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stepper /servo motor torque requirements

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  • stepper /servo motor torque requirements

    I was having trouble finding what sorta actual stepper torque requirements are needed for a small knee mill's x and y axis, so I did a couple experiments. so with a sharp 1/2 inch end mill 1/2" deep and moving at about 1.5 to 2.5 inch per min. travel 500 rpm cutting hot rolled steel I was seeing about 20-25 inch pounds of torque on a small torque wrench. using a 5/8 at 5/8 deep at say 2- 4 in per min seeing about 35 inch pounds. ( the kinda speeds and feeds like I was paying for the cutters) the next test was a 1/2 dull end mill .500 deep and 500 rpm and all the feed I thought it would take going from heavy blue black chips to cutter glowing and sparks basically just short of breaking the cutter off( like some one else was paying for the cutters) so with that it was about 100 inch pounds. I have like a 3/4 scale bridgeport, a 9x 36 enco. so this would be about all one would reasonably load it up too. Chris
    As an update the data was gathered using an acme lead screw they are about 25to 30 percent efficient . Using a ball screw would be 90 percent efficient or be roughly half that torque for the same forward thrust of the table.
    Last edited by chrsbrbnk; 03-18-2015, 07:25 PM. Reason: more data

  • #2
    I retrofitted a manual series one bridgeport years ago and used 906 oz/in steppers. They worked very well, no complaints. Your 100 inch pounds converts to 1600 oz/in. I had a 2:1 belt drive to the ballscrew which doubles the motor torque, so 906 oz/in X2 = 1812 oz in for my retrofit. As you can see my steppers had a reasonable comfort zone above your measurements.

    As a side note, I used the 906 oz in rather than the more common 1200 oz in because their inductance was much lower which gives far less torque roll off at higher speeds. The small motor produced much more torque than the larger motor over 1000 rpm. Steppers are rated for holding torque, zero rpm, torque drops off as speed increases. The inductance spec for a stepper greatly effects the higher rpm performance. Bigger is not always better. It pays to look closely at the specs.

    It takes extra power to accelerate a axis up to speed than at steady state cruise. Kinda like a car, little more gas to get up to speed, then less to maintain that speed.

    All the retrofitting I have done since that first bridgeport have been servo motors. Servo motors have much different characteristics than steppers.

    Last edited by Sparky_NY; 02-25-2015, 02:51 PM.


    • #3
      Smaller VMCs use 900-1000W servos on their axes. A shop saber router a guy had used 400w servos to drive the gantry and 200w on the gantry itself. I use 1kw servos on my mill and they are overkill (What I had lying around)