turns per inch vs threads per inch

[pminmo]

A really dumb question
a 4 turns per inch, 2 starts, 8 threads per inch ACME lead screw.
To get 1 inch of travel, I assume it needs to be rotated 4 times? Or is it 8 times?

Phil

[marvinstov]

Looks like 4 turns per inch. You have a .25 Lead. (1/4" per turn)

Marv

[BigDaddyG]

Phil,
Not a dumb question at all. 4 turns per inch is just that, the amount of turning to get the nut to travel one inch. The threads per inch, again are just that (as in any common screw thread ie. 1/4-20 is a 1/4" Dia screw with 20 threads per inch). The "Starts" refer to engagement of the threads. When the nut engages the threads, it does so 180 deg's apart. You can have different amounts of starts (usually even numbers). So, a 4 start will engage the nut in four places (90 degrees apart) and so on.
Regards,
Glen

[High Seas]

BigDaddyG - I'll bite!
So, a 4 start engages at 4 places - got that.
Question 1:
If its 5 threads per inch (on a 4 start) is that the same as a 20 TPI - normal/single start?
Question 2:
So on the threaded rod: 4 start 5 TPI - the nut rotates 20 times in an inch? How is that different from a 20 TPI thread rod?
Question 2a:
Or, does it rotate 5 times per inch - because it takes a bigger bite?
Question 3?
Do I get tighter tollerances on the 5 TPI/4 start for the same travel and therefore better resolution?
Question $:
Same price - highrer?

Final tounge in cheek:
Maybe you can get me straightened out (untwisted) here?
Jim

[BigDaddyG]

Jim,
Q1 - No!
Q2 - No!
Q2a - No!
Q3 - Yes, and Yes, Higher price usually!

[BigDaddyG]

Jim,
Standard threads you see on a bolt, the Thread is a single start. If you look at the end of a 1/4-20 x 1" you will see one thread end (or blend) off the end of the screw / bolt. That is one constant thread that twists 20 times in the inch. Thus a single "Start"!
In the case of an "Acme" thread you will see the end is totally different. If you have a piece of Acme, machine it with a flat on the end, it will have more than one thread that "Ends" on that flat, Thus 2-4 + Start's. Its the geometry that makes it different than standard thread.

Yes, the more "Starts" on the thread, the more engagement you get on the nut thus the more or tighter tollerances. But again, the more starts, the Less Turns per inch you get. It is a give and take.

Hope I answered your question, assuming your not pulling my leg!
Glen

[High Seas]

... assuming your not pulling my leg!
Glen

No Glenn - really serious here - sometimes I kid a bit but not now
SO,I've seen reference on the site to using multi-start threaded rod to increase the speed/IPM on a machine. That is why I asked the ?'s regarding how that works -- well - now I get an AH HAAA - of course - you can use a 10 TPI threaded rod - get twice the speed of a 20 TPI and if the 10 TPI is multi-start, you also get better resolution - ie tighter tolerances! OK if I got that right Thanks - else give me another heading change.
Thanks Man! Jim

And OH yeah - better usually cost a bit more

[BigDaddyG]

Jim,
Cool man! I am an engineer and sometimes its hard to explain things with out having the literature right in front of me. If your in the market, I can recommend "Nook" lead screws. You can find them at thier web site or at MSC (which have distribution all over the usa). They make a precision lead screw that is very affordable. I bought a 36" x 1/2-10 section of thier "Precision" lead for $22.37, a precision nut - $20.05 and the flange was $30.20. So the entire thing cost me less than 75.00 and the screw is rated at +/-.003 per foot. Almost as good as any ball screw for a fraction of the cost. I am alway looking for stuff/deals like this. Like I said for 75 bucks I got the precision screw, nut and mount and you just cant beat the price.

Good luck man, Glad to help.
Glen

[marvinstov]

Phil,
I have attached a pic of the explanation from McMaster Carr. It says:
Starts — The number of independent threads on a screw. Multiple-start screws provide faster nut travel than single-start screws. For example, a single-start, 10-turns-per-inch, 1/2" dia. Acme lead screw with 10 threads per inch requires 10 revolutions for the nut to travel one inch. A multiple-start, 2-turns-per-inch, 1/2" dia. Acme lead screw with 10 threads per inch and five starts only requires two revolutions for the nut to travel one inch. Most single-start screws are self-locking and help resist backdrive so a braking device is not necessary. Multiple-start screws are not self-locking and may require some sort of braking device to hold them in position.

To Determine Number of Starts— Place a pencil tip in your thread's groove and turn the thread one revolution (see diagram, left). If the end of your pencil mark is in the next groove, you have a single-start screw. If there's a thread groove between the beginning and end of your pencil mark, you have a two-start screw. If there are two thread grooves between the beginning and end of your pencil mark, you have a three-start screw, and so on. Another way to determine the number of starts is to look at the end of your screw (see diagram, left). The end of a single-start screw is an offset circle. The end of a two-start screw is shaped like a football. The end of a four-start screw has four corners. You can easily count the starts on screws with five or more starts.

Marv

[pminmo]

Thanks all.
I got the starts, it was thread per inch vs turns per inch that threw me a little.

Phil

[marvinstov]

Phil,
As long as it is only one start, Thread per inch and Turns per inch are the same but when you have more than one start, you should go by TURNS per inch. I am using some 1/2 - 10 THREAD per inch, 2 start ACME which has a lead of .2. That is 5 TURNS per inch. Confusing I know.

Marv

[drawbar]

Threads per inch / Number of starts = Turns per inch