On Mon, 8 Apr 2013 17:14:22 +0000 (UTC), Cydrome Leader
wrote:

RogerN wrote:

I'm close to milling my AR lower receiver, this "95%" machined receiver
needs a pocket for the fire control group and 3 holes cross drilled for 2
pins and a selector/safety.

I found some instructions for machining the pocket using a DRO, they
recommend drilling the selector switch hole, machining the pocket then
drilling the trigger & hammer pin holes. Why not drill all 3 cross holes in
that 1 setup and then machining the pocket? I would think the drill bits
might be more likely to walk as they start through the 2nd side. I don't
see a benefit to set up for drilling, turn turn the part for milling, then
turn back to the first setup to drill 2 more holes, any idea why they would
do, or recommend this?

https://colfaxtactical.com/docs/Fire...%20a%20DRO.pdf

RogerN

does anybody make plastic unfinished lowers, just for machining practice?

It would seem like a shame, and expensive to be using trashed lowers for
the next aluminum casting project.

Real AR-15s and M-16s have 7075-grade uppers and lowers. You can't
cast that with home hobby equipment.

I've read that some of the aftermarket is using 6061 for lowers. But
the people making those comments probably don't really know. Yield
strength of 6061 is roughly half that of 7075.

6061 is a wrought grade, but it can be cast without doing anything
special.

--
Ed Huntress