Thursday , February 22 2018
Home > Shop Tips > Single Point Threading: Set Compound at 29.5° or 31°?

Single Point Threading: Set Compound at 29.5° or 31°?

This is one of those concepts that I thought I had a handle on until I dug a bit deeper.

I was taught to set the compound (or top slide for those of you across the pond) to 29.5° when cutting a 60° thread. My understanding was that this would allow for you to advance the threading tool at a 29.5° angle into the workpiece so that only the leftmost side of the threading tool would remove chips.

Image from

Recently I learned (like so many other machining operations) that the method I was taught wasn’t the only method out there. It turns out that there are people out there that not only don’t use this method, they’ve never even heard of it! That lead to a forum post that I encouraged Jerry to write about the topic. The purpose was to discuss the method in greater detail and gain some clarity along the way.

Then a forum member mentioned that if the intent was to give relief to the back side of the threading tool, the angle needed to be 30.5° and not 29.5°. This made me curious and I proceeded to draw the concept up in Sketchup to either confirm or deny that 29.5° was the proper angle.

It turns out that it does seem that 30.5° might make more sense (if you’re truly trying to prevent the back side of the threading tool from cutting). Setting the tool at 29.5° actually causes the right side of the tool to remove material (albiet a lot less material than the left). While setting the tool to 30.5° looks as though it would result in the desired relief on the right hand side of the tool.

The image below shows (exaggerated in red) the amount of material that would be removed if you were to set the compound to 29.5° degrees (but it’s actually set to 25° to exaggerate the concept for clarity).

29.5° (exaggerated for clarity)

While this image shows how the right hand side of the tool would have clearance if you set the compound to 30.5° (again, exaggerated for clarity by setting the angle to 35°).

30.5° (exaggerated for clarity)

So what do you think? How were you taught?

I put together a YouTube video that illustrates the concept and shows what I discovered when I drew things up in Sketchup. If you’re already familiar with the concept, skip ahead to 4:20 in the video, which is where I start talking about my Sketchup findings.

If you have experience with this method and can explain why my Sketchup results were counter intuitive, please feel free to leave a comment and enlighten me (and everyone else) via the forum topic connected to this post.

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About Tyler

Tyler is a hobby machinist and 3D printing aficionado. He teaches computer programming and web development at Highline College near Seattle. Tyler founded Projects In Metal in 2008 because he was frustrated by the lack of free plans available for hobby machinists.

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  1. Tyler: my shop classes were back in the '60's and we were taught 29.5.

    I haven't done a great deal of threading on a lathe , but what I did , I did at 29.5.

    That worked real good if I didn't mess something else up !

    All other machinists that I have known , including gunsmiths and machine shop owner/operators ,

    were using 29.5 to the best of my knowledge.

    As to 30.5 I dunno , never heard of it.

    That doesn't make it wrong , only different.

    If it works and you're happy with the results , so be it.

    The math of machining was almost over my head 50 years ago so I can't help you there either.


    PS I almost  missed the math problem to post this reply !


  2. 29.5 or 30.5, both will lead to a nice cut on the advancing (headstock) side of the cutting tool, so that the face of the thread facing AWAY from the headstock will be one single surface.  However, if you think about how the trailing edge of the tool will cut the other side of the thread, 30.5 degrees will produce a different result than 29.5 degrees. 


    Because the trailing edge of the tool still cuts all along the thread face on every pass, albeit a thin shave, both faces of the thread will be smooth.  However, when you set the compound at 30.5 degrees, the face of the thread cut by the trailing edge of the tool will be made by the incremental tool advances made each pass.  In other words, that face of the thread will be a series of steps, each step as long as the amount that the tool is advanced for each pass.


    Now, as Jerry has pointed out, there may not be any advantage at all to setting the compound at an angle, and that you might very well be better off to cut your threads by advancing with the cross slide.  Perhaps I'll cut three threads, one advancing with the cross slide, the other two with the compound at an angle (29.5 and 30.5 degrees), and I'll post photos of the results.


    “Rules of thumb” are convenient heuristics that make it easier for us to complete tasks without having to know everything about what we're doing.  The problem is, the “rules” themselves are completely divorced from the actual problem that one is tackling.  Indeed, that's the whole point; I don't need to know exactly what is going on if a general-purpose rule of thumb will give me a “good enough” solution to my problem.  The thing is, these rules are developed at a particular time and place, and the rules get “locked in”.  Such rules generally long outlive the times in which they were born, with people treating them as gospel even though they no longer are necessary or even apply.


    I couldn't tell you whether or not the compound method of thread-cutting is better than the cross-slide method.  I can think of good arguments in favour of both, and as an engineer, I can assure you that the models that we have of how tools remove material are approximations.  At the end of the day, the only way to know if one method is better than another is to directly compare them.  Like I said, I'll do a comparison on my lathe.  I'd encourage someone with a 7x lathe to do the same, as well as a 9x lathe owner, and owners of more “robust” lathes, too.  We may find that the compound method is better suited to machines of lesser rigidity (what I would expect if there is any advantage to it at all), while more rigid machines can use either method with equal ease.



  3. Tyler, your killing me with your math.  I believe your using the wrong reference point.  In threading the reference point is usually the “Y” axis thus the offset of 30 degrees is consistent with the reference used to measure thread angle.  Using the “Y” axis as our reference, moving the compound slide from zero degrees to 29.5 degrees will result in the leading edge of the tool engage the material and the trailing edge will be clear of the material.  Moving the compound slide to 30.5 degrees will result in both edges of the tool engaging the material, the trailing edge may or may not remove any material depending on the specific grind of the tool.



  4. @ironhorse69: I know, the math to post keeps getting harder! Yell

    @ironring: yea, that makes sense now. I hadn't actually considered how little the amount of material being removed on the right hand side was. And the benefit to removing it is (after your explanation) clearly understood. By removing a little bit of material on the back side the thread will have a smooth v threadform on both sides, not just the leading left side. I think I might revisit my sketchup file and see what percent of the overall V threadform is made by removal of material from the left of the tool vs the right. I'd guess it's 95+ %. 

    @Jim: You bring up another topic that could probably use some clarification. Lathe Axis Definitions.

    Axis definitions for a manual lathe:

    Z: The axis of the rotating spindle. To cut along Z you use the carriage/apron feed dial or power feed.

    X: Perpendicluar to the Z axis. Controlled with the cross feed dial – controls the diameter of the part being turned. 

    Y: ?? Tool height? I hear differing definitions on what Y is. I've even heard Y is the compound/top slide. 

    I made my angle change on the angle of the compound, changing it 29 and 31 degrees from perpendicular to the Z axis. Where should I have changed the angle?

  5. Hi Tyler, good point on the what the proper definition of a lathe axis is.  Not sure how they ever arrived at the rotating part being the “Z” axis, I'm sure there is a good reason.  But, for us simple folks who look at the problem as though you were looking at a print the spindle becomes the “X” and the cross-feed the “Y”, right or wrong it works for me. 

    Back to the point of this discussion and my comments.  I tried following your google sketch example and find it very confusing, I did my own to verify my math and validated my previous observation.  29.5 degrees from perpendicular using a 60 degree tool set to perpendicular to the work will result in the leading edge cutting and the trailing edge not cutting, or rubbing. 



  6. So it sounds like we are talking about the same axis, just using different names for it. I drew my sketchup file as if I had rotated my compound (not my cross slide since that doesn't rotate) at 29 and 31 degrees, and then again at 25 and 35 degrees to get a more clear result. 

    So you did the same thing and got a gap when setting the tool to 29.5 degrees? Hmm. I'll have to check my sketchup file again because that's what I would have expected to see. But it was hard to see at 29.5 degrees, which is why I exaggerated it to 25 degrees. At which point I expected to see a gap between the right hand side of the tool and the workpiece. But I didn't see that. I saw a gap only when setting the tool at 31 (or an exaggerated 35 degrees). 

    I do know that both tools in my sketch are exactly perpendicular to the work and parallel to each other. But I'll double check my overall setup. I might even start over from scratch to see if I get the same results. I could have very easily made a mistake in my drawing. 

  7. Hi Tyler, you might want to make your model a little simpler by just using a “Vee” to represent a single thread form.  Make it large enough to fill your monitor screen.  And then of course there is the math, 30 – 29.5 = 0.5.



  8. The reason for turning the compound rest 29.5° from perpendicular to the spindle axis is to lessen the
    horsepower required to make the cut. This reduction in horsepower is
    accomplished by reducing the length of cut by only using the left side of the
    threading tool. This is most important for those of us who use small
    underpowered lathes. If you set the compound at 30.5° the right side of the
    tool will engage the right flank of the thread thereby increasing the length of
    cut and horsepower requirement.


    The problem with the drawing
    presented was that either the cross slide was advanced OR the angle was not
    correct. Please keep in mind that ALL depth of cut is put on with the compound
    and NOT with the cross slide. The cross slide is only used to get you back to
    the same place in the X axis every time.


    This is not even worthy of mention
    on a tiny thread like ¼-20 which could be accomplished as easily using the
    cross slide to set the depth of cut. On a large thread it surely makes a
    difference because the larger the thread, the longer the cut and therefore the
    more horsepower is required.


    I teach 29.5° from perpendicular
    and so does every Journeyman Machinist I ever met. I believe the 30.5° business came from
    someone who got confused about which way to compensate half a degree after it was explained to them a long time ago similar
    to the JFK assassination being carried out by one individual or the Gulf of
    Tonkin incident being real, that sort of thing, and it has just been so often
    repeated that it has gained some validity that it does not deserve.


    Please note that the finish pass
    (which is way lighter than the roughing passes) is put on using the cross slide
    alone causing both flanks of the thread to be finished at once. This is
    possible on the finish pass because the depth of cut is so light. 

  9. Okay, so if we dissect the method into two parts.

    First, the point of setting the compound at and angle is, as you say, to have the cutting action on one side of the tool bit to relief pressure on the cutting edge.

    Second, the actual angle of the compound has to be set so that the angle of the finished thread is dictated by the cutting tool, and not by the compound. This means that if the angle of the compound is greater than half the angle of the finished thread, you will end up with a thread that is bigger than 60° and not symmetric. To get a better picture of what i mean, you can try to cut a thread and exaggerate is by going as far as 45° or even 60° with the compound. 

    Now as for the point of cutting only on one side, the small chip on the right side that one, or one half degree constitutes has very little impact on the cutting force relative to the chip forming on the left hand.

    Further more, cutting below 30° will make a superior finish on the right side as opposed to cutting at 30° or over as you cut the entire face on the thread instead of small steps.

    With this method you reduce the forces on the tool, make it easier to grind the toolbit (as you use almost only one cutting edge) and ensure that the thread is formed by the tool and not the compound. 


    In short, cut below 30° Cool

  10. Hi Barry! It's good to hear from you!

    I'll have to re-visit my sketchup calculations. I was overly careful, but that doesn't mean I didn't still screw them up. I know I didn't advance the tool in X (ie by using the cross slide) because I have made (and learned from) that mistake in the past. But I'll re-work the drawings and see if I get a different result.

    By the way, is the Y axis on a manual lathe the height of the tool? I think that's another topic that could use some clarification because I see lots of different opinions on the web, and it gets even more muddled when you include CNC machines.

    And thanks Jrolands for your explanation as well. 

    It's good to know that it is supposed to be 29.5, as well as why it's supposed to be 29.5. 

    29.5 was the method that I was taught (by you Barry) and I've used it successfully many times. It wasn't until recently that someone mentioned 30.5 that I started to question the theory behind the method. 

  11. Hi all, just joining with interest into your deep discussions on single point threading.

    I agree that 29.5 is the accepted angle where the compound slide offset method is used.  Not only does it require less hp, but it reduces the load on the tool and the work piece and when set to this angle the tool also cleans up the RH side of the thread form.  The setting of 29.5 degrees (in Aussie terms anyway) is “a fanny hair off 30 degrees” and is  set by eye.


    The alternate threading method is to leave the compound slide at zero degrees and advance the tool using the cross slide, however this would result in a plunge cut where shavings from both sides of the tool will meet in the middle resulting in much tearing and a poor finish.  To over come this I have always taught students to advance the compound slide parallel to the work piece axis, the amount of movement is not defined and changes with different thread sizes anyway.  The trick is to advance the tool just enough to prevent the RH side of the tool from cutting.  This method is really a roughing process and when full thread depth is approached the machinist needs to move the compound slide backwards to clean up the RH side of the thread.


    In either method, the machinist needs to calculate the depth of thread.  A simple trigonometry calculation which for most threads is a guide only, as the final fit is determined by testing to the mating part or test nut.  If on the other hand you need to cut a thread for a mating part that cannot be brought to the machine and no test nut is available, then the determination of thread depth becomes a very much more precise arrangement and one to avoid is possible.


    It is worth noting that when cutting external threads, the outside diameter of the finished thread will always be smaller than the theoretical diameter (for example for a 1/2″ UNC the theoretical diameter would be 0.500″ ) and the root diameter is always a little smaller and its exact value is found by subtracting two calculated depths of thread from the theoretical diameter.  These two under size diameters provide clearance to the matting part.  The actual fit of the thread being cut is determined from the sides of the thread  and not the diameters.


    Out of curiosity, how many thread cutting machinists out there track their threads by using a recognised tracking method, like the chasing dial, and how many use the old reliable favorite, reversing the spindle?




  12. Sorry have been away for a while but my college course has been getting quite tough lately. That aside I did get some time to contemplate this question concerning the compound angel and I have had the chance to start making some experimental threads. Just as Chris has requested I made three threads one with the crosse slid and two with the compound set above and below 30 degrease, also I got permission to use my schools tool room lathe to cover the absolute highest end of the lathe spectrum. I got some vary interesting results and as soon as the mid terms end I should soon complete a report.         

    Image Enlarger

  13. Also, the question should not 29.5 vs 31 (or 30.5) if the issue is which axis you are measuring relative to; it would be  29.5 vs 60.5. 




    PS, no apologies for the math; math is what makes figuring this out possible!


  14. When single point threading, I was always taught to set my compound at 29° or 29 1/2°. This will leave a planing cut on one side of the material and a full cut on the other. The closer you get to 30° the more likely you will leave a stairstep pattern on one side of the threat. If you think about it, your Illustration will show that every time you move the Cutting tool in, the tool is moving away from the opposite side of the thread. This will even stairstep pattern on the thread. Whoever taught you Used the correct Method but They had it backwards. If you are a good machinist and careful with your set up you can set your compound at 30°. But most of us cannot set the compound that precisely and we should not try. You can Achieve very good results using the 29° method.


  15. If you set the compound at 30 degrees (and confirm the accuracy of the setting) you can put some side rake on the toolbit and cut a very smooth thread without interfering with the right flank of the thread. Got this info from a professional machinist’s discussion on the internet.

    If you think about it, any deviation from 30 degrees will result in a drunken thread…not a great deal of inebriation , but it depends on the desired precision. Many machinists in the past have ground tool bits with only the advancing edge angle to make the cut (see a book on turret lathes), with the back edge clearing the right flank of the thread. The 29 degree angle of infeed works well with a flat top tooolbit: great if you want to tear your thread bodily from the workpiece. Try some top rake and see the difference.

    Several machinists I have known used a straight infeed with the compound set at 90 degrees to the workpiece axis using a flat top toolbit. You can make this work if you take your time.

    29-1/2 degrees is specified by all of the lathe manuals I looked in, but I still get my best results at 30 degrees and using about 5 degrees top rake  behind the left hand cutting edge. This geometry does not gouge up the right flank to any discernible extent.

    If all you want to do is clear the right hand side of the cut, use a 55 degree toolbit and set the left edge at 30 degrees to the workpiece, avancing the top slide at 30 degrees. Lots of clearance then. This will prove to you that clearance should be on the cutting surface of the toolbit, not from cocking the compound.

    This has produced great results for me for over 50 years, but, then, I’m not a machinist; I’m a gunsmith, and I fit the threaded part to another part, not to a gauge.

  16. Hi John!

    Thanks for the tips. I was just threading last night in fact, and having trouble using my thread dial.

    I was making a copy of my spindle thread (on the back end of the spindle) and I needed to make a 39 x 1.5mm thread. For some reason I couldn’t get the thread form to repeat when using the thread dial. I know with some threads the number(s) you start on matter, but I was starting over on the same number each time (#1), so I was stymied.

    I ended up leaving the half nut engaged and reversing the spindle to return to the starting point of the thread (after backing the tool away from the work a bit). 

    That worked ok for the outside threads, but I need to cut internal threads on a blind hole for the mating piece, and I don’t have the space for a huge thread relief, so I really need to be able to disengage the half nut for the other part I’m making. But I can’t seem to figure out why it isn’t restarting in the same spot when I use the dial. 

    Any ideas?

  17. Hi Tyler, I believe you will find that the lead screw dial will only repeat on inch threads, not metric.  For outside threading I use a retractable threading tool and then reverse the motor direction.  Absent that tool you should zero your cross slide and at the end of the thread back the tool out away from the material and reverse out.  Works on inside or outside, especially if you have the zero backlash nut and screw.Smile  It’s even easier with a DRO.



  18. I have to hand it to you guys on your precision.  Do you have people that can sharpen that accurately?  I guess I look forward to cutting my first threads a bit reluctantly.Confused



  19. When I was rebuilding the DR mowers I thought I had to make some new spindle shafts.  One of the threads was a left hand thread which should have been about the same except in reverse.  It was good that I found two new ones on ebay for $70.  If I would have read this article I might have been scared off.  Otherwise, I would have probably just done it.Laugh

    It is obvious that I have a lot to learn.



  20. Hmm, thanks Jim! I’m not sure if I knew that at one time and forgot, or if I never realized that you can’t use the threading dial on the G0602 for metric threads. But either way I know now. Thanks for the heads up!

    I imagine if the leadscrew was metric it would work, but since it’s 8tpi that’s the problem?

    Anyway, Modela, to your question. Sharpening a thread form on a HSS bit is actually a lot easier than you might think. You use a thread gage (or “fish tale”) to check the thread form by holding it up to the light. Adjust left or right to get it very close to 60 degrees. I’ve never actually measured precisely, but I’d imagine it’s within a half a degree of perfect without much effort.

    Or you can always buy a relatively inexpensive pre-sharpened HSS tool, or a threading tool with carbide inserts (although on a smaller lathe with less HP I try to avoid carbide).

    Don’t be scared off by threading. It can be fun … when it isn’t frustrating. Yell

  21. I remember threading years ago on my dad’s Logan.  I look at the Logan now with its additional box of gears and all the tools it came with and it brings back memories.  Come to think about it I remember the thread gauge.  I think I know where one is in my dad’s old box. 

    I remember always wanting to cut an Acme thread.  I don’t remember why.  It just looked nice.

    Most of my time in business I was working against time or dealing with fragmented bits of time. There were things I wanted to do that I sent off to expedite the job. Most of the time I was fixing or rebuilding old tools or working on business systems like network systems or point of sale systems.   

    With this Mill rebuild I am declaring myself free of fixing old tools.  If I want a new tool I am going to buy it new.