AR upper receiver lapping

[Shadow9mm]

Ok, first lapping job done. Came out fairly decent I feel. I did end up needing to use my drill. Even well lubricated the lapping bar was tight in the action. This is an aero precision upper. got 3 grits 600, 320, 220. Tried 600 and it was not quite touching the finish. 320 got the job done in 4 passes of about 10 seconds each at a slower speed. Henld the drill in 1 hand upper in the other. Figured if I put it in the vise i would end up applying uneven pressure, this way it would king of float a bit.

First Pass

Second pass

Third pass

 

[Shadow9mm]

and Final Pass

 

[FrankenMauser]

Get a good grease coating on there before torquing the nut.
It will help prevent creating a corrosion cell in the future, should water (and god forbid, salt) get in there.

 

[Shadow9mm]

did a quick lap on my colt upper today. was able to use the 600 grit instead of 320. cleaned up in 2 ten second passes. top lapped first and the rest followed quickly, it was very close to true.

Barrel is back on, got a new hand guard and it is a TIGHT fit. heated the rail with a heat gun and it did not want to go on. Got the upper in the fridge for a couple minutes. gonna warm up the hand guard, pull it out of the fridge, and see if i can get it together.

 

[MarkCO]

Anyone done it? Thoughts?

When I did not know what I was doing, I did a few times. I've cleaned up a damaged receiver a time or two as well. Thoughts...waste of time and money. Only a very low quality receiver would need it, and then you are better off spending another $20 on a better receiver.

The receiver is aluminum. With anodized layer removed, it is pretty soft. The lapped area is load bearing. It transmits recoil force from the barrel to the receiver. Bedding may be a better choice, if I ever want to tinker with it.

Yes, the anodizing does create a little surface hardness. And, the portion on which the anodizing is removed is now raw aluminum. Mate that to stainless...oh no! Even on carbon steel, you will create corrosion cells that, in a little time with the thermal cycling, will create more stress risers than you eliminated making it "flat".

Best bet is buy 7075 receivers, use the proper anti-seize, and torque at the high end, 70 to 90 pounds for final. The harder extension, with proper seating by low torque with successively higher torque, will move the aluminum more than your lapping creating a rigid joint, with full corrosion protection. It is highly rare that won't get the best accuracy that the barrel is capable of. If you have a really out of spec receiver you must use, get a foil ring to use between the receiver and extension. Way better.

If you are searching for supreme accuracy, like in the 1/4 MOA range, undersized 7075 receiver or oversized extension, and thermo-fit to complete with torque in the 90 to 100 ft-lb range.

I've tested all of this with lab equipment as I was building my barrel manufacturing business up, and in consulting for other barrel manufacturers.

 

[MarkCO]

and Final Pass

I see nicks and grooves on the raw receiver face. You have successfully made your receiver worse than it was.

 

[Shadow9mm]

maybe maybe not. Based on the removal patters i had a bit of a high spot at the bottom. while the surface may have some small grooves in it, it is also now flat. it may be an even trade.

I did also notice some wear marks/grooves in the bar itself after lapping this upper. so that's not great. Im calling it at this point. I only used the 600 grit on my colt upper. it looked much cleaner. But based on this upper and the way the tool looks, its getting returned.

 

[HiBC]

It looks like a steel tool. Steel is harder than aluminum. The way lapping works,the grit embeds in the softer material . Its the harder material that gets cut.
Interesting. Thanks.
Did it get dry? A little galling maybe?

I'm not criticizing. I'm trying to learn from what I see.

MarkCo's comments are interesting,too.

 

[Shadow9mm]

It looks like a steel tool. Steel is harder than aluminum. The way lapping works,the grit embeds in the softer material . Its the harder material that gets cut.
Interesting. Thanks.
Did it get dry? A little galling maybe?

I'm not criticizing. I'm trying to learn from what I see.

MarkCo's comments are interesting,too.

Im trying to learn too, no worries. Just keep in mind im no expert. Just sharing what im doing in the hopes i get some good advice and others can learn from my experience.

steel tool. Did not get dry as best i can tell. The lapping compound was kind of a thick paste. Had to mix it up. Had separated. Looked like it had grease or petroleum jelly in it from the consistency. I worked in about 10 second passes. Applied fresh compound each time. Im wondering if the 320 grit was too coarse. Also i think wheeler uses silicon carbide, which can embed. Apparently there are non embedding lapping compounds, which i would consider if i was looking to try this again.

 

[tangolima]

Found some 0.01" soft lead strips (originally for fish sinker). Cut a thin strip and used that as gasket / crush washer under the barrel shoulder. While I was at it, I applied blue loctite to the barrel shank. Torqued the nut back to proper timing. Torque went up by about 10lbf-ft. Will see whether it will help. Probably not much as it is a very cheap upper. Once a while, when everything is perfect, it did shoot sub moa 10-round groups, with peep sight mind you. Hopefully with the mod, such delighting moments happen with more regularity.

The idea is similar to lapping. The soft lead conforms to the harder steel and aluminum so that the force is even all around.

-TL

Sent from my SM-N960U using Tapatalk

 

[stagpanther]

I pull barrels quite a bit off of well-used rifles, I've found that the area around the chamber and the extension where it fits into the receiver is a cauldron of high-pressure heated gas which mixes with combustion residue and residual cleaning solvent--it's going to find a way into any joint. The other thing about lapping that I became nervous about it was that I was unsure of how it might affect headspacing and lug lock-ups--I suspect that even minute depth changes could make a difference. The extension pin slot may or may not also be affected--I had a barrel once that absolutely refused to work with any bolt and I eventually figured out the extension pin was slightly out of clock with the gas port and lugs. I'm not saying it's a bad thing you shouldn't do--but I suspect that it's not as straightforward as blueprinting a bolt action receiver. I'd like to hear from an industry expert to weigh in on this. My opinion about any receiver to barrel mating that involves shims is that is inherently going to cause problems sooner or later--always has on any rifle I've used them on.

 

[tangolima]

Headspacing is in the barrel extension. Lapping / shimming should not affect headspacing. It does change the location of the lock lugs by a few thous. Between the bolt and bolt carrier there is about 1/4" margin built in the design to delay the locking / unlocking. A few thous change shouldn't be problem. Lapping / shimming is perpendicular to the pin slot. It doesn't change that either.

I prefer shimming (actually not quite shimming in normal sense) to lapping for a few reasons. I generally don't want to remove metal from parts if I don't have to. Shimming is reversible. Spending money to buy another tool is another reason.

There was a bit of residue on the barrel shank. When I wriggle the barrel I could feel a slight slop there. The loctite hopefully will fill that.

Will shoot some group to see how it works.

-TL

Sent from my SM-N960U using Tapatalk