[jpwatkins]

I'm diving into the idea of building afew cnc machines (a good friend of mine thats in fab wants one too) and there is so many ways of going about doing it. I just have some questions, pardon my ignorance.

I am sure most of you have seen that engadget article, and well..how accurate can you get with normal screw mechanisms like that? Now I don't mind gearing down the steppers to gain more persision, it also means that I dont need as powerful ones to get the job done-a win win as long as the controller mods can be made without being an ee. Any advice? Oh, and if I was to build a machine out of crap parts like that could it be able to cut acme screws or similar with the perscision necessary?

How much horsepower wound it take to cut various materials (aluminum, mild steel, tool steel)? I just have no clue.

Modding the life out of everything I get my hands on is a common trend. I am no stranger to fab and jury-rigged work. I want to build a small test machine to get experience on the engineering side of it. There's this crappy 1/2 horse drill press that I bought awhile back to use as a poor man's lathe turning down pvc for a custom exhaust valve. I plan on using the motor on that sucker just because I can...small xy table, easy vise mounting, though I am unsure of how to set up a z linkage. Off the top of my head I'm thinking counterweight and drill an axle hole for the stepper or some such. Any concerns advice on this one?

I'm just looking to build a passable (+-.002 would be nice) machine just as a test project at first, for wood or whatever the motor is up to doing. In the trifecta of good-fast-cheap, its fast that is the odd man out for me. I don't care how fast it cuts as long as it works without too much interruption needed.

Well thats all I have for starters, back to researching. I appreciate any help I can get.

[cbass]

I am sure most of you have seen that engadget article, and well..how accurate can you get with normal screw mechanisms like that?

Do you have a link? When you say "Screw Mechanism" I think of about 100,000 different kinds.

Carlo

[jpwatkins]

Well I'm concerned with the threading i.e. off the shelf at Lowes or Acme or whatnot. Heres the linkage to see what they did with it:
Its a good build on the cheap using scavanged parts.

http://www.engadget.com/2006/06/29/h...achine-part-1/
http://www.engadget.com/2006/07/04/h...achine-part-2/
e12.engadget.com/2006/07/11/how-to-build-your-own-cnc-machine-part-3/

[thkoutsidthebox]

I just have some questions, pardon my ignorance.

Im afraid I dont think I'll be any use to you here, but I'm very interested to see how this all pans out. Good luck with your build.

PS: Not having any questions to ask is a sign of ignorance, to ask questions and seek knowledge and self improvement, is a sign of intelligence.

[cbass]

If I understand correctly, you are thinking of using standard threaded rod to move your axis, and you're wondering if you'll be able to get .002 accuracy out of it.

I highly doubt it. For one thing there's going to be a lot of backlash between the rod and nut. Even if you deivse an anti-backlash nut (spring loaded for instance), how consistant do you think the thread will be over a given distance? Will it be to within .002" ? I doubt it. .002" is out of reach even for some entry level

The other drawback to using the threaded rod is that it is pretty inefficient in transfering power. As far as what it will cut, that will depend on a lot of other things (spindle, feedrates, stiffness of the machine etc.).

I'm not trying to discourage you from using the rod, just trying to predict its shortcommings. If low cost and quick assembly is your ultimate goal, then go with it...

Carlo