Advice needed for spindle Servo on lathe


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Thread: Advice needed for spindle Servo on lathe

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    Default Advice needed for spindle Servo on lathe

    I have a CQ6230 lathe that I converted to CNC about 10 years ago. The lathe works well, but I recently had an issue with the spindle VFD and since I had an extra servo motor and drive laying around, I decided to try a servo driven spindle. My initial results are good and I'm starting to revel in the possibilities - parametric gcode etc. Then I started to think about the whole servo thing. My questions are: What size servo motor should I havee used to replace a 1.5 hp single phase motor? I had already changed to a 2.2kw 3 phase motor, because of the benefits related to speed control. When using the lathe at 1:1 gearing, I suspect that my 1kw servo is sufficient, but what about at lower rpm ranges? To take this all the way, in a 50 rpm gear the power from my single please motor was magnified by a factor of 30. Is this necessary? The servo will still produce 1kw at 50 rpm, which seems sufficient to me, but I don't want to have issues due to under powering the lathe down the track.

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    Community Moderator Jim Dawson's Avatar
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    Default Re: Advice needed for spindle Servo on lathe

    My lathe is about the same size as yours and I am working on the designs for CNC. I have been wrestling with the same questions. You have an advantage in that you have already installed a 1KW servo and found it to be OK....so far. My existing motor is also a 3 phase 2.2KW and it is adequate for what I am doing. But, to get the same torque as the 2.2KW motor geared down to 70 RPM is going to require a much larger motor running direct drive. I am considering using a 4KW servo geared down 1.5:1. That would still give 2500 RPM maximum speed. Would lose some torque but I think it would work for the most part. Been trying to figure out how to use the existing gearbox and make it zero backlash, but I don't have a good idea yet.



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    Quote Originally Posted by Jim Dawson View Post
    My lathe is about the same size as yours and I am working on the designs for CNC. I have been wrestling with the same questions. You have an advantage in that you have already installed a 1KW servo and found it to be OK....so far. My existing motor is also a 3 phase 2.2KW and it is adequate for what I am doing. But, to get the same torque as the 2.2KW motor geared down to 70 RPM is going to require a much larger motor running direct drive. I am considering using a 4KW servo geared down 1.5:1. That would still give 2500 RPM maximum speed. Would lose some torque but I think it would work for the most part. Been trying to figure out how to use the existing gearbox and make it zero backlash, but I don't have a good idea yet.
    Backlash control isn't so important on the spindle when you just use the lathe for turning and threading etc. It becomes an issue when you think of the other possible things you could do with it. 😕. I'm not sure but I suspect that the torque output from a servo is fairly constant through the rev range.



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    Community Moderator Jim Dawson's Avatar
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    Default Re: Advice needed for spindle Servo on lathe

    Yes the torque curve on a servo motor is pretty flat through its operating range. The full load torque on a 3 phase 2.2KW motor is about 51.5 in/lbs, a 1KW servo is in the 40 in/lbs range so pretty close. But this depends on the servo motor. A 1 KW servo could be rated for higher speed but lower torque or higher torque but lower speed. Just depends the servo rating. To get the combination of speed and torque you have to go up in power rating.



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    Default Re: Advice needed for spindle Servo on lathe

    As Jim said, the torque curve on a servo motor is pretty flat. Since power is a function of torque and speed, the slower your motor is turning, the less power it is putting out. This affects how much metal you can remove in a given amount of time.

    For instance, the rule of thumb for steel is that you can remove 1 cubic inch per minute per horsepower. So your old lathe could remove about 1.5 cu/in per minute at any speed. With the new 3 hp servo, you can remove 3 cu/in per minute at the rated speed but maybe .1 cu/in per minute at the low end using the 30-1 ratio you mention.

    If you can do it. I would recommend a two speed setup.

    bob



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    Quote Originally Posted by rowbare View Post
    As Jim said, the torque curve on a servo motor is pretty flat. Since power is a function of torque and speed, the slower your motor is turning, the less power it is putting out. This affects how much metal you can remove in a given amount of time.

    For instance, the rule of thumb for steel is that you can remove 1 cubic inch per minute per horsepower. So your old lathe could remove about 1.5 cu/in per minute at any speed. With the new 3 hp servo, you can remove 3 cu/in per minute at the rated speed but maybe .1 cu/in per minute at the low end using the 30-1 ratio you mention.

    If you can do it. I would recommend a two speed setup.

    bob
    I will have a 3.5kw servo that I ordered for the mill by next week. I will give that a try on the lathe first and if o.k. I will order another the same.



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Advice needed for spindle Servo on lathe

Advice needed for spindle Servo on lathe