Does any one know anything about multiple lead threads?

Actually, the easiest method IS to turn the work 120 degrees. I've done two-starts by marking the spindle gear, then disconnecting the gears, flipping the spindle and reconnecting. Worked fine. I've seen driver plates with multiple tapped holes so a work piece could be threaded between centers, then advanced to the next position to chase the next thread. Your thread settings for a three-start should be OK, you want a lead 3 times as fast as the apparent lead. So 6 tpi should be right for an apparent 18 tpi lead. Depth of cut depends on what thread form you want, a 60 degree sharp root and crest will be different from a Unified form. And those will be totally different from square or Acme. Probably the "root" of your troubles. The mating female thread is handy to have, too, you can figure out how deep you need to cut the thing to clear the threads on it.

Stan

But why would you _not_ want to do it the easy way, which is to either re-index the work 120 degrees per start, or to use an indexed bull gear or back plate to slip the change gears by the requisite amount?

Those really are the easiest ways to do it, and get accurate results.

As soon as you get past that, then you can use your cross-slide and compound for advancing the tool the way it was meant to be advanced. A smidge of trig will tell you how far to advance the 29-degree compound to get the full root depth. Tables will tell you how far that is.

LLoyd

On a triple lead thread you will need more clearance on the side of the threading tool because of the 6 tpi helix angle. The helix angle is the angle the thread makes in relation to the cross section plane of the shaft you are cutting. If you dont have enough the tool will rub on the side and push, giving you a poor thread. Just grind away some of the leading side of the threading tool until you can see it isn't rubbing.

John

snipped-for-privacy@gmail.com wrote: >I calculated my depth of cut for 18 TPI and got .041.

Does you cross slide measure diameter or radius change? ...lew...

Depending on the specific TPI and leads you're going for, and your particular lathe, it may be possible without much effort at all. For example, I do 4 lead 36 TPI threads. I set the gearing for 9 TPI and it just happens to work out that each lead is cut from each of the four mark on the threading dial. I was lucky in that it just works out well.

So if it does not work so well, there is another very easy method (with the compound at 30 degrees) involving just a touch of trig. When you cut the first lead, you set the compound and cross slide both to 0 where the tool just touches the work. After cutting a lead to full depth, bring your compound and cross slide back to the 0 points. Then adjust the 0 points for the compound and cross slide both by the amounts defined by the sides of a 30-60-90 triangle. In your case, with 18 TPI, you want to move the starting point 1/18 or .0555" laterally. So, just move the compound out by twice that (.1111") and the cross slide in by .0962 (.0555 * sqrt(3)) and reset that to your 0 points. Or, move the cross slide out by .1111" and the compound in .0962"; either way gives the same result. Cut second lead and repeat once more for the third lead.

- Bruno

Or, move the compound in by .1111 and the cross slide out by .0962,

Oops, forgot to proof read. Corrected above.

snipped-for-privacy@gmail.com wrote in news:1180973925.828954.126250 @q75g2000hsh.googlegroups.com:

There is nothing wrong with your method. The calculated depth seems deep, but you say it is shallow, so are you sure you moved .041" radially? Or diametrically? IOW, does your cross slide dial read the actual amount moved or half that amount?

Another problem could be that the helix angle could be quite large depending on the diameter involved. So you need to calculate the helix angle and make sure that your tool has that much side clearance on it. On very coarse threads on small diameters, it's not at all unusual to have to tilt the tool in order to have a strong enough edge AND the proper clearance.

Also, not knowing your lathe, I would ask you to check the thread pitch. It could be the gear combo isn't right and you are cutting at a faster lead than 6TPI.

D Murphy wrote in news:Xns9945EC1133771BW12BU20MU38SY@130.133.1.4:

I should clarify that I'm coming up with .0343" radial depth. .0686" in diameter difference.

My cross slide is a 1 to 1 ratio so it removes the amount dialed off of the diameter of the work. The thread depth 0f .041 I calculated by using .75/18. I'm using a south bend that was specifically designed for a training environment. I checked the finished 18 tpi with a thread gage and it fits.I had to change the gears to acheive the 6 TPI so I am sure the machine is geared right, but I should have checked the first lead I cut to ensure it was cutting 6 TPI.

If this is an external thread, the compound is set to 30 degrees, and you are using a sharp V form tool, the depth of cut measured on the compound should be 7/8 of the pitch, or .0486. For an internal thread, the depth of cut should be 3/4 of the pitch. This is for a standard

1/8 flat on external threads and 1/4 flat on internal threads.

With the compound parallel to the work and all the advancement on the cross slide, then the depth of cut should be .0421.

-Bruno

These are external threads, I'm using a 60 degree V form tool and I cut them to .041 and the just looked really flat on top. I have a question about your explaination of the 30 degree method and the

30-60-90 triangle. How do you deal with the backlash of the compound when moving it out away from the work and also do you then use your compound to dial in the depth of cut for that lead and zero it and then move it back out and the cross slide in? Basically I'm asking what you use for depth of cut.