[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

[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?