Multi-start Threading?

Putch · Jul 11, 2012

I was just thinking about cutting a multi-start thread for the first time and was wondering if a single start nut would work on one? From what I understand, a multi start thread's purpose is for speed and they have greater cross-sectional strength than a normal thread. Can a single start nut start on each of the 4 leads on a 4-start thread? I guess I'll figure it out once I start cutting, but thought some of you guys would have some useful info on the topic. Thanks

locoguy · Jul 11, 2012

Never have I cut one, but a single thread nut will not work on a multiple start thread.

67Cuda · Jul 11, 2012

Putch said:
I was just thinking about cutting a multi-start thread for the first time and was wondering if a single start nut would work on one?

Hmm, no.

Sometimes machining requires visualization, just think about about what you posted.

Tom

willbird · Jul 11, 2012

You COULD make a single start nut that would go on say a 2 lead thread two different ways, but it would have a very funny looking thread sort of like this

/\__/\__/\

gorrilla · Jul 12, 2012

Putch said:
I was just thinking about cutting a multi-start thread for the first time and was wondering if a single start nut would work on one? From what I understand, a multi start thread's purpose is for speed and they have greater cross-sectional strength than a normal thread. Can a single start nut start on each of the 4 leads on a 4-start thread? I guess I'll figure it out once I start cutting, but thought some of you guys would have some useful info on the topic. Thanks

The short answer is no.
The long answer involves why "No". The multi-start thread is a faster pitch, as in the helix around the part is at a steeper angle than a single start thread. The "nut" on a multi start thread traverses the "bolt" at a faster rate, longitudenally, than the equivalent single start thread. Another way of putting it, if you count the threads on a multi-start thread, and then on a single start thread of the same threads per inch count, the multi start will thread into its nut by the multiple of the thread faster than the single part. An 8TPI double start will thread into it's nut twice as fast as a single start 8TPI. At first glance, they LOOK the same, but if you look at the helix, and the gap between threads of the same lead, they are drastically different.
I used to cut a 5 lead buttress thread in a down-hole oilfield tool. Watching that SL7 Mori Seiki run that explained it all better than a book full of words.

willbird · Jul 12, 2012

Gordon B. Clarke said:
I believe I've seen that done but can't remember where or why.

Pop bottle screw on caps ?? maybe not a multi lead, but a thread made in segments so the plastic injection die can make it ? Looks sort of like a "skip tooth" tap in some cases.

swarf_rat · Jul 12, 2012

It seems to me that you could make a multistart thread that would work with a standard nut. I'm not sure it has any utility, but just for the pedantic exercise: why couldn't you take a normal threaded stud, cut a new thread start halving the existing threads at the same pitch? The thread form wouldn't be as strong (half as high). But a standard nut would go on perfectly engaging the available thread depth, and further it would start on either of the two thread flights. If measured with a thread gage it would look like double the TPI, but the pitch would be the same. It's kind of like Willbird suggested, except changing the stud instead of the nut.

wlbrown · Jul 12, 2012

HELLO,
on multistart threads, the pitch is two time faster.this is on a
2 start thread.if the pitch is 11 tpi, then the gear box must be
set at 5 1/2 tpi. one some machines to cut the thread you proceed
just like normal, but the thread dial is the main function.one the first pass,
you close on number1, and on the second pass you close on number 3.this
is on a dial that has 4 numbers on it.this puts the second cut in the center
of the first cut. this may not work
on all machines. you have to try it to make sure it works. this works
very well on manarch 10EE machines.
good luck to all.
wlbrown

Toms Wheels · Jul 12, 2012

Wilbrown you are incorrect in your thread dial suggestion. There would be only a very small # of threads that could be split with the threading dial. If cutting a 16tpi double start thread, no matter where on the dial you engaged the half nuts you will end up cutting the same path each time. The cutting tool would need to be offset .0625" in the example given to cut the second thread.

No way would a single start nut thread on a double start thread. A double start 16, has the thread depth of a 16 pitch thread, and the advance of a 8 tpi thread. So the minor diameter of an 8 pitch nut is to small to fit the thread depth of a 16 pitch, and a 16 pitch nut has the wrong helix angle to fit the 8 tpi angle.

BR3 · Jul 12, 2012

As I think about it...you could cut one thread the same as one of the multi start threads and then continue to cut all other material that was not part of the single thread form away. The only practical reason I can see for doing this much work would be for a gage to check each start individually. That would only work after the mating part was finished though. Seems kind of pointless to me. Or you could use it as a teaching aid.

swarf_rat · Jul 12, 2012

Toms Wheels said:
No way would a single start nut thread on a double start thread. A double start 16, has the thread depth of a 16 pitch thread, and the advance of a 8 tpi thread. So the minor diameter of an 8 pitch nut is to small to fit the thread depth of a 16 pitch, and a 16 pitch nut has the wrong helix angle to fit the 8 tpi angle.

I'm not suggesting that a "standard" double start would fit a standard nut, just that one could be cut that would fit, and properly too, and would start on both flights - unless I am missing something fundamental. Might be hard to cut on a Monarch 10EE, but I think pretty easy on my CNC!

Stuart Caruk · Jul 12, 2012

We do hundreds of different varieties of multi start threads a month for very specific applications. The most common one is a fast acting pilot valve. If you measure the thread pitch with a thread pitch guage it would measure as a 24 TPI thread.

In reality, it is 3, 8 pitch threads stacked 120 degrees apart. It's a triple start 8 pitch thread. If you try to thread on a standard 8 pitch or 24 pitch nut, it will bind up. If you thread on the matching triple start nut. it will travel an inch in 8 turns. It's just that there are 3 sets of threads sharing the load, so you can use shallower threads.

Multi start threads are common in submarines, space products, and certain military products (think explosive decoupling bolts). On a CNC lathe they are simple to turn, for a triple start, you simple adjust the start point 1/3rd of a pitch away from the origin. Extend the thread so they all match if you need to. On a mill you program the same thread, but start it 120 degrees from the initial and subsequent start.

It's a hard concept to wrap your head around. I remmeber my first double start years ago. I simply didn't believe it was possible. So I made a test part. Then it was super clear. It's a great conversation piece in your shop, because few other machinists have ever heard of, much less seen a multi start thread.

Mike C. · Jul 12, 2012

Old school way to do a multi start thread is to run the work between centers and put the tail of the drive dog against the jaw of a four jaw chuck (for 2 and four lead threads) or a three jaw for three start threads. You can use faceplate slots, make a disc and drill holes in it, slot it, whatever to provide an index. Some lathes, specifically the Hendey T&G, had an index system built into the spindle to allow multi starts.

Mydrrin · Jul 12, 2012

2 start threads have double the feed rate of a 1 start thread. The feed on a 4 TPI - 2 start thread would be 0.5" / rev at 90 deg separation. So no you cannot, you need to make another gauge. You can use wires.

Putch · Jul 12, 2012

wlbrown said:
HELLO,
on multistart threads, the pitch is two time faster.this is on a
2 start thread.if the pitch is 11 tpi, then the gear box must be
set at 5 1/2 tpi. one some machines to cut the thread you proceed
just like normal, but the thread dial is the main function.one the first pass,
you close on number1, and on the second pass you close on number 3.this
is on a dial that has 4 numbers on it.this puts the second cut in the center
of the first cut. this may not work
on all machines. you have to try it to make sure it works. this works
very well on manarch 10EE machines.
good luck to all.
wlbrown

exactly what I was thinking - 2 times faster or else the pitches would run over themselves. Didn't really have a use for this but my mind's always going and figured I'd cut one so I have a leg up on those who haven't.

4GSR · Jul 12, 2012

Putch,

Some of them trepanners down at Boring Specialites had double pitch lead screws on them, at least when I was there. I know, I threaded a couple of brass nuts for them!

Ken

johnoder · Jul 12, 2012

Ferguson, a UK gent of old, had a breech loading flintlock rifle, the breech of which was a multi start thread. This threaded plug was vertical, and attached to the trigger guard. One turn of the trigger guard would drop the plug sufficiently to open the breech for loading ball and then powder. I can see making the plug not much of a problem, but the mating female hole - supposedly somewhat gas tight - must of been a pain. George Wilson can tell about it better.

When in the apprenticeship, back in the sixties in the Connecticut days, I tried for the plug on the loose change 13" X 5 foot South Bend in my basement. 9 starts on a 7/8" diameter with 1" lead.
It looked a little raggedy.

J. Randall · Jul 12, 2012

John, I have read about the Ferguson rifle, just from memory it seems it might have been an 8 start thread. A fascinating piece of work for the time, I think revolutionary war era.
James

WILLEO6709 · Jul 12, 2012

the way i was taught was to set the compound rest on the infeed angle, calculate a triangle where the cos was the pitch. remember the lead is some fraction of the lead. you are in effect cutting multiple synchronized threads. you have a compound move at the hypoenuse of the triangle and a cross slide move of the sine. Layout dye the one lead when complete, makes it easier to tell which one to measure. you can do it from drive dog on chuck jaws, monarch even had a special faceplate for it.... but low tech is the compound rest and counting turns.

hachu · Jul 13, 2012

we used to have a job for a cooling place
several different sizes of these things
anything from 4 to 8 leads
was done on a danichi lathe
it had some code for it
i did one on a manual
like these guys said twice the feed for the pitch
it was only a 2 lead thread
cut 1 thread
then put an indicator on compound move it forward the proper amount and cut the 2nd
later on
do 1 thread on the numbers of thread dial
the other on the lines

Multi-start Threading?

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