Couple threading questions

[Richrd]

I read about using the compound gauge when threading. What is the purpose and why 29.5 degrees.

Also, when do you cut internal threads instead of using a tap?

Thanks

Rich

 

[3strucking]

30 degrees is half of 60 degree thread form. I internal thread down to .375" anything smaller I usually tap.

 

[Chipper5783]

The 29.5 is so that most of the chip is on just the leading cutting edge, the trailing side give a little polish (only getting 1/2 a degree of advance). The straight in approach also works fine - perhaps a little harder on the tool bit? I've threaded both ways - I prefer with the compound just a bit below 30° (I don't sweat the 29.5° thing - the actual number does not really matter, so long as it is steeper than 30°, I'm cutting to a thread mike value, so the advance number is just a guideline.

For internal threads that are not standard, or you don't have a tap - then cut 'em single point. If I need to cut internal threads and I happen to have that tap (and I have a pretty good selection) - then I use the tap because it is quick and easy.

 

[Bob Korves]

If anybody is using their lathe's factory cross slide quadrant to set their compound for threading and think they can set up their threading tool by half (or less!) degree increments and have them be anywhere near reality, please send me a PM, I have this bridge I want to sell you...

"IF" I wanted to achieve something like the fabled 29.5 degree cut for threading, and had to set the compound angle using the factory quadrant, I would use a number closer to 27 or 28 degrees to make damn sure the right side of the cutter was cleaning up that side of the cut with every pass. It is way better to have too little compound angle than a little too much. Or, I would simply dial in the cross slide for the last light passes, as noted above, to clean up the right side of the cut.

If you have actually tested and qualified your cross slide quadrant and know for sure what the real numbers of the indicated angles are, you are a more anal machinist than even I am...

 

[mikey]

It's funny that we worry so much about 29.5 degrees on the compound but just eyeball the fit of the threading tool in a fishtail gauge that is moving around on a round work piece.

Oddly enough, I actually measured the accuracy of my cross slide index marks with a Starrett vernier protractor to see if 29.5 degrees was actually 29.5 degrees, and it is. Sooo ... I are anal, Bob!

I also grind my threading tools so that the left 30 degree angle is accurate relative to the side of my threading tool so I can just slip an accurately faced cylindrical thing between the side of the tool and the face of the chuck. I do this in self defense; as I've gotten older, my vision is not as good as it used to be and I can't be all that sure I got the tip of the tool centered in the fishtail notch. You younger guys are probably okay, though.

 

[British Steel]

The engraved "protractor" scale on my Holbrook is more accurate than my eyes, although I've only checked it at a few points!
With a loupe screwed in it's within a couple of MOA when measured with a sine bar so I'm pretty confident 29.5 is +/- 0.05 degrees - if I really squint with the right glasses!

Dave H (the other one)

 

[RJSakowski]

The engraved "protractor" scale on my Holbrook is more accurate than my eyes, although I've only checked it at a few points!
With a loupe screwed in it's within a couple of MOA when measured with a sine bar so I'm pretty confident 29.5 is +/- 0.05 degrees - if I really squint with the right glasses!

Dave H (the other one)

Alas, my G0602 protractor isn't built that way. It's divisions are actually 2.5º apart so instead of splitting a space in half, I actually have to estimate 1/5th of a division. In addition, the protractor is buried under the the compound clamp so it takes a flashlight and some bending and squinting to even see the scale. Finally, I'm don't completely trust Chinese manufacturing to mark things correctly.

Fortunately, as Bob indicated, getting the angle dead on 29.5º isn't critical. The purpose of the less than 30º is to insure that the advancement of the compound is causing the back side of the cutter to act as a form tool cutting tiny saw teeth. Any angle less than 30º will work, as we see in people stating the 0º works just fine for them.. It just depends on how much you want to cut on the back side of the thread.

Unfortunately, I happen to be somewhat anal myself. Not being satisfied with the potential inaccuracies of using the machine's protractor, I made my own gage. It was made to an accurate 29.5º and setup is super easy. Loosen the compound clamp, rotate the compound to fit the gage, tighten the clamp. It can be done in the dark.
http://www.hobby-machinist.com/threads/29-5-deg-angle-tool-for-lathe.34541/#post-299346

 

[Bob Korves]

Oddly enough, I actually measured the accuracy of my cross slide index marks with a Starrett vernier protractor to see if 29.5 degrees was actually 29.5 degrees, and it is. Sooo ... I are anal, Bob!

There is a seven step program for that, Mike...

 

[Bob Korves]

I also grind my threading tools so that the left 30 degree angle is accurate relative to the side of my threading tool so I can just slip an accurately faced cylindrical thing between the side of the tool and the face of the chuck. I do this in self defense; as I've gotten older, my vision is not as good as it used to be and I can't be all that sure I got the tip of the tool centered in the fishtail notch. You younger guys are probably okay, though.

Excellent idea, Mike!

 

[Bob Korves]

Unfortunately, I happen to be somewhat anal myself. Not being satisfied with the potential inaccuracies of using the machine's protractor, I made my own gage.

I should have known that other machinists would be at least as anal as I am! As both you and Mike pointed out, a fixed compound setting gage for 60 degree threading is quick, easy to use, and you know where you are.