Angles and threading

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Was that you watching over my shoulder, DoN?

The only thing you missed was the condition of the lathe, which was in good repair, extremely well cared for, oiled regularly, and not very old. I was running a machine I had never been assigned to before that day, so I didn't recognize the quirks it displayed. The fellow that ran it routinely, who is likely the highest skilled lathe man I ever met, was totally familiar with the machine and never had the same problem with it.

The problem was exactly as you described, with the drunken condition perfectly timed with the revolution of the hand wheel. The error in the thread was not visible to the eye, but due to the long engagement of the mating part, the thread, while in tolerance pitch diameter wise, would not assemble with the mating part because the drunken thread generated altered the lead. Overall, the thread was correct, but internally, from error point to error point, it was not. It takes precious little to render threads unable to mate when long engagements are involved due to the minimal amount of clearance between male and female threads.

buyer, Thiokol. I recall that we were able to simply recut the thread, removing the miniscule amount of deviation that made the thread tight. The opposing faace of the thread, which was already cut away, was minor enough that the part was accepted. I remember this job fairly well because of the extrordinary circumstances, plus the fact that it was rejected. I rarely turned out scrap, thanks to my anal approach to machining.

Guilty as charged!

Just out of curiostity, this either a fairly coarse pitch thread, or an acme thread? I trying to see if my guess of a large amount of cutter engagement bears true.

Jim

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It seems to me that the larger side force required of the leadscrew, when cutting only on the leading side of the tool, would tend to make this problem *less* likely to happen under his circumstances.

This is of course not to say that it positively would have avoided it - that would involve seeing how much force the handwheel 'thump' would have generated, and comparing it with the force of the (30 deg feed) cut.

And of course, as everyone had suggested, the handwheel imbalance problem is constant during the threading operation, and the force of the cut is basically zero to start out with - which implies that even threading at an angular infeed the thread will start out a bit incorrect, but will tend to correct itself as it deepens.

I think maybe there was some rust in Harold's machine hidden away where he couldn't see it! :^)

Jim

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threading

I'm not sure I would want to take this thing to the ridiculous. Would anyone expect to have a perfect thread if you had a perfectly balanced hand wheel but you stood there and thumped the carriage even with your fist? Any unbalanced component in the hand wheel drive train will systematically try to advance then retard the motion of the carriage, driving halfnut to lead connection tighter then looser. The thumping just tries to do it all at once.

I think it's all a mater of degree. A minor vibration will show up on a turned surface, or push the fixed jaw of a vise with your finger and you can detect it bending with a fine enough indicator. So any imbalance will translate to errors but luckily usually to insignificant to show. I don't know about thumping though.

Maybe but if you keep thumping at the same place it might get worse.

Thanks again for your take on this.

I find it hard to blame a process for a machine fault. Aside from that, my point is, and always was that the plung method is practiced to varing degrees by most all of us and we would be remiss in not teaching it. (With whatever warts, it is seen to have!)

I think we all know that we can pretty much take what a fellow like Harold say, to the bank. Even in doubt, they are probably more right then wrong. That's as it should be.

It don't mean you should follow blindly though.

I have been a little leery of experts ever since ,,, well let's just say for a very long while.

I try hard to sort the wheat from chaff and understand any subject " my mouth puts my foot in". (O:

Bill

(as the old Indian says) We will endeavor to persevere!

failing

Thanks Hoyt

I also thought it was pretty convincing illustration that there is no magical power that would keep on pushing the carriage and cutter. Never mind go ahead of the previous speed. But no one but you seemed to pick up on it. I cannot make people acknowledge a thing if they don't believe it.

One other little tid bit that I picked up on was from a guy speaking about having learned threading on a 125 RPM lathe. He was speaking to another guy who had never threaded faster then 25 RPM and the slow guy was wondering how people handled such speed.

Anyway the "fast guy" simply said "You gotta learn to be quick on the half nuts."

It tells me for sure that that the fast guy knows from experience that the single point cannot keep on pulling the carriage forward by itself. He bets his lathe on it every time he does it. (In all fairness I have to say that the fast guy never mentioned how his compound was set) But I'd bet a nickel he would not hesitate to do what he always does regardless of the compound set up. Just fast on the nuts!

Thanks again

Bill

Hardinge has that sort of threading on an HLVH as well, the carriage will trip the clutch on the leadscrew and this can be set for repetitive jobs. Works great.

However hardinge figures that the user is going to be feeding with the compound anyhow, because their fast retract is on the compound dial, not the crossfeed.

Could be. Then again it could be from a personal experience he had in the past. This is one reason I suggested that the novice at threading (in another thread) start out feeding in only a thousanth at a time; when I was just starting out I did indeed set the whole job up, took a scratch cut, and then tried to take the whole thing at one whack.

Didn't work too well. A thing like that will make a man gunshy for years afterwards.

Jim

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I, for one, value his helpful advice far more than your fussing about it.

Gregm

Newbies occasionally use CNC also. But for that matter, I once used a Cinnci with about 10-12" swing, manual machine, and a device called a Snap-Tap, which automatically disengage at the right point. This was intended to be used by plunge-feeding the cross-slide, and it cut perfect threads.

I think Harold's precautions are fanciful, the product of a brain that can see only one 'right' way to do anything. It may be a common failing among trained machinists, which is one reason why I consider myself so lucky to be so largely self-taught: there's always more tha one way to skin a cat and usually I find it. The idea that the thread being cut can drive the carriage is largely vapor. Bill Darby showed us that by his thought experiment of releasing the lead screw while in the thread. Does anyone think the carriage will continue to follow the thread being cut? Seriously, that went out with hand-activated thread chasers. As for Harold's asserting the trailing edge WILL drive the carriage, esp at larger pitches, I can only remark that the tool in those cases needs more bottom relief, to prevent the thread form from rubbing the flank of the threading tool.

Both. Coarse, and Acme. I recall it to be a 2 pitch thread Acme, perhaps it was 2-1/2" diameter or so. Length of 6" or so, more than one revolution of the carriage hand wheel for sure. Damn, Jim, this was back when you was a pup, had to be in late '66, because we were still in the old shop at United Precision. I'm having a hard time remembering my address, so it shouldn't come as much of a surprise that I don't remember the fine details. The actual failure was embarrassing enough that I remember it well.

Harold

Having plonked this dude, I am grateful to Bill for having quoted his self incriminating statement:

which is one reason why I consider myself so

I rest my case. What better way to strengthen poor procedures in one's mind than to use them endlessly. Doing something wrong time and again won't make it right.

What better way to learn a procedure than to go to one that is self taught, no guidance by those that went before him, certainly too clever to take advice from one that may have practical experience.

I take credit for one thing, and one thing only, which is to have learned from people that had practical experience in the field of machining, most of whom were well trained in the missile and aero-space industry. I didn't consider myself to be so clever as to be smarter than the total of those that went before me.

It has been my experience in machining that the most clever of novices are certainly much more talented and skilled than those guys that do it daily. After all, what could someone like that know that some clever person doesn't? People like that remind me of teenagers. What better way to learn than to hire one to teach you everything he knows while he still knows everything. Most of them outgrow it. Apparently some do not.

Harold

Well that's humbling. You were cutting two pitch acme threads at about the time I was incarcerated in first grade.

Jim

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I don't quite believe that (Jim in grade one) but that was about the time I was starting my second hitch in Germany. A hell of a lot better then the boys stuck in Viet Nam though !

Bill

Well, I wouldn't mind hearing what he has to say either. But I know what Moltrecht has to say about it since he uses 1/2-13 as his example of how to set infeed.

He says that after doing a couple thou scratch pass, to do the next two passes at 10 thou, then work down from there until the final pass is 1 or 2 thou to hit the exact minor diameter.

As Harold mentioned, on a light springy lathe, a 1 thou cut can be pretty iffy, so I like to work it out so the final few cuts are equal and more than a thou. That way I can predict how much spring I'm going to have to compensate in order to hit final diameter.

Gary

As I said several times, arrogant bastid!!! Nothing wrong with self-taught, it got me most of what I wanted in life. Certainly doing things wrong endlessly would be a shame, but fact is I've always been the one in any shop to see an alternative that was usually easier, almost always faster and almost always produced less scrap. That was partially due to refining techniques I'd seen others use, partially due to keeping my nose centered on the grindstone, and partially due to being frankly inventive.

By golly, you make it sound like machining is/was HARD!! I hate to labor the obvious, but it isn't rocket science.

IOW, trained to do what a million others had done before, and never try to do it another way. Here's a secret: there's absolutely no progress in that sort of learning.

Obviously, you weren't. You weren't even as smart either.

Ah, you do understand!!

My best guess is nobody can teach you anything.

I'm not going to quote any of it hoyt, it's not worth quoting, and I think mostly bullshit. I've been taught by a lot of men, all better than you can ever hope to be. Your "experience with the drill jig", only makes you what we call a shitrunner, someone that is in too big of a hurry to CHECK his work, turning out box after box of substandard material. PLONK!

I think I had better respond to Harold's assertion that my statement is not true before anyone gets the idea that I think he's right. With all due respect I do not see any understandable argument to support his assertion. I see only a series of words that say nothing that would support any such conclusion. Just words.

As best I can understand in the case of the thumping handwheel the stock was 2.5" in diameter, the thread was 6" long with 2 TPI. It was plunge threaded from right to left with the threading cutter properly mounted strait ahead such as to produce identical sides on the thread.

So lets examine the travel of the cutter: In one single threading pass the 2.5" stock would rotate 12 times. A surface distance of 94.25" and the cutter would travel 6" to the left.

The Arc tan of that should be about 4 degrees (3.64 degrees)

Sketch a 6" by 94" rectangle then run a diagonal to represent the actual cutter path as it progresses forward and to the left. From the sketch it should be possible to visualize the cutter on it's diagonal path while it still points strait ahead. In fact the cutter's attitude is skewed 3.64 degrees to the right of the path of travel through the metal, thus exposing more of the left side and less of the right side of the cutter faces to the resistance of cutting as it progressed along it's diagonal path . Thus trying to push the cuttet to the right.

This explanation is not unlike trying to push a boat in a current approaching from the front left. The left side is more exposed to the current so a constant force from the right is required to counterbalance the additional current on the left side.

All of which (I think) supports my contension that:

The leading face of a plunge cut threading cutter has always got a more significant load then the trailing face.

Bill

Yeah, but only by about 4%. Doing the infeed at 30 degrees gives you about 8 times greater loading against the leadscrew. That's much more likely to keep all the lash loaded out of the system.

Gary

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Huh????? What, this is a contest?

This whole thing is simple..........*real* simple. People with far greater reasoning powers than you or I can hope to have long ago determined that there are certain risks to plunge threading on engine lathes. Their findings are well documented and published.

One of the negatives of plunge threading is the premature failure of the tip of the threading tool. Like it or not, it's not good for the tip, and had it not been for advancements in the shock and tensile strength of carbides, it would likely still be nearly impossible to thread that way with carbide. Even HSS is subject to early failure by plunge threading, especially in the tougher grades of steel of all kinds.

Try not to forget that while you were serving your country in the military, I was actively engaged in cutting these threads you seem to know so much about, and had been doing it at that point for more years than you have been machining. My work was for a customer that had more than enough intelligence to know what good threads were. I've had my threads inspected by almost every means possible, which quickly taught me to not add risk of failure to the job. That's what you do when you plunge, and the error increases drastically as the pitch decreases. Not only are tolerances tighter on fine pitch threads, but the pressure propelling the carriage is reduced to almost zero, permitting even lower transient forces to effect the carriage.

Whether you wish to not consider the increased risk of creating an uneven lead when threading doesn't concern me nearly as much as you're seemingly endless quest to encourage others to bypass learning the basics. That, to me, is very troubling. For anyone that has enough skill to thread properly, there are NO advantages to plunge threading, not even a gain of time. Certainly not an improved thread. Why then would anyone wish to increase their possibility of making scrap?

Chip flow is a serious problem when plunge threading, a problem that is well addressed by proper feeding by compound. That is the lesson that the new guys should be learning. When one has learned good and proper procedures they can then make decisions about altering a given process to advantage, should they find it necessary. I strongly encourage new guys (and gals) to learn to walk before they try to run. Seems to have better results.

I see nothing good coming from this discussion. Those that have an interest in learning proper procedures will likely go to the books and learn what is recommended. The others will do as they please, likely using both procedures until they land on the one that suits their quality demands. Any further comments from me will serve no good purpose, so I am finished with this discussion.

Harold

And a good morning to you Gary

Thank you! ........ I will take what I can get. I have no idea where you got the specific 4% figure from but it supports what I said.

Bill