Before the cash flows...

[Adler]

Hi all.
I, like so many others, am considering building a CNC cutter for thin wood (balsa and lite ply) and foam sheet. I've read lots of threads here and am sure I need a machine. Besides, I like to build stuff also. Looks like my weak spot will be writing the code. I've built vacuum formers before so adding vacuum to the table shouldn't be too difficult if required.

I dont want to buy one of those pre-fabed hobby CNC machines.. I need something a little larger and more versatile.

I'm considering the following hardware and would appreciate comments or suggestions.

Most of the guides and associated hardware will be purchased from Mcmaster:

1. 1/2" stainless steel pre-drilled shafts, 48" and 24" for X & Y axis.
2. Full length aluminum supports for above.
3. 1/2" I.D. steel open bearings with pillow blocks to support the platforms riding on the stainless steel shafts.
4. 32 degree pitch rack and pinions for motion on X, Y & Z axis.
5. Ceramic coated aluminum 1/2" shafts will full length aluminum supports for Z axis. (not stainless because easy for me to cut to required length). Will use open bearings also.

Since the material being cut is thin... I'm planning on using a pin router or something similar.

The big question is E-bays' steppers or servos... accuracy is important because I'm cutting kit components that have to fit together.

Any thoughts or suggestions greatly appreciated. I think I'll be in this forum for a while.

[mxtras]

Sounds like you have done your basic homework - good for you!

I don't think I would run Z with a rack....doesn't sound like a good idea to me. I think a screw would be better as it is less likely to fall upon power failure and has a higher mechanical advantage than a rack.

Accuracy? Servos are the answer in my opinion, but - what are you going to use for controls? You can encode your motion mechanically to get equal results with a stepper system. Cost is of course the primary concern with building a closed loop motion system. If sized and set up properly, steppers alone can offer excellent results.

Oh yeah - in order to post here, you must share lots and lots of good quality photographs. This is a requirement.

Scott

[2muchstuff]

If you want your cutting area to be 48" x 24" then your rails will need to be longer to compensate for your bearing spacing.

[ignatz]

Have you seen these bearings? Might fit your need.

[Adler]

Okay, accuracy is a concern therefore closed loop it is.. I have this crazy idea about replacing the router with a sodium laser at a later date so if the accuracy is there.. might be worth a shot. I think digital over analog control would be better in the long run. I agree with screw drive (worm gear) on the Z. Thanks for the insight. Have you built a machine? Do you have some background in commercially available servos / controllers ?

I understand the principles of operation...used to work on F-14s in the Navy and they do have servos for control surface command and position feed back. Computer controlled, hydraulically actuated systems..

A little fighter jet insight here... the force feedback in the control stick that gives the pilot the "feel" of flying the aircraft is actually provided mechanically and not aerodynamically. The mechanical force comes from the hydraulic actuators that move the control surfaces.
There is the equivalent of a bar magnet floating in a magnetic field (coil) in the hand grip of the control stick and any lateral motion of the magnet within that field (pilot inputs) generates the input signal to the flight control computer. Those signals are amplified and sent to the individual control surface computers. Thats where the servos come into play. These signals are nulled in the flight computer and therfore are closed loop also. Good stuff. :-)

It's not the same feeling as driving a car.. anyway, back to CNC ... to put something together with off the shelf servo components interfacing with a PC.. once again... I can use any info / advice you have to offer.

Looks like the bill is gonna hit 2.5K+. Not an issue if the end result is a quality machine. I dont expect perfection but I'd really like to do this just once :-).

Nails.. :-) we're talkin stainless baby..bolts it is !! nice.. very nice :-).
Appreciate the heads up on the bearing clearance. I may go with 48" on both X & Y. That gives more than enough clearance over the material size being used.

Ebay shopping... Thank you for showing me the good deal.. but I think I'll stick with the full supported stainless shaft over the chrome plating. Thing about e-bay is you have to watch tax if locally purchased and of course the shipping. Now if you see some good servos.. send 'em my way :-).

Got a fine digital camera so I'll post a step by step. May be a few so dont hold yer breath... I'm still in design phase. I dont want any "awe shytes" when I start assembling.