Stepper suggestions

[Ben Thomas]

Hi there,

I plan on making a CNC machine that has a work area of 1metre x 2metres that can cut HDPE and teflon mainly, both are very soft plastics. If possible I would love the ability to cut aluminium aswell but this isnt essential. I plan on getting the HobbyCNC pro combo kit with the 305oz-inch steppers. Will these steppers be good enough to cut the HDPE and teflon at least and what about the aluminium?

Thanks

[BrianV]

From Ben:
Hi again,
Can you please tell me what rpm the 305oz-in steppers are rated for/capable of?

[BrianV]

Max RPM is not typically a spec provided for a stepper motor. Let me give you a great response from an older Yahoo post that is very well written:

The answer is: There are many variables which determine the top
speed of a stepper motor. For purposes of this discussion I am
going to assume that whatever program/computer combination being
used can always output step pulses at a rate greater than the motor
can handle. This eliminates pulse frequency as one of the variables
in determining the TOP speed of a stepper.

In the following explanation, I’m going to simplify and
take “poetic” liberties in an effort to make the subject
understandable (to a limited degree) by a person with little
knowledge of electrical circuits. So don’t all you EEs out there
jump on me for leaving holes in my explanation.

Most of the controllers used in today’s “hobby” style CNC setups are
CURRENT limiting devices. That is, they do not control the voltage
fed to the motor, they control the current. Many of the controllers
control the current by “chopping” the feed to the motor coil at a
very high rate (typically 20,000 times per second or more). Each
time the current is turned on in one of those 20,000 time periods,
the current flow is monitored by the controller and when it reaches
the desired current flow (i.e. 3 amps for a 3 amp motor), current
flow to the coil is shut off (chopped). (Do confuse this with the
step signals sent to the controller.) This “chopping” takes place
even when the motor is not moving.

Stepper motors are inductive devices, thus they resist any change in
current flow. So as the controller is sending current to the
stepper motor coil, the coil is resisting the flow of the current
because the current is constantly changing in volume. When one of
the 20,000 cycles per second starts, the current is trying to rise
from 0 to the desired value. The motor coil is not just resisting
the simple flow of the current (by the resistance value of the wire
in the coil winding) but also the CHANGE of the current flow from 0
to the desired value.

Here’s where more voltage comes into play.

Voltage is the “pressure” trying to “push” the current through the
coil. The higher the voltage (so long as you stay within the limits
of your controller), the faster the rise to the desired current will
be. This allows the motor to reach it’s rated torque faster during
each of the 20,000 cps thus giving the motor a higher “average”
torque than would be provided at a lower voltage.

There’s a lot more to this story. You would think that when the
current is “chopped” off, the torque of the motor would fall to zero
until the start of the next cycle. Not so. Without going into a
detailed explanation, the collapsing magnetic field created by the
movement of the current through the coil releases some “stored”
energy into the motor which helps to maintain torque during
the “off” stage of the chopping cycle.

The inductive value of the coils of a given motor are giving in the
value “millihenries” (mh). Motors with a high mh value will not
have as high a top speed as similar motors with a low mh value.
However, there is a benefit in that higher mh values result in
higher torque at low speeds.

Hope this helps a bit. Motor speed is a complex subject that is
difficult to explain in a few paragraphs.

Now all you EEs out there, feel free to correct me if any of my
basic premises are wrong.

Regards,
Gary

[Author]

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