Threading - 29.5/30 vs 90
- Thread starter Susquatch
- Start date
Brent H
Ultra Member
@Susquatch : here is a PDF for you to have a gander at :
A comprehensive read of the document you provided included the following statements:
Donald of gingery_machines BBS points out: “The usual argument against this method (i.e. in favor of setting over so you're feeding effectively parallel to one face) is to avoid the cut material from both sides of the tool bit from crowding; this crowding causes an increase in pressure and can lead to chatter or digging the cut off material into the face and pushing the work piece out of alignment, leading to a tapered thread.”
Russ of atlas_craftsman pointed out that with the cutting force on both sides of the cutter, there is no consistent pressure on the leadscrew and halfnuts. The result is that any backlash will show up as a wandering thread.
Doc of atlas_craftsman wrote – I believe the helix on the lead screw provides constant pressure towards the headstock , eliminating backlash or a drunken thread ...the straight in bit has pressure perpendicular , but there is pressure to the left by the driving of the lead screw....
So I guess Russ and Doc will have to go outside and settle the matter
This treatise provides two views on whether or not loading the leade screw by cutting at 30/29.5 degrees is important but makes no attempt to validate either point of view. The author leaves the reader to decide.
I will only say here that I think the choice of view is always the right of the reader. However, it is my opinion that the quality of the machine should have an impact on that choice because the level of force required to keep the so called drunken sailor on course depends on how drunken he is...... That is to say that 30/29.5 might be a better choice than 90 for lower quality or well worn machines. The size of the thread might also be important.
Thanks Brent.
I was going to draw this up for you but Brent's attachment above has a perfext drawing and description of 90 vs 30/29.5 starting on page 5.
I just do what Tony does...
Who is Tony?
This Old Tony... I blame my entire descent onto this bottomless rabbit hole on him...
Never mind..... @140mower twigged it for me. If using 29.5/30 you infeed with the compound Doh.
Rereading the threading section of "BASIC LATHEWORK FOR HOME MACHINISTS" by Stan Bray. No where is that detail stated.
I don't have a copy so I can't help with that Craig.
But that reminds me that I should look to see what I do have though.....
140mower
Don
If you are going to use the cross slide, it makes no difference where your compound faces, but unless I am missing something important, once committed to a method you need to stay the course.... Ie... If you are facing the trailing edge of the thread with the point of the cutter and the leading edge of the thread with the forward face of the cutter as in the 29.5 method, but try finishing up with a 90 plunge, you are going to remove the backside of the thread you just cut.....
I'm having a bit of a day, so I apologize if that description made things worse..... I think I know what I am trying to say. Lol
Former Member
Guest
I learned 29.5 (or there abouts as long as its not 30) to prevent stepping. I think the rest of the world uses 30 as the number as long as its a shade less.
The 90 method I definitely agree is from insert tooling (CNC) influence and is beginning to become popular as it now better documented.
Now which to use becomes the question based on you machine and tooling.
The 90 method I definitely agree is from insert tooling (CNC) influence and is beginning to become popular as it now better documented.
Now which to use becomes the question based on you machine and tooling.
Theoretically, that should not be required using the 29.5 degree method. But it will be necessary virtually every time if you mistakenly use 30.5 instead of 29.5. It's a very easy mistake. Page 6 (or shortly therafter) in the paper that Brent provided assesses the reasons for this quite well. It repeats what I did many many years ago when I was trying to use a compound that didn't have 29.5 on it so I needed to go back to basics to work it out. I wish I had @Brent H s paper back then.
That said, yes, a straight plunge using the compound will clean up all the faces. I have done this many times so I can vouch for it. Once the trench (plank might be a better word) is laid there are no worries about drunken sailors anymore. They just fall in where they are supposed to.....
I confess I also do this when I have to pick up a thread to take a few more thou off or cleanup an old existing thread. It probably isn't the best practice but it has worked for me.
PS - we should all get @140mower drunk so he is happier. He might make more sense then. I have no idea what he was trying to say! LOL!
Last edited:
VicHobbyGuy
Ultra Member
On my 7x lathe, smaller cutting 'working faces' produce less chatter. So plunging cutting both sides of the 'V' in a thread is not as easy as cutting one side using the cross slide at 30 degrees.
Similarly a thin parting tool works better than a thicker one, chamfering cuts using the side of the tool are difficult etc...
Back to our regularly scheduled program....
On my 7x lathe, smaller cutting 'working faces' produce less chatter. So plunging cutting both sides of the 'V' in a thread is not as easy as cutting one side using the cross slide at 30 degrees.
Similarly a thin parting tool works better than a thicker one, chamfering cuts using the side of the tool are difficult etc...
Back to our regularly scheduled program....
I think that's a perfectly fine post for the topic. And it's my thread so it's my call!
But ya, according to sandvik, you are supposed to just cut less each pass as the going gets tougher. I dunno - I think sometimes a less rigid machine has trouble with that.
Also, as @gerritv pointed out earlier, the size of the thread matters. A little 32 TPI cut is easy to cut no matter how you do it or what machine you have (unless it's really sloppy). But 4 TPI might be impossible on a small lathe no matter which way you cut it. If I had to do it with a small lathe, I'd prolly make a special hss tool that would allow me to cut the thread from both sides in stages - maybe make a double sided staircase that I clean up afterward.
Just another followup to my reply to @VicHobbyGuy .
This thread is about 29.5/30 vs 90 degree threading. It isn't about a particular size or class of lathe.
I invite opinions about the problems, solutions, disadvantages, advantages, pros and cons, for each from everyone. I'm also interested in what others might know about the history no matter what size or class of lathe they have. It might well be that size matters, but that will be ok! It may well be that this might be the last chance for many of us to know the history of this matter before it is lost in the complexities of today.
This thread is about 29.5/30 vs 90 degree threading. It isn't about a particular size or class of lathe.
I invite opinions about the problems, solutions, disadvantages, advantages, pros and cons, for each from everyone. I'm also interested in what others might know about the history no matter what size or class of lathe they have. It might well be that size matters, but that will be ok! It may well be that this might be the last chance for many of us to know the history of this matter before it is lost in the complexities of today.
Dabbler
ersatz engineer
I missed this thread somehow, but @Susquatch pointed me to it, so blame @Susquatch ...
I have zero skin in this game, and have zero intention to comvert any of you to "my way of thinking", so this is what I do, for reference, with a update that happened 3 years ago.
I was taught by a toolmaker to set up things a particular way, and it has worked for me, despite the fact that 80% of machinists do it another way:
I set my compound to 30 degrees. as precise as a tenths indicator can make it. It takes me about 5 minutes, and then I lock it down. For years. Only when I loosen it to cut a custom taper, then I spend that 5 minutes again to get it to 30. I cut threads with no discernible rubbing or laddering even under a 20X loupe.
But that was 42 years ago, and 3 years ago (was it longer?) I watched a video by Stefan Gotteswinter that made me question reality. It was on his old lathe, a 9" very light lathe that he made extensive tailstock mods to. Even on this sub-600 lb lathe he cut his threads straight in. On a solid toolpost - the compound was stored in a drawer somewhere.
I'm always up for something new, so I tried it - on my sub-700 lb 12X36 lathe. Worked like a charm. Clean threads, and much more straightforward. (well at least to me). Now that I have bigger lathes, I can't wait until I have the next threading job...
@Susquatch I have to disagree with the opinion about parting tool method in @Brent H PDF -- the movement in the carriage is enough to keep backlash at bay, regardless of single side cutting or double cutting, as in the European method.
I have zero skin in this game, and have zero intention to comvert any of you to "my way of thinking", so this is what I do, for reference, with a update that happened 3 years ago.
I was taught by a toolmaker to set up things a particular way, and it has worked for me, despite the fact that 80% of machinists do it another way:
I set my compound to 30 degrees. as precise as a tenths indicator can make it. It takes me about 5 minutes, and then I lock it down. For years. Only when I loosen it to cut a custom taper, then I spend that 5 minutes again to get it to 30. I cut threads with no discernible rubbing or laddering even under a 20X loupe.
But that was 42 years ago, and 3 years ago (was it longer?) I watched a video by Stefan Gotteswinter that made me question reality. It was on his old lathe, a 9" very light lathe that he made extensive tailstock mods to. Even on this sub-600 lb lathe he cut his threads straight in. On a solid toolpost - the compound was stored in a drawer somewhere.
I'm always up for something new, so I tried it - on my sub-700 lb 12X36 lathe. Worked like a charm. Clean threads, and much more straightforward. (well at least to me). Now that I have bigger lathes, I can't wait until I have the next threading job...
@Susquatch I have to disagree with the opinion about parting tool method in @Brent H PDF -- the movement in the carriage is enough to keep backlash at bay, regardless of single side cutting or double cutting, as in the European method.
Good stuff @Dabbler !
Perhaps you didn't notice that the paper Brent provided presented two views. One being the same as yours and one suggesting that it needed a bit more.
Your comments about two smaller lathes are quite compelling. I had thought perhaps the size and quality of the lathe might dictate a preference in method. But perhaps not.
I assume you are quite happy with the quality of your threads using the 90 degree plunge cut. Do you also reduce the amount of infeed as the depth of cut increases?
I can't help but wonder if your view of plunge cutting is influenced by the amount of effort you took to setup your compound for a very precise 30 degree cut. If it were me, I'd abandon your previous method in a heart beat if I found something so much easier.
But my practice (at least till now) has been to cut at 29.5 degrees with virtually no precision in the angle. I just eye-ball it and start cutting. I have never found the angle to be critical. It always worked just fine as long as it was slightly less than 30.
Unlike your experience, I found 30 to produce poorer results. But I confess that I didn't set the angle with any precision so perhaps I was not really set to 30 but maybe 30.x instead. That would explain a lot.
I may yet go @gerritv and your way on this. We will see.
Dabbler
ersatz engineer
Yes. even on a larger lathe.
Not at all. I set up the compound only once every year or 2. No big deal, and it keeps me in practice.
That could be one problem. The other might be chip clearance. I was taught to use LOTS of lube oil when cutting threads, to stop chips from crowding and jamming on the trailing edge. As the cut gets deeper it is harder to clear the chips without good lube.
For me, the attraction wasn't anything more complicated than having only one dial to worry about. I dislike jumping from the cross slide for extraction, back to zero, then jumping to the compound for next depth of cut. I prefer to extract, then dial to the next increment. One thing to remember, and one thing to calculate, all on one dial. Just my preference. They both make nice threads. I was taught the compound method, but now prefer the straight in method. The bonus round is the possibility of removing the compound until it is really necessary, which is very rare.
gerritv
Gerrit
@Susquatch re size of lathe, I use the plunge method on Unimat and Taig (with thread followers to act as leadscrew), and now a used King 1022. None I would call excellent condition or stiffness. I do keep the tool overhang to a minimum, and use fresh inserts/honed HSS esp as the pitch gets coarser.
As confirmed by a few videos I watched last night (in Dutch and Flemish), plunge is used when pitch is small (as in less than 1.5mm/14tpi), after that they use A) the 29 or so method or B) set the compound parallel to axis and dial in 2, across 1 on each pass. (as in .002 infeed, .001 toward spindle). The B method has the same result as offsetting the compound. Of course A and B don't work without modification for 55 or 47.5 or other thread angles, but the plunge method does
The big advantage on the non-29 method is that you read your depth off the dial, no real need to keep measuring. You can see that in Stefan's video, no measuring.
My reco, try it out on a few threads. Too much theoretical chatter is not good for making progress.
As confirmed by a few videos I watched last night (in Dutch and Flemish), plunge is used when pitch is small (as in less than 1.5mm/14tpi), after that they use A) the 29 or so method or B) set the compound parallel to axis and dial in 2, across 1 on each pass. (as in .002 infeed, .001 toward spindle). The B method has the same result as offsetting the compound. Of course A and B don't work without modification for 55 or 47.5 or other thread angles, but the plunge method does
The big advantage on the non-29 method is that you read your depth off the dial, no real need to keep measuring. You can see that in Stefan's video, no measuring.
My reco, try it out on a few threads. Too much theoretical chatter is not good for making progress.
Last edited: