The Making Of A Glock Race Gun

A compensator is going to see FAR more heat and stress than an AR upper.

Test, if 7075 AI is good enough for an AR upper than it is good enough for me.

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AR upper characteristics and a compensator characteristics are not the same, like it has been pointed out.

I am all about people's freedom to do things like this.

If it does not work out, I suggest that you try these:

- 6061 alloy

- Thicker material between vents.


Reagarding the malfunctions, you do need modifications. These pistols operate by recoil force. Your compensator reduced that recoil. It retards barrel's rear travel speed by pushing the forward end with the gas, and that barrel must travel with the slide for it to unlock. So, it needs modification to cycle on less recoil.

Why would you suggest a weaker alloy? From memory, 6061 has a tensile strength of 18k while 7075 has a tensile strength of 40k.
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You're thinking of 6061-0.

6061-T6 is a different story. Also, it is mostly because of higher melting point.

As well, I don't have the opportunity to make another because the class I made this in has ended. As well, do you think a 13# spring would be a good place to start or should I go lighter?

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I can't tell without knowing exactly how much recoil is dampened. You'll have to experiment with various springs and loads.

I see, but the correct name I have is 7075-T651. I just never added it because I always thought AI was always t6 unless specially ordered. And 7075-t6 is less heat resistant than 6061-t6?

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According to this data, that would be the case:

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6

I also belive that is the reason commercial makers who use Aluminum for compensators use 6061. There are commercial ones with Aluminum, but I've never seen a case with 7075.

I had about .03" wall thickness on my 1/8" endmill (was hoping for .05") because a 1/8" endmill will apparently wiggle about .02" when it is .9" in a part and sticks out over 1.5" in its tool holder! The design change is shown below.

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Not if you're doing it right. Crap collets, sloppy quill, junk cutters, slop in the vise or table, or your speed & feeds ain't right. I do deep cuts with 1/8 (and smaller) cutters all the time, and tolerances are within .001".

Always spec your cutters, know the proper speeds and feeds for the machine and material, and remember that final passes, CNC or manual, need to be at higher speed and lighter load for a precise, clean finish. All of my final passes are no load and with the cutter, not against it.

I also belive that is the reason commercial makers who use Aluminum for compensators use 6061.

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Probably has more to do with cost; 6061-T651 is a whole lot cheaper than 7075-T6. It's also easier on cheaper HSS cutters.

Personally, I'd much rather work with 7075-T6, though. I use carbide cutters almost exclusively, and it's far less prone to galling during dry machining. Only real downside to 7075 is that you can't weld it.

Quite interesting. But how hot does a gun barrel get after a day of uspsa?

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That is a wrong question.

The correct question is "How hot is a propellent gas blasting on a compensator?"

I think it can reach the 7075 melting point.

Test, can I ask how you got to your answer that the propellant gas will reach the melting point? How many shots will it take?

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Propellants are hot enough to melt an AR-15 barrel when fired non-stop.

Normally, the barrel only survives because it is allowed to cool in between shots.

Now, for materials that are softer and has a lower melting point than a steel barrel, like your compensator, the heat itself probably won't make it blow up like a barrel. However, you're more likely to encounter a heat erosion, the material being eaten away by the gas, if 7075 happens to have a low of a melting point for the application.

If the gas itself can melt a steel barrel, it can potentially melt your Aluminum alloys too, however, it is just a matter of degree.

Barrel itself is always getting eroded away by a small degree, each time it is fired also, but at a much lower rate than a softer matarial that melts more easily.

It is just a matter of degree.

I can't say how many shots, because there is no definite failing point. Even one shot will caus some degree of erosion, no matter what the material, but I am saying that if the melting point is too low, than the degree of erosion would be higher.

I think it can reach the 7075 melting point.

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For a couple milliseconds. But then, it also exceeds the melting point of 6061, 440 stainless, 1095 carbon steel and myriad other alloys. It just doesn't do it long enough to actually melt the material. I can blow through 2" AR400 plate with an oxy-acetylene torch, but it won't even poke through aluminum cans in the amount of time that a flame is present at the muzzle of a firearm.

Also of note, 6061 melting point is not even 100*F higher than 7075.

Mach, I don't know if it makes a difference but I had a lot of chatter on the part because it was sticking out a good 6" (not that I wanted to do that, but was told I had to because 7075-T6 is so expensive that I had to save as much of it that I could). I'll send you the DOC and all that stuff in a pm if you are interested.

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7075 is more than 6061, but it's not exactly platinum. I pay $20 for my 8x16" sheets of 3/8" 7075-T6. Your 2"x13" bar stock would be $25-$30 as a single piece order. 6061 would be about $15-$20.

When producing in quantity and not needing the tensile strength or hardness of 7075, 6061 is far more economical. But for doing small, one-off pieces, I generally subscribe to the overbuild theory. Doesn't matter to me if I can get away with using $7 worth of 5052 for a part, I'll still use $20 worth of 7075.

Anyway, I have no idea what kind of machine and vice you were using, but a piece of stock that hefty should not really exhibit chatter 6" out from where it's clamped if speeds and feeds are right and the equipment is up to the task. But if the gibs aren't snug on the ways, if the quill is extended too far, if the vise is insufficient, etc, you'll have unwanted movement that causes chatter and affects tolerances.

I used to run on a small benchtop mill drill that was of mediocre quality, and it was always a challenge to keep things locked down tight enough to avoid chattering or even cutters walking over a corner. Putting a compound tilt/swivel vice on the tiny table only exacerbated the situation. I kept the gibs very tight to minimize that slop, and I have a good bit of carpel tunnel from running the X-Y handles with so much resistance. But I now run on a Lagun FTV2 with a 6" Wilton swivel vice (about 80 pounds) and a 12" Troyke compound cross slide rotary table (about 165 lbs). When your equipment is rigid and heavy enough, things stay where they should and cutters do their job right. Having said that, even as robust as this 3,600 lb vertical mill is, I still bring the knee up as far as possible, using quill travel only to move in and out of pockets on the work piece.

Mach, it was a Haas VF-4 using Haas's 4th Axis with .02" DOC and .0005 tooth load on a 4-flute endmill. It was at....5500 rpm with a feedrate of 6 ipm (all of these recommended by teacher)

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Hmmm. There should have been virtually no run-out or chatter with cuts that light and speeds that high. I assume that is a CAT40 or CAT50 tool holder system? Those pull in hard, but if the tool isn't tight in the collet, it doesn't matter how hard the collet locks into the quill. Or were you using a fingered chuck for the tiny end mills? If so, yeah.......Jacob's type chucks and end mills just don't mix.

On my manual machine when running 1/8 cutters through 7075, my DOC is usually almost twice that (assuming the material is that thick), but feed rate is only about 1.5-2 IPM. I run at 1,200-1,500 RPM for rough cuts with as much as 100% radius engagement until final passes, where I'll back it off to .002" per pass at ~2,500 RPM. Of course, my mill only runs up to 4,200 RPM.

For future reference, one or two flute cutters are gonna be more rigid, and are generally the better choice in small diameter cutters. I try to avoid 4 flute cutters smaller than 3/16" unless 2 flute mills are not giving me a smooth enough cut, in which case the 4 flute is only used on final passes.

I guess the real question should be why the cutter was 1.5" out of the collet? With a 4th axis, there's no good reason to have it out any further than the max DOC + 0.050" or so, or right up to the start of the flute cuts, whichever comes first. I'm assuming the outside of the comp to the inner bore is no more than 1/4", and most 1/8 cutters have no more than .500" flute depth, so that's all the further I'd have had it sticking out. You make the cuts on one side, then rotate the workpiece 180* for the opposite side. Even good carbide cutters taking light passes at high RPM will flex with that much shank hanging out of the collet. Those sort of super deep cuts with small diameter mills require multiple low load passes, and are best left to high speed machines (with lubrication) if available. And no, 8,000-10,000 RPM is not high speed; HS VMCs can run as much as 60K. The machine you are running on is very similar to the Fadal 4020 machine my friend and mentor has; Haas actually bought Fadal, IIRC. Good machines, just a little outdated.

Having said all that, I'm an amateur level manual machinist, can't tell you a tremendous amount about CNC equipment. Lots of folks who can better answer those questions over at the practical machinist forums.

Quite interesting. But how hot does a gun barrel get after a day of uspsa?

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That's a good question. Heat treated aluminum like 7075 loses strength (permanently) after exposure to elevated temperature. Either one good run at a high temperature, or cumulative exposures to not quite so high temperatures will degrade strength. The former is pretty unlikely for a handgun, but repeated heating to 300-400F could have an effect over time (would take a lot of shooting, though).

Looks good, but you should bevel or soften those corners. Will improve aesthetics, and -more importantly- prevent holster gouging and injuries from handling. Believe me, if you leave a sharp corner on an implement, you'll find it with your most sensitive parts.

Heat treated aluminum like 7075 loses strength (permanently) after exposure to elevated temperature.

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The temper matters. 7075-0 is not heat treated/no temper; it will only be weaker at the elevated temperature. T6, T7, T651 and RRA will be affected to different degrees, but uncontrolled heating and cooling may anneal them, which will reduce hardness. Uneven heating could cause stress fractures if the temperature differential is enough. T651 is more stress corrosion cracking resistant than the others, though.

Of course, tempered steels are also affected, just at much higher temperatures.

Having said that, his comp unlikely to see the 775*F temperatures where 7075-T6 anneals. The material does begin to show increased fatigue under stress at 350*, but it's also not a stressed part in this application; won't really matter if it loses it's hardness or if it's tensile/yield/elongation properties are reduced. 1000 series die cast aluminum could perform the function of a compensator.

The muzzle blast temperatures are high enough that some erosion may become evident in time, particularly at the hard corners, but that won't affect function.

I'm bettin' the comp itself never even hits 200*. Yes, gunpowder burns at temperatures hot enough to melt most alloys, but not for nearly long enough durations. If it did, aluminum used for cartridge cases would be very problematic. As it were, we see more metal fatigue in cases from work hardening than temperatures. In point of fact, brass case life is increased if you do anneal them.

One other point, Aluminum (no matter the specific alloy) is a much better conductor of heat and can dissipate heat much faster than Steel can. The powder blast may be very hot but the Aluminum will get rid of that heat quickly. I agree that erosion could be a problem over time.