Threading with compound

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liveaboard
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Re: Threading with compound

Post by liveaboard »

My compound doesn't actually have any scale numbers that far around; I measured with a protractor and made a mark with a paint pen.
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Harold_V
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Re: Threading with compound

Post by Harold_V »

John Evans wrote: Sat Dec 19, 2020 12:44 pm I don't bother with the scale on the lathe. Rather I use a 30-60-90 triangle and a 1-2-3 block off the chuck face.
It should be understood that the angle isn't really critical, so long as it is less than ½ the included angle of the thread in question. The purpose for the less than ½ angle is to ensure that the feed causes the tool to continually wipe the opposing face of the thread. If one chooses a reduced angle, it simply takes a deeper wiping cut. With a sharp tool, that is of no consequence. The resulting thread will still have the proper form (assuming the threading tool is properly ground).

Plunge cutting threads works, of that there is no doubt. Some prefer to generate their threads that way, but there's a reason why it's not recommended. Watching the chip being generated should provide a clue why that is.

The tip of a threading tool is quite fragile. The wedging action of a plunge cut puts tremendous strain on the tip, so if the initial passes are relatively deep, there's risk of damaging the keen edge. It will still cut, but the shearing action that is so desirable is lost, creating what is akin to negative rake. The deep pass will overcome the resistance, but when it gets down to light passes, often used to improve the finish on the thread, cutting pressure is too great, so there is no cutting action. As one takes successively deeper passes, trying to overcome the resistance, when the pressure is great enough to overcome cutting resistance, all is taken at once, resulting in a rough and now undersized thread.

When one feeds via the compound, the vast majority of the chip being generated is from he advancing edge of the threading tool. That lowers chip flow conflict, reducing pressure on the threading tool tip.

For those math geniuses on the board, who think that the proper pitch diameter can be achieve by the depth of the compound, I have some less than good news for you. Unless EVERYTHING is perfect (the major diameter, the flat on the tool bit, an errorless pickup of the surface of the part in question), the chances of hitting the proper pitch diameter is close to zero, or, at best, reduced to chance. That's due to the tolerance of those features allowing for greater error than the typical pitch diameter tolerance.

With the above idea in mind, learn to use thread wires. They offer a far better opportunity of hitting the pitch diameter. Do that even when it doesn't matter (in your mind). By doing so, you assure interchangeability of your work.

To be clear, I have been known to use a nut to fit a thread. Not the best practice, I admit. :roll:

H
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John Evans
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Re: Threading with compound

Post by John Evans »

Yes I have tried plunge threading and noted exactly what you say. And yes I learned the hard way that if the major dia. is a few thousands to big your "nut" won't fit !! I cheated and bought 0-1,1-2 import thread mics ,also have several sets of thread wires and triangles. But usually cheat and use the mating part if possible ! LOL But when that is not possible I break down and use the right tool. I have "Top Notch" threading inserts in several sizes with matching boring bars and turning holders. Aquired over the years on the cheap !! just like me LOL
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Re: Threading with compound

Post by SteveM »

Harold_V wrote: Sat Dec 19, 2020 5:14 pmFor those math geniuses on the board, who thin that the proper pitch diameter can be achieve by the depth of the compound, I have some less than good news for you. Unless EVERYTHING is perfect (the major diameter, the flat on the tool bit, an errorless pickup of the surface of the part in question), the chances of hitting the proper pitch diameter is close to zero, or, at best, reduced to chance. That's due to the tolerance of those features allowing for greater error than the typical pitch diameter tolerance.
I'm not a math genius, but I play one on TV,

I had to cut a thread a couple of weeks ago and I was wondering this very thing.

Is there a direct relationship to the change in

I used a thread mic and it got me to a good fit, although in retrospect, I should have used wires to validate it.

Steve
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Re: Threading with compound

Post by mcostello »

Did that once around 1995 on a very expensive part and have never forgotten it.
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Re: Threading with compound

Post by Harold_V »

I'm not a fan of thread mic's (based on attempts to use them satisfactorily). For that reason, I won't comment, but I can tell you in all honesty, I wouldn't use them. Better than the compound measurement, for sure, but still questionable in my mind. YMMV, however.

I could be wrong, but I don't think they're acceptable for calibrating B thread gauges. Maybe not a good enough reason for me to not like them, but that's how I feel.

H
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Re: Threading with compound

Post by Steggy »

Harold_V wrote: Sat Dec 19, 2020 5:14 pmFor those math geniuses on the board, who think that the proper pitch diameter can be achieve by the depth of the compound, I have some less than good news for you. Unless EVERYTHING is perfect (the major diameter, the flat on the tool bit, an errorless pickup of the surface of the part in question), the chances of hitting the proper pitch diameter is close to zero, or, at best, reduced to chance. That's due to the tolerance of those features allowing for greater error than the typical pitch diameter tolerance.
I said as much earlier. :D
A go/no-go gauge or a nut with a class 3B fit is your friend when single-pointing, as working only from tool depth rarely produces a properly-sized thread due to variables such as tool, work or machine deflection under load.
I've come close on occasion, but have never hit it dead nuts, and I've got some 50 years of machining experience behind me.
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Harold_V
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Re: Threading with compound

Post by Harold_V »

The one distinct advantage of using wires is you don't operate blindly, as you do with gauges, and that includes the use of a nut. Pitch diameter can be measured while well oversized, giving the operator enough input to be able to work to a given mark on the dial before having to check size again. Can't do that with a nut or any of the gauge types. Thread mic's offer the same advantage.

That said, gauges are really good for inspection after the fact. They're fast and easy. If they provided a sense of size before they fit, they'd be more useful on the machine.

H
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liveaboard
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Re: Threading with compound

Post by liveaboard »

I have found that even starting off with the shaft correctly sized, the thread cutting often raises a tiny little ridge that makes nut fit unreliable.
None the less, I'm a nutter.
If a nut is available for test sizing, then that works for me.
A light brush with a fine file takes the ridge off; that was how I saw it done, so I do it that way, but I don't know if it's "right".
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Harold_V
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Re: Threading with compound

Post by Harold_V »

I'm inclined to use strip abrasive to eliminate the raised ridge, as a file tends to generate another, only on the flanks of the thread. I don't know that there's a right or wrong--so long as the ridge is eliminated fully.

H
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Re: Threading with compound

Post by NP317 »

Harold_V wrote: Sun Dec 20, 2020 5:36 am I'm inclined to use strip abrasive to eliminate the raised ridge, as a file tends to generate another, only on the flanks of the thread. I don't know that there's a right or wrong--so long as the ridge is eliminated fully.
H
That is comforting to hear from Harold.
Exeprience has led me toward that same conclusion. Now I will trust it.
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Re: Threading with compound

Post by John Hasler »

Cratex abrasive rubber sticks are great for that. They also get down into the threads.
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