[ger21]

I guess if most of your cutting is going to be straight lines then steppers will be fine. I gather a servo will do nice rounds.

This is not a fair statement. Both steppers and servos will give you the exact same results up to a certain point. That point being when you need more power, or higher speeds than steppers are capable of.

Servos been a DC motor have a smooth motion. Steppers move by a series of jumps.

If you command either a stepper or servo to move 500 steps, they will both accelerate and move 500 steps. Tell them both to move 1 step, they'll move 1 step. My steppers move very smoothly, and seem to spin just like any other type of motor I've ever seen. Even at very slow speeds, there is no stop and start jerking, just a slowly rotating motor.

Sometimes, steppers can suffer from resonance problems. With microstepping, this can be mostly eliminated. And Gecko drives, I believe, have some additional features to reduce resonance problems.

The bottom line is, a properly set up and designed stepper system should perform flawlessly. And, a properly set up and designed servo system should perform flawlessly. And generally, stepper systems are cheaper. So, if a stepper system can provide the performance your system requires, why bother with the extra expense and complexity. jmo

[vacpress]

Another note on this debate - the big expensive CNC we use at school\work is driven by stepers without encoders. It has run dozens or programs, and hasnt had any reports of lost step related failure. These programs are usually very complex and large 3d shapes. I have seen almsot nothing like them on the forum. The device is used constantly for finishing 3d surfaces. It does this just fine. I think the bigest problem we have is ocassionaly breaking a bit by running to fast in a material we dont know. If you setup the stepers right they seem to work well.. Some notes on right:

1-a good power supply is key - no switching computer supplies for obvious reasons.

2-the faster you go the less torque, the more likely the thing will stall.. a stall is the worst because it can trash a part if nothing stops the program. we have never stalled our machine, running unto 180ipm in foam.. 85-120ipm in REnboard.

3-good wiring, grounding, etc.

[radio-op]

I agree that a closed loop system is better, and I haven't understood why there aren't more stepper systems with encoders. I have been working on small robot projects for a wile now and it doesn't seem that costly or hard to close the loop for a CNC table. Is the problem the added cost or the control software? Or is it a case of " if it anit broke..." ?
I'm still working on my table so I haven't played with TurboCNC yet, but I thought that it could handle encoder input. I'm not really planing to add encoders right away, just want to get things spinning first. But would it be worth the extra work of adding a feedback system?
Bill

[ger21]

None of the inexpensive "hobbiest" software can handle feedback from an encoder. I'm sure that is the limiting factor here. MaxNC sells closed loop stepper systems as well as open loop. The cost difference is about $1000. Everything else is the same.

[radio-op]

Yea that would explain it! Thanks
Bill

[imserv]

Originally posted by ger21
None of the inexpensive "hobbiest" software can handle feedback from an encoder. I'm sure that is the limiting factor here. MaxNC sells closed loop stepper systems as well as open loop. The cost difference is about $1000. Everything else is the same.

In the case of the DeskCNC servo drives, the PID servo loop is closed in the software contained on the driver board, rather than within the PC. The drivers work with brushed DC servo motors with quadrature encoders, not steppers. The system provides precise positioning from step and direction signals. They are very competitively priced with equivalent stepper systems. Under $150 per axis for the Kit-2 which includes drivers, servo motors w/ encoders, mounting plates and a paid license for TurboCNC (parallel port).

http://www.cadcamcadcam.com

Fred Smith - IMService

[vacpress]

hey Imserv- do you offer just the servo amps?

[imserv]

Yes, just the drivers, just the motors, just one axis, w/PS kit, extra and combined 6 ways to Sunday, due to popular demand.

Fred Smith - IMService

[vacpress]

imserv -

tell me, if you would be such a good fellow, would your drivers make this guy:
http://www.automec-direct.netfirms.c...r_details.html

move well if it was given an encoder?

[imserv]

Short of buying one and trying it out, I will qualify that it probably will work very well with the DeskCNC servo driver. It's too bad that encoders are so expensive. $10 for a motor and $35 for an encoder.

The web site does not list the duty cycle. If they are only showing peak load, it may not be the right motor for a CNC milling or routing machine.

They may also be too powerful for some desktop machines, they weigh 3 lbs. I am evaluating some new motors to replace the surplus Pittmans that we use with our kits. They only weigh 2 lbs and look enormous on a Sherline mill.

Fred Smith - IMService

http://www.cadcamcadcam.com

[vacpress]

ah sweet. well. i ordered one, and have some encoders from some oldschool okidata dotmatrix printers which had big servos and encoders... kinda wish i knew how to hack the driver on that thing... anyhow, perhaps i will order a ims drive... thanks for the information.

robert

[HomeCNC]

mysterious:

Have you seen Bloy2004's post on his shoptask conversion? It has lots of good information in there. He went with Servos on his system. I also like Servos over steppers.