Thread Cutting on the Lathe, Part Four - Setting Up the Lathe and Cutting the Thread

[Tom Griffin]

Thanks. I need to get it put back before my dad discovers it's missing. :shush:

Tom

 

[Vince]

Thanks Tom, That was a good video. You partially answered one question I have had for a long time.
That is, when you are threading whether it be external or internal I have always wondered how deep the threading tool is being advanced using the compound. If one doesn't have the wire gages or a blade micrometer to measure and you are threading say 32 TPI you would need to figure out at what point you cease to advance the compound and do a clean up pass with the cross feed.
Would you mind elaborating on the formula again for root depth versus pitch
I just so happen to have to thread a few pieces. 3A 1.062 x 32 TPI both internal and external.
In the past I have relied on luck, using the cosine x the amount of advance of the compound, and a lot of trial fitting. (I was never really sure if I was doing this right or not)
Also, is the amount of movement on the compound at 1:1 or 2:1 like the cross feed.
I get so confused with this.
Thanks again, Vince

 

[Tom Griffin]

Thanks Tom, That was a good video. You partially answered one question I have had for a long time.
That is, when you are threading whether it be external or internal I have always wondered how deep the threading tool is being advanced using the compound. If one doesn't have the wire gages or a blade micrometer to measure and you are threading say 32 TPI you would need to figure out at what point you cease to advance the compound and do a clean up pass with the cross feed.
Would you mind elaborating on the formula again for root depth versus pitch
I just so happen to have to thread a few pieces. 3A 1.062 x 32 TPI both internal and external.
In the past I have relied on luck, using the cosine x the amount of advance of the compound, and a lot of trial fitting. (I was never really sure if I was doing this right or not)
Also, is the amount of movement on the compound at 1:1 or 2:1 like the cross feed.
I get so confused with this.
Thanks again, Vince

Vince,

The common formula, .75/TPI for a sharp tool or my formula for a tool with a flat, P/1.5 will both get you close, but that is not good enough to properly size a thread, especially a 3A thread. You need to measure the pitch diameter using a thread mic or wires to get it right. The Pee Dee thread wires that I used in the video only cost about $25, so they are well worth having.

The cross slide dial reads diameter (2:1), but the compound dial is direct reading (1:1). In other words if you move the dial .010", the compound moves .010". This is useful when cutting threads because all the thread depth calculations apply to the thread profile, not the part itself.

I'm glad you enjoyed the video.

Tom

 

[rangerman]

The compound angle really has nothing to do on which side of the cutter's edge is cutting more, The cutter will always cut more on the side of feed direction. If you are moving the cutter from right to left, the cutter edge on the left side would cut more since that is the direction it is going to. If your are moving the cutter from left to right (like in the case of LH thread cutting) the right edge of the cutter is the one that is cutting more.
For me the different angles that we hear about from many machinists on how to "properly" set the compound angle are just arbitrary personal preferences or habitual practices passed on from their previous mentors and nothing more.
What's really critical and important is to make sure the cutter "V" is even and square/perpendicular to the work. That should be measured with a center gage.

For me the 30° or 60° ( depending on compound protractor orientation) compound angle setting is the only one that makes the best rational sense because it enables the dial feed setting to be translated exactly into an actual feed that is only half of the dial setting
The Sine of 30° or Cosine of 60° is exactly .500, a very convenient number, so when you set the dial 0.010" feed you are actually just moving the cutter 0.005" actual feed to the work.
You can choose any larger angle than 30° for even lesser feed but you won't be able to come up with a convenient number that's easy to calculate.

Can anybody please shade some light on the history or mathematical explanation of why some choose 29° or 29.5° compound setting for thread cutting or how it came so casually practiced?

 

[12bolts]

Can anybody please shade some light on the history or mathematical explanation of why some choose 29° or 29.5° compound setting for thread cutting or how it came so casually practiced?

By running in at something "just less" than 30* the main cutting is done on the left edge of the tool, leaving the right edge to clean up the thread flank with a very fine cutting action. This helps maintain a smooth surface on the, usually, main bearing surface of the thread.

Cheers Phil

 

[rangerman]

By running in at something "just less" than 30* the main cutting is done on the left edge of the tool, leaving the right edge to clean up the thread flank with a very fine cutting action. This helps maintain a smooth surface on the, usually, main bearing surface of the thread.

Cheers Phil

If you have the compound set at 30° that means the tip of the cutting tool moves in at 30° and following perfectly right along the profile on the right side flank of the thread "inverted V". There would be no cutting because that side is just sliding or "skating" along and parallel to the surface and not digging in since it's also slanted at 30°.
Furthermore, since the profile of the cutting tool is ground with 60° V and square or perpendicular to the axis of the threaded rod then both sides of the cutting tool V should be in perfect fit with the inverted V of the thread.
There couldn't be any more or less cutting else a proper thread inverted V wouldn't be created with a 60° included angle to match the cutting tool V.
After all, the thread V is determined by the cutting tool else there's no reason why we have to labor on grinding the tool with a perfect 60° V ( for Imperial and ISO threads anyway).

It's either 30° or straight in at 90°, but 30° has an advantage of not creating too much cutting stress since only one side cuts as opposed to both sides for 90° .
I have yet to find a book that explains the 28°, 29° or 29 1/2° compound setting with good mathematical logic.

Until it's satisfactorily explained I would just consider it a machinist myth.

 

[12bolts]

You ask the question and then debunk the explanation with nothing but gibberish. Slow down and think again about the angle of approach given by using 29.5 * (or something just less than 30*)

Until it's satisfactorily explained I would just consider it a machinist myth.

So youre saying that pretty much ever since screw cutting lathes were invented, that the methods taught, and the educational literature printed for single point threading on a lathe is all complete guff?

Cheers Phil

 

[rangerman]

You ask the question and then debunk the explanation with nothing but gibberish. Slow down and think again about the angle of approach given by using 29.5 * (or something just less than 30*)

What is gibberish about what I have said?
To help you understand or make it even simpler, how are you going to maintain the cutting tool to cut only at the tip and not any part of the right side edge ( as you have previously claimed the reason for having a less-than-30 compound angle of approach) if the angle of tip movement is less than the angle of the side of the cutting tool V?
A 30° angled cutting tool edge and tip moving at an angle less than or more than 30° could not possibly be moving in parallel or in the same direction as the surface or profile of the inverted V that is supposed to be 30°. Likewise I would suggest to give it more thinking also.

So youre saying that pretty much ever since screw cutting lathes were invented, that the methods taught, and the educational literature printed for single point threading on a lathe is all complete guff?

Not everyone teaches or supports the less-than-30° compound angle setting. In fact, many books do not even mention it. In my opinion, the reason that old meaningless practice persist is that it does not really create much of a drastic or perceivable difference to the final thread cut profile.
All the previous small discrepancies created by the wrong less-than-30° angle setting would just be wiped out by the next cutting step due to the fact that the cutting tool is actually a form tool ground to a 60° included angle ( symmetrical and 30° on each side). That 60° profile would just clean up any slight discrepancy created by the previous cut.
So, my point is, why not just use 30° and do it correct the first time?

I really would like anyone, including you, to offer an intelligent explanation that would support this questionable old traditional practice of setting the compound less than 30° and still justify the reason or claim that it makes the tool cut only at the tip and the left side and none on the right side edge of the cutting tool
The reasons I've heard or read so far just don't support it logically in closer scrutiny using mathematical/geometrical analysis.

 

[SE18]

that stop you have is amazing and makes for quick, safe work!

I've cut just a few threads and had a nut handy and when the nut fit I knew I'd cut deep enough. Also, I only used the crossslide for feed. Now I know the right way! I just have to write down those formulas you gave of when to stop feeding with the compound and begin with the cross slide or find them in my machinery handbook (I doubt they're there)

BTW, I may have missed it but I didn't see you centering the tool bit on the work. How is that done? I may be doing it all wrong but I try and get dead center putting a credit card between the tool and the work and when the card seems to be sticking straight up I know it's centered.

I'll be looking forward to inside threading video

 

[eightball]

If you feed in at 30 or 31 degrees and looked at the thread it would look like a stair step. Like Tom says, Feeding straight in towards the end cleans up both sides of the thread so it dosent matter. It takes the form of the tool. the back side of the thread is where you get your engagement. If you feed in at 29 or 29.5 The leading edge of your tool does most of the work and you take a light cleanup pass on the back of the threads. I have cut threads and never look at what the compond is set on. A lot of componds are off a little bit anyways and you can check that with a little trig. I personally feed in on 29.5 and get really close using the formula .750/number of threads. Then i always feed in on the crossslide taking spring passes and checking with thread wires, or fitting to a nut. I file my threads toward the end and mic over them to make sure there are no burs left on them. When i use wires on something this small, I usally stick them in a piece of dum dum and hold the front with one hand and mic with the other. On bigger stuff i use rubber bands or o rings to hold the wires so it frees up both hands. when you use. rubbers to hold the wires you dont want them to bend the wires, just hold them snugly so you can measure. I've never owned a set of thread mics, I use wires exclusively.