Using Servo Motor as Spindle Drive, Is it OK?

[elanman99]

I have an X1 mini mill and I am thinking about improving the spindle drive so getting rid of the plastic gears etc. In fact I would discard the motor/contoller and the box that contains it all.

What I have is a (bit oversize) BLDC Electrocraft servomotor and BRU drive and am considering a belt drive by mounting it behind the column so that the belt passes either side of the column. I anticipate that the motor will be bracketed off the head casting which means that I will need some sort of splined shaft on the mill spinndle to accommodate the 30mm or so quill movement. The motor I have runs up to 4000 rpm and I intend to have a 4:1 pulley ratio to give me 16,000 rpm max.

I have set the drive up so that the motor speed can be adjusted with a potentiometer so in some ways its just like a VFD on an induction motor but maybe its not not as good as a VFD as there is no built in accel/decel feature?

I know many people have modified their machines to belt drive, usually using the original motor but I would be interested to hear any comments on my scheme.

Ian Phillips

Cheshire UK

[Al_The_Man]

I have used both DC and BLDC servo motors for a spindle on small machines, are you using the drive in a velocity mode? As it is best if you have some kind of speed reference feedback to maintain a close to constant rpm.
You do realize your motor torque will be reduced by 4 due to the reduction?
Al.

[elanman99]

I have the BRU Advantage software and have set the motor up in velocity mode so that the speed is controlled by an anamlogue voltage. The motor and drive I have is 2 Kw so I have too much torque for this size of machine anyway and the high speed will be much more use.

The motor has a 4000 line encoder which I suppose will control the speed rather well.

Ian Phillips

[Al_The_Man]

I realize it may be controlled by analogue, but the feedback is important, unless it uses the commutation as tach feedback?
i.e. closed loop velocity as opposed to open loop.
Al.

[elanman99]

I realize it may be controlled by analogue, but the feedback is important, unless it uses the commutation as tach feedback?
i.e. closed loop velocity as opposed to open loop.
Al.

The drive manual states that the velocity is 'Digital PID with less than +/-2rpm ripple in 1000 rpm'.

I am not sure whether the motor commutation is derived from the shaft encoder or whether it uses the stator windings for power and position sensing. Either way the motor speed seemed to be very stable a when I tested it by loading the motor shaft.

One of the options shown in the manual is to control the drive with another shaft encoder, it appears that the motor will mimic or follow the encoder position. That gives me an idea to use the machine for cutting threads with a tap 'effortlessly'. I am sure I could break lots of taps!

Ian

[SCzEngrgGroup]

What're you going to use for spindle bearings to deal with that kind of RPM? 16K don't come easy....

Regards,
Ray L.

[elanman99]

What're you going to use for spindle bearings to deal with that kind of RPM? 16K don't come easy....

Regards,
Ray L.

You are right, I've not given it any thought yet. The spindle has two cheap 61905 ballraces (42x25x9wide) as standard. I will see what is available in that size and set the machine top speed according to what I can get.

Ian