Re: [CAD_CAM_EDM_DRO] Digest Number 1465

At 07:08 02/07/2001 +0000, you wrote:

>Hoping for some answers for the hopefully not too irritating
>questions:
>
>MOTOR SPECIFICATIONS:
>No. of phase : 4 phase PM Type (pulse modulation?)

Permanent Magnet

>Step angle : 7.5deg+-5%
>voltage : 5
>winding resis: 70 ohm

5V, 70 Ohm means about 70 milliamps per phase. This is WAY too small for
CNC use (see below)

>winding induc: 30mH (1Vrms 1kHz)
>start pulse rate: 600pps
>slew pulse rate: 610pps
>holding torque : 180g-cm
>no. of wires : 6
>QUESTIONS:
>
>What is the difference between 2 phase and 4 phase steppers?

2-phase motors have 4 wires, attached to 2 windings & can be driven only
with bipolar drives. 4-phase motors either have 4 separate windings (8
wires) or 2 centre-tapped windings (6 wires). These can be used with either
unipolar or bipolar drives.

>Are 7.5 deg. steppers good enough for CNC work?

The significant thing is the power, rather than the step angle, but most
7.5 deg motors tend to be too low power to be useful for CNC work. Motors
used in CNC applications tend to be 1.8 degree step angle - sometimes 0.9 deg.

>What is starting/slewing pulse rate? What significance does it have
>for motor controller?

Starting pulse rate I would take to mean the max pulse rate that will allow
the motor to start from rest. Slew pulse rate I would take to mean the max
pulse rate that the motor can handle.

>1g-cm = ??? oz-in (sorry, but someone on the list will be able to do
>this conversion without having to hunt for conversion tables)
>Will the motor be compatible with those CCED motor controllers
>(gecko, camtronics, etc.)

1 oz is 28.4 grammes; 1 inch is 2.54 cm. So 1 oz-in is approx. 72
g-cm. Your motor is therefore about 2.5 oz-in holding torque. To give you
an idea of what is required for CNC, the entry-level desktop CNC mills
(MaxNC, Taig, Sherline...etc). generally use 20 TPI leadscrews driven by
1.8 degree steppers in the power range 80 oz-in to 200 oz-in, depending on
which machine and conversion you are talking about. For example, my Taig
has 200 oz-in motors, rated at 9.2v, 1a per phase.

Short answer: if you want to build something with useful CNC capabilities,
you wouldn't start with a motor like the one you describe. You should be
looking at a motor with a holding torque of 100 oz-in or more - choosing
the power of motor to suit the size of machine that you plan to drive.

Hope that helps -

Regards,
Tony