Math help needed for threading...

[jcp]

Put your compound mid stroke and zero it. Advance the cross slide and touch the tool to the work.
Move your carriage off the end of the work. Advance the cross slide the depth of thread and lock it in that position.
Now back off the compound move over and touch it to the work and adjust for your first depth of cut and continue cutting till you
reach the previously set zero. you will then be at the depth of cut you set initially.
I usually set zero on the cross slide also and use it to back off for clearance when traversing after a cut.
No trig involved.
.

This is the method I've used for several decades.

 

[AJB]

A thread dimensions calculator can be found at: http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx

It doesn't speak to depth of compound feed, but it does provide some information without having to do the math. Just passing it along as semi-related.

 

[chips&more]

I don’t need math when threading. I don’t need the compound set at any concerned angle. Because I don’t feed with the compound. I feed in with the cross slide only. Been doing it this way most of my life and would not do it any other way…Dave.

 

[benmychree]

I don’t need math when threading. I don’t need the compound set at any concerned angle. Because I don’t feed with the compound. I feed in with the cross slide only. Been doing it this way most of my life and would not do it any other way…Dave.

In my experience, that method can create poor finishes due to the chips crowding up in the middle of the cut, the chips do not flow off the tool smoothly that way, and chatter is likely to occour , especially at coarser pitches. Also one needs to keep track of each cut when feeding the cross feed in, and subsequent cuts change the position of the cross feed handle making it more clumsy to pull out at the end of each cut; I like to park the handle at about the 10:00 position and set the dial to zero, for me this position makes it less clumsy to back the tool out, whether or not you are cutting into a recess or pulling out of the cut without recess.

 

[AJB]

MACHDADDY,

Please check your math again. I don't think .054 is correct for thread depth for 24 TPI. That might be the double depth.

If you think of a thread as a 60/60/60 triangle, all the sides are equal in length. Therefore, if there were no root or crest flats, the depth of the compound feed (set at 30 degrees) would be equal to the pitch. For 24 TPI, it would be .04166 (1/24 = .04166) and coumpound feed would also be .04166. With root and crest flats, some amount must be subtracted (roughly equal to the width of each added together).

This is my rough math to put you in the ballpark for compound feed, however someone with more experience than me may want to jump in...

 

[Machdaddy]

Thank you for all your responses! Catsparadise gets the award for best answer. I just wanted the formula to get real close and I can't test in the intended nut without altering my gripping points
Boy you folks know how to get sidetracked!
Interesting discussion though!

 

[Machdaddy]

Posted at the same time as AJB. Now I'll have to recheck my #"s...
You're right, I had double depth!

 

[Holescreek]

I've threaded every which way and like most use the method that works best for me. That said, I have never ever threaded to a specific depth based on a table.

I quit threading when the pitch diameter of the thread is in spec using either thread wires or pitch mics.

 

[chips&more]

In my experience, that method can create poor finishes due to the chips crowding up in the middle of the cut, the chips do not flow off the tool smoothly that way, and chatter is likely to occour , especially at coarser pitches. Also one needs to keep track of each cut when feeding the cross feed in, and subsequent cuts change the position of the cross feed handle making it more clumsy to pull out at the end of each cut; I like to park the handle at about the 10:00 position and set the dial to zero, for me this position makes it less clumsy to back the tool out, whether or not you are cutting into a recess or pulling out of the cut without recess.

John, to each his own. I’m glad you are happy with setting the compound @ 30°. To me that setting/method does not take a cleaning cut at all on the one side. I have absolutely no problem with chatter or any other cut problems even with a coarse pitch, doing just the cross. You also have even cutting/tool pressure on both sides of the cutter. Carbide is happier with that set-up. I also like how the center of the “V” cut stays on point and does not travel to the left or right as would happen if you fed the compound. I can then put a full radius stop very easily at the end of thread cut, instead of seeing a wedge pointed end cut. I drill a 60° pointed drill at the end of the first cut. And every subsequent cut also ends in that 60° hole leaving a nice rounded end on the thread cut. Also just using the cross is much faster for me…Cheers, Dave

 

[benmychree]

John, to each his own. I’m glad you are happy with setting the compound @ 30°. To me that setting/method does not take a cleaning cut at all on the one side. I have absolutely no problem with chatter or any other cut problems even with a coarse pitch, doing just the cross. You also have even cutting/tool pressure on both sides of the cutter. Carbide is happier with that set-up. I also like how the center of the “V” cut stays on point and does not travel to the left or right as would happen if you fed the compound. I can then put a full radius stop very easily at the end of thread cut, instead of seeing a wedge pointed end cut. I drill a 60° pointed drill at the end of the first cut. And every subsequent cut also ends in that 60° hole leaving a nice rounded end on the thread cut. Also just using the cross is much faster for me…Cheers, Dave

So far as cleaning up the backside of the thread is concerned, I was taught to take a slight drag on the carriage handwheel when taking that last cut; there is enough spring to take a tiny cleanup chip on the backside. My teacher in high school and Junior college was a Mare Island Navy Yard apprentice in the late 1930s and taught in the apprentice school there during WW-2 and was a great teacher; I did not truly appreciate his talent until I experienced others; he required everyone's full attention and there was no horseplay in his classes, and any unusual sound coming from a student's work had his full attention immediately! All the journeymen at my apprenticeship shop at Kaiser Steel in Napa Ca. threaded this same way; perhaps we should say that there is more than one way to skin a cat, you can do it your way, and I, my way.