Interpret this thread cutting advice for me

[seal killer]

All--

Some time ago, I posted a YouTube video regarding one of my first thread cutting efforts. The topic here on the forum was "Cutting a 3/4"-10 Class 3A Thread in Aluminum". Yesterday, a gentleman left a comment with thread cutting guidance on my YouTube page. Because of my inexperience, I can't figure out why what he is telling me is better. Will you explain it to me?

Bill your thread is wounding on you if you are just learning , learn the right way don't get into bad habits right from the start, run all but the last .020 of depth with your compound and finish it with the cross slide then you will get a true angle cut and you will not get a taper in your thread millions of threads are cut this way and I think it is the only way ,but I have been doing this for 52 years so what do I know ,but everyone thinks you should start and finish threads with the compound try it the other way you will like it ,also I have cut metric threads on my 709 and you can't disengage your split nut so just step on the brake to stop it then reverse it to back you carriage back to the beginning also get away from that water you use for coolant it will contaminate your lower gears in saddle and really give you some problem's down the road I use cutting oil in my coolant tank believe me water and oil don't mix sooner or later it will get into your oil and screw up your clutches ,I have always keep my hand on my carriage wheel to take up the slack when threading and start your threading tool back a little more some day you will hit the wrong number or line and you will not have enough time to react on your mistake and scrap the part.

Thanks.

--Bill

[JHenriksen]

I learned to thread using the compound set at 29.5 degrees and just advancing the compound. I have done threading just suing the cross slide and dont feel as comfortable with that method. I think that is a personal practice preference.
I do hold my hand on the cross slide wheel to keep it from drifting. Again that is practice on my old worn out Rockwell lathe.
Often i will l do most of the cut passes without holding the handwheel and only hold it the last few passes. This is something that will vary with each persons old worn out lathe problems.
His advice to start the thread feed a bit farther from the work piece is good, especially when you are cutting the lower thread per inch threads or using too high a spindle speed.
I like Trim Tap heavy for my thread cutting. Its a honey consistancy cutting oil that doesnt stink nor sling off while cutting.

[GlennW]

He's saying to infeed with the compound slide until about the last .020" of depth, and then feed straight in with the cross feed.

I would disagree with that up to a point, as I do the same, (as recommended by Harold) but I only take the last couple of thousandths of infeed with the cross slide. .020" is way overkill and not necessary, (unless possibly if you were cutting a 2 tpi thread) as all you are trying to do is clean up the flanks a bit. If the tool is good and sharp, it leaves a really nice, smooth, finish on the flanks.

Water (soluble coolant) getting inside the carriage isn't a concern of mine, as it has never happened. But not all lathes are created equal and some may not be sealed as well as others. I use thread cutting oil for threading...

Starting a thread with the tool an inch or so away from the part is a good idea when possible as it will give you a bit of time to get disengaged if you missed the mark or realized something was not set correctly.

On my smaller lathe I used to apply a slight drag to the carriage handwheel when I engaged the half nuts to eliminate any gear train backlash before the tool contacted the part. On my larger lathe I haven't found the need to do so as it is much heavier and smoother operating.

[BadDog]

Glenn saved me a lot of typing. He said everything I thought when I read your post.

[seal killer]

JHenriksen and Glenn and Russ--

Thanks for the "interpretation." I learned something; cutting the last few thousandths with the cross slide. I don't know why he thought I was using water as a coolant. Maybe I did not announce in the video that it was WD40.

--Bill

[Harold_V]

I only take the last couple of thousandths of infeed with the cross slide. .020" is way overkill and not necessary,

That's precisely my stance on this issue. The purpose of feeding with the cross slide is to ensure that the thread form is complete (if markings on a lathe are off, and they can be) when you choose 29½° for the compound, you may well be set beyond 30°. I routinely set my compound @ 29° in lieu of 29½°. There is a fringe benefit of the plunge cut, assuming it's not a deep pass---the finish is generally improved on the thread.

A setting beyond 30° leaves steps on the trailing flank, and introduces minor error to the thread form. By plunge feeding until the entire thread form cleans up, that possibility is eliminated. The resulting form will be as true as the threading tool, and, if light cuts are taken and the tool is sharp, the resulting finish will be improved, often remarkably.

I do not recommend a large amount be left to be removed with the cross slide, as there is serious conflict with chip flow when the depth of cut is extreme. That, in and of itself, creates problems.

How much to leave before starting the cross slide feed?

I've found .005" is generally enough. I take, at the deepest, .002" off the diameter per pass, and prefer less. For a final pass, if the machine responds to light cuts, I often take only a couple tenths, with the cut well oiled.

Harold

[seal killer]

JHenriksen and Glenn and Russ and Harold--

I'm glad I asked this question!

--Bill

[Carm]

I do not recommend a large amount be left to be removed with the cross slide, as there is serious conflict with chip flow when the depth of cut is extreme. That, in and of itself, creates problems.
Harold

Further, trying to remove large amounts with what now becomes a fully engaged form tool will cause grief especially on a pea-shooter lathe.

[scmods]

Bill,

Also one other thing. If you're going to try the stop and reverse method, like one of my friends does because his little SB doesn't have a chasing dial, make sure you back off the tool bit before reversing. This prevents breaking the point off the carbide tool insert and canceling your hundred "attaboys".

While a final plunge tends to clean up both sides of the thread groove, it can also lead to the workpiece climbing the tool. Not fun!

Bill Walck

[Jim Jakosh]

I have been cutting thread for 45 years and learned at Western Electric. I have always cut threads by stopping the cross slide on a zero and advancing the compound until the pitch diameter is reached. I never got a tapered thread and the threads are clean and usable. If you use the little plate with the the v grooves in it to orient the bit to the part at set up, your thread form will be accurate. And I believe I learned the right way like you did!!

[Harold_V]

I have been cutting thread for 45 years and learned at Western Electric. I have always cut threads by stopping the cross slide on a zero and advancing the compound until the pitch diameter is reached. I never got a tapered thread and the threads are clean and usable. If you use the little plate with the the v grooves in it to orient the bit to the part at set up, your thread form will be accurate. And I believe I learned the right way like you did!!

Your method is, indeed, the acceptable method, but don't discount the use of the cross slide for the last few passes in lieu of feeding with the compound. I make mention BECAUSE the thread form may NOT always be proper. I'll say a little more about that, below.

The graduation marks on any given lathe may or may not be proper. If, on the outside chance the marks are off a half degree or more (certainly a possibility, especially with import, inexpensive machines), even when setting the tool with a fishtail, the thread can still be wrong. The reason that is so is because feeding beyond the 30° angle (due to error in the markings on the lathe) will produce a stepped trailing flank. That alters thread form and can be the cause of scrapped parts, assuming the error is great enough. Beyond that, the trailing flank of a thread tends to get roughed up by the chip produced by feeding exclusively with the compound, so feeding the last few thou, taking light passes, generally cleans up the trailing flank and ensures thread form.

So then, assuming all conditions are correct, and you are adept at chasing threads, sure, use nothing but the compound if you desire---but if you hope to turn out the best possible thread, if you're threading on a manual lathe, where surface speeds are restricted due to the degree of skill required to thread at high speeds, it's hard to improve on the method I suggest.

Like you, I was trained in industry, with my experiences beginning in 1957, in a missile facility, where every feature of a part was inspected 100%. Working for the defense and aero-space industry tends to demand the highest level of performance. I learned how to produce threads that were acceptable to stringent QC requirements. Even our threading tools (which were hand ground) had to be approved by QC before making parts.

A comment on keeping a slight drag on the carriage hand wheel while threading. Do it! Make it a habit. If, by chance, the hand wheel is not well balanced, as it moves beyond center, it has the potential to bump the carriage, causing a drunken thread. Light pressure prevents that from happening. That's a lesson I learned the hard way.

Harold

[Harold_V]

While a final plunge tends to clean up both sides of the thread groove, it can also lead to the workpiece climbing the tool. Not fun!

Bill Walck

While I concede that that is a possibility, it is my opinion that if that's a problem, you have greater problems than the plunge cut.
To be clear, I chased threads for many years, threads that were subject to critical inspection. The part climbing the tool isn't an issue, not if you're not chasing a thread that is too long (and should have a center), or if your machine is reasonably rigid. A small, light weight machine (such as a Craftsman 109), sure the increased surface contact is likely to create issues.

In parting, I'd suggest to you that if you have not tried this method, do not form an opinion, assuming you have a reasonable lathe. If you keep a sharp tool and enjoy threads that rival threads that come from a thread grinder, you may be pleasantly surprised to discover this method works exceedingly well. Give it a go with aluminum, or even stainless. You'll be amazed at the quality of the final product.

Harold