Monday, November 1, 2010

The New Project Continues

This weekend, I removed the bulk of the metal from the forging's fire control pocket, and now I "just" need to clean up the resulting shape with the end mill.  Some of the cleanup has started.  It looked like this when I had to quit last night.
The lighting in this picture is borderline awful, I had to really bring up the mid-range, but you can see that the majority of the pocket is ready to clean out.  You can't see that it's the full depth (1.249") and that I've smoothed out some of the sides already.  The trigger slot is currently two drill holes, but it will be easy to remove the little web of metal between the holes.  The rest of the lower receiver parts (a DPMS kit) might be here by Friday, but UPS hasn't promised, yet. 

I'm finding that my approach is probably slower than just using a manual mill and cranking the handles, but for things like drilling all the holes out, the CNC automatic peck drilling cycle sure is nice.  For the rest of cleaning out the fire control pocket, I need to find the location of the corners where the end mill stops (the radius of the cutter short of the ends in X, and the end points in Y), and a reasonable depth of cut, then just write the tool path in a text editor.  This is hand coding the G-code that runs the mill.  As an alternative, the fixture I bought has a top plate that will allow you to run (the non-cutting portion of) the end mill along the edge of the jigs, like a routing template.  It can be done on a drill press, as this CNC Guns tutorial shows.  A milling machine is not necessary.

More on this as it goes along. 


  1. I'm really intrigued by this operation, thanks for posting it! I'd never thought about doing something like this before, but the more I think about it the more I think this may be something I have to try sometime. I've got quite a bit of experience hand-coding G-code programs for our Sherline machine at work, and given a dimensional drawing I could probably knock out a simple program relatively quickly. Keep us updated on the progress, it really fascinating!

  2. It sounds to me like you could do this with no extra work. Do you have a few end mill holders? I've used 1/8 and 3/8" for most drilling, and a Jacobs chuck to hold the 5/16" drill for the trigger slot.

    This is simple to do. When it comes time to engrave the label, I'll use a program to generate the code.

    There are dimensioned drawings of the lower online. If nothing else, Colfax Tactical sends one on the CD that comes with the forging.

  3. I've got a 3/8" quick change holder and a 1/4" collet, as well as the Jacobs chuck for our machine so I can handle just about any type of end mill or bit. My only limitation right now is that I don't know if I have a vise big enough to hold the lower and the jig. I have step-block hold downs and the little Sherline vise that opens to about 2 1/8" but anything bigger than that and I'd have to engineer a method to keep it on the axes (which could be done pretty easily now that I think about it).

    Of course, I still have to let the budget recover from the Garand that I just picked up through the CMP before I launch into the next firearms adventure. :-)

  4. Use your step-block hold down kit.

    In my first post on the AR lower, I mentioned that I needed to modify the fixture so that the hold down set from Sherline could work. It's not a big thing, I just put two 1/4-20 bolts at about 0.5 in and 0.5 up from the bottom of the fixture.

    If I had it to do over, I'd lower the one on the front side of the lower, next to the magazine well, maybe drop it 2/10, to 0.3 up and 0.5 in. It fits as is, but the hold down bar hits the mag well before the full end is over the bolt. I have an aversion to just engaging the tip of the hold down.