"Ed Huntress" wrote in message
...
On Mon, 8 Apr 2013 22:04:03 -0500, "Terry Coombs"
wrote:
"Ed Huntress" wrote in message
news
On Mon, 8 Apr 2013 19:24:51 +0000 (UTC), Cydrome Leader
wrote:
Ed Huntress wrote:
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.
I meant once you bungle a partially made lower, you're finished, and
it's
just expensive scrap metal, of a useless shape probably only good for
tossing into the aluminum scrap pile for some backyard casting. It seems
like a shame.
Well, it is a shame, but I wouldn't toss a high-zinc alloy, which also
has around 1.5% copper, into a pile I was going to use for backyard
aluminum casting. Zinc and copper will hake it hard to make a casting
without internal shrinkage voids.
--
Ed Huntress
Neither would I . I'd toss it into the pile for ZA alloy casting . Ed ,
where did you acquire your knowledge about casting and alloys ? Some of
the
stuff you've posted is directly opposite what experience and hobbycasting
yahoogroup has taught me .
Mostly as Materials Editor at _American Machinist_. I had to study
basic metallurgy for a couple of years to be able to handle the tech
papers coming in from Alcoa, U.S. Steel, and so on.
I'm hardly an expert but I know the sources. I'm very curious about
what the hobbycasting group is saying that's directly opposite. Does
it concern 7075? It's notorious for having lousy corrosion resistance.
There are some 7xxxx alloys that are much better, but not 7075.
As for copper in aluminum, it makes it extremely hot-short and prone
to porosity. That's why welding 2024 is not recommended except by some
specialized techniques. It cracks like crazy, and if it happens not to
crack today, it may in a couple of weeks, as the HAZ age-hardens.
Anyway, what is opposite?
--
Ed Humtress
Not that 7075 is corrosion-resistant , just that it can indeed be cast by
a home shop foundry . I've cast a couple of pieces with no problem - in fact
it casts nicely and machines well if you chill it in water straight out of
the mold then let it age for a couple of weeks . 6061 is very gummy
especially when freshly cast , and 356 is sweet to work with .
As far as copper in aluminum , we try to avoid it . It does indeed cause
problems , mostly in the machinibility of the items . A trace is no problem
, but get too much in the melt and you'll find that it's nearly impossible
to machine , even with carbide tooling . Some of the best bronzes are mostly
copper with small amounts of al , ampco 45 for example my bike rides on
swingarm bushings made of it ... and they too are difficult to machine .
I'm not set up to weld aluminum , and cannot speak to the weldability of any
of the alloys . When I need that knowledge I'll be looking for info !
--
Snag