Shafts or rails?

[sjmarsha]

I stumbled across this forum last week and I've been spending most of my free time looking at the various designs everyone has come up with. I'm starting to design a machine of my own, and I've run into a few things that I'm a bit unsure of.

I guess I should give a little info on my plans first. I'm shooting for a 4'x4' table with roughly 5" of z-axis travel. Aluminum frame and gantry. I'm planning on using it mostly for MDF and plywood, along with a little bit of foam here and there. I'm hoping to use it mainly for making speaker cabinets and subwoofer enclosures, so I don't need anything insanely accurate, but 1/64" or better would be great.

My first question is whether I should use shafts and pillow blocks or rails. It SEEMS like rails would be a little bit cheaper, but I really have no idea which would work better. I'm still not too sure what diameter shafts or what size rails I would ned either, so it's a little hard to compare prices.

Second this is whether I should use Acme rod or something else. It seems like a pretty standard and reliable way of doing things, but if there is a cheaper solution like timing belts or rack and pinion that will do the job then I'm all for it.

Keep in mind that I'm trying to keep the costs as low as possible, I'm a college guy making 10.50/hr. I should be able to get most of the aluminum machined where I work, but the rest of it is still going to cost me a kidney or two. It'll pay for its self right quick when it's all setup, but until then I'm short on cash.

Any other advice would also appreciated.

Scott

[samualt]

I'm a complete newbie so keep that in mind....

I use THK rails because I believe them to be more accurate than shafts. I very much doubt they are cheaper. My 99" THK's cost $500 (that was pretty cheap), and my 53.5" THK's cost about $269. All came in pairs with carriages. All from Ebay. As far as size...I just got what was out there for those lengths. Not much choice when buying used/surplus. I've heard shafts are much cheaper and work as well.

Acme rod is cheap. I doubt you can do cheaper and still be as accurate. I'm using a Ball screw and rod from Nook for my Y-axis. They are pretty expensive but are cut to order. I'm using rack-n-pinion for X-axis (99"). It's just too big for screws. The problem with rack-n-pinion and belt-drives is that you have to usually use some form of large gear-reduction. My servos run at 2000 rpm. In order to run my CNC at 150 IPM (inches per minute) I have to have a reduction of around 40:1 on my rack-n-pinion (or something close). So I have to build a gear-reduction device. Gears, belts, bearings, mounting plates and the rest get very expensive and time consuming, for a gear-reduction device. You usually don't have such a big reduction using acme screws and such. It's easier not to have to mess with large gear-reductions.

If I were going to build a 4x4 I would use acme rod, and shafts with pillow blocks. Cheap and easy. I don't know much about the shafts, but for the acme screw I think you want at least 3/4" thick, if not 1". That keeps it from whipping about or bowing since it's 4 feet long. Dual bearings on each end might be a good idea also. I read somewhere that it keeps it from bowing.

Well, that's my 2-cents. And worth every penny you paid I'm sure!

[ger21]

Don't use unsupported shafts, except for a Z-axis. Either supported round shaft, or THK style will work. THK's seem to be a little smoother, and can carry larger loads. And a Z-axis with THK's is a lot easier to build versus unsupported shafts.

Are you planning on using steppers? I'd try 1/2-10 5 start acme, you won't have to spin it that fast so whipping shouldn't be a problem. You can always change it later, but I don't think you need to go to 3/4" or 1". 5/8"-5 would probably be fine.

[sjmarsha]

I'm planning on using steppers. I've heard that I need at least 100in/oz and preferably larger. I won an auction for 7 assorted 1.8 degree steppers for $20 and a couple were 150+. If at least one of them is in working condition then that'll be helpful.

It looks like I'll be using the Xylotex controller with a 24v power supply I'm snagging from work.

Can anyone advise me as to what diameter I would need for supported shafts or what size THK rails (do they vary in size?) I would need?

Also, if there's anywhere besides Reid, McMaster and E-bay where I should be looking for parts, I'd like to know.

[samualt]

For places to buy stuff, try:
My Haphazard CNC-Links

[ger21]

If you're building 4ft x 4ft, Unsupported shafts are a BAD idea. I'd say at least 1-1/2" to 2" Diameter, but even those will still sag and flex quite a bit. For the Z-axis, you can get away with 1/2" or 3/4", but you can probably get some 12" thk for around $100 on ebay.

For THK's, anything over 15mm should be fine. They look small, but are very strong. You might also want to look into DualVee systems from here. http://www.bwc.com

If you're using the Xylotex, I'd look for 200-250 oz-in motors, although you can try the 150's and upgrade later if they don't give you enough speed.

Check out http://www.mscdirect.com , or http://www.use-enco.com