Making hole bigger on recoil lug

[Dud]

I don't think you would need to open it much so I would be tempted to use an adjustable reamer if I wasn't set up to bore it on a lathe or with a boring head on a milling machine.

The hole shouldn't need to be super precision but an adjustable reamer should self center somewhat. If you can find the right size cheap, it would be a lot less hassle than making a lathe jig that will hold it precisely.

Any way, that would work for several recoil lugs but if I had to do a lot of them, I would probably do a jig on a mill and cut with a boring head.

 

[gunsandgunsmithing]

Main reason for rethreading would be out of alignment. The threads hold the barrel against the face of the action. Unless they are at a skew angle, or badly off center, they don't need to be changed. 9/10 machinists make threads too tight. Barrel threads should be relatively loose so the barrel can settle against the action.

I do run a 1 1/16-16 tap into them to clean out any burrs.

There's nothing to be gained from a tight thread fit in barrel work but there is the risk of too tight a fit causing problems..so it's a no brainer..A little loose is better than a little tight. A true machinist should thread to the proper thread class fit. There is very often about .004" acceptable tolerance even within a class! A class 2 is generally safe but plug gages are great to identify the class of thread you should thread to. It can be tough to accurately measure pitch diameters of the "B" part(femele). Making a thread too tight does not make anyone a better smith. Just the opposite really, as it shows that said smith doesn't truly understand thread classes and proper clearances. Another example is thread crests. There's virtually no practical use for a male thread that is left at a sharp vee. Look it up in Machineries Handbook. The crest should have a flat on top equal to 1/8th of the pitch, wide. A burr here, or it causing interference with the female major diameter will ruin your day, especially in a very fine and/or tight fitting thread. The top of the thread(hopefully) does nothing, by design.
Notice I saide wide and not tall. On a 32 pitch thread the flat should be 1/8th of 32..so .004 wide but on a 60° thread, that works out to be about .005-.006 in height. So, a proper finished major od for a basic 7/8 major 32tpi thread should be .869-.870.

I ran into this when I made the change to .875-32 on my tuners. A very few really reputable smiths tore the o-ring up trying to put the tuner on. The reason is that I spec'd the o-ring groove just deep enough for the o-ring to pass over a "properly" cut 7/8-32 thread, which has a finished major of .869-.870. There is a distinct difference in basic vs finished major diameters.

I don't really fault someone that ran into this. There are some very good self taught smiths that do this. Mostly, they were just taking pride in a nice, pretty thread, that they didn't have to machine the tops down on. I get that and can relate but pride of workmanship doesn't equate to proper workmanship, in this instance.

I simply clarified the need for the flat or finished major to be .869-.870 on the print and it seems to have stopped the issue. Sometimes it's the little things that can matter but a thread crest should have clearance and there is a spec for it.

 

[dfrosch]

If you machine the action threads and want the barrel threads to be tight, I would think that it would be easier to slightly truncate the barrel threads than machine the lug.

 

[GenePoole]

I used to just open up R700 actions enough to clean up the threads, but that necessitated a custom barrel thread for each action. While this might be fine for 90% of R700 owners, most of my customers are fellow shooters who put thousands of rounds down a tube each year.

I decided to standardize on the same thread for every R700 action so I could cut a new barrel sight-unseen for any action I'd ever done. For historical reasons, this ended up being 1.080x16 3B; mainly, that is the max I ever had to go to cleanup an R700. I made a go/nogo thread gage set for this.

I always use full-profile inserts for both internal and external threading so I can be sure that the finished profile is in spec WRT crests, etc. and that when I cut an internal thread that mates with my gauge, I can be sure that it's pitch diameter is consistent.

This is why I need a custom recoil lug diameter as well. I settled on the Badger Ordnance 5/16 lug and use it nearly exclusively so a single fixture is all I need.

 

[AWLEAPHART]

Gene,
What your doing with the actions with threaded go and no go gauge is what I’m wanting to do. I was think 1.077 on action and 1.075 on the tenon.
A Leaphart

 

[DaveTooley]

Gene,
What your doing with the actions with threaded go and no go gauge is what I’m wanting to do. I was think 1.077 on action and 1.075 on the tenon.
A Leaphart

Your numbers don't reflect pitch diameter which is the key number for threads.

 

[GenePoole]

You can consult a Machinery's Handbook or use http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx to find that a 1.0770x16 thread definition would give a 3A (external) pitch diameter range of 1.0323 - 1.0364 and a 3B (internal) pitch diameter range of 1.0364 - 1.0417. As you can see, an in-spec internal thread will always accommodate an in-spec external thread.

Use a thread mic or thread wires to achieve an in-spec pitch diameter.