To legally manufacture a silencer/sound suppressor in the United States, it is imperative that one submits an ATF Form 1 with fee, waits until it is approved, and receives the approved form with attached tax stamp, before any construction is started
The first cuts on this project will be taken on the thread mount, the “entrance” end cap on the silencer. I’ll be cutting 17-4ph stainless steel, making an internal thread of 1/2″-28 with a tap, and using the single-point threading method on a machine lathe to cut the external thread of .900″-28tpi. Once all that is done, I will apply a helical knurl and a slight roundover to the cap!
Let’s just start at the beginning! So, I decided to just chuck up my sizable piece of stainless into my lathe and supported the end with a steady rest. This is a little helper to keep longer pieces of stock rigid in the lathe when you’re working on one end of them. It made more sense to me to use this method than to cut off a piece and try to figure out how to hold it in the lathe. After facing off the end to be perfectly flat, I turned down the outside to .900″, the outer diameter of the threads we’ll cut to screw into the tube. This cut was made to 1/2″ back from the end, and took several passes. I utilized a carbide bit, which is an extremely hard material and great for this sort of thing.
Once that diameter was turned, a groove was required at the very end of the turned down area. This groove will give our threads a place to end, avoiding the phenomenon sometimes noted with factory threaded items where the threads sort of just spread out into the shoulder, preventing a tight and smooth fit. To make this groove, I’m using a thin parting blade. This tool is thin with a square tip and is plunged straight into the workpiece for grooving operations like this, or as you’ll see soon, cutting off an entire section of the metal. To help prevent binding, one trick I’ve used in my lathe is to hold the parting blade upside down and turn the workpiece in reverse. Due to the type of cut made in the parting operation, there is a tendency to grab the work and cause the tool post to bind. With this method, the tool and tool post can be pushed out of the way in the event of the workpiece grabbing the tool!
With the groove successfully cut to a depth of .860″, the next task was to drill and tap the inside of the mount. With progressively larger drill bits, passes are made on the lathe until the tapping size for a 1/2″-28 hole, which I use a 29/64″ drill. This is the part that screws onto the firearm so making sure it is a tight and straight thread is crucial.
Sharp drill bits were key in this; the stainless is machinable bit still very hard. Truth be told, the bits I had sharpened with my drill doctor just wouldn’t cut very well, so after touching the cutting edges up properly on a grinding wheel, they cut the material very easily. This was a small but valuable lesson to me in the importance of learning how to grind a drill bit versus buying a machine to do it for you.
With the holes drilled, I used my adorably small boring bar to turn a taper on the inside portion of the bore. This will of course be what faces towards the inside of the silencer tube, so this taper will assist the expansion of gasses, as well as reduce the overall weight of this individual piece.
Tapping the mount for 1/2″-28 was probably the easiest thread cutting operation of the entire can. I am lucky to have found a reasonably priced tap in this thread size, so all that is required is putting the tap into the lathe, and turning the headstock by hand. As expected, the tapped threads came out perfect and only took a few minutes of grunting as I turned the workpiece around the tap manually, with plenty of cutting oil of course.
Moving to the external threads that will screw into the tube, these won’t be quite as simple. Cutting threads on the lathe, referred to often as the “single point method”, is one of the most important operations on a metal lathe. The cutting tool is driven across the work at at the correct ratio to the work’s turning speed in order to make the specific helix required for the thread pitch. 28 threads per inch is a comparatively fine thread pitch, so the tool doesn’t move across quite as quickly, and the total depth of cut won’t be as deep as some other thread pitches. There are a number of considerations to take in making the threading cut, multiple things that need to be given attention simultaneously, and it can be a nerve racking process, but I got through it relatively unscathed.
This next paragraph is a bit of an esoteric machining soliloquy. My shop note on accomplishing these threads was that it would have been more optimal do it by the numbers. While there are a couple of different techniques that can be utilized in the single point process on the lathe, the most accepted one is to disengage the cutting tool from the lead screw (which is what is driving it across the workpiece) after each pass, back the cross slide off of zero away from the workpiece, reset the tool (like a typewriter) put the cross slide back on zero, advance the tool on the compound rest ever so slightly (to deepen the thread by .001″-.002″), then re-engage the lead screw when the rotation dial is in the right place. This method is what every teacher and book recommends, and is what I should have done. The method I used was one recommended to folks with smaller lathes or unsure of the tool rigidity, which I’ll admit I was unsure of to a degree. Instead of disengaging the lead screw and backing the tool off, the lathe is simply reversed after being stopped at the end of each cut. I should note that this did work and the threads are perfectly smooth, true, and tight, but in this method, it left scratches on some of the threads and in my opinion made them uglier than necessary, despite still being functional. That’s enough of my abstruse rant.
Out of the metal turning frying pan and into the metal turning fryer; another operation that many machinists don’t exactly look forward to is parting. As I was describing the parting tool earlier, we’re basically just going to cut the thread mount off of the long bar stock now. I do that just by turning the work slowly, and driving the parting tool straight into the part. It takes a while, especially with hard material such as this, but it works!
And it leaves a cut surface needing only a small amount of touch up; I put the threaded portion of my newly minted part into some aluminum tubing to protect the threads from the chuck jaws, and turned the part around to face off what I’d just cut.
And since I happen to have such a cute boring bar, I put an equally cute chamfer on the side of the threads that will be twisting onto a barrel.
At this point the part is basically functional. I took a file and ran it over the rear corner a number of times to give it a nice, round edge
Then I put a gripping surface on the part, since this is where the whole silencer will be gripped when screwing it onto a gun. I did this with a method I’ve been experimenting with that I refer to as a Helical Knurl. I made a YouTube video about the process a few months ago. Basically, the part is placed on the bolt that mounts the tool post (I used a custom bushing to make sure it fit snugly but could still rotate) and a large tap is mounted in the chuck and supported with a center in the tail stock on its other end. The tap is turned by the lathe, as the part is slowly moved in on the tool post. As the tap starts to cut, it will rotate the part. After feeding the part in, it has a relatively deep series of vertical and slightly angled cuts that not only look unique, but also provide an easy to grip, concave surface. I’ve done the more traditional forms of knurling, and I tend to prefer this one when I can get away with it.
After a quick pass with a file and some emery cloth in the lathe to clean everything up, that’s the thread mount!
The next part to be made will be the forward end cap, which will be made out of 7075 aluminum. If you’d like to see some of these machining operations in action, you can check out the video I made: