New CNC Machine - Building Stage!

[JavaDog]

Wow, tons of great information there. I am going to digest the link you posted there, thanks again.

Originally Posted by arvidb So timing belts are better for fast motion with less force demands. Not too good for aluminum, although it's surely possible with oversized servos. Arvid

So, you're telling me belt-drive isn't going to work for whatI want to do.

I guess I am back to the ($$$) ballscrews then. I just want to make sure I do it right the first time. I can go larger, and keep my good servos and drivers, down the road.

[High Seas]

Thinking sideways - and - just wondering...
hmmmm, What about a rack and gear setup? Is the "answer" the same as with belts? (I'm thinking about the, " force demands" as quoted. Is that a result of gearing and slack in the belt? - Or just the torque/rpm.) If a rack and gear setup has the same properties of a belt - why use a belt? But if the rack is better - and not too dear checking at McMaster-Carr - maybe thats an approach! Like to hear the thoughts. Jim

Arvid - still hoping some clever chap will do those pic for that great text!

[arvidb]

Theoretically, a timing belt used for linear motion will work just like a rack and a pinion. The rack is the belt and the pinion is the pulley. So the same math and physics applies, I'm afraid.

It's all about getting the power from the motor to the part or tool. No matter how you do it, if you want say 100 mm/s feed and 500 N cutting force, you will need 50 W of power at the table (P = F*v = 500*0.1 - and I just realised that my CNCMechanics document says P = F*s, which is wrong ).

You can get those 50 W there with, say, a 100 W motor running at full power output (nominal RPM and max load) with a 50% efficient ACME screw with a lead giving you exactly 100 mm/s feed at this motor RPM,

or with the same motor running at half its maximum power output (nominal speed/2 and max load) driving a geared down timing pulley along a belt at a speed that gives you 100 mm/s - this has the advantage over the previous alternative in that you could actually go twice as fast if you only wanted to,

or with a cheaper 50 W motor having the same nominal speed as the 100 W above but with half the torque, and a ball screw with the same lead as the ACME above,

or... well the choices are endless. If you had a motor with twice the nominal speed but the same power output (and therefore half the torque) as the ones above, you could just add gearing 2:1 to the above choices and still get the same result. I have assumed here that timing belts and ballscrews are 100% efficient, which should not be too far from the truth.

Arvid

[JavaDog]

So, am I actually going to run into problems running servos and geckos - driving a belt-drive CNC machine - and doing plastic (80% of the work) and aluminum and copper (other 20%) work??

I am happy to go either way, but since I have a source for belts and gears, the belt drive will be cheaper for me. Cheaper only matters, though, if it can do what I need it to.

For the alum, I jsut need to be able to cut oout custom brackets out of 1in stock (with multiple passes). Copper, I am making waterblocks for CPU cooling. I know the 2 1/2HP PC Router I am going to use can handle that stock.

[arvidb]

JavaDog,

I'm not saying it won't work, or that you will run into problems (more than anybody else trying to build his/her own CNC machine ), just showing the possibilities and that some things might work better than others. If you have access to cheap belts and gears, you might want to try that? Do you really need the full power of your servos? You might do very well with a dual 3:1 reduction (9:1 in total), and since timing stuff is cheap for you that reduction would also be cheap.

It's all about weighing pro's and con's together.

[edit: I also want to add that when I'm talking about motor power I'm talking about continuous mechanical output power. Many/most DC motors are marked with peak electrical power which might be 10 times the continuously available mechanical power. Industrial servos are often marked with continuous mechanical power, though.

Electrical power is U*I, mechanical is (rotational movement) ω*M or (linear movement) v*F, where
* U is voltage
* I is current
* ω (omega) is angular velocity [rad/s], 10 RPM = about 1 rad/s
* M is torque [Nm]
* v is velocity [m/s]
* F is force [N]

If you had a 100% efficient motor you would get 1 W of mechanical power for each watt of electrical power you fed it.]

Arvid

[ger21]

Use steppers, and you won't have to gear down nearly as much. Since you don't think your going to need high speeds, I don't see where the servos give you an advantage (other than more torque from the gear reduction, but that comes with the added compexity of the, gear reduction). There is nothing wrong with steppers, if done properly. At least a couple times a week, I read about guys with stepper powered Bridgeports that run just fine. Get some 300-400 oz steppers, and Gecko 201's, and you'll be all set. Just my personal preference.

[JavaDog]

Just wanted to update. I am working on getting some THK rails as we speak, should know by tomorrow if I got them or not.

If I do manage to get them, my machine will be 30" x 30" with a 10" Z-Axis. I'm keeping my fingers crossed...


EDIT: Nevermind, I was hoping I had a steal on my hands, but the bidding shot out of control (and my price range!). See: Qty:5 30in Rails with Blocks.

[JRoque]

Hi. Check the auction again; it says *not* THK. They put THK in the title so the search engine picks the auction out when you search for THK. AFAIK, THK doesn't make aluminum rails, only steel.

JR

[JavaDog]

Originally Posted by JRoque Hi. Check the auction again; it says *not* THK. They put THK in the title so the search engine picks the auction out when you search for THK. AFAIK, THK doesn't make aluminum rails, only steel. JR

Oh, I knew they weren't THK. I checked out the mfg that makes those, and they look pretty decent. I'm sure IKO and THK are the best, but if I could have picked up all 5 30" rails for under $200, it would have been a steal!

[JavaDog]

I have a question for y'all.

Would a matched set (2) of THK ballscrews (14mm with 4mm pitch) with the
cylindrical ways, and aluminum stages and platforms for $250, would I be getting ripped off, or would that be a deal? They are in PERFECT used condition. They are 26in long (should be about 20 odd inches of travel, right?).

This would be a HUGE start for my project, but I want to make sure I am getting a good price. Thanks again for the continuing help everyone!

[buscht]

JavaDog, that sounds like a good deal to me.
Trent

[JavaDog]

Originally Posted by buscht JavaDog, that sounds like a good deal to me. Trent

Ok, that is what I was thinking, from prices I have seen everyone mention.

Can I expect 24in of travel, realistically?