# new to threading and have questions



## pete1972 (Nov 2, 2014)

hey guys, been playing with the lathe doing simple turning but would like to try threading but got a few questions.its more about the compound function really. im trying to understand  why the compound needs to be set at 29.5 degrees for threading. if its like that and you use the compound screw to advance after each pass then it seems to me that each time you advance while at that angle i would be cutting into the thread and ruining it. seems like you would advance each pass with the cross feed instead of the compound therefore the 29.5 would mean nothing. what am i missing??????????


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## Tony Wells (Nov 2, 2014)

Since the standard UN thread is 60 degrees, and your tool is ground to 60 degrees, moving it along a 29.5 degree line into the cut throws the majority of the cut on the leading (Left) edge of the tool. Many people take this route believing that the cutting force is primarily on that edge and the trailing (Right) edge only cuts the 0.5 degrees left from the compound movement angle, yielding a better finish. This could be true since you can do better in some material with a positive rake, and you really can't practically grind both leading and trailing edges to cut with positive rake. The more positive the leading is, the more negative the trailing edge. If you are using a neutral rake tool, there really isn't as much difference, IMO. It is a point that many will argue, but straight in feed, 29.5 degree feed in.....they both have advantages that are sometimes determined by the machine and material.

If you are feeding in straight, with the cross slide as you mentioned, yes, the 29.5 degrees is immaterial since you won't be moving the compound. In that case, the chipload would be fairly equal on both flanks of the thread, depending on the grind of the tool. Make sure you are understanding that the 29.5 degrees is from the face of the part, not the axis. Not all compounds are marked from the same reference.


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## GK1918 (Nov 2, 2014)

Not being rude Pete what your missing is dont question the teacher.  This has been the rule of thumb for 100 yrs.  Allthough some prefer feeding  straight in which is ok.  But
when you have to do a lot of threading, for me its the thread stop and feed with the compound because you get a fine feed using the compound.  No need to get into the math
as far as compound feed - just go with it and sneek up to it.  Don't worry after you do a couple hundred thread jobs all the math goes away you will get into the look listen
and feel mode. Be patient with yourself it will come but not overnite and with all  my overnites  I can just look at a thread and tell you all about it with no charts no math
because I've done enough of them.  Hope this helps little..however I'm old school and I am now older than the school I went to.
sam


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## 12bolts (Nov 2, 2014)

GK1918 said:


> Not being rude Pete what your missing is dont question the teacher.


Sam I have to disagree.If we all followed in the footsteps of our teachers without question then we would all be educated to the same tune and nobody would have discovered the earth was in fact, round.

Pete,
the following is a bit long winded, (it is from another post) but have a read and see if it makes sense.


12bolts said:


> As I suggested, you are missing the point about the relationship of movement established by having the compound set at 29.5*. Yes the tool form does establish the thread profile, but by having the tool cut progress at not quite 30* it takes the majority of the cut on the left (leading) edge, and the right, (trailing) edge takes very light cuts. In the pic the V cut by the tool form is 60* but as can be seen by the dashed line the movement of the tool when set at less than 30* causes it to move in a marginal amount as well as left a greater amount. Causing the tool to take a heavier cut on the leading edge and a very fine cut on the trailing edge.
> View attachment 68387
> 
> 
> ...


Any questions, please ask

Cheers Phil


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## GK1918 (Nov 2, 2014)

Oh you are right Phil, I looked out the window all through high school years, and when I walked out the door I clicked my delete button.  My teachings came from old time
machinist mentors not in public schools....rather the school of hard knocks.....a big LOL


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## higgite (Nov 2, 2014)

Great post, Phil. 

Pete, in addition to Phil's explanation, this illustration helped me get a better handle on the 29.5 degree way of thinking. 
(Note: Remember the 29.5 degree compound angle is measured from the X (cross slide) axis.) HTH.

(Edited to correct axis reference. Thanks, John. My bad.)

Tom


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## pete1972 (Nov 2, 2014)

great info guys!!!!!!!! really helps me sort things out. have another question if i may. how do i determine thread depth. i have a thread gauge in my tap and die set but i never see people using them when they do videos. is this just because of experience and just knowing?????? or just keep checking till the nut screws on? which would be a problem if you didn't have a nut at the time of threading.  so many questions! but this will make me a wiser man   pete


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## John Hasler (Nov 2, 2014)

I believe that is conventionally the X axis on a lathe, with the Z axis being the spindle axis.


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## higgite (Nov 2, 2014)

Thanks for pointing that out, John. My bad. I have corrected my original post.

Tom


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## Rangerjoe2 (Nov 2, 2014)

Pete,
I had a lot of the same questions when i started machining.  Watch the threading video on YouTube by the lazy machinist.  By far it's the best explanation I've seen so far on the subject and talked a lot about the reason for an angle less than 30 degrees.


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## 12bolts (Nov 2, 2014)

Pete, it is common for threading on the lathe to cut a run out groove at the finish of the threaded area. So if you turn the major OD for your threading blank, and then cut the run out groove to the minor od as you cut the thread progressively deeper when you are at the correct depth you should just see the tool tip scratching the surface of the run out area. Provided you have the correct angle on your tool you would also see the thread come to a sharp vee point at this time too. Basic trig for a right angled triangle will tell you how many divisions on your compound (which is the hypotenuse) equals the distance "in" that you need to cut. 
This is also where single point threading using the crosslide and the compound set at 90* is beneficial in that you can see directly on your dials how deep to cut according to your class of fit.
For threading on the lathe it is also common to run a die nut over the finished thread. This will give a much smoother finish to the thread and also check your diameters are correct.

Cheers Phil


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## pete1972 (Nov 2, 2014)

awesome info guys !!!!!!!!!!!!! this really really helps me out. i'd rather ask know and learn the proper methods and be able to understand the whole process like you would do in a classroom. im pretty happy i joined this forum!! great bunch of guys willing to help a new guy out, it really means alot. im thinking i will do some trial test runs on pvc pipe i have sitting in the corner. seems like a good idea. any thoughts?


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## Tony Wells (Nov 2, 2014)

Just for fun, I'll illustrate some of the difference in hobby work and industrial here. Phil is spot on with his how to, but that won't work in many commercial settings with industrial specifications/requirements. By far, the majority of threads do not allow a thread relief groove. CNC is capable of very consistent tool pull-out, so it isn't necessary. However, like so many things, there are exceptions. 

 One exception. In highly stressed thread, a rotary shouldered connection used in oilfield tubular components for example, a generous radius is required in the corner of the shoulder and major diameter of the thread. Sometimes the required radius is so large that an undercut is needed to accommodate it. So in effect, there is a relief groove, but not so much for that purpose. There is a problem inherent here though. The surface finish comes into play with stresses, so that these reliefs and radii must be very smooth. And since threads are simply spiral, relatively sharp bottomed scratches, the reliefs must be smaller than the minor diameter so they can be without any scratch, including a thread runout. The same principle is used, but not quite as commonly seen in internal threaded tool joints (RSC). 

  Additionally, there are specific dimensional requirements that must be met in commercial threading. There are tolerances for every feature of the thread. In the end, depending on the application, various gaging and inspection procedures and instruments are used to qualify the thread as acceptable to drawing requirements. It is difficult to compete with CNC equipment when running most threads on a manual machine. The CNC simply has the speed and repeatability that no human can match. I've had some fun trying, but can't keep up for long, even on a simple thread. 

  All that aside, to make the best threads possible is a good goal for any machinist, since perfectionism is often a part of the mindset of a machinist. So the more you know about threads the better off you will be, and the sooner you will be cutting threads you are proud of.


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## chuckorlando (Nov 2, 2014)

A thread pitch gage just tell the tpi. If your measuring thread depth you need thread wires or thread mic. A little math, or if it's common the machinery handbook will have the math done.

Or cheat and test it with a nut till the nut fits

Though if your just learning your only cheating your self if you dont at least learn how to do the math. I like to practice math with a problem thats already been answered so I know if it's right


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## pete1972 (Nov 2, 2014)

great stuff tonywells & chuckorlando. very informative. can never know too much!  there must be a chart somewhere that tells the exact diameter for the start of the shaft size and end of the shaft meaning depth size no???????? lets say i start off with 3/8 rod  but need a 1/4- 20  thread on the end  what would i turn the shaft down to and what diameter to turn the shoulder relief down to so the thread blends in? is there a chart somewhere to do simple calculations???????? im really lousy at math!!!!!!! but mama tried!


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## chuckorlando (Nov 2, 2014)

http://americanmachinist.com/thread-cutting-values-calculator


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## pete1972 (Nov 2, 2014)

i should be all good now guys. will report back later  pete


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## NightWing (Nov 2, 2014)

pete1972 said:


> awesome info guys !!!!!!!!!!!!! this really really helps me out. i'd rather ask know and learn the proper methods and be able to understand the whole process like you would do in a classroom. im pretty happy i joined this forum!! great bunch of guys willing to help a new guy out, it really means alot. im thinking i will do some trial test runs on pvc pipe i have sitting in the corner. seems like a good idea. any thoughts?



Just don't breathe the fumes.  PVC is nasty stuff.


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## chuckorlando (Nov 3, 2014)

No he aint joking. Your best bet is dont breathe any fumes or smoke from anything if you can help it. Alot of old timers have died way to early and alot more still to come from fumes that they thought would never hurt them.

And some things only take a small amount to do instant damage.


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## pete1972 (Nov 3, 2014)

i have to say im totally screwed then. have smoked for 30 plus years and a welder for over 25. maybe the pvc will be the straw that broke the camels back. its all coming to a stop someday for all of us so im not to worried about a little practice threading on PVC.  thanks for the warning guys


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## 12bolts (Nov 3, 2014)

Pete,
For your application, (threading on the lathe) you should be fine. Use very sharp tools and have at it. PVC is very forgiving on the lathe. As long as you are not overheating the work you wont have any problems with fumes.

Cheers Phil


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## chuckorlando (Nov 3, 2014)

I'm not saying you have anything to worry about. I'm just saying it's smart to not breath in anything you dont have to. I dont care what the label say's. Better off safe then the first reported case.


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## JT. (Nov 4, 2014)

Tony Wells said:


> Since the standard UN thread is 60 degrees, and your tool is ground to 60 degrees, moving it along a 29.5 degree line into the cut throws the majority of the cut on the leading (Left) edge of the tool. Many people take this route believing that the cutting force is primarily on that edge and the trailing (Right) edge only cuts the 0.5 degrees left from the compound movement angle, yielding a better finish. This could be true since you can do better in some material with a positive rake, and you really can't practically grind both leading and trailing edges to cut with positive rake. The more positive the leading is, the more negative the trailing edge. If you are using a neutral rake tool, there really isn't as much difference, IMO. It is a point that many will argue, but straight in feed, 29.5 degree feed in.....they both have advantages that are sometimes determined by the machine and material.
> 
> If you are feeding in straight, with the cross slide as you mentioned, yes, the 29.5 degrees is immaterial since you won't be moving the compound. In that case, the chipload would be fairly equal on both flanks of the thread, depending on the grind of the tool. Make sure you are understanding that the 29.5 degrees is from the face of the part, not the axis. Not all compounds are marked from the same reference.


so move clockwise 0.50dg from the 30dg  (ore 60 ) ?


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## Tony Wells (Nov 4, 2014)

Viewed from the top, the move should be CW, so that the heel of the tool will still advance into the cut. If you move it CCW, you will get a stepped flank. In reality, the closer to 30 degrees you get, the better your results should be.

For a mental picture, forget the 60 degree tool and think of a straight sided groove tool. If you feed it in straight, no steps on either side (flank) of the groove. But if you feed with the compound at 29.5 degrees, what happens? The left (leading) side strays straight, like the tool, but the right (trailing edge) yields a angled groove wall. The angle will not be smooth though.....it will show the steps along the angled tool path


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## higgite (Nov 4, 2014)

JT. said:


> so move clockwise 0.50dg from the 30dg  (ore 60 ) ?



When looking down on the compound from above, yes, you move it 1/2 degree clockwise from 30 degrees. See the illustration in post #6. Tony described it right, he just said it wrong. I've never done that. :whistle:

Tom


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## Tony Wells (Nov 4, 2014)

Yep Tom.....I blame it on the 7 drug cocktail my back doc gave me. I should be in a coma about now. 

Thank you! Correction edit made (I think so, at this point anyway)

R. Sparber, as usual, has an excellent writeup about it, with an explanation of even another workable method of cutting threads.

http://rick.sparber.org/spt.pdf


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## Brandon1 (Nov 12, 2014)

Tony Wells said:


> Yep Tom.....I blame it on the 7 drug cocktail my back doc gave me. I should be in a coma about now.
> 
> Thank you! Correction edit made (I think so, at this point anyway)
> 
> ...




Thanks for that info!

I am really wishing I had paid more attention to the math class instead of the hot little redhead in class.  Oh well you live you learn.


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