Re: Light machine CNC

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Alex Fraser <beatnic@...> wrote:

>
> I got a good price on a Light Machines CNC mill/Lathe. I have the
> controller box, but not the pc card. I figure I'll have to buy some
> more modern electronics to drive this little cuttie. BTW It uses a
> Sherline mill/lathe. I have one of the steppers off of the machine. It
> has 8 pins, but when I looked at the controller cable, which plugs on I
> noticed that they only used 4 pins in the plug. I also don't know the
> voltage these steppers run at. My two questions for the list are;
>
> Why use only 4 of the possible 8 pins and how do I check out this
> stepper to see what kind of controller I need to drive them.

It's probably because 4 pins are all you need for a bipolar drive. Most 4 wire, 6 wire or 8 wire motors can be used with a 4 pin bipolar driver. You MUST use a 6 wire or 8 wire motor for unipolar drives. if you connector only has 4 pin s to the drive it's a Bipolar driver. It's all about how you hook the motor up. If the motor has 8 wires that is 4 separate coils. They can be wired in parallel or series. It affects the current draw to the drive and certain acceleration and upper RPM parameters.

Steppers are not voltage devices. They are current devices. A motor has a specified current rating based on how it is wound and the magnetic structure. Modern drives limit the peak current on each pulse to a point at the motor specs. The time to charge each coil is dependent on the motor inductance (reluctance) and determines how fast you can move from one pole to the next. The more voltage the faster the charge (up to a point) and the faster the motor will spin. Since the coil reaches it's current point quicker the current waveform is shorter.

If you have a motor with a voltage rating on the plate it's just an indication of the DC resistance (meaning if you apply this DC voltage continuously it will draw this much current). From that you can get an idea of the wattage of the device but all the motion is done with non-constant DC waveforms (square waves with a varying duty cycle) so simple ohm's law formulas don't tell you much about magnetic structures (transfomers, coils, motors, etc) and with all AC theory you have to get into current vectors and frequency considerations.

You can find a white paper on steppers on the GeckoDrive website.

>
> Any links would be appreciated.


>
>

You may find it to be less frustration to just replace the controller (and maybe the motors). You will spend a lot of time figuring out the controller you have and how to interface it to a PC based software control (like MACH or EMC). A lot of advances have been made in small scale electronics and motors in the last few years. Prices have come down and you can buy turn key boxes to run anything from a desktop mill or lathe all the way up to Big Iron!

TOM Caudle
www.CandCNC.com