Stepper suggestions

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  • #1888
    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?



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


    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.


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