Threading and the compound angle.

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SmithDoor

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I posting because a lot confusing information.

The photos are on a 9A lathe. One lathe is a barn name and one is clone of same lathe.

Which do you like

Dave

Screenshot_20240125-185111_Drive.jpg
Screenshot_20240125-185558_Drive.jpg
Screenshot_20240125-165224_Drive.jpg
 
I leave my compound parallel to the bed when threading.
 
I leave mine parallel too but only because my ML7 can't swing round that far. It does mean a heavier cut but I can live with that.
Rich
 
I have threaded both ways, at 29 degrees and perpendicular and the final results can be similar. For the most part I leave my compound set at 29 degrees and then adjust any angles of cut not involving threading by pivoting my QCTC. I just find it easier to be more precise when cutting threads with the compound set at 29 degrees. Like many operations, various users will have different methods of accomplishing the same result, none of which are necessarily wrong.
 
I prefer the angle method, but if for some reason I need to use the parallel method, I advance the top slide by half of the radial cut depth.
This is equivalent to setting the topslide at 26.5° so it is close for 55° threads, and helps with 60° ones.
 
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I have always fed the compound @ 29 degrees. That way you are only cutting on the leading edge.
Also your cross slide goes back to zero every time if you feed with the compound.
But I have also seen machinist just plunge cut with no top rake and still make excellent threads.
 
I used to cut at 29 degrees but when I got a QCTP, I eliminated the compound and replaced it with a solid riser block for the QCTP. I honestly don't miss the compound and just plunge the threads straight in. The carbide threading inserts and carbide threading bars seem to cut fine plunging in, so long as I respect the limitations. Most of the threads I cut are 24 pitch or finer, so I don't challenge the tooling or workpiece at all. And always plenty of oil.
Lloyd

P.S. I think the first paragraph in the first picture in the first post says it all, "maximum production."
 
Another advantage of having your compound at 29 deg. is you feed the compound until you reach 75% of the pitch of whatever thread you are cutting. (60 deg threads only) Your cross slide will always be returned to zero. A lot of older lathes you can set the cross slide to mechanically stop at zero.
If you don't have a way of measuring the pitch this method will get you very close.
And, you won't have to keep track of the cross slide like you do when plunge cutting.
 
I used to cut at 29 degrees but when I got a QCTP, I eliminated the compound and replaced it with a solid riser block for the QCTP. I honestly don't miss the compound and just plunge the threads straight in. The carbide threading inserts and carbide threading bars seem to cut fine plunging in, so long as I respect the limitations. Most of the threads I cut are 24 pitch or finer, so I don't challenge the tooling or workpiece at all. And always plenty of oil.
Lloyd

P.S. I think the first paragraph in the first picture in the first post says it all, "maximum production."
I have use both way in past.
I have not found any difference.

I had read a article from before WW1 where on light duty lathe the 29½° would help the threading on the lathe.

Also shrapping you a high rake angle on one side for some types of materials the 29° or 29½ would be used better. The high rake angle helps light duty lathes too.

Dave
 
Another advantage of having your compound at 29 deg. is you feed the compound until you reach 75% of the pitch of whatever thread you are cutting. (60 deg threads only) Your cross slide will always be returned to zero. A lot of older lathes you can set the cross slide to mechanically stop at zero.
If you don't have a way of measuring the pitch this method will get you very close.
And, you won't have to keep track of the cross slide like you do when plunge cutting.

is it possible you have mis-understood the geometry of traditional threads ?
changing the depth of cut alters the pitch-circle-diameter of the threads,
if you don't go deep enough the work will not fit into a tapped hole,
if you go too deep the work will be too loose in a tapped hole,
and IIUC what is meant by 75% threads is what size *tap drill* is used
before tapping the hole, so IIUC if you used a 100% (smaller) *tap drill*
before tapping but you still wanted only 75% thread engagement you'd
have to start with undersize bar stock before cutting external threads into it ?
IMHO, YMMV, VWPBL, yada, yada, yada...
 
IMHO and experience, the only way to get smooth finish threads when single-point cutting on a lathe is to start with leaded-steel or leaded-brass, in my experience everything else leads to unacceptable crap (and either way 30-deg, 29.5-deg, and 29-deg doesn't seem to matter, I stick with 90-deg plunge or 30-deg side face for simplicity), hence for other materials I'm forced to use a die, perhaps I'm doing something wrong ?
 
is it possible you have mis-understood the geometry of traditional threads ?
changing the depth of cut alters the pitch-circle-diameter of the threads,
if you don't go deep enough the work will not fit into a tapped hole,
if you go too deep the work will be too loose in a tapped hole,
and IIUC what is meant by 75% threads is what size *tap drill* is used
before tapping the hole, so IIUC if you used a 100% (smaller) *tap drill*
before tapping but you still wanted only 75% thread engagement you'd
have to start with undersize bar stock before cutting external threads into it ?
IMHO, YMMV, VWPBL, yada, yada, yada...
Drill size is the thread major diameter minus the pitch. The attached chart shows drill size for a 5/8-11 at 75% will be 17/32 (.531)

11 threads per inch has a pitch .091 (1 divided by 11)
.625 minus a pitch of .091 is .534 Chart says drill size is .531(very close).

So if your are at "home" and don't have a thread mic or a set of PD pins to measure the actual pitch diameter you take the thread nominal OD (.625) and subtract the pitch. This will determine the drill size.

If you set your compound to 29 degrees you will feed in 75% of the pitch (nothing to do with a 75% thread.)
Pitch of 11 is .091 75% of .091 is .068. .068 is how far the compound moves to finish size. Cross slide always returns to zero.

The proper way cut a thread is measuring the actual pitch diameter.The Machinist Handbook will give you the actual pitch diameter of several class of threads.
 

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Drill size is the thread major diameter minus the pitch. The attached chart shows drill size for a 5/8-11 at 75% will be 17/32 (.531)

11 threads per inch has a pitch .091 (1 divided by 11)
.625 minus a pitch of .091 is .534 Chart says drill size is .531(very close).

So if your are at "home" and don't have a thread mic or a set of PD pins to measure the actual pitch diameter you take the thread nominal OD (.625) and subtract the pitch. This will determine the drill size.

If you set your compound to 29 degrees you will feed in 75% of the pitch (nothing to do with a 75% thread.)
Pitch of 11 is .091 75% of .091 is .068. .068 is how far the compound moves to finish size. Cross slide always returns to zero.

The proper way cut a thread is measuring the actual pitch diameter.The Machinist Handbook will give you the actual pitch diameter of several class of threads.


cosine (30-deg) == 0.866, sine(30-deg) == 0.500, neither is 0.750, it is the sine or cosine that determines the depth of cut based on moving a compound slide at an angle.
Also a 60-deg triangle has all equal sides, so the pitch of the thread and the depth along the direction of the compound cross-slide are necessarily equal by geometry, there's no 75% here.
I think you are confused and are going by some "rule-of-thumb" that isn't correct or accurate, did you actually learn geometry and trigonometry in school ???, do you care to re-think this ???
 
Got my journeyman's card in 1980 after serving an apprenticeship of 4 years, and 2 years machine shop school as an aircraft machinist. I retired last year after 40 years repairing industrial valves. Have cut thousands of threads over the years.
I am not rethinking ****, chuck up a part and cut a thread using my method, then you can challenge me. I have never needed basic trig to cut a thread.
I use the machinist handbook. PD pins/triangles don't require trig, neither does a thread mic or go no gauges for internal threads.
Your math doesn't allow for crests and roots. Threads have a crest.
Also are you assuming the O.D of a 5/8 thread is 5/8? Wrong!
Machinist handbook says a 5/8-11 UNC Class 2A thread has a major diameter of .6234" to .6113". I always cut the major diameter .010" smaller.
Take your junior high school math and bother someone else.

"yada, yada, yada" (your quote)
 
Got my journeyman's card in 1980 after serving an apprenticeship of 4 years, and 2 years machine shop school as an aircraft machinist. I retired last year after 40 years repairing industrial valves. Have cut thousands of threads over the years.
I am not rethinking ****, chuck up a part and cut a thread using my method, then you can challenge me. I have never needed basic trig to cut a thread.
I use the machinist handbook. PD pins/triangles don't require trig, neither does a thread mic or go no gauges for internal threads.
Your math doesn't allow for crests and roots. Threads have a crest.
Also are you assuming the O.D of a 5/8 thread is 5/8? Wrong!
Machinist handbook says a 5/8-11 UNC Class 2A thread has a major diameter of .6234" to .6113". I always cut the major diameter .010" smaller.
Take your junior high school math and bother someone else.

"yada, yada, yada" (your quote)


From Machinery's Handbook, table of dimensions for National Unified Threads
10-TPI, pitch 0.100", depth of external thread Sharp Vee 0.0866,
depth of external thread National Unified 0.06134

so the pitch is 0.1000", if your cross slide is at 30-deg and you feed in 0.075"
on the cross slide you'll get a final depth of 0.065", not equal to either the
Sharp-Vee or the National-Unified depth (and 29-degree goes even deeper
and more off)

so I'm confused, it still looks to me like you're using a "rule-of-thumb" that is
close but not exact ? and on top of that, all of this depends on the shape of
the tip of your thread cutting tool, you didn't mention that in your original post,
what shape tip are you using ?, what shape tool are you recommending ?
Curious minds want to know !
 
From Machinery's Handbook, table of dimensions for National Unified Threads
10-TPI, pitch 0.100", depth of external thread Sharp Vee 0.0866,
depth of external thread National Unified 0.06134

so the pitch is 0.1000", if your cross slide is at 30-deg and you feed in 0.075"
on the cross slide you'll get a final depth of 0.065", not equal to either the
Sharp-Vee or the National-Unified depth (and 29-degree goes even deeper
and more off)

so I'm confused, it still looks to me like you're using a "rule-of-thumb" that is
close but not exact ? and on top of that, all of this depends on the shape of
the tip of your thread cutting tool, you didn't mention that in your original post,
what shape tip are you using ?, what shape tool are you recommending ?
Curious minds want to know !
What do you not understand about edit someone else?
I am tired of reading your edit.
"Edit
 
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Got my journeyman's card in 1980 after serving an apprenticeship of 4 years, and 2 years machine shop school as an aircraft machinist. I retired last year after 40 years repairing industrial valves. Have cut thousands of threads over the years.
I am not rethinking ****, chuck up a part and cut a thread using my method, then you can challenge me. I have never needed basic trig to cut a thread.
I use the machinist handbook. PD pins/triangles don't require trig, neither does a thread mic or go no gauges for internal threads.
Your math doesn't allow for crests and roots. Threads have a crest.
Also are you assuming the O.D of a 5/8 thread is 5/8? Wrong!
Machinist handbook says a 5/8-11 UNC Class 2A thread has a major diameter of .6234" to .6113". I always cut the major diameter .010" smaller.
Take your junior high school math and bother someone else.

"yada, yada, yada" (your quote)
I do agree with cutting 0.01" smaller
I only cut threads in a lathe over 3/4". The rest I like using dies.

Dave
 

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