Single Point Threading 101

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rmac
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Single Point Threading 101

Post by rmac »

Once you get the lathe itself configured for single point threading, the procedure I've seen described most often goes something like this:
  1. With the tool just touching the work, set the cross slide and compound dials to zero.
  2. Move the carriage to the right so the tool clears the work.
  3. Make a scratch pass and check that you're getting the thread pitch that you expect.
  4. Withdraw the tool from the work using the cross slide.
  5. Move the carriage back to the right for the next pass.
  6. Set the cross slide back to its zero position.
  7. Set the compound a little deeper for the next pass.
  8. Make a pass to make the thread a little deeper.
  9. Repeat Steps 4 through 8 until the thread is complete.
Fair enough. That works. But it seems like messing with the cross slide twice per iteration is just wasted motion. Why not withdraw the tool from the work in Step 4 using the compound instead of the cross slide, and then skip Step 6 entirely? Is it just so you don't forget where the compound is supposed to be?

-- Russell Mac
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Harold_V
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Re: Single Point Threading 101

Post by Harold_V »

If you're threading to a shoulder, I highly suspicion that your compound move would prove to be a disaster. Even pulling out without a thread relief (not always an option) would be difficult. The reason is that the handle of the typical compound is too small to be really functional. You're also busy enough with your right hand opening the half nuts.

One thing you may have overlooked here is the handle orientation of the cross slide. If you make a habit of placing the handle @ 10:00 for external threads, it's a simple downward motion to extract the tool from the cut. Choosing other locations work, but it requires that you keep in mind what you must do to extract the tool. That can get a bit confusing if you choose random locations.

Assuming you like the 10:00 idea for the cross slide handle, you then would consider using a 2:00 setting for internal threads. Same deal--- a swift down push of the handle while you're opening the half nuts with the other hand. Bob's your uncle.

People have been single point cutting threads like this for a long time. I suspect that re-inventing the wheel isn't a good idea. :wink:

Harold
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Bill Shields
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Re: Single Point Threading 101

Post by Bill Shields »

Unless you have a lathe with a quick return cross slide lever..

Very handy things
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Technical Ted
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Re: Single Point Threading 101

Post by Technical Ted »

Harold_V is right on. That's the way the guys taught me when I was learning. Always pushing down on the cross slide handle if you run into trouble can save your workpiece/tool if something goes bad. It becomes automatic; a reaction. You do it without having to think.

Further, some tool room lathes have a built in cross feed stop which allows the operator to set a positive stop for the cross slide. I made a dove tail clamp on stop for my South Bend lathe that I use if I'm going to be doing a lot of single pointing. Otherwise, I just put the handle ~9 o'clock and set the dial on zero.

Ted
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rmac
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Re: Single Point Threading 101

Post by rmac »

Technical Ted wrote: Harold_V is right on.
This is the typical state of affairs.
Harold wrote: I highly suspicion that your compound move would prove to be a disaster.
I'm not seeing any potential for disaster.
Harold wrote: Same deal - a swift down push of the handle while you're opening the half nuts with the other hand. Bob's your uncle.
Aha! (Maybe.) This could be what I'm missing. I was taught (perhaps incorrectly?) to open the half nuts, then withdraw the tool from the thread. Besides being slower, is this wrong for some other reason?

-- Russell Mac
Last edited by rmac on Wed May 19, 2021 11:10 am, edited 1 time in total.
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Bill Shields
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Re: Single Point Threading 101

Post by Bill Shields »

Whatever spins your prop and works. Many lathes have stops that trigger the half nut disengagement.
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liveaboard
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Re: Single Point Threading 101

Post by liveaboard »

I don't open the half nuts; since I don't have a threading dial (and I'm not even sure what that is) I have one hand on the cross slide crank and the other on the motor control lever.
I pull the cross slide, reverse the motor, adjust the compound in, reset the cross slide on zero, repeat.
I used to crank out the compound a full turn at least, to avoid the approaching shoulder or whatever.
That's how I learned from my Indian machinist.

Adding the VFD to my lathe has made it much easier due to the braking action, and the ability to go extremely slowly if I want. I can also speed up the motor for reversing if it's a long thread.

On the other hand before I had the VFD, a quick touch in reverse would stop the motor dead; but the slightest error would reverse it prematurely, and that broke my cutting tool a few times as I was getting the hang of it.
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Harold_V
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Re: Single Point Threading 101

Post by Harold_V »

rmac wrote: Wed May 19, 2021 7:25 am
Harold wrote: Same deal - a swift down push of the handle while you're opening the half nuts with the other hand. Bob's your uncle.
Aha! (Maybe.) This could be what I'm missing. I was taught (perhaps incorrectly?) to open the half nuts, then withdraw the tool from the thread. Besides being slower, is this wrong for some other reason?
Yes! Absolutely wrong! You should synchronize the action such that the tool leaves the cut at the same time propulsion ends, or, if anything, even slightly before. However, even that isn't acceptable if you're threading to a shoulder.

Reason?

Try removing the half nuts with the tool still engaged when you're machining a coarse pitched thread and you're near full depth. What you can expect is a broken tool and/or a scrapped part. If neither of those happen, do you really want circular cuts at the end of your thread, each in a random location, when there is no undercut permitted?

It is critical to success for one to be able to stop a threading cut at a specific location. A good example is threading to a shoulder, with no undercut.

See what I mean?

H
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Bill Shields
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Re: Single Point Threading 101

Post by Bill Shields »

Harold...

It depends if the tool / thread is finishing in a groove or not.

If the tool is in space (groove) -> it matters not how disengagement occurs.

You are absolutely correct you are cutting while retracting...
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rmac
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Re: Single Point Threading 101

Post by rmac »

Harold_V wrote: Try removing the half nuts with the tool still engaged when you're machining a coarse pitched thread and you're near full depth. What you can expect is a broken tool and/or a scrapped part. If neither of those happen, do you really want circular cuts at the end of your thread, each in a random location, when there is no undercut permitted?

It is critical to success for one to be able to stop a threading cut at a specific location. A good example is threading to a shoulder, with no undercut.

See what I mean?
Oh, my. Yes, I see what you mean. Somehow I have inadvertently dodged bullet after bullet and not yet been bitten by the situations you describe. To wit:
  • I have yet to encounter the need to cut a really coarse thread.
  • At the same time I learned the incorrect procedure, I was also taught to provide an undercut at the end of the thread. I guess it's obvious now why my teacher thought that was important.
  • In situations where I don't want an undercut, I go really slow and use a dial indicator on the carriage to show when to disengage the half nuts. So I do get a circular cut at the end of the thread. But I can reliably stop within a few thousandths of the desired spot, so fortunately I don't find myself breaking tools or ending up with multiple, random grooves.
  • When I have needed a thread right up to a shoulder, I have always cheated, either 1) by using a die to cut the thread, or 2) by making the part in two pieces, with a threaded part screwed into a tapped hole at the shoulder, or 3) by stopping the lathe early and turning it by hand for the last part of the cut.
So it sounds like I'm going to need a little practice to do this correctly. (It also sounds like a good use for some of that cast HDPE!) I think I understand what I should be doing, including the ten o'clock/two o'clock positioning of the crossfeed handle. But I do have a couple more questions:
  • When you're threading up to a shoulder, what do you look at to know when to stop the cut? Do you watch the tool approach the shoulder? Or do you set up a carriage indicator like I mentioned above? Or something else?
  • Brain fart. I can't remember the other question at the moment. Maybe later.
As always, thanks for the help.

-- Russell Mac
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liveaboard
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Re: Single Point Threading 101

Post by liveaboard »

I use a relief groove, run the machine at ultra low speed, 20 or 30 rpm, and stop by eye. The groove gives a tiny little bit of error room, and the small braking power of my VFD will stop in time.
Pulling out the tool in that situation is difficult, due to the temptation to do it too soon, resulting in a not quite complete thread.

This choice of technique is due to my particular machine and experience; other people with other setups do it differently.

Without a relief groove, you need to pull the tool out very close to the exact same point each time.
I tried to do that, and succeeded at first, then broke a couple of cutter tips.

Sometimes I put a paint mark on the workpiece to help indicate the end point.
Sometimes, especially when internal threading makes it impossible to see the cut, I count off the rotations.

As an untrained amateur, I'm very pleased with myself when I manage to cut a decent internal thread.
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Harold_V
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Re: Single Point Threading 101

Post by Harold_V »

rmac wrote: Wed May 19, 2021 10:05 pm When you're threading up to a shoulder, what do you look at to know when to stop the cut? Do you watch the tool approach the shoulder? Or do you set up a carriage indicator like I mentioned above? Or something else?
I tend to use a combination of the shank of the tool, and the tip. The shank and tip when threading to a shoulder, but the tip, alone, when threading to a specific stopping point. Doesn't make much difference if I'm threading with or without an undercut. It's more about getting a rhythm --- a sort if "feel" when to open the half nuts and retract the cross slide. That's the method I used for this part, a replacement auger for Susan's wheat grinder. The original, made of cast iron, broke. It's a three pitch lead, created by starting the cut in a groove, but ending by pulling out at the appropriate time. The amount of tool contact was quite large, so it would have been very unforgiving of an overrun.

Auger5.JPG

If you are curious, the part shown was made from 17-4PH stainless. The color is the result of heat treat to condition H900.

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. If you're threading internally to a shoulder, it can get you in trouble quickly, especially with a coarse thread.

For internal threading, the one thing that remains constant is the relative position of the tool tip (and end) as it relates to the part. When I'm faced with that situation, what I do is determine where I must disengage the half nuts, then I use a pencil to make a line on the top of the threading tool. When the line is even with the end of the part, I open the half nuts. Works a charm.

I strongly encourage you (everyone, really) to pursue threading as if it's always up to a shoulder. By doing so, you gain the necessary skill required to generate threads to a specific stopping point. If you rely on an undercut, which may or may not always be possible, you don't get the necessary practice it takes to become proficient.

A comment about a thread relief. It stands to reason that the home shop type is free to cut one. The choice is usually his/hers alone. However, where I was trained, that was not the case. And, when there was one permitted, it had rigid specifications so the undercut didn't lend to the problems that come from sharp corners. Undercuts, like all other features, were provided by engineering and held to tight tolerances as a result. I make mention so you'll understand my comments. I had no choice but to learn to thread to shoulders, generally without a thread relief.

H
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