[Chris Tucker]

trying to size a ball screw for my first cnc machine. does anyone know how many steps makes up one inch. example: if it is 200 steps per inch then i would need a one tread per inch lead screw or a one inch per revolution ball screw

is this correct is this correct or not ???? (need some help)

thanks

[ger21]

Steps per inch depends on what motors (and encoders if servos),drives, screws, and gearing. You can use any screws you like. Most stepper motors are 200 steps per revolution. If you use a Gecko 10 microstep drive, that gives you 2000 steps per revolution. If you use a 5 turn per inch screw, the you have 2000 x 5 = 10,000 steps per inch. If you use a 2 turn per inch screw you'd have 2000 x 2 = 4,000 steps per inch. You then tell the control software how many steps it takes to move YOUR setup 1 inch.

[DennisCNC]

^^^Like he said, it can be from 200 to ∞ .

I would say start with 5 tpi screw direct to stepper and 10uS drive.

[ger21]

^^^Like he said, it can be from 200 to ∞ .

It could be less than 200 too.

[Oracle_9]

I will be a first time cnc builder, and this thread was really informative. Thanks.

So a 200steps/rev + Gecko 10uS + 5tpi ball screw + Direct Connect = 10,000 steps/inch.
And if the Stepper is reduced 4/1, then its simply 2500 steps/inch?

Now I would tell the software I need 40,000 steps for 1 inch? If so, wow, thats good resolution, eh?


To further this.
Is this true?
If I wanted to go 300 inches/minutes with the setup above (moving in a straight line for example), then the software would have to send (40,000 step pulses) X (300 inches) = 12,000,000 pulses (steps) in 1 minute. This would mean 200,000 pulses (steps) per second. If we using the Geckodrive 201A 10uS driver, this would be the speed limit, since its "step frequency" is 0-200kHz. ???

Oh boy, now I am confused....again.

[ger21]

To get 200K steps per second you'll have to get a G100 and wait until Mach4 is done. I don't know anything else that can output steps that fast.

[Evodyne]

So a 200steps/rev + Gecko 10uS + 5tpi ball screw + Direct Connect = 10,000 steps/inch.

Yup!

And if the Stepper is reduced 4/1, then its simply 2500 steps/inch?

If you use a pulley arrangement to reduce the speed of the screw relative to the motor, then the motor has to turn that much more to drive the screw the same amount. A 4:1 reduction (with everything else as you meantioned) would give 40,000 steps/inch, not 2500. Along with that you would increase the torque at the screw and lower the top RPM of the screw (by a factor of 4).

What are you planning on making and how much do you want to spend? 10,000 steps/inch is 0.0001" of travel per step. Unless you use top quality parts ($$$) and good building techniques, the machine itself won't be accurate to that level (slop in the screws, misalignments, etc). If you are planning on doing woodwork this is overkill. 1000 steps/inch or 0.001" travel per step is likely all you need, even for metals.

Remember, there is no free lunch. Everything has trade offs. Too much resolution you probably can't use anyway and your rapid movement will be slow.

Evodyne

[Oracle_9]

Thanks for the replies.

lol, Evodyne...I was just "openly thinking" of the possiblities..hehe. It would be neat though, assuming the mechanical devices can be made to support it.

Concerning my project, I am designing a cnc plasma cutter with an interchanged head to allow Oxy/A cutting and maybe a small router for my r/c hobby. But the main intent is for plasma. I would like an accuracy of 0.002-005 or lower if cost permits. My cutting area will be 4ft x 4ft (will upgrade to larger table later on). Will use CampbellDesigns breakboard, Gecko drives, and 640oz steppers (2 for x-axis, 1 for y-axis) and maybe manual for Z-axis or a small stepper for it (havent decided on it yet). I will be using either linear bearing on tool steel rod or a HiWin HG type linear guide block + rail (if the cost is not too high on it) I am thinking of using a rolled ballscrew which is like 0.002"/ft. accuracy. Or some people mentioned rack and pinion, but I read the backlash is not good, like 0.005-0.010". Maybe for plasma thats ok, but I would be doing some routing too....thats why I am thinking of ballscrew...wouldn't hurt. I originally thought of using direct drive for ballscrew, but maybe I'll put a reduction belt drive of 3:1 - 5:1. The problem I see is figuring out what this setup would give me in speed. Sorry for getting of topic here a bit. This thread did help me understand some more about step/in, that I did not know before.

[Evodyne]

Oracle_9,

Poke about the sight and look at some of the machines using belt drives. Essentially a long toothed belt is fixed to the table at both ends. Your servo (usually with reduction) or stepper is mounted to the moving axis and turns a toothed gear. Guide pulleys ensure that the belt wraps around this drive pulley. Once the belt is pulled taut there is no freeplay. It's basically another play on the rack and pinion. Looks like it could be very cost effective too.

Good luck!

Evodyne

[smarbaga]

the printer port of a pc opperates at a bus speed of 10 megabytes
this means it can put out 8 different data bits 10,000,000 times a second.
directly connected drive transistors it could drive 8 motor phases at that speed,
whatever that is , theory etically. (lol)

[highoctanecnc]

I saw this thread and thought I'd let you all know, in case it will help you... I wrote an article about calculating steps per inch and also published a video about How To Calculate Steps Per Inch of a rack and pinion drive axis with a single-stage gear reduction. If it helps anybody out there, here goes the links:

Do-It-Yourself CNC Tutorials for CNC Routers and CNC Machines

Josh Glenn

[Hirudin]

Oracle_9,

Poke about the sight and look at some of the machines using belt drives. Essentially a long toothed belt is fixed to the table at both ends. Your servo (usually with reduction) or stepper is mounted to the moving axis and turns a toothed gear. Guide pulleys ensure that the belt wraps around this drive pulley. Once the belt is pulled taut there is no freeplay. It's basically another play on the rack and pinion. Looks like it could be very cost effective too.

Good luck!

Evodyne

A machine using the belt setup Evodyne is talking about is here: New Machine Build.... The person building the machine has posted a couple videos on YouTube
[ame=http://www.youtube.com/watch?v=9QgnD-bgd_4]CNC machine belt drive axis test[/ame]

This one shows the belts real well

[ame=http://www.youtube.com/watch?v=dGwZiFQFnvo]CNC machine belt drive all axis test 2[/ame]
[ame=http://www.youtube.com/watch?v=Wn-9DeWCQW4]CNC Machine sketch test July 4th[/ame]

P.S. Hey look at that, the forum automatically linked to the actual videos, cool!

[kawazuki]

Thanks for the replies.
I would like an accuracy of 0.002-005 or lower if cost permits. My cutting area will be 4ft x 4ft (will upgrade to larger table later on). I am thinking of using a rolled ballscrew which is like 0.002"/ft. accuracy. Or some people mentioned rack and pinion, but I read the backlash is not good, like 0.005-0.010". I originally thought of using direct drive for ballscrew, but maybe I'll put a reduction belt drive of 3:1 - 5:1.

From what I understand, your accuracy of 0.002-005 is meaningless as it is compared against nothing! Your quoted ballscrew has 0.002/ft so will be 0.008" out on your 4ft table so could be outside your parameters.
Stepper useful rpm's top out at around the 2,400 - 3,000 mark so with a 3:1 reduction screw rpm will be 800 - 1000rpm and with 5 threads per inch your axis travel will be 160 -200ipm therefore you wouldn't achieve your desired speeds and certainly not with any force on the gantry? Just my 2c worth?