JGRO Build nearly complete, motor help please

[tmills01]

I have just finished building my CNC and managed to actually do a little engraving. However I am really disappointed with how smoothly my machine works. I am 99% convinced that it was my choice of $4 stepper motors. They are 12v motors with 47.5 ohms per coil and 7.5 degree steps. That's all the data I had. I got them from a surplus place. It gave me an easy chance to practice with setting up the machine, and it does sort of work.

However I want be more precise in movement I am going to get some of the 205oz motors from HobbyCNC.com. These should work with the simple stepper controller I built which is not a chopper board as far as I know.

However they are a lower voltage so here is my question. Do I need to over volt these motors and how much? I have found a couple of threads that leads me to believe that if I do over volt the motor that I need to use a chopper board to control current through the motor. Also I have a 15v power supply that can source 13amps, would this work?

I am really sorry for all the noob questions. If there is a FAQ I should read please point me there. I did some searches for this info but I couldn't find a clear answer.

Oh and I am having an absolute blast in this new hobby.

Thanks in advance.

Tommy

[BobF]

I believe you do need a chopper board. Look at the hobbyCNC kits. It includes everything you need except the transformer and an enclosure.

[tmills01]

I appreciate the answer. I may have to go that route. I have an enclosure so no problem there.

Is there a FAQ anywhere about how much you should overvolt your motors? What size transformer will I need? I have an electronics background so I understand the idea behind overvolting. The inductors in the motors are going to resist the voltage input until the magnetic field develops, then the resistance drops. The only way to overcome this would be to increase the voltage. I was concerned that just increasing the voltage would dramatically increase the wattage across the winding and might damage the motor. From what I have gleaned, the chopper boards prevent the current from increasing too much and keep this from happening. Please feel free to correct me if I am wrong.

This site has been an invaluable resource so far. I have built a full blown machine and got it working just on info I have found here. BTW I am using the LinuxCNC EMC2 program for control.

One other question I have. Is there a recommended program to convert autocad drawings to gcode?

Thanks again
Tommy

[BobF]

I haven't seen a FAQ, just a sort of rule of thumb about 20 times over rating, but that may be too high in some cases. Most motors have an inductance rating also. Some also come in more than 1 version with different ratings for amperage and inductanc. You may even find a time constant published. This is the best thread I have seen on the subject.
http://www.cnczone.com/forums/showthread.php?t=31815