Servo System Set Up

[Gorillakilla]

4 other people and I are making a CNC milling machine for my senior design class and I was put in charge of the electronics. Initially we were going to use a stepper set up but the sponsoring professor seems to prefer servo motors. From what I've read on the forums I will need the following components for a servo system: interface board, drivers (1 per axis), amplifiers (1 per driver), servo motors, encoders, coupling for motors to screws, and a power supply. Since I am new at this I have a few questions.

1) I read somewhere that a 1:1 gear ratio is adequate when connecting steppers to screws. Does this apply to servos as well or is gearing reduction necessary?

2) How much torque is enough torque for the servo motors? We plan on building our machine out of steel, with 18" X 12" X 6" travel, so I supposed it will be a relatively big machine. Would a servo like this be enough?

3) With a budget around $1500.00, including building the machine, would a servo system be feasible or should I go with steppers?

4) Would the power supply for a brushed DC servo be different than one for a brushless AC? If so what do I look for in a power supply?

That is all I can think of for now, but I am sure I will have a lot more questions as this project goes on. Any replies would be greatly appreciated.

[ger21]

1)Since servos have a much higher rpm range, and constant torque throughout that range, servos are usually geared to provide more torque, and higher resolution.

2) Those may be on the small side, but you'll also find that drives for them will eat up a lot of your budget, if not all of it. Rutex at the low end are $200 each. Anything else will be at least $400-$500 each, unless you get some Ebay deals. A few inexpensive drives are being developed right now, though.

HappyServo - Open Source Brushless Servo Controller
New sinusoidal AC servo drive development

3) Depend on what kind of price you can find the servos for. A set of Geckos and a power supply for DC brushed servos will be close to $500-$600 without the motors.

4) Not necessarily, but it really depends on the motors and drives.

Almost forgot, servo drives and amplifiers are usually the same thing.

[carlnpa]

For a first machine I think steppers make a good choice and should keep you within budget. Xylotex and Hobbycnc are first choices. I have both type systems (servo and stepper), Hobbycnc was the first I built and it works very well http://www.hobbycnc.com/cncpackages/...g/4aupcpkg.htm
package with 3 steppers $275

You will also need control software ie Mach 2/3 and a cad/cam program.
I use mostly sheetcam with Mach2/3 http://www.sheetcam.com/ $275 for package. You can use a turbocad program from ebay for around $20 to get you going.

Another opinion, for the size machine you plan, build in aluminum and extrusions ie 8020 will be more appropriate for cutting wood and plastics. If you want to cut steel convert one of the mini mills (around $500).

You have alot of time to invest on the software side.

[Gorillakilla]

Thanks for the replies. Initially we were looking into a stepper set up, but my professor strongly prefers servos; he does work with robotics so I think he deals with servo motors a lot. I do not believe software will be an issue. We have access to AutoCad, Pro-E, Ideas 9, and Solid Works through our university.