cutting my first thread on a south bend heavy 10. before this i cheated and used dies. the thread to be cut is 1.00-14. material is TGP something (i forgot to mark what it is; bought it about 3 years ago). turned the TGP down from 1.250 to 1.00. using a brand new factory insert from A.R. Warner. watched Tubalcain video. adjusted lathe properly. started the cut, did everything right; and from the beginning the cut was very rough and finished rough; and, the nut won't go on. Oh, by the way, this first piece is a "practice piece". so, thinking that it is the steel, i went to a piece of 12L14. same results. the od of the round was down to .985. think, think think. tubalcain's video states to set the compound at 29.5 degrees, which i did. AND, to feed with the compound which i did. BUT, SouthBend book "HOW TO RUN A LATHE" says to feed with the cross slide. i did this first with the leaded steel; perfect threads. next did it on the excess length of the TGP. perfect threads. what is happening here????. its much eaiser to feed the cutter by using the compound and not the crossfeed dial. oh, after reviewing my post; i didn't mention that the thread cutter is also AR Warner thread cutting insert
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Thread cutting dilema?
- Thread starter porthos
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- May 10, 2014
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If the plunge cut with the carriage created a good thread, but the compound did not create a good thread, it feels like the compound was not set to the correct angle. I have made this mistake myself.
Many threads on the forum about threading issues, one example.
Threading help on HM
Many threads on the forum about threading issues, one example.
Threading help on HM
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SCLead
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Your thread cutting tool is absolutely square to the work, and on center (height-wise)?
The purpose for 29.5* compound is because you then only cut on one face of the tool. Tough to explain clearly, but if you look at the compound from directly above, the angle of movement should be DIRECTLY in line with the left/headstock cutting edge of the tool (ok, not directly. 0.5* angled "into" the work). The purpose for this, instead of a solid 30* cut, is so the right/tailstock side of your tool doesn't contact the thread at all, and in fact has a wider clearance nearer the toolpost than at the tip of the tool. When you feed with the compound, you're cutting the full thread profile (up to that depth) with the left side of the tool, which theoretically reduces chatter as opposed to cutting the full thread profile on both sides of the tool simultaneously, where chips will be constantly crashing into each other on top of your tool. The SB manual likely "predates" this concept, which is to say feeding with the cross feed worked perfectly well for many people, for many years. If you find it works better for you, there's no harm in using this method.
So if you're getting better threads cutting with the cross slide, I would first look at your setup. Tool is perfectly square to work. Tool is correct height - on center is the "one size fits all" answer. Everything is tight - tool is tight in post, post is tight on machine, compound angle is tight, cross slide gibs are tight. You shouldn't see a ton of springing with 1" OD, but are you supporting the tail end of the work with a center or anything? What's your cutting speed? You're not trying to take hog-depth cuts, right? Double check the angle on your compound, you're not trying to take 29.5* or 30.5* cuts, are you? I know I've mixed up a couple compound angles in the past.
The purpose for 29.5* compound is because you then only cut on one face of the tool. Tough to explain clearly, but if you look at the compound from directly above, the angle of movement should be DIRECTLY in line with the left/headstock cutting edge of the tool (ok, not directly. 0.5* angled "into" the work). The purpose for this, instead of a solid 30* cut, is so the right/tailstock side of your tool doesn't contact the thread at all, and in fact has a wider clearance nearer the toolpost than at the tip of the tool. When you feed with the compound, you're cutting the full thread profile (up to that depth) with the left side of the tool, which theoretically reduces chatter as opposed to cutting the full thread profile on both sides of the tool simultaneously, where chips will be constantly crashing into each other on top of your tool. The SB manual likely "predates" this concept, which is to say feeding with the cross feed worked perfectly well for many people, for many years. If you find it works better for you, there's no harm in using this method.
So if you're getting better threads cutting with the cross slide, I would first look at your setup. Tool is perfectly square to work. Tool is correct height - on center is the "one size fits all" answer. Everything is tight - tool is tight in post, post is tight on machine, compound angle is tight, cross slide gibs are tight. You shouldn't see a ton of springing with 1" OD, but are you supporting the tail end of the work with a center or anything? What's your cutting speed? You're not trying to take hog-depth cuts, right? Double check the angle on your compound, you're not trying to take 29.5* or 30.5* cuts, are you? I know I've mixed up a couple compound angles in the past.
If it s an asian lathe the angle setting is different then an american machine. The 0 degrees has your compound in line with your shaft. So if so it may be set closer to sixty then thirty. An american machine 0 degrees has your compound at 90 degrees to your shaft. Whatever you set your angle tomyour shaft being threaded. Also it s not critical to be exactly 29.5 degrees. Just less than 30 and more than 29. Some may argue but in really doesn t matter.
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How far from the chuck are you working? was there centre support?
Any chatter? These things can all contribute to bad turning finish.
When you say "rough" do you mean the surface finish?
Or do you mean the threads looked "wonky" with a different angle on each side?
If the later, double check the compound angle.....some lathes call zero degrees at parallel to the lathe centre axis, while some call zero degrees perpendicular to the lathe centre axis.
Here's an example of setting to the wrong angle:
https://www.hobby-machinist.com/threads/threading-help.49488/post-416851
Notice the slopes on the leading and trailing edges are not the same; one is steeper than the other.
Also one side is "stepped" and not one flat surface.
Here's an example of "double tracking" where the half-nut was engaged on the wrong number:
https://www.hobby-machinist.com/threads/threading-something-went-wrong-here.33772/post-284757
Some photos of your set-up and results would help to debug it.
-brino
Any chatter? These things can all contribute to bad turning finish.
When you say "rough" do you mean the surface finish?
Or do you mean the threads looked "wonky" with a different angle on each side?
If the later, double check the compound angle.....some lathes call zero degrees at parallel to the lathe centre axis, while some call zero degrees perpendicular to the lathe centre axis.
Here's an example of setting to the wrong angle:
https://www.hobby-machinist.com/threads/threading-help.49488/post-416851
Notice the slopes on the leading and trailing edges are not the same; one is steeper than the other.
Also one side is "stepped" and not one flat surface.
Here's an example of "double tracking" where the half-nut was engaged on the wrong number:
https://www.hobby-machinist.com/threads/threading-something-went-wrong-here.33772/post-284757
Some photos of your set-up and results would help to debug it.
-brino
- Joined
- Nov 5, 2016
- Messages
- 1,419
You've been given a lot of good advice. Here's my take: if it cuts fine feeding straight in with the cross slide, but not when feeding in with the compound, the first thing I would check is to verify the compound is set at the proper angle.
I have two SB lathes (see pic of my 13"). I have both set at the proper 29 degree angle for threading and leave them there most of the time. But notice that they are set at 61 degrees on the graduations to achieve this 29 degrees of actual movement needed.
Ted
I have two SB lathes (see pic of my 13"). I have both set at the proper 29 degree angle for threading and leave them there most of the time. But notice that they are set at 61 degrees on the graduations to achieve this 29 degrees of actual movement needed.
Ted
everything that i did so far is done properly. however, i like Techinal Teds remark about 61 degrees on the South bend. i'll try that this afternoon and get back to the site with results. thanks for all the help so far
i'm back!! set the compound at 61 degrees as per TechnicalTeds instructions. PERFECT threads... a little confused as to why everything that I have read states to use 29.5 degrees. i know that 61 degrees is the same; but, from a different direction. its not quite the same; it is at a slightly different angle on the compound. i'll bet there have been a lot of bad threads cut because of the miss information out there and not telling a novice about this issue with different types of lathes. THANKS TED
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You will find often that the markings on the compound don't match up with the actual angle you want to turn or feed.... you need to do the math at times.
It can be a little confusing. When you set the compound per the markings to 90 degrees it is set perpendicular to the turning axis of the lathe and feeding in that direction will face off a part at 90 degrees the way it should be faced. When you are threading, if you look at a diagram of a 60 degree sharp V thread profile it shows the thread at being 30 degrees on both sides for an overall angle of 60 degrees.
But look closely and you'll notice the 30 degree is actually off from the 90 degree setting on the lathe. So, 90 degrees minus 30 degrees is 60 degrees. Back it up 1 degree so you're on 29 and that equals the compound being set at 61 degrees per the compounds markings.
But, thread wise, you will be feeding in a 29 degrees.
So, in the case of threading, the compound's 90 degree setting (per the marks) is actually 0 degrees (feeding straight in). You need to shift the compound 29 degrees off from this 90 degree position to get the correct angle for threading.
Hope I'm explaining that well enough.
Great job glad you got things working.
Ted
It can be a little confusing. When you set the compound per the markings to 90 degrees it is set perpendicular to the turning axis of the lathe and feeding in that direction will face off a part at 90 degrees the way it should be faced. When you are threading, if you look at a diagram of a 60 degree sharp V thread profile it shows the thread at being 30 degrees on both sides for an overall angle of 60 degrees.
But look closely and you'll notice the 30 degree is actually off from the 90 degree setting on the lathe. So, 90 degrees minus 30 degrees is 60 degrees. Back it up 1 degree so you're on 29 and that equals the compound being set at 61 degrees per the compounds markings.
But, thread wise, you will be feeding in a 29 degrees.
So, in the case of threading, the compound's 90 degree setting (per the marks) is actually 0 degrees (feeding straight in). You need to shift the compound 29 degrees off from this 90 degree position to get the correct angle for threading.
Hope I'm explaining that well enough.
Great job glad you got things working.
Ted
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- Dec 27, 2014
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- 4,059
The logic says "set the angle at 29.5 degrees," Not "set the dial on 29 1/2 degrees. " Many of us don't realize the difference. I know I had trouble with that on a new (to me) lathe.