Lead, pitch, start?

[sixpence]

Hi
could anybody out there please define these terms for me?
1.Lead
2.pitch
3.start

I have general idea but the explanations I have gotten before on other things are so good and concise it saves me a lot of time by cutting out the volumes written in technees.

Also what is the difference between a stepper motor and a servo motor?

[ballendo]

Hello,

Lead is the distance a screw(or nut) moves axially in ONE revolution.

Pitch is the distance from the crest of one thread to the next.

Start is referring to how many individual "helixes" are going around the shaft.

Nearly all common bolts and screws are single start, and the pitch will match the lead. Multi start threads have a relationship between pitch and lead that is best understood by example:

(All in inches)

1/2-10 single start- pitch .100, lead .100
1/2-10 double start- pitch .100, lead .200
1/2-10 quad start- pitch .100, lead .400

Next,
A stepper motor is a synchronous motor run by square waves. Huh? (I wrote that so you don't get the idea that a stepper is some kind of magical device...) BUT:

A stepper motor is OPTOMISED FOR running on square waves, which means it moves in discrete steps, if the windings are energised in the "correct" sequence. There are several "correct" sequences, which result in the motor moving more or less for each step, and with varying amounts of torque.

A servo motor is just a motor. Nearly any motor can function AS a servo, because the thing that makes it a servo is the feedback loop, which involves the drive and feedback device (most often a rotary encoder in inexpensive systems.) BUT:

A servo motor is OPTOMISED FOR running with a feedback loop. Which means it has the ability to accelerate and decellerate quickly, and does not "prefer" one place in its rotation over another. This is called "cogging". You can feel this with any cheap dc "toy motor" when you turn it by hand. Motors designed for servo use have low cogging.

Steppers have nothing BUT cogging, as that is what the "steps" are; places where the motor "prefers" to stop its rotation.

Tjhe above is only a bare scratch of the surface of this subject, and experts will find things over-simplified. But it should get you on the right "page" with regard to understanding the diffeence between steppers and servos...

I wrote quite a bit about this recently on my DIY-CNC list at:

http://groups.yahoo.com/group/diy-cnc

And have written more in the past at the CCED group, also on
Yahoo...

Hope this helps,

Ballendo

Originally posted by sixpence Hi could anybody out there please define these terms for me? 1.Lead 2.pitch 3.start Also what is the difference between a stepper motor and a servo motor?

[duluthboat]

Ballendo,

Good response, it should be in a FAQ section.

Gary

[CNC Brute]

I wonder if you guys could also cover these related items:

How teeth per inch are related to all of this?

And how the size of shafting relates to the size of the machine?