Math help needed for threading...

benmychree said:
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.
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Relying on slop/play in a machine and varying hand pressure to finish up a cut in my opinion is asking for uncertainties in accuracy.
 
We are not always looking for extreme accuracy; read the last sentence of my post. I have been at my trade since I was 19 years old, I am now 73, and have done it my way--- I would not do the drag on the handwheel in making a thread plug gage, but would likely feed in with the crossfeed for the last few thousandths until I reach the desired pitch diameter. Doing it my way makes most threads "perfect enough"
 
benmychree said:
We are not always looking for extreme accuracy; read the last sentence of my post. I have been at my trade since I was 19 years old, I am now 73, and have done it my way--- I would not do the drag on the handwheel in making a thread plug gage, but would likely feed in with the crossfeed for the last few thousandths until I reach the desired pitch diameter. Doing it my way makes most threads "perfect enough"
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Good for you and I can tell you are set in your ways, please continue. So will I. I can cut a finished thread in about 5 passes. And use the graduations on the dial for reference and accuracy with no guessing on backlash or needed hand pressure. I will have both hands free to do other things. And the bottom of my “V” cuts will not have a wider path because I did not move the carriage to cut just one side on the last cut.
 
chips&more said:
Relying on slop/play in a machine and varying hand pressure to finish up a cut in my opinion is asking for uncertainties in accuracy.
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For best-possible accuracy, most very highest-end English toolroom lathes (Holbrooks, DSG toolroom models and I think CVA (the "English 10EE")) have a plunger or lever to disengage the carriage handwheel or even carriage rack and pinion for this exact reason, to prevent "operator input" - their leadscrews can be trusted, particularly if you're advancing the tool with the topslide at half-angle-minus-a-half as the load will be on the leading edge of the tool and take up the (already minimal with toolroom lathes) backlash in the threading train.
Holbrook sent their leadscrews to be measured at the NPL for accuracy though, and supplied lathes with charts in millionth-inches of leadscrew pitch inaccuracy, not something likely on hobby lathes?

Dave H. (the other one)
 
Tozguy said:
I agree with post no. 3. It is understood that the compound is set at 29.5 deg from perpendicular to the spindle axis.
Also, because the tops of the finished threads should be rounded or flat, starting diameter of the work might be less than the nominal diameter (less than 1/4'' in this case). So just a caution that calculations might not give you the exact amount of compound travel that is needed to get a good thread fit. I believe in sneaking up on the final dimensions by trial fits.
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Especially so if you use a flat on the point of the tool in cutting US standard threads; using a sharp vee tool, especially for coarser threads, the compound travel thing won't work. Using a small flat on the tool will increase tool life, as the tool will not have so much tendency to burn off at the point; I have been at this trade since I was 19 yrs old, now going on 74, and I favor the "seat of the pants" approach to threading on the lathe, formulas do not necessarily lead to accurate threads. To achieve accuracy, I use a thread micrometer. So far as exact ODs are concerned, for most work, an undersize OD makes the job easier as you don't have to worry so much about burrs on the crest of the threads; if you have exact ODs you invariably have to file them off to remove burrs, which undersizes the OD anyway.
 
The Cos of 29.5 degrees is .870
Thinking of it as a ratio may help. If you infeed a tool at 29.5 degrees from the compound the actual change is .87 of the dial reading or .00087" per .001" of actual infeed. In order to increase the cut depth .01" a compound move of .01 / .870 will be required or .0115" on the compound.
 
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I agree with all the above.

You know, it might not be a bad idea to get a machine shop practice book and read about threading on the lathe. One that has some of the math in it. That way you have a consistent source and can try actually cutting threads their way and see how it works out. Trig is great for 100% threads, but gets confusing when you do less than that. And don't forget about helix angle. For books, just stay away from the Brit stuff, they don't speak English very clearly.

There are plenty of less formal and 'seat of the pants' methods, but maybe learn the conventional methods first and then vary off of that? Dials probably have engraved increments for a reason, maybe use them, at first anyway.

While I'm at it, thread micrometers for itty bitty threads are a pain, as are the three wires. A nut is your friend.

Just my $0.02 (keyboards no longer have a 'cents' key)
 
OT, but what the hey ...
On a Mac, ¢ = OPT-4
Easy way to remember ... SHIFT-4 = $ (another currency symbol)
 
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