Multi Start Screws

[TMurch]

What is a multi start screw? I think it may also be called a multi lead screw. I want to make a machine that requires a 3 or 4 start screw, but first i've got to find out what that means.

[Geof]

It is a screw with more than one thread; a two start has two threads 180 degrees apart at the start, four start has four at 90 degrees.

A four start thread with a lead of 0.400" (lead is the distance the nut will advance in one revolution) will have threads that are spaced 0.100". The thread spacing is called the pitch so it is obvious that the lead is the number of starts multiplied by the pitch.

The thread form is identical to a single start thread with a lead of 0.100" which shows one of the advantages of multi-start threads; the thread form does not goes as deep.

For instance if you want a fast lead on a smallish diameter you have to use a multi-start thread. A single start thread with a lead of 0.100" on 3/4" outside diameter has a thread depth of around 0.05" and a root diameter of about 0.65"; this is a standard 3/4" NC thread. If a single start thread with a lead of 0.400" was cut into the same 3/4" diameter the thread depth would be around 0.200" which would be impractical because the root diameter of this thread would be 0.3" or in other words less than half the outside diameter and greatly weakened. A four start thread has a thread depth of 0.05" so the root diameter is 0.65" just the same as a standard 3/4" thread with the same strength.

[Mcgyver]

Originally Posted by TMurch I want to make a machine that requires a 3 or 4 start screw, but first i've got to find out what that means.

A sensible sequence of events.

[Delw]

Multi start Screws are mainly used for fast threads that need to be very accurate. used in projects the huble telescope the old and first( in the early 90's cd burners) some ball screws super high precision Micrometers for certain applications etc.

Basically lets say you have a .5 dia. thread and you want to turn1-3 full turns to run 1 inch in length and be very precise. the multi start thread is your best bet.

it can be done on a lathe both internal and external parts then you MUST Lap them in together. Its very important you have a tapered thread on the outside to squeeze the multi start female thread to get the proper tension on the thread as with out this you just got a sloppy useless thread thatyou have no control over.
you can also use a tap to make the female part. I have done it both ways, the best is to thread grind the male thread, then do the female on a lathe ( a tap will wallow it out a tad to much and you will have too much lapping work to do causeing your multistart thread to be a little sloppy and not 100% true.)
of coare this depends on how big a thread you got on the femal side as you may be able to thread grind the female as well.

I made them buy the thousands at a gage place I used to work at in the late 90's and early 90's, I was also then one that suggested we do it on a lathe vs thread grinding /tapping. it was a project that we would have had to turn down if we didnt find a way to make it work.

At the time I made them on a 10 year old sloppy machine and a brand new hitachi seiki, tooling for small ones are a big key due to clearance issues and lead. Our od size was around .375 and it was a 3 start. I got a few laying around if your doing something that small I can show you.
we used 440c for the male and 06 for the female(barrel) both heat treated as 440c and 06 are very predicatable when heat treated. you also need to cut out the root in the barrle if your lapping it as you don't want the crest to hit the root.