Ball Screw and Lead Screw

[ljoe1969]

OK, so here is my Situation and Question.

I am planning on making a cnc router. I am gonna get the stepper motors and controlers from "STEPPERWORLD.COM" as a complete kit.

I am gonna design and build my own table. (i am a mechanical drafter in a factory :-) )

so the question is, How do i know what pitch of ball screw to get that will work best with my stepper motors. so that the machine is calibrated to cut one inch = one inch in the drawing. I was told that there was a formula to find out.

Also if someone could tell me how to figure out which rack and pinion system would work i would appreciate it.

i emailed stepperworld.com and i haven't heard anything from them.

[CNCadmin]

I used screws and balls from Roton 317-821-4400 the part numbers are
59321-36 ball screw 29.95 ea
19193 ball nut 19.40 ea
to make them pre loaded 9.65 ea

[balsaman]

It doesn't matter what the pitch etc. is. This can all be easily setup in the software.

1/2-10 acme screws work quite well. Even cheaper regular threaded rod works too. It's all about what you want and how much money you want to spend.

Eric

[HomeCNC]

A nice pitch ball screw is .200. The de-facto standard according to some on a different group is the 5/8 .200 pitch ball screw and ball nuts.

[ljoe1969]

Thanks.

This information helps alot.

[ljoe1969]

ok. so the next question is.

how do i determine the lead for a screw?

how far does a 1/2 - 10 go in one revoluton?

how far doew a 1/2 -20 go in one revolution?

etc..

i bought the xylotex 3 axis driver and the 116 oz. motors. i will have a cut of 24" x 36".

i was just wondering what size lead screw to use.

[balsaman]

1/2-10 goes .1" 1/2-20 goes .05"

Eric

[bb99]

My understanding is that the first number (1/2) refers to the diameter of the screw and the second the number of turns to go one inch. So, 1/2-10 goes .1 and 1/2 - 20 goes .05.

If I'm wrong, pelase correct me.

--bb99

[Chagrin]

I'm trying for a more methodical approach to calculating required torque or screw pitch from the education I've gleaned from various sources - I'm by no means an expert. Everyone feel free to stomp on my blatant inaccuracies

This site will calculate the torque required to give you the force you need for a given screw pitch and pitch diameter:
http://www.racointernational.com/screw_formula.htm
(For practical purposes, you can ignore friction in your leadscrew.). For those people that already have their motors and are trying to match screws to them, well, just kinda work backwards

The forces acting against you are the mass of the gantry and and the router bit chewing into whatever you're cutting. I figure the best way to guesstimate these are to feel how much pressure you have to use to move your gantry and similarly with a router freehand in a piece of wood. Obviously very light forces on either the gantry or router would move them, albeit slowly, so you want to push with as much force as you need to make each move as quickly as you want. A spring scale would be helpful for comparison in getting a feel for how much pressure you're applying.

So, with an estimate of the force you need, there's one more thing that you need to consider before choosing your stepper motor: stepper motors lose power as their rotational speeds increase. Only a spec sheet for the specific motor you're looking at will show you those curves, but there's a few example curves at http://sanyo-denki-online.com/nema23.htm (for basic NEMA 23 motors). These graphs have "pulses" along the X-axis, so, for example, a 1.8 degree step motor would be 200 pulses per rotation (200 * 1.8 = 360 degrees).

...and finally, you try for a motor three times larger than you think you'll need.

HTH. Of course, in review, I think I missed the actual question!

[marvinstov]

Don't forget that you can have lead screws with more than one thread. Example: A 1/2x10 with one thread start will have a 0.1 lead, a 1/2x10 screw with 2 thread starts will have a 0.2 lead.