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Gunsmithing small milling cut in 7075.

jacq220

jacq220

Sergeant
Full Member
Minuteman
Dec 5, 2007
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Indianapolis, Indiana
www.indygunworks.com
I know this would be a super easy job for most of you, but I want to make sure i do it right.

material is 7075 aluminum anodized. Its the side of an AR-15 billet lower reciever. I am machining some improper engraving (not atf related) off the side.

it will need to be 2.5 inches long, and around .20 deep.

A 5/8 end mill will cover the width i need it to cover.

What speed feed would you use, what lube?
is a HSS 5/8th endmill going to be an appropriate tool?

I plan on taping the rest off when finished and blasting the machined area and trying some aluma black to recolor it. If that doesnt take then the whole reciever is going to be recoated.

I have been struggling w/ the whole machining aspect of gunsmithing so i want to make sure i get this one right, it will be a huge confidance booster.
 
Re: small milling cut in 7075.

.2 is awful deep for removing engraving. Did you actually mean .02?

It's going to depend in large part on how fast your spindle can go and how rigid your machine is. Also, how many flutes on that endmaill and were you wanting to do the .20 in a single pass or multiple? Unless you have access to some fairly heavy machinery, .2 in a single pass is probably not going to turn out too well. On smaller machines usually multiple light passes is the trick. That's easy to do with a CNC where you just program in what you want and let it run, a bit more of a pain if you're cranking things out by hand.

If you can get me these details I can tell you what my speed/feed calculator recommends.
 
Re: small milling cut in 7075.

4 flute.

Wells index 9x42, max spindle speed is, 2850.
I will do a bunch of passes if neccessary.

not .02, its CNC engraving so maybe not .2, but its deep enough i dont think ill get it all on one pass.

machine probably weighs close to a ton
 
Re: small milling cut in 7075.

Think of it in terms of fractions, 0.2 is over 3/16" of an inch, thats DEEP! 0.0625 is 1/16 and probably even deeper than engraving would be. 0.2 and youre going straight through the receiver.

With aluminum I dont really think you can screw it up if you take your time. Id use a smaller endmill 3/8 maybe, two flute, probably 1200 or so, 0.01 depth per pass, and keep it lubed well or the flutes will clog up. WD40 will work if you have nothing else, you dont need it as a coolant just to keep the flutes lubricated to keep the alum from sticking to it.

If someone with a better name chimes in take their advice because I am no expert by any means, but again, aluminum is pretty easy.
 
Re: small milling cut in 7075.

I am having a hard time picturing where you would need to cut a big ol' horse trough....but that aside, make a test pass on a piece of stock. You may find that you want to bump the head out of tram a fuzz to give you the finish you want....assuming its on the money now.
 
Re: small milling cut in 7075.


Any pictures?
The hardest part of this operation will be getting that receiver clamped down on the same plane as the surface you want to cut.

For speeds and rates its time to just put a piece of aluminum in that milling vise and start chomping. The machine will tell you what it does and doesn't like. WD40 works good on aluminum. Going slow you can do about anything to aluminum.

Even modern CNC shops make practice runs. You can practice on wood and plastic too. It's Etch A Sketch with a depth knob.

EtchASketch10-23-2004.jpg
 
Re: small milling cut in 7075.

Am I the only person to raise an eyebrow concerning the hardcoat anodizing?

I've never found a way to cut through that shit "nicely". Even with carbide tools, it still chips and buggers my tools.
 
Re: small milling cut in 7075.

How far does that 5/8 endmill stick out of your spindle? Assuming 1" then @ 0.02 depth of cut you shouldn't see any endmill deflection. You could go as high as 0.10" depth of cut with only 0.0001" deflection, which should be fine (in my limited experience) but I personally prefer to do many fast light cuts over 1 heavy one (that suits the characteristics of my relatively light-weight CNC.)

As an example, according to GWizard, with a 0.05" depth of cut...

@2850rpm, 13 IPM plunge, 52 IPM feed
@2000rpm, 9 IPM plunge, 36 IPM feed
@1250rpm, 5.7 IPM plunge, 23 IPM feed
@750rpm, 3.4 IPM plunge, 14 IPM feed


Remember, this advice is worth what you paid for it.
 
Re: small milling cut in 7075.

incidentally, going slower probably won't ruin anything on your workpiece (if you can keep the aluminum lubed up) but it will dull your endmill a lot faster because if the feed rate is too slow instead of chomping off chips the teeth will just be rubbing against the outside edge of the last tooth's cut.
 
Re: small milling cut in 7075.

Take a carbide scribe and outline the area to be resurfaced. Do this just as if you were engraving a Purdy shotgun.

Once you have bare metal, then you can proceed.

Use a die grinder to remove the anodization, then proceed to mill.

Nothing will guarantee a chip free result, but if you start small and work with patience, you will be rewarded.

Remember, firearms used to be made with files.

Key word, PATIENCE!!!!
 
Re: small milling cut in 7075.

No your not. The first thing that came to mind was the anodized surface. Aluminum oxide is one tough mother, probably why they use it on sand paper....
 
Re: small milling cut in 7075.

Your HSS endmil is going to die as soon as it touches the anodize. It will be very dull and most likely you will get a bad surface finish. Mag well area i assume? This is also very flimsy as far as cutting goes. The 5/8 is going to take a large bite and make matters worse. I would use a 1/4 carbide em and start from there.
 
Re: small milling cut in 7075.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: heatseekins</div><div class="ubbcode-body">Your HSS endmil is going to die as soon as it touches the anodize. It will be very dull and most likely you will get a bad surface finish. Mag well area i assume? This is also very flimsy as far as cutting goes. The 5/8 is going to take a large bite and make matters worse. I would use a 1/4 carbide em and start from there. </div></div>


Yep!
 
Re: small milling cut in 7075.

sorry guys, typo, i did mean to say .020.

I have a 3/4 carbide endmill i can spin at 2850 rpm.

Will that work? should i plunge through? Or just find the surface then come in from the edge?
 
Re: small milling cut in 7075.

Buy a smaller end mill! If anything run the 3/4 end mill after you grind through the anodized surface, would be best just to stick with a smaller end mill though. At best the anodize will be 0.005" thick.
 
Re: small milling cut in 7075.

You need to grind the anodize off first in order to gain access to the aluminum underneath. Use a die grinder (like a dremel) or an angle grinder or something with an abrasive disc and then once you're through the anodize put it on the mill and make your cuts.
 
Re: small milling cut in 7075.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JACQ</div><div class="ubbcode-body">How is the smaller end mill going to far any better in the anodizing than a larger endmill? </div></div> Not that it wouldn't be nice to have the technical answer, but if anyone is the voice of experience on working with 7075 it would have to be Seekins.
 
Re: small milling cut in 7075.

The anodize is going to eat whatever you touch it with. Even carbide will be toast when your done, but it will have a better chance of breaking thru the anodized surface. You can bead blast the anodize off in that area. Just mask it out with some electrical tape (something thick) and blast away.

Smaller EM wont take as bit of a bite. Its all about the surface feed (see machinists handbook) and RPM. Your going to have a crap load of chatter with the larger EM because its going to take a larger bite out of a not so stable surface and you will most likely be done as soon as the sparks fly from the coating.

My advise, sell what you have and buy something else. Forged lowers are cheap and way cheaper than 1 carbide 3/4" em.
 
Re: small milling cut in 7075.

Its not a forged lower... Its a mega billet lower that a customer brought me.

I can definatly blast the anodizing away, but if it can be blasted off that easy, how is it going to be so difficult to machine?
 
Re: small milling cut in 7075.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JACQ</div><div class="ubbcode-body">Its not a forged lower... Its a mega billet lower that a customer brought me.

I can definatly blast the anodizing away, but if it can be blasted off that easy, how is it going to be so difficult to machine? </div></div>


Because aluminum oxide (the anodized surface) is a very abrasive, and hard material. Again thus why aluminum oxide is used on sand paper. You can blast through it with a sand blaster because the base material (aluminum) is soft. Do as Glen said, he machines and works with aluminum every day. From the looks of his rings on my TRG he seems to know what he is doing.
 
Re: small milling cut in 7075.

ok, well, its done, i dont know what i was so nervous about w/ the anodizing, i just dont do much w/ aluminum at all and read that anodizing could be a bitch.

I used a chinese 2 flute 1/2 inch cutter, and pluged in while moving to the side just a little. once it cleared the anodizing i finised plunging until i was through the anodizing (around 10 thou maybe 15).... then moved the cutter latterally to get rid of the rest. taped it off, blasted it, and alumina blacked it back out. looks like it came from the factory this way.

TOTAL time 15 minutes.

I guess i was making a mountain out of a mole hill.

thanks for all the input guys.
 
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