Single Point Threading 101

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earlgo
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Re: Single Point Threading 101

Post by earlgo »

While not intending to detract from the conversation about stopping in time, I have found that in my situation the easiest way to thread close to a shoulder is to set up the tool as follows and reverse the direction of the lathe spindle. Unfortunately the cutting pressure pushes against the tailstock center, but it works for me on threads that are small enough to clear the compound on the 12" Atlas.
reverse threading.JPG
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rmac
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Re: Single Point Threading 101

Post by rmac »

earlgo wrote: I have found that in my situation the easiest way to thread close to a shoulder is to set up the tool as follows and reverse the direction of the lathe spindle.
You can also do something similar without working from the backside of the part by just mounting the tool upside-down:

https://www.youtube.com/watch?v=Z-dqOi_z5bk

I've never tried this myself because I have a screw-on chuck and don't want to risk having the chuck unscrew itself if something gets hung up while running the lathe in reverse. I also wonder what (if any) problems result when the cutting forces are trying to lift up on the compound and cross slide instead of pushing them down. I've never seen a discussion on that point.

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rmac
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Re: Single Point Threading 101

Post by rmac »

rmac wrote: When you're threading up to a shoulder, what do you look at to know when to stop the cut?
Harold_V wrote: I tend to use a combination of the shank of the tool, and the tip. The shank and tip when threading to a shoulder.
Can you guess how much of a gap remains between the shoulder and the shank of the tool at the end of your cuts? I'm assuming that you don't actually let them touch.
Harold_V wrote: It's more about getting a rhythm --- a sort if "feel" when to open the half nuts and retract the cross slide.
Harold_V wrote: As far as a stop goes, it generally isn't of much value, as the relative position of the tool tip is ever changing due to the compound feed.
Yikes. That's another thing I have never considered, and another thing that's happened not to bite me because I've always cheated when threading to a shoulder. But it seems like the stop might still be useful with a little bit of pre-planning to set the stop (or indicator) according to the compound's eventual position when the thread is at full depth.

I'm kind of partial to watching a carriage indicator rather than the tool itself because it makes it easy to feel that "rhythm" you're talking about as the dial goes 'round and 'round. But liveaboard's trick of putting a mark directly on the part might work just as well to help establish that rhythm. Plus, it would be a little quicker than setting up an indicator.

Lots more stuff here to think about. Thanks.

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Bill Shields
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Re: Single Point Threading 101

Post by Bill Shields »

The last set of threads I did to a shoulder was with a .040 groove at the end of a 32 pitch thread.

...maybe .010 to the shoulder when the tip and back edge of tool is clear of thread.

I can pop the half nut quicker than spin the compound.

Back side reverse threading is a commonly used solution to the problem for those saner than I.
Too many things going on to bother listing them.
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Harold_V
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Re: Single Point Threading 101

Post by Harold_V »

rmac wrote: Thu May 20, 2021 3:08 pm Can you guess how much of a gap remains between the shoulder and the shank of the tool at the end of your cuts? I'm assuming that you don't actually let them touch.
Depends on the requirements of the part. If a grip is permissible, I'll pull out a safe distance, but still try for uniformity. It's called practice, which is the very thing that creates the ability to do these things reliably.

One thing you might consider is what happens when your tool hits the shoulder. A lot depends on the material you're machining, and the amount of contact the tool makes with the part. You generally lose the part, but it can be a lot more serious, depending on the machine. Some machines bind up if there's excessive pressure on the lead screw/nut assembly, so you can't release the half nuts. When that happens, damage to the machine is sure to be the result. In fact, my good friend Jake (Dennis Jacobs) allowed a less than talented person to operate his 17" Mori Seiki and he did just that. He had to replace the lead screw.

That said, I've had to thread to the shoulder with full thread, so in that instance you relieve the threading tool on the right hand side (that's the side that approaches the shoulder) until the amount of thread form of the tip is just barely longer than the full thread depth. You pull out at just the right moment. It sounds hard, but if you do it time and again it isn't all that difficult. Of course, the bulk of my experience in doing this was when I was assigned to a new Monarch EE, which makes thread chasing very easy. It may not be as easy on other machines. I know my Graziano isn't anywhere near as user friendly as an EE (not complaining---it cost a lot less, too!).
I'm kind of partial to watching a carriage indicator rather than the tool itself because it makes it easy to feel that "rhythm" you're talking about as the dial goes 'round and 'round.
What ever works for you. Personally, I don't like taking my eye of the cut when threading. I want to see the chip as it develops, as I am overly concerned about surface finish of the thread. It also reveals how sharp the tool is. As I don't use inserts for threading, I need to know the status of my threading tool, especially when approaching size. I often take shallow cleanup cuts (by plunging instead of using the compound), and I need to know that the tool is up to the task. If, when threading abrasive material (like chrome moly), the tip often dulls prematurely, so light passes don't work, and the resulting finish is often a compromise. I've been known to remove the threading tool for sharpening when I'm near size. That, too, is a good exercise, as it teaches one how to pick up an existing thread.
But liveaboard's trick of putting a mark directly on the part might work just as well to help establish that rhythm. Plus, it would be a little quicker than setting up an indicator.
If I'm not threading to a shoulder, I always make a mark on the part (for external threads). A pencil does a good enough job---just anything to let you know that you've achieved the desired length. I determine where it must be and place a pencil on the rotating part. The line is best when it circles the entire part. Once you've established a reasonable depth, you can simply pull out by watching the generated thread, as the line tends to go away, anyway.

H
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Harold_V
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Re: Single Point Threading 101

Post by Harold_V »

Bill Shields wrote: Thu May 20, 2021 8:15 pm I can pop the half nut quicker than spin the compound.
That's why I talk about the setting of the handle (10:00 for external threads). If you do that routinely, you can pull the tool and open the half nuts simultaneously, all in one smooth motion. That often saves you when the shoulder isn't very large and you don't have the option of the thread relief.

H
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earlgo
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Re: Single Point Threading 101

Post by earlgo »

Harold wrote:"...and you don't have the option of the thread relief."
Please explain to this ignorant mechanical engineer when it is necessary for no thread relief.
I am unable to understand, even from a stress analysis standpoint, why the extra thread is important. It seems to me that the part designer needs to re-adjust their thinking. Sort of parallel to the car designer that puts the spark plugs up against the firewall.
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GlennW
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Re: Single Point Threading 101

Post by GlennW »

earlgo wrote: Fri May 21, 2021 9:09 am Please explain to this ignorant mechanical engineer when it is necessary for no thread relief.
Sort of parallel to the car designer that puts the spark plugs up against the firewall.
Just look at that spark plug, for one!
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rmac
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Re: Single Point Threading 101

Post by rmac »

Harold_V wrote: Fri May 21, 2021 1:51 am Lots of stuff.
Thanks again, Harold, for taking the time to explain all this. It really helps.

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liveaboard
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Re: Single Point Threading 101

Post by liveaboard »

Harold_V wrote: Fri May 21, 2021 1:51 am
If I'm not threading to a shoulder, I always make a mark on the part (for external threads). A pencil does a good enough job---just anything to let you know that you've achieved the desired length. I determine where it must be and place a pencil on the rotating part.
H
I was referring to putting a mark along the work, not around it.
Then I count off the rotations as the mark passes; this way, I'm able to get the tool out at close to the same point each time.

It requires a lot of concentration; but doesn't everything?
earlgo
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Re: Single Point Threading 101

Post by earlgo »

GlennW, I'd bet spark plug threads are rolled and not cut. I'd also bet that if you cut away the last thread or two next to the shoulder on a spark plug, it would not blow out. I don't know what chamber pressure is in a firing gas or diesel engine, but stress calculations might be an interesting exercise.
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GlennW
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Re: Single Point Threading 101

Post by GlennW »

From what I have seen, things in tension are far less likely to have a relief groove than things in shear.
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