Thread relief on tenon

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eww1350

Guest
I have notice that many barrel fitters are cutting a thread relief on the tenon when chambering new barrels...I have had friends ask me to set back a barrel to freshen the throat and that thread relief sure leaves an ugly uneccessary gap...I do not cut a thread relief just for that reason...you never know when you need to remove several threads to change a chamber without making the barrel too short....
Take a LV barrel at 21.750" and you want to set it back .500' or so every 300-500 shots and you can see where a thread relief ruins the usability of that fine barrel...

Just some observations...


Eddie in Texas
 
If the action does not require a relief I don't cut one on the barrel... but you have to on some actions...
 
Some purists look at a relief cut as a way to avoid the extra effort needed to thread a barrel without one. Maybe so, maybe not. I usually do not cut one but I have all the time in the world to thread without one, or to fix things up if I happen to hit the shoulder.

I have set barrels back when they had a relief cut and I don't think it hurts anything to have that gap. After all, it is hidden and you can always use the old machinist's excuse of, "Hey, that's exactly how I wanted it to look." ;)

Ray
 
Please explain, how do you thread full depth hard to a shoulder with NO relief ?
Are you tapering out to crest dia and relieving the first few threads in your reciever ?
Or what am I missing ???
 
Most receivers have some relief in them. Either that, or the action has a recoil lug or other spacer. No further relief required.
 
Vinny

It's not possible to cut a full depth thread hard to a shoulder, but if you are sober, turn off the Tv, and lock the door, it is possible to come pretty darn close. And, the pucker factor isn't too high since a wreck can be easily fixed.

But, as 4Mesh said, it isn't really necessary, regardless.

High School Machine Shop teaches you to always cut a relief on any thread that comes up to a shoulder, but a full thread looks more professional. And your friends will be impressed. Never underestimate the importance of peer acknowlegement.

Ray
 
Several years ago before I started doing my own, I had one set back 3/8". It now had a .390 thread relief. They did not have to pick up the old threads. Everybody shook their head, but it shot lights out. It doesn't take but a few threads to hold it sufficiently. I thread to the shoulder or about .040 from it.
2q2jm8m.jpg

Not a good photo, but you can't get much closer without crashing into the shoulder. It really is easy as I have travel dials and set it to "O" when I want to disengage the half nut. It ain't hard and it will not hit the shoulder if you watch your Trav A Dials or indicator. It ain't necessary to thread to the shoulder but it looks better.
Butch
 
I recently fitted a barrel to an '03 Springfield. It has square threads @ 10 TPI and really can't be done right with a thread relief. So I threaded it to the shoulder. I set up a dial indicator on the ways where I wanted to turn off the motor, let it coast to a stop (from 35 rpm, which didn't take long), then pulled on the drive belt by hand to finish the last half turn or so to the shoulder. I used an HSS bit to cut the threads, carbide would probably have shattered cutting to a stop like that, but with HSS it worked great.

When I'm cutting normal "V" threads I just flip the spindle into reverse (I have a 3 ph drive motor on a rotary converter, do not try this with a single phase lathe!) and back off the cross slide. Works for me.

Fitch
 
Butch Lambert said:
Fitch,
What would happen if you just disengaged your halfnut and didn't put it into reverse.
Butch
Click to expand...

Flipping it into reverse with the half nuts engaged keeps the tool pretty much in the groove if my timing backing off of the cross slide is off a tiny bit. Disengaging the half nut stops the carriage but the spindle keeps turning so there is the risk of making a circumfrential cut. Flipping it into reverse, with the tool (cross slide) retracted (I use a cross slide stop when threading, do the feed with the compound) I just let it run, crank the cross slide in after I clear the work, advance the compound a few thou, flip it into forward, and make the next pass.

Truth be told, I didn't give it a lot of thought, I got started flipping it in reverse cutting metric threads that wouldn't pick up again on my imperial lead screw if I disengaged the half nut. In fact I converted the lathe to 3ph for that very reason - doing metric threads. A single phase motor won't plug reverse, it has to slow down enough to engage the starting capacitor.

Fitch
 
OK, let me just get this right.....Fitch you just HIT REVERSE with a couple hunnerd pounds in motion? (I realize that reversing force is probably only 20-30ft/lb but hey..... if it was hooked to my nose it'd hurt)

I know of hydrostatic drive heavy equipment where that's normal but that's because of bleeders and popoffs.. and routing. Does a three-phase offer such a smooth cushion that there's no shock or what?

al
 
Does a three-phase offer such a smooth cushion that there's no shock or what?
Click to expand...

Yup, just like power tappin.

Al, ya sayin ya never, oh gosh

3ph will direct reverse.
 
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Al,

That doesn't mean you can just run em full throttle and hit reverse like you can with a 3 phase dc motor. Nothing really enjoys that sort of treatment, and I've even seen drives fail on big dc stuff when that's done too much. I know the one lathe at work has a spec on it that says what rpm you can reverse it at. Seems to me it's around 300 rpms. On any motor, a stalled rotor condition causes current to go through the roof. That's what you have when you hit reverse. Just don't be surprised if you do it at too high a speed and everything comes to a coasting halt after the breakers blow. Done at proper speeds, you should never see that.
 
I don't see myself ever doing it. But for production work in a machine shop environment I can see applications.

In this vein I used to rent a lot of equipment doing my own stuff. I BROKE a lot of rented equipment, generally by being too "gentle" with it. I'm now a perty good operator, I can get work done without every real equipment guy in the area getting knotted up just lissening to me work. :) I've grown up around machines, machine shops and machine people and can state as fact that I've actually learned how to drive certain rigs by watching the expressions of the other drivers........ so I'm fairly confident that there's a right way and several wrong ways to "hit reverse." :)

al
 
alinwa said:
OK, let me just get this right.....Fitch you just HIT REVERSE with a couple hunnerd pounds in motion? (I realize that reversing force is probably only 20-30ft/lb but hey..... if it was hooked to my nose it'd hurt)

I know of hydrostatic drive heavy equipment where that's normal but that's because of bleeders and popoffs.. and routing. Does a three-phase offer such a smooth cushion that there's no shock or what?

al
Click to expand...

Al, trust me on this, it works. Really. Why? Well, at the risk of TMI, I'll tell you why it works.

Well, first of all, the heaviest thing in the drive path is my 8" 4J chuck. I don't know what it weighs, but it isn't a lot, so the rotational inertia (Wk^2) is relatively small.

Second, I only do it when I'm threading which I seldom if ever do at more than 50 rpm, usually at 35 rpm, so there the angular velocity squared term is small.

Third, I'm hooked to a rotary converter which be default limits the current to the generated leg compared to a power company supplied three phase source.

Finally, if you look at the drive train, the gear ratio between the 1725 rpm motor and the spindle is about 34.5:1 when the spindle is set for 50 rpm. Inertia is reflected from the spindle to the motor inversly as the square of the ratio, which means the motor is seeing ~0.08% of the spindle inertia under threading conditions - in other words, the motor is seeing conditions not much more than if it was reversing itself. The inertia of the pulleys and the first couple of gear shafts are way more significant than the spindle, but they don't have a lot of inertia to them - and even the driven pulley will see a reduction of something like 1/9. Combine the small spindle inertia (that the motor can't even see much of) with the slow speed (very little kinetic energy) and the limited current and it is pretty much a non event.

I did it from full speed a couple of times as a test for the converter a couple of decades ago when I built it. Lathe slows down over several revolutions then accelerates back in the other direction to full speed. It probably puts about as much heat in the motor as starting it twice at full speed.

I'm running my 2hp 3ph spindle motor off a 5hp idler that's balanced (using run capacitors) to give a voltage of 108% of line between each line and the generated leg and a power factor correction cap that results in about 5 amps of idle current (no load). The whole thing is powered off a 20A 220V single phase circuit and will run the lathe and mill at the same time if I wanted to do that - and I've done that when I was flycutting the surface of a piece of metal in the mill feeding it as slow as the feed would go - mill motor doing almost no work.

If I wanted to make the reverse snappier I could turn on my mill (also 2hp 3ph) and let it run unloaded while running the lathe. That effectively turns it from a perfectly (my opinion) balanced 5hp converter into a poorly balanced 7hp converter with more surge capacity. I don't do that because there is no reason to. But I tried it after I built the converter because I wanted to know the limits of the system.

Anyway, if all this hasn't put you to sleep, it works a treat for threading if the drive motor is powered by a converter, or if it is powered from the power company 3 ph line. I've never tried it with a 3ph inverter (variable speed) drive. It might work, but I'd think it would have to be a programmed ramp down and back up, and it might not like it much - but it might work at low speed.

There ya go, everything you ever wanted to know about plug reversing a small lathe.

Fitch
 
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If you have a chuck that is threaded to the spindle, it can get mighty interesting when you hit reverse. We used to do tyhat in high school machine shop just for laughs, until we got caught.
 
OK, Thank You.
I dont see myself ever being concerned with it, as it isent effective for my needs or equipment.
But I understand what you'r refering to and can see how it makes good practice.

Cheers, YV
 
Cut away from the shoulder with the HSS tool upside down and run the spindle in reverse. You can also cut to the shoulder using a Hardinge HLV-H or similar. There are quick retracting atachments that are similar to the Hardinge available.
 
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