3 start acme screw

The depth will be the same ,The lead(how far the nut advances per turn) will be 3 times that of a single start thread.I don't know the pitch or lead yor trying to cut So lets say you have a pitch ( number of threads per inch) of 6 threads per inch and a 3 start thread. Figure 1inch divided by 6 =.1666 x 3 =.5 so your lead is .5 inches per turn or 2 threads per inch. To set your lathe up for this set the compound slide to 0 deg.set the thread gears to 2 threads per inch.Cut the first start of threads to the depth you want (.200) use the same number on the thread dial for all 3 threads . After the first thread is cut.Move the compound slide .1666 inch this will give you the start for the 2nd thread .When finished move the compound slide another .1666 inch for the 3rd thread.Again start the thread on the same number on the thread dial. Hope this helps Tim Kallam

Forgive my ignorance. When you say set the compound slide to 0 deg., I thought that would stay at 14.5 deg? Also, in what direction is the compound being moved .1667 in.? I thought the compound just moved in or out on the y axis or rotated from say 14.5 to 30 deg. etc. I am just a newbie and have only moved the compound in or out to cut threads and have only had it in 14.5 deg. or 30 deg. positions. Please clarify and thanks a lot. Bob

If you wish to leave the compound at 14-1/2 degrees you still must cut the first thread complete.
To cut your second and third threads, the gearing must be disengaged and the spindle advanced by 120 degrees. Re-engage and cut the second thread.
Repeat process to cut your third thread.
HTH

Donald Nichols scribed in :
Another way is to use the screwcutting dial for the offset. only works on some pitches though.
I needed a 36tpi dual start thread and experimented a bit. I found that at 18tpi if I cut one thread on the '1' mark, then cut again at '1.5' mark (the unnumbered mark between numbers on my dial), I go a perfect 2 start thread. this way I could cut both grooves at the same setting, then infeed, then cut again and so on. was real quick. final part was in polypropylene for a tension adjuster on a sewing machine
I doubt it will work for 3 starts though, but someone with math instruction might be able to figure out which pitches it will work for
for which parts can probably be purchased at much less than they can be made IF and only IF the makers time is factored in.
swarf, steam and wind
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