Re: Threading on a lathe with EMC

Chris:

I don't see the difference. A VFD used in a loop would be the same as a
servo motor. The computer only outputs a voltage to control torque and
reads position and speed. If EMC was told the acceleration of the
spindle "servo" was very slow EMC would not try to change it's speed
more rapidly than it could respond. EMC would then adjust the carriage X
axis to match the spindle position which it has some but little control
over.

Could EMC handle such an axis? The speed would be very fast but the
acceleration of it very slow. It would kill the idea if EMC had to stop
spindle motion between each move but I understand it does continuous
contouring.

What am I missing. :-) Thanks.

Hugh Currin
Klamath Falls, OR

On 29 Oct, ccs@... wrote:
>
>> Is should be possible to fit a Variable Frequency Drive to the lathe
>> spindle. Then one could control the torque using a voltage output from
>> the computer. This is then very similar to a servo loop. If one fitted
>> an encoder and tach to the spindle these would feed postition and speed
>> back to the computer which would adjust torque to complete the loop.
>> Could this be used with appropriate control loop variables in EMC to
>> deal with the spindle as simply a rotary axis? It would be very slow
>> reacting but the variables in EMC for servo loops might be able to deal
>> with it.
>
> EMC expects every axis to position perfectly, so any error caused by a
> spindle servo loop which is 'slow reacting' will show up in the part.
> There are certainly spindle drives that can keep position accurately,
> but it is far above the normal requirement of simply keeping a steady
> speed. This is why there has been some talk in small machine circles
> of simply using a hefty servo for a spindle motor. Make it powerfull
> enough and you have indexing capability, too.
>