multi-start thread callout

[specfab]

I am going to need to supply a locking ring with some other parts that need to be built, and the locking ring currently has incomplete specs on the drawing for a multi-start thread. I need to get complete spec for the thread from my customer if possible; my guess is they will just want to send me a sample... In any case:

1. Multi-starts have a TPI callout typically, I think. Does the TPI spec for multi-start represent the thread count for all threads included, for example, a 4-start thread at 16 TPI is essentially a 16-pitch thread run at 4 TPI setting, then the 16 TPI spec adds all the separate threads in the pitch count?

2. Is a lead angle ever required for multi-start spec?

3. Seems like the easiest way to make this on a conventional lathe is using the same thread dial number/mark every time and indexing the compound at zero degrees by the TPI number. (?)

 

[chad883]

I have only ran these on a CNC.
1. Yes 4TPI would be the correct pitch for a 4 start 16 TPI

2. Not sure on this one.

3.On CNC I ran one thread complete, then started again at different Z start point, so that sounds about right.

 

[Dan from Oakland]

Normal practice for multi start threads is to disengage the spindle from the gear train and reengage it 90 or 180 degrees for a 2 start or 4 start thread respectively. Obviously, the number of spindle gear teeth need to be an even multiple of the number of starts. If you are between centers, another way is to use faceplate slots or chuck jaws for the indexing, but that's not going to work in your case.

Your method will work, but you loose the ability to feed in at 30 degrees, and its not as inherently accurate as indexing, but if its a 1 off, it probably does not matter.

No lead angle needed in spec- specify either # of starts or lead and TPI. Thread class gives the tolerance info, and for a external thread, a measurement over wires is always nice info.

 

[BSCustoms]

Multi start threads are usually designated as you stated. Say a Ø.750-16TPI 4 Start( or 4 Lead depending on the draftsman).

I have not seen the lead specified on a drawing.

The compound method is likely the easiest on a manual machine and most commonly done. Yes advance the pitch you are cutting divided by the number of starts. So for this example it would be .0625

 

[wheels17]

I think this is an easier method, which has worked for me. Quoting Lathefan from this site:

Threading dials are marked in half inch intervals because all even number threads will pick up at half inch intervals. This makes every other line(the numbered ones) a one inch interval and all odd number threads will pick up at one inch intervals. For multiple threads you will have to use a marking pen and mark engagement points between the existing lines. For two start thread with an even tpi lead the pick up point for the second thread will be exactly half way between the existing half inch interval marks. For a two start thread with with an odd tpi lead the first thread will start on a numbered one inch interval line and the second thread will pick up on the unmarked half inch interval lines. For four start threads with an even tpi lead you make three evenly spaced marks between each half inch interval mark, label the existing half inch interval marks "A" and the marks you drew in between with labels "B" "C" "D". The first thread will pickup on all lines marked "A", the second thread will pickup on all lines marked "B", the third thread picks up on "C", The fourth thread on "D". For four start threads with an odd tpi lead, draw a line between each existing half inch interval mark. Label the existing numbered lines "A", Label the next line that you drew "B", label the next existing un-numbered line "C" and label the next line that you drew in "D" The First thread will pick up on "A", the second thread on "B" the third thread on "C", and the fourth thread on "D",.The above information will work for an 8 tpi leadscrew or multiples there of (like the sixteen tpi leadscrew on 9" chinese lathes). It will work for two start threads on a lathe with a four tpi leadscrew, but not for a four start thread because of the lack of extra engagement points between the half inch interval marks.

The full thread is at How do you cut multiple start threads? . Well worth reading.

 

[AlfaGTA]

You guys are working too hard. Just need the right lathe.
On my manual "Dong Yang" ,Stupid name beautiful machine! (licensed copy of an Okuma) The spindle nose is marked around its circumference :0,2,3,4,5 and 6.
There is a reference mark on the front retaining ring for the spindle (fixed part on the lathe)
To cut a multi lead thread you rotate the spindle by hand to the "Zero" mark...disengage the tumbler gear (feed reverse) .
Rotate the spindle forward to the number for the starts you want, and reengage the tumbler gear..
Cut the next start as with the previous start....same settings cross and compound ....repeat for the required starts....works a treat.
Not sure why this is not a feature on all manual lathes....

Cheers Ross

 

[specfab]

Thanks to all of youse for generally confirming my "suspicions". As it turns out, the actual TPI info came my way during a telephone design review. It's a 3.75-12 4-start internal thread. I'm not sure my lathe has a 3 TPI gearing....In any case, thanks for the answers.