a good practice job.