Wednesday, December 1, 2010

Receiver fun

Now that we're past the rest of the previous noise.......

On to the receiver!

Honestly, there wasn't a whole lot to the receiver. Let me begin by saying that the overall length is arbitrary, well, as long as it isn't any shorter than say, two feet long or so. Any shorter than that, and I would be concerned about having the muzzle blast a bit too close for comfort.

The Bill Holmes guide calls for a three foot long piece of 4130, 2.25" outer diameter and .12" wall thickness. This gives a .005 inner clearance when a 2.000" inch piece of 4140 is used for the bolt, which has shown to be more than sufficient for clearance. The bolt will move back and forth and turn freely, especially when given a little bit of oil to lube things up a bit.

Anyway, I originally bought a 3' section of 4130 for the receiver. I had no issue with a 3' long receiver at first, until I realized I really didn't like the overall look of the gun as the original plans outlined. I finally got around to photoshopping the whole thing and then I realized I preferred the receiver right around 28" long. So, back in the lathe the whole assembly went, and I parted off about 8 inches of receiver. I did a rough reassembly of the barrel and receiver blank tube and decided I was pretty happy with the overall length and appearance of it.

In the original plans, Bill Holmes outlines a number of slots that are to be cut into the end of the receiver, mainly for looks. I liked the slotted look on his receiver, but affixing a large (2.25" od) tube into my milling machine is a bit of a pain, as I don't have a vise that can effectively grip a tube of that diameter. I can do it, but it takes a heck of a lot more work than I was interested in doing, given that the whole slotting thing is arbitrary, and really is done just for looks. After mulling the whole thing over, I decided to just drill a series of holes in a similar manner as the slots were supposed to be milled.

I took the original drawing from the Holmes manual and got the necessary dimensions from it, then opened up my copy of Deltacad (my favorite CAD system) and redrew the whole receiver and measured out the series of sequential holes that I wanted. I first inserted the .5" holes in the drawing where I wanted them after calculating the given distances to make sure everything was even and equidistant, then I put a smaller circle inside the center of the .5" hole so that I would have a place to center a punch.
After that, I measured and illustrated in my CAD drawing where the slots for the bolt handle and the bolt access needed to be. I also made a set of marks in these, as I didn't want square corners to all the slots I had to cut, I wanted radiused corners instead. So, I guesstimated what radius  I wanted and from there marked the center of that so I could drill a hole and complete the inside of that corner.

Once I had all that done, I printed the whole thing out at a 1:1 scale, which gave me about 4-8x11 sheets to join together on the receiver. I then took the receiver and sprayed 3M spray-on adhesive on the receiver and on the printed out drawings. Here I made a mistake, although not a crucial one. When I applied the drawings to the receiver, I managed to misalign the ones with the series of holes to drill. I didn't notice this until after I had drilled them. Dammit.
Thankfully it isn't off by much, and it really doesn't make much difference. Thankfully it is not altogether too noticeable, but I know it's there, and it irritates me when I pick up the rifle and see it.

Once the template was in place, I got a punch out and marked the center of all the holes to be drilled, both the series of sequential holes (speed holes, as Homer Simpson calls 'em), as well as the holes in the slotted areas' corners.

Next came the fun part. Drilling a LOT of holes.
By the way, Harbor Freight is good for a handful of things, including cheap prices on 10 packs of HSS drill bits. I picked up 2 packs of 1/16", 1/8", and 1/4" drill bits in anticipation of this process. Glad I did, because HSS drill bits do NOT last long against 4130 tubing, even with ample cutting oil.
I started out by clamping the tubing to the mill/drill table and making sure it was centered for the drill bit. (Side note here: clamping the tubing to the table in preparation for drilling is not hard. Getting it clamped to the table or the vise securely enough to be milled is something else entirely.) I have a few different centering drill bits that are normally used in a lathe to drill a centered hole in stock that is to be mounted between centers and turned. I used one of these bits to do the initial drilling to mark the hole, then switched to 1/8" bits to finish the hole boring. Once that was done, I switched to a 1/4" bit to enlarge the hole, then a 1/2" bit to finish the job. I have a small carbide tool that is used for cutting and clearing any shavings that remain from drilling holes in metal, so once everything was drilled I went ahead and trimmed out any remaining shavings.

This is what I was left with

And, after adding a barrel and barrel nut for mockup,

I also drilled holes about 3" in front of the chamber area in which the tapered barrel retainer would reside. Twelve 5/8" holes were drilled in two rows of six holes around the circumference of the receiver. The tapered barrel retainer would be plug welded through these holes once the chamber and a handful of other things were machined and set in place. However, that is for another entry......


  1. It's definitely looking like a rifle. I was looking at that last picture and trying to imagine the receiver 8 inches longer and I have to agree. The esthetics are better at 28 inches.

  2. Yeah. I agree.
    The pictures of the original rifle built from plans aren't necessarily downright ugly, but I just thought it looked a bit long.
    Don't get me wrong, I would've been perfectly happy with the original design, but given that I have a hard time building something that looks exactly like every other Bill Holmes design, I made some changes.

    Of course, if I screw something up and upon the first shot it flings the bolt out of the back of the receiver and across the parking lot, I guess that is certainly my fault. :)