Machining Question

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%20Control%20Pocket%20with%20a%20DRO.pdf
RogerN
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
RogerN wrote:

I drill all the holes in one setup long before milling the FCG cavity. I suppose one theory for drilling after the FCG cavity would be to reduce the chances of the drill getting off course, but that doesn't seem to be an issue in my experience. The Ray-Vin guide is the best reference IMO.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Pete C." wrote in message

That's more what I was thinking, drill the cross holes first in one setup and then mill the fire control group. I have the Ray-Vin guide and a blank lower also, the "95%" machined lower should get me shooting right away and the 0% won't be rushed. Do you know how long it takes to completely machine a lower using the Ray-Vin guide?
RogerN
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
RogerN wrote:

Nope, but close. I use the 30% lowers from TM, so that saves the magwell and decking steps. The 30s take most of a day to machine on a manual mill working at a careful pace with lunch and dinner breaks. A good chunk of the time is the 8 or so setups and getting each one indicated and edge located.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Pete C." wrote in message

<snip>

I have one lower raw forging now and 4 more that have been on order since early February. I'm hoping the other 4 come in and I can machine them as a batch of 5. I would like to machine them all to the 80% stage so I could legally sell some if I wanted to. I could complete however many I think I wanted to keep. I now have one rifle and one carbine less lowers, would like to also get a 300 blackout, I hear barrel and ammo is the only difference from the .223/5.56. An area gun shop had a Bushmaster 450 upper, I'm not sure what cartridge they shoot.
RogerN
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Know where a complete drawing is? Would 6016-t6 bar be a suitable peice to start with?
Remove 333 to reply. Randy
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Randy333 wrote:

There are a bunch of AR prints on the 'net, just search around a bit. 7075-T6 is what you want.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Mon, 8 Apr 2013 17:14:22 +0000 (UTC), Cydrome Leader

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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Mon, 8 Apr 2013 19:24:51 +0000 (UTC), Cydrome Leader

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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

So it's a total loss then.
How much might it cost to make some really crappy molds (we're talking 1970s Hong Kong grade stuff) to make these out of plastic?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Mon, 8 Apr 2013 20:54:33 +0000 (UTC), Cydrome Leader

Well, if you have enough aluminum to sell your scrap from time to time, you could toss it in there.

First off I'd look at the redesign of the plastic lower that those 3D printing guys have designed. Just duplicating the aluminum one in plastic led to a very short life, so they've redesigned it.
As for the molds, it depends on what plastic and how many. It also will depend somewhat on the configuration and drafts. For machined molds, aluminum (2024 or 6061) is used for injection-mold prototyping and very short runs but some plastics require a chrome plating. After that, it's mold steel, and big bucks unless you're machining it yourself.
If you're using a pourable resin, like epoxy or rigid polyurethane, you can make molds out of plaster of Paris. They'd be stronger than most injection-moldable plastics but they're much more expensive, too.
--
Ed Huntress

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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 .
--
Snag
Cast 5 ingots and
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Ok, a little deeper digging: 7075 develops brittle microstructure and intermetallics when cast. There have been many professional experiments with it, including rapid chilling and squeeze-casting (not really full-melt casting), but they have been abandoned as far as I can tell.
An application that requires wrought 7075 requires yield strength on the order of 73,000 psi or higher. The best figure I've found for cast alloys in the 7xxx series is 30,000 psi.
I have some more checking to do when I get a chance. But what I think you're getting with your castings is an alloy that may machine well, but which has low strength compared to any wrought 7075, plus 7075's limitations: poor corrosion resistance and low ductility. Intermetallics frequently aid machining. They also destroy strength.
The alloy was developed by Sumitomo for making fighter aircraft in the 1930s, similar to our earlier development of 4130 steel. Since then, other aluminum alloys in the 7xxx series (and 7xx.x for casting alloys) have been developed, and they deal better with individual limitations of 7075.
That's not to say you can't make useful castings with it. If it works for you, great. The copper level is not high, so it may avoid the hot-short issue and internal voids that make 2024 so difficult. Porosity is an open question. But it seems unlikely that you're getting a result with any more strength than 6061, and with less ductility.
I'll let you know if I find anything else. Not today, though. The trouble with the information available is that 7075 is not a casting alloy so you aren't going to find a lot of scientific information on it. And I don't think most hobby casters do enough testing to know what they have.
--
Ed Huntress

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Cydrome Leader wrote:

It depends on exactly what you bungle, but most mistakes in the machining can be remedied/worked around to make a perfectly serviceable lower.
On the 7075 vs. 6061 thing, operationally it won't make much difference since the AR receiver basically guides parts and have very very little stress placed on it due to the fact that the bolt locks into the barrel extension and no into any portion of the receiver. I believe the first M16s were 6061 and they only changed to 7075 due to corrosion issues in the 'Nam jungles.
With the lower forgings running in the $25-$80 range depending on completion state, the financial risk isn't really much more than the financial risk of breaking an end mill or reamer in the machining.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

hmm, I've not stumbled across the $25 ones, but that would be worth it just for the practice.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Cydrome Leader wrote:

I haven't looked recently, but that's what 0% forgings were going for last I looked. The 30% TM ones I got for $45 in qty 5+, I think the broached magwell is worth the $20.
One key thing is to have a lower parts kit when you start the machining so you can test fit many of the parts as you go before you tear down each setup. It's also helpful to have a regular factory AR for visual sanity check reference, but you can use good pictures as well.
On the first one I did I grabbed the wrong drill and drilled the hammer and trigger pin holes oversized. After staring at it and scratching my head for a few minutes I re-drilled the holes a bit larger to give a reasonable size for a bushing, grabbed a piece of brass rod, threw it in the lathe and made a set of four bushings to bring the holes down to the correct size. I assembled the lower with a set of anti-rotation links to act as bushing retainers and all works perfectly, you can hardly even see the bushings under the AR links.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.