UPDATE (From Sebastian): If you choose to make a lower from Delrin, you’re safest using Food Grade Delrin, which is highly x-ray detectable. While I believe ordinary Delrin should be dense enough to be detectable by the modern equipment used for airport screening, it is unlawful to make a firearm, the major component (which includes the receiver) does not show its true shape on the types of x-ray equipment commonly used for airport screening. Be careful about the types of plastic you choose. Jason did not mention this in the original post, but I am updating now as a warning. You are responsible for compliance with federal law.

After finishing the aluminum bolt together lower, I went back to trying to make a single piece lower out of Delrin.

The process described below is my third attempt. In the first two I had alignment problems which made the right and left sides not quite match up and gave an egg shape to some of the holes.

I also switched from PyCAM to Deskproto. While the people working on PyCAM have done a nice job so far, it needs more work. For the kind of things I do its just barely useable. Deskproto has all the things that drive me nuts in PyCAM fixed, and while its commercial, the cost, especially for a hobby license, is quite reasonable (less than an AR-15 parts kit) and it doesn’t use any nasty DRM, so after you’ve purchased it you don’t have to get permission from the mother ship to reinstall it or install it on a different computer.

In Deskproto, I added support bridges to the model that run through the buffer tube hole and magazine well.

This way I could use the outside edges of the Delrin block for holding down the part while machining the left and right sides, but don’t need to do extra machining steps later to remove the supports.

Update: The 3d model of the lower is originally from www.cncguns.com.

I used an inexpensive Chinese import band saw to cut the block of Delrin down to an appropriate size….

Then bolted the block down on the mill and started machining the right side.

I did a roughing pass with a 1/4 inch flat end mill, and contouring pass with the same 1/4 inch flat end mill, a contouring pass with a 18 inch flat end mill, and then a finishing/polishing pass with a 1/8 inch ball nose end mill.

Next I milled a slot along the back and top of of the block that went almost all the way through the block, and then used the band saw to cut away the excess material beyond the slots. Since I know the exact location where it milled the slots, this gave me good surfaces for aligning things when I flipped the block over.

Here is the block flipped over and mounted back on the mill.

Next I did the same procedure on the right side for the left side. What you don’t see from the other pictures is that the process is rather messy. Here is what things looked like before I vacuumed up the debris.

and cleaned up…

Now back to the band saw to cut the outside frame away from the part. Next was the buffer tube mount hole. The Taig mill in its default configuration doesn’t have enough clearance in the Z direction for the lower to fit with its back surface perpendicular to the spindle, but it turns out that is easy to fix. If you loosen one bolt the whole spindle assembly slides off.

You can then unbolt the mounting plate and bolt it back on higher up.

Then slide the spindle back on the mounting plate, and keeping it as high as possible tighten the dovetail bolt. Now there is enough clearance for the lower and and a vise to fit.

The lower was held in place by clamping the vise down on the remnants of the support bridges. They provide a nice flat surface and there is no risk of scratching or damaging the lower.

After machining the buffer tube mount hole..

Next was the bottom half of the magazine well and the slots for the trigger guard. Again the lower is held is place by the remnants of the support bridges.

While the lower was mounted upside down I took the opportunity to drill the hole for the selector spring. Then I used the band saw to cut off most of the remaining support bridges, and positioned the lower right side up for machining the top half of the magazine well and the fire control area. There isn’t enough support material left to provide a clamping place, but the trigger guard area has a flat surface that can be used.

At this point the lower is finished, except for a few holes that need to be drilled.

First I did the hole for the front take down pin spring.

Then I tried the bolt catch hole.

But it turns out you can’t drill that hole from the front. The bulge in the magazine well gets in the way.

You have to go in from the back. I should have drilled this hole while I was machining the buffer tube mount hole. Now without the support bridges there isn’t a good way to mount the lower with the front facing down. But by moving the spindle mounting place I was able to do this…

and drill the hole.

Next came the rear take down pin spring hole.

Then the trigger guard holes.

And finally the hole for the pistol grip.

Here is the fully assembled lower..