Double Lead 12-28 Thread Question

[Parlo]

Why not single point the thread? Left, right, multi-start, all doable on a basic threading lathe. To start 180 out of phase, just leave your half nuts engaged and move the tool over by 1/2 pitch distance for the second start measuring with the compound. No limits.

I think you need to move the compound a full pitch of the finished thread.

 

[Chewy]

Looks like full pitch is correct according to chart and videos. Also Tubalcaine #128 & #129 covers this and has a demonstration with a dial indicator.. Gotta love them rabbit holes!!

 

[RJSakowski]

The thread depth stays the same whatever the number of starts, just the lead angle differs.
Starting a standard tap or die 180 degrees out of phase will not produce a double start thread as the lead is wrong.

To attempt to cut a 12-28 double start thread, using a 14 tpi die,the lead angle will be correct but the geometry is wrong. Th drawing below illustrates the the problem. The first pass is depicted by the blue thread and the second pass, 180º out of phase, by the red thread. As can be seen, the moist of the thread cut by the first pass would be removed by the second pass, resulting in an unusable thread.

 

[savarin]

The Giant Binocular

Great to see you working on the Bino again Savarin !

www.hobby-machinist.com

see from message 248 how I cut my 4 start threads

 

[Parlo]

To attempt to cut a 12-28 double start thread, using a 14 tpi die,the lead angle will be correct but the geometry is wrong. Th drawing below illustrates the the problem. The first pass is depicted by the blue thread and the second pass, 180º out of phase, by the red thread. As can be seen, the moist of the thread cut by the first pass would be removed by the second pass, resulting in an unusable thread.View attachment 415275

The depth and width of thread doubles using a 14 tpi die, removing the material in between the threads required for the second start. The purpose of a multi start thread is to have the advantages of a longer lead without increasing the thread depth.

The diagram actually shows a 2 start thread of the correct pitch, lead and depth produced by the shaded area, ignoring the root radii, it is just too small on the diameter.
I didn't expect to see that, thanks for the drawing.

 

[RJSakowski]

The depth and width of thread doubles using a 14 tpi die, removing the material in between the threads required for the second start. The purpose of a multi start thread is to have the advantages of a longer lead without increasing the thread depth.

The diagram actually shows a 2 start thread of the correct pitch, lead and depth produced by the shaded area, ignoring the root radii, it is just too small on the diameter.
I didn't expect to see that, thanks for the drawing.

To further clarify the error of attempting to cut a 12-28 double start thread with a 12-14 tap, the area depicted by the fine cross hatch would be missing from the thread.

 

[Parlo]

To further clarify the error of attempting to cut a 12-28 double start thread with a 12-14 tap, the area depicted by the fine cross hatch would be missing from the thread.

View attachment 415347

Yes, it confirms how the thread diameter will be too small. Remove everthing except the coarse cross hatch and it shows a 2 start thread at 28 tpi.