Tuesday, January 25, 2011

.50 BMG striker assembly and more work on the bolt

Well, as my last post may have indicated, I haven't gotten a whole heck of a lot done on my .50 BMG rifle. I did manage to pick up some M2 drill rod the other day from my local Fastenal. I must admit that I have done a fair bit of business over the last few years with Fastenal, and as soon as I found out that I had one locally, I knew that they would carry some specialty materials I use on occasion.
One material I use from time to time is O1 tool steel, sometimes referred to as drill rod. I went to Fastenal this last week while I was out running errands and getting parts to repair my truck hoping that the local Fastenal kept O1 tool steel in stock, as the store in my old home town did. I must admit I enjoy using O1 for a lot of things, as it machines well and is easily hardened by oil quenching. Unfortunately, the local chain did not have any O1, but they did have some M2 cold roll high speed steel drill rod, which I purchased a small 3' length of in 5/8" diameter. M2 machines well, but the most efficient way to do so is with carbide cutting bits as they are hard enough to take the abuse. M2 is pretty tough stuff in its annealed state. No way I could have made decent cuts in this stuff on my old lathe. However, the new-to-me Enco 13x40 ate it up.
I only got to work on some stuff today, but something is better than nothing.

I started out with some 5/8" OD M2 solid round stock (they were out of 1/2" OD) and since I wasn't exactly sure how long the overall firing pin/striker assembly was going to be, I just ran the whole 3' length through the headstock of my lathe and faced off the end of it. I then center drilled it and extended about 4" of material from the chuck and locked a live center in place to support the center drilled end. I then cut a 2.25" section down to a overall outer diameter of .124" so it would slide smoothly in the drilled end of the bolt. I then stepped up a small radiused section to .25". The stock was then extended further out of the chuck and a 6" section behind the firing pin was turned down to .45" OD. I then ran the machined end through the end of the bolt to get an idea of how long this thing needed to be.


 I found that I needed about another 6" past the .124" OD section, with just a little bit of that being "just in case". If there is anything I have learned in my experience working with metal, it's that it is easier to keep a little extra material on the part to be made than to trim to exact size right off the bat. Odds are this will allow for fixing small screw ups and prevent you from having to make an entirely new part, which wastes even more material than leaving a little extra in the first place.
I kept about 1.25" of the .45" OD section, and turned the remainder down to .30" all the way to the end. The last 1.5" was then turned down to .244". This was done in preparation for the threading that needed to be done next. I then turned the lathe speed up to about 1300 RPM so I could sand and polish the whole thing. I had to use a file and sand paper to get the end of the firing pin to the rounded shape I wanted.
I started cutting the threads on the end while it was chucked in the lathe before I realized I was wasting my time. Threading something that small on the lathe is still a bit of a chore due to part flexing. I remembered that I do own a tap and die set, so I got a 1/4x20 die and threaded the end the whole 1.5" length that was turned to .244".
I dug around in my parts box and found a spring that will probably work (at least for now) and put it in place on the firing pin. I had to take a few measurements to figure out what length to trim the spring to, hence the difference in the next few pictures.
Ignore the tap, it was just laying there when I took the pic. That is a standard 1/4" nut holding the spring in place for the moment.



Next, I needed a "washer" of sorts that would be pinned in place and capture the firing pin spring. I started off with a section of 4130 1.5" OD round stock that had to be faced off. I only needed a .375" long piece 1.36" in diameter, so I started facing the end before center boring to .350", turning the outer diameter to 1.36" and parting off the .375" section needed.


This piece was tapped in place with a hammer into the sections of DOM I had threaded onto the bolt that act as sleeves.
I then spun the rear outer sleeve onto the bolt as far as it would go before putting a 8" C clamp on the bolt to retain the "washer". The threaded end of the clamp went inside the outer sleeve and kept pressure on the "washer" made of 4130 1.36" OD. I had to do this because this piece was going to be pinned in place by an off-center hardened roll pin and drilling anything off-center is a bit of a pain. With a C clamp around the whole bolt keeping pressure on the inner "washer", I tightened up the bolt in the milling machine vise, took some quick measurements, adjusted the XY table of the mill, and chucked up a .25" center drill in the mill in order to make the initial drilling. This is an absolute must. A normal drill bit would wander off center and eventually snap. The center drill is only about 1.25" long, is very rigid, and has a very small but reinforced cutting end.
After the hole was started, it was a piece of cake to finish the hole and the drill it out to .1875 in preparation for a hardened roll pin. This pin retains the "washer" which holds one end of the firing pin spring.


The threaded end of the striker/pin assembly protruding from the end of the bolt sleeve.

Ignore the coffee mug behind the bolt. Yes, that is a Pyrex coffee mug. Courtesy of my girlfriend this Christmas. Found it here.
You can see the firing pin protruding just slightly from the bolt head.
I also drilled and tapped the rear sleeve area for a retaining screw to keep the sleeve from rotating on the bolt itself. The sleeve was then counter-bored for the round allen head of the screw.
Not a whole lot of progress, but better than nothing.
I am getting closer and closer every day. I even took the liberty of ordering some ammo the other day, I found a great deal on some surplus .50 BMG ammo for 17 bucks for 10 rounds. That should be enough to test everything with once I get the parts back from being hardened.
That's all for now, hopefully I'll get more done tomorrow.

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