Internal Threading

I found this diagram:

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It confirms the compound orientation and the angles.
Of course it is completely wrong because the tool orientation has not been adjusted to be square to the work!
Robert
 
I think I am wrong about the Trigonometry. Cos 30 is .866 (not 1/2). So the depth of cut should be compound travel x .866.
I like using 24 TPI since the depth of cut is very close to .024" and that is almost exactly 1/4 of .1" (one handwheel turn on cross slide). Easy to remember without consulting a chart.
Robert
 
It really does not matter if the tools come to a fine point so far as cutting the threads, the national thread forms allow a flat on the crest and root of threads, and the flat, to specification makes the tool stronger and longer wearing. I use Aloris HSS threading tools, and come in different sizes for fane and coarse threads; for the finest, I just stone a tiny flat on the point, for the larger ones, I use a larger flat. Brown & Sharpe made a circular thread tool with vee slots around its circumfrence marked with TPI with the appropriate flats in the root of each thread pitch so that one could, after sharpening the tool to a sharp point, the tool point could be dubbed off to the appropriate width of flat.
 
Personally, I do not use compound depth in threading, one, because I use tools with a flat at the point, and also because I fit to a gage, or I use a thread mike for fit, which, I think makes for the best possible fit. On the opposite of accurate, years ago, I threaded a lot of square shafts for tillage discs, it was 1 1/8 – 7, and the threads were cut in just five passes; an initial nearly half depth cut was taken, then with the tool set at the same depth, the number on the dial was skipped and the half nuts closed on a line, which took its cut behind the first, the subsequent cuts were taken on a numbered line so that they were only cutting at half the apparent depth you would normally see with the conventional method; I was told that this method was done in the oilfields back in the day. It may not be pretty, but it was quick, and oh! The ODs were turned round and undersize to minimize burrs.
 
In my very limited experience with internal threading I have never been able to have the compound angled towards the headstock due to interference with the chuck. Also the tool bit has to stick out of the holder by at least double the distance. So I have had to use the same set up as for external threading. I can't tell any difference in the threads. I have often thought about rotating the tool bit 180 degrees and cutting on the far side of the hole. Are there any problems with doing this?
 
I feel like using the compound is more important on a light lathe because it reduces the cutting force on one side of the tool. I just like to know the thread depth to get in the ballpark.
I'm gonna try the back side of the compound next time. I will report back.
Great-now I need to buy a thread mic...
Robert
 
mickri said:
In my very limited experience with internal threading I have never been able to have the compound angled towards the headstock due to interference with the chuck. Also the tool bit has to stick out of the holder by at least double the distance. So I have had to use the same set up as for external threading. I can't tell any difference in the threads. I have often thought about rotating the tool bit 180 degrees and cutting on the far side of the hole. Are there any problems with doing this?
Click to expand...
Cutting on the backside is a good method, especially if you are working in a blind hole, the tool cutting face is upward and you start in a recess in the bottom of the hole and feed outwards wit reverse rotation; if you have the tool facing down, there is the tendency for the tool to lift in the cut, especially in light machines, another thing is that if cutting with the tool facing up in reverse rotation with a threaded on chuck, it can loosen and start coming off the spindle.
rwm said:
I feel like using the compound is more important on a light lathe because it reduces the cutting force on one side of the tool. I just like to know the thread depth to get in the ballpark.
I'm gonna try the back side of the compound next time. I will report back.
Great-now I need to buy a thread mic...
Robert
Click to expand...
Actually several thread mikes, in zero to one inch, there are three to cover the range of pitches encountered.
 
Sigh...I'll probably just go back to tap and die....
R
 
rwm said:
Sigh...I'll probably just go back to tap and die....
R
Click to expand...
Don't let yourself be intimidated, its not all that hard to do; practice makes (almost) perfect. I see so many on this site acting like everything must be perfect! Put that aside, there is such a thing as "perfect enough" That is why on drawings of parts, there are tolerances.
 
I recently made a chuck keeper (wrong terminology) to prevent the chuck from unscrewing if I had to run the lathe in reverse. Have not tried it yet and am more than a little leery of testing to see if it works. My thought was to have the threading tool facing down. My threading bar does flex. I can see it flex if I take too big of a cut. I limit my threading cuts to .005.


IMG_3943.JPG

I'll 2nd not trying for perfection. I always strive to do absolute best that I can do, not what someone else can do and accept whatever I achieve. Seems to work for me.
 
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