how do i choose the right lead screws?

[dansal]

hi,

i am building a cnc wood router and have some questions about lead screws.
My table will be around 1m x 1m. I have a driver set up that will do 1600 steps
per revolution. I want around .005mm acuracy, what lead screws should i get?

[IJ.]

Do you mean .05 mm accuracy?

[ger21]

What are you going to be cutting? What size notors will you be using?

[dansal]

yes sorry .05mm. I will be cutting light wood (ply and balsa). my motors are NEMA 23 280 oz/in and i'm using xylotex 3 axis board.

[kong]

Out of interest, what software will you be using for the machine? At 1600 steps/rev, Mach2 will turn your screws at a maximum 28 revs per second, divided across the three axis. If you use the xylotex board in a 1/2 step mode or less, your motors will be even slower. You may want to go for quite a course leadscrew if this is the case.
My estimations were based on Mach2 output of 45Khz, which is not possible on some computers.

[IJ.]

Dansal: I wasn't being a smartarse or anything it's just that I've recently completed a retrofit and shooting for .005mm would work out very expensive!

I'm sure one of the "Wood" guys could jump in and reccomend a suitable tolerance for woodwork, you may be able to save a huge amount of cash building a machine to suit your purposes.

[marvinstov]

Are you sure it's 1600 steps per "revolution" or are you using 1/8th micro step which gives you 1600 steps instead of the normal 200 for a 1.8 degree stepper motor?

Marv

[marvinstov]

Oops, I see what you are saying now. Sorry.
Marv

[marvinstov]

Kong,
If he has the board set for 1/8th micro step (1600 steps per rev) and changes to 1/2 step wont the same pulses generate more revs per second? Or am I backwards?

Marv

[dansal]

yes i'm using the xylotex board in 1/8th micro step giving me 1600 steps per rev instead of the normal 200. Can anyone tell me how i work out what lead, size (diameter) lead screw i need? Also how do i work out what speed is reasonable for my machine to move at? What is an average speed i should be
looking at?

Dansal

[sendkeys]

[chuckknigh]

Well, think about it this way. You need to come at it backwards.

How many steps per second or minute can your parallel port produce? Let's just say it's 4000 per second...that makes the math fairly simple. (This information is estimated by TurboCNC on the start up screen, so it's easy to obtain)

If you have 1600 steps per revolution, like you said, then that represents 2.5 revolutions per second...pretty slow. Let's say you don't use microstepping, so you have 200 steps per revolution. That's 20 revolutions per second. A bit faster.

OK...now, let's play with some numbers. How far do you want it to move in a second? 1" is a nice, round number, so let's use that. (BTW, that's 60 inches per minute...or 5 feet per minute! That's reasonably fast, for a cutting tool) So, 20 revolutions per second to move an inch...that means the lead screw has to have 20 threads per inch. The most obvious choice would be 1/4" x 20 threaded rod, or something else with that number of threads.

Job done...your lead screw is 1/4" x 20...or anything else you can find with that thread pitch.

Simple, right?

-- Chuck Knight

P.S. If you shop around, you can get all different thread pitches on all different diameter screws. Balsaman has a 1/2" diameter ACME leadscrew that has 2 turns per inch...pretty coarse, so the movement would be pretty fast.