Tuesday, December 7, 2010

Machining the chambering area.

On to the next piece of the puzzle, the threaded chambering piece, or "barrel extension", that attaches to the barrel.

(I think Bill Holmes refers to this as the barrel extension in his book, as it is not the actual "chamber". I couldn't think of a decent term for this piece, so barrel extension is fine. Whatever.)

So, yet again I chucked up some more of the 2" diameter 4140 solid round stock and began the arduous task of parting it off in the lathe. Trust me when I say, parting off 4140 round stock with a HSS cutoff blade on an old Logan lathe is a real PITA. That took me at least a couple of hours or so.
Thankfully I have a new lathe lined up for purchase, and as soon as the money comes through in the next few weeks, I should have a much newer and larger lathe.
So, once I was done standing there and listening to the squeal of the metal being parted and the roar of the back gears on my lathe, I had a >2.5" long section of 4140 ready to be machined. When I was done parting the material, I faced off both ends of the section of solid round and began the next long task, boring the whole thing out to a 1" inner diameter.

I don't know who coined the term "boring" to describe drilling out large holes, but they were right on the money with that particular word. Geez, was that boring.

And messy. That took a lot of cutting oil for the drill bits and HSS boring bar.

I didn't bother taking any pictures of that process. A 1/8" bit to start, and slowly worked my way up to a 1" drill bit to bore through the whole thing.

 Once I was done with the drilling, I had to bore one side out to 1.3" inner diameter and 1.75" deep in preparation for threading.

Now, the inner .75" was bored out to 1.5".

Next, the outer 1" of that side was now bored to 1.3" and was then threaded to 8 t.p.i.

I cannot stress how difficult this was to do, doing a smaller bore and thread on the outside with a larger bore on the inside. It also had to be measured and cut precisely in order to get a round to chamber properly. The largest reason for the smaller threads was because the M3 barrel I bought was already threaded on the chamber area, and it had the most unconventional sized threads I have ever seen. In retrospect, I wish I had had a normal barrel blank to turn instead of the surplus M3 barrel I bought. Oh well, live and learn. It seemed to work out just fine in the end, however it took quite a while to do this.

The bored and threaded piece.

Next, this same piece was reversed in the lathe chuck and a dial indicator was used to check for runout to make sure I had faced it off properly. After I was certain it was, I removed it from the lathe and took it to the workbench for some paperwork.
Yes. Paperwork.
As in, drawing a section that mated with the face of the bolt I had previously machined in my cad program, printing it out, and gluing it to the other side of the piece I had just bored and threaded.

You can see that there are inner and outer lines. I used the outer lines as a guide for a punch. The dashed lines were there to represent areas that did not need cutting.
I went back with a punch and outlined all the areas to be machined, and after that removed the paper in preparation for machining.
All the punch marks in the corners and inside edges were drilled with a 1/16" bit first before clamping the piece in the mill. I then took a 1/4" four flute endmill and trimmed the rest of the inner "ear" area out. As you might have guessed, the inner circular area was the 1" bore from earlier, so all that had to be milled out was the inner "ear" areas. These "ears" took several passes with the endmill, and when I was within .01" or so of the edge limit, I stopped milling. I then took a bastard cut file and finished the part until the bolt snugly fit into this piece.

Once the bolt fit snugly in place in this "barrel extension", I threaded the whole assembly on to the barrel.
Here's the extension in place on the end of the barrel with the tapered barrel retainer in place as well.

Now, here is the entire assembly including the bolt in place. It is a very, very snug fit.

It was a heck of a lot of work to get to this point, but it was well worth it to see that part come together.

Notice that the bolt only has the outside machined, and I will say now that there was a lot of work involved in getting that particular item drilled and bored as well.
With that out of the way, its getting late here and it's time for me to call it a night.
More later.....


  1. Don't want to jump the gun but are you machining the bolt internals (firing pin, extractor, retaining pins) or using factory parts?

    I'm drooling over this thing and can't wait to see the finished gun.

  2. Yes to all of the above.
    It has an action similar to the way the action in most any other bolt style receiver works, wherein the firing pin is held by a sear under tension and when the trigger is pulled, the sear releases and the pin springs forward.
    I have a post on all the little items you mentioned lined up. Stay tuned.

    Glad you're enjoying this, there's more on the way.