Spindle options for desktop mill G0704

I have a grizzly G0704 which is a popular conversion to cnc but most people use Mach. As a Linux systems administrator I prefer Linux.

I would like to add a position controlled spindle motor. The G0704 comes with a 1HP brushed DC motor. I would like something around 1.5-2kW (2-3HP) that can do rigid tapping and will spin at 5000 rpm. I would like to spend $500 but could stretch to $800 if needed. From my research I have a few options

1) Use a brushed DC motor (treadmill motor) and add an encoder. LinuxCNC can use the encoder to position the motor. Cheap and easy. Can LinuxCNC cope with a pulley ratio on the spindle? IE if I setup a belt drive with a 1:2 ratio can I inform LinuxCNC of that and the encoder pulses will be read accordingly?

2) Use a brushless DC motor and a controller. There are a lot of e-bike and electric vehicle components that look perfect for this. The problem is most of the motors are about 8 inches wide which is too big to fit on top of my mill. I need something about 4-5 inches. It's also really hard to find a 2kW motor, there are lots of 500 watt motors and 5kW motors with nothing inbetween.

3) Use a full on servo motor with drive. I really don't know much about this despite spending over 10 hours researching it. I think I could use an AC brushless servo motor and the Mesa 8I20 card but I'm not sure what else I need. Seems like I need a 200-400v DC power supply too. The whole matching servo motors to drives seems expensive and if something breaks I would possibly need to replace both.

4) Use a VFD and a 3 phase motor with an encoder on the end. The problem with this is the 56 frame motors seem to be the only ones out there and they are too large. Is there a smaller frame size?

I'm leaning towards the servo motor with the Mesa 8I20. I can pickup a servo motor on Ebay, not worry about finding a matching drive, and I have a PC that can run a 4Khz servo thread. I'm just not sure what else I would need

If anyone else has done a position controlled spindle on the cheap I would love to hear about it. I can do 120v or 220v power but not 3 phase for the input power.

library wrote:

I have a grizzly G0704 which is a popular conversion to cnc but most people use Mach. As a Linux systems administrator I prefer Linux.

I would like to add a position controlled spindle motor. The G0704 comes with a 1HP brushed DC motor. I would like something around 1.5-2kW (2-3HP) that can do rigid tapping and will spin at 5000 rpm

You do not need a positioning servo to do rigid tapping.
I do it regularly on my Bridgeport mill, with a standard VFD and induction motor. I do have
a spindle encoder with index rigged to the machine. I regularly do 4-40 and have done as
small as 2-56 taps (these are under 2mm size) with excellent results.

So, unless you have a different need for spindle positioning, you don't need a positioning servo
for threading or tapping.

I have used one of my servo amps with a DC brush motor on a minimill I drag around for
demos, and that is also capable of rigid tapping. The key point is that the Z axis
slaves to the spindle, rather than the spindle is slaved to the Z motion.

Jon

I appreciate the feedback.

I know the VFD with a 3 phase motor will work but it's just too big. I have a power drawbar that also has to go on the head and the whole head is about 5 inches wide by 9 inches deep. I've actually considered building a bigger head but that's at the bottom of my list. If I could use a VFD to drive a servo motor I would do that but I'm pretty sure I can't.

I would like to use a brushless motor if possible due to the better performance and not dealing with brushes. The servo motors are just the right size. Tall and skinny. If I could drive a servo motor like a normal brushed motor I would be all over that in a heartbeat. If they sold a tall skinny brushless DC motor then I could power it easily with an e-bike controller and an old Dell Blade Chassis power supply.

I suppose I'll probably fall back to the brushed dc motor with an encoder on the end. I just hoped there might be a better option that I've overlooked.

Is there anything the motor would need to do for rigid tapping? Does it need to stop quickly? Start quickly? What's really needed other than a simple encoder. Do I need spindle speed control or linuxcnc just slaves the z axis to whatever rpms the motor is going at?

library wrote:

I would like to use a brushless motor if possible due to the better performance and not dealing with brushes. The servo motors are just the right size. Tall and skinny. If I could drive a servo motor like a normal brushed motor I would be all over that in a heartbeat. If they sold a tall skinny brushless DC motor then I could power it easily with an e-bike controller and an old Dell Blade Chassis power supply.

Well, you can get a variety of drives for brushless servo motors. Just because the motor says
"servo" on it does not mean it needs a SERVO drive! All these permanent magnet brushless
motors are very much the same inside. I make brushless servo amplifiers that could be
used to drive a spindle up to about 1 HP or so, depending on the motor's voltage.
It could be controlled with my Universal PWM Controller.

Is there anything the motor would need to do for rigid tapping? Does it need to stop quickly? Start quickly? What's really needed other than a simple encoder. Do I need spindle speed control or linuxcnc just slaves the z axis to whatever rpms the motor is going at?

library wrote:

I would like to use a brushless motor if possible due to the better performance and not dealing with brushes. The servo motors are just the right size. Tall and skinny. If I could drive a servo motor like a normal brushed motor I would be all over that in a heartbeat. If they sold a tall skinny brushless DC motor then I could power it easily with an e-bike controller and an old Dell Blade Chassis power supply.

Well, you can get a variety of drives for brushless servo motors. Just because the motor says
"servo" on it does not mean it needs a SERVO drive! All these permanent magnet brushless
motors are very much the same inside. I make brushless servo amplifiers that could be
used to drive a spindle up to about 1 HP or so, depending on the motor's voltage.
It could be controlled with my Universal PWM Controller.

Is there anything the motor would need to do for rigid tapping? Does it need to stop quickly? Start quickly? What's really needed other than a simple encoder. Do I need spindle speed control or linuxcnc just slaves the z axis to whatever rpms the motor is going at?

No, too rapid starting/stopping of the spindle will make it hard for the Z axis to follow!
You want a somewhat gradual reversing at the bottom of the hole so the Z keeps
up with it. If the spindle can stop within one revolution, that may be as rapid as
you want it. You mostly just want it to stop quickly enough so you don't drive the
tap in so far the shank chews up the hole. (Rigid tapping blind holes is a bit
too nerve-wracking for me!)

You don't actually need computer spindle speed control, but it is easy enough
to do now that you might as well do it. Also, this allows the software to control
RATE of spinde reversal, which is desired. The Pico Systems brushless PWM
servo amp and PWM Controller can handle this, that't how I did it on my
minimill (although I kept the BRUSH DC motor on mine.)

Jon

Well, just as an update I decided to go with a VFD and a 1.5HP motor. I'm pretty sure I can make it fit because I'm adding a spacer between the column and head of the mill to increase the y axis travel and this will give me the few inches I need. I'm also changing my power drawbar plans to use a hydraulic ram instead of an air cylinder so it can be narrower. With those two parts in place I'm pretty sure I can make it work. I would love it if large servo type motors were cheap and easy but honestly VFD's and AC induction motors are the cheapest easiest way to go.

I found a screaming deal on a 2HP Emerson Commander SK VFD (the exact same one Tormach uses) and I wanted to go with a 2HP motor but they are too heavy. Also, I decided to be realistic and 2HP is probably excessive compared to the rigidity of the mill. The motors I'm looking at have 1000:1 constant torque so they will provide full torque throughout the entire low range. I'm planning on mounting a cheap rotary encoder on the back of the motor.

So let's clarify the encoder part. I'll more than likely be running the motor with a 1:2 pulley. BigJohnT indicates that I need the encoder to match the spindle speed, not the motor speed. I did some more reading and it appears what I really need is the index pulse to match the spindle speed, the A/B quadrature output from the motor encoder can apparently be scaled. I could probably put an index only pulse on the spindle, that seems like an easy electronic part to make.

library wrote:

Well, just as an update I decided to go with a VFD and a 1.5HP motor. I'm pretty sure I can make it fit because I'm adding a spacer between the column and head of the mill to increase the y axis travel and this will give me the few inches I need. I'm also changing my power drawbar plans to use a hydraulic ram instead of an air cylinder so it can be narrower. With those two parts in place I'm pretty sure I can make it work. I would love it if large servo type motors were cheap and easy but honestly VFD's and AC induction motors are the cheapest easiest way to go.

I found a screaming deal on a 2HP Emerson Commander SK VFD (the exact same one Tormach uses) and I wanted to go with a 2HP motor but they are too heavy. Also, I decided to be realistic and 2HP is probably excessive compared to the rigidity of the mill. The motors I'm looking at have 1000:1 constant torque so they will provide full torque throughout the entire low range. I'm planning on mounting a cheap rotary encoder on the back of the motor.

So let's clarify the encoder part. I'll more than likely be running the motor with a 1:2 pulley. BigJohnT indicates that I need the encoder to match the spindle speed, not the motor speed. I did some more reading and it appears what I really need is the index pulse to match the spindle speed, the A/B quadrature output from the motor encoder can apparently be scaled. I could probably put an index only pulse on the spindle, that seems like an easy electronic part to make.

I am using one on Jons pwm-servo amps to drive the stock 1 hp motor, but using a 126 volt supply good for around 20 amps surge. So its likely I'm peaking at around 2 hp.

I tried to make an all optical encoder disk but quantization noise from the number of slots available destroyed low speed, hammering the motor so bad it sounded like the spindle bearings were destroying the ball cages or had square balls.

So I bought a 1000 line Omron encoder, and made a short shaft extension to drive it off the top of the motor. Then I put a notch in the shift knob rim, and added a couple switches so LCNC knows what gear its in. Then I added some counter stuff to my hal so I could determine the gear ratios. I and also using the index pulse from the optical encoder, which gives LCNC a one full turn reference. A mux4 tied to the gear knob tally's selects the encoder scale, so the tach is still correct. I took advantage of the lack of a gear tally to feed an offset into the mux4's unused inputs so the motor if on, is getting about a 30 rpm at the motor signal, so when the gear is between home positions, the gear change is smooth as it meshes into each gear, so i don't have grab the spindle by hand and turn to mesh the gears.

Counting a A pulse for 100 turns of the spindle gave me the two "scale" figures. I use one in the .ini file, and scale it by the ratio when in the other gear. The numbers are just over 7000 and just over 14000. Quantization noise is gone, and the Pgain is raised from 1.5 to 20 and its still dead stable. So speed control is extremely stiff and I am unaware for how hard its working until I hear the chirp from the motors iron as Jons amplifier goes into current limit at about 17 amps. The motor is rated at 9.7 amps, but I've not hurt it in better than 2 years. I do have a limit3 in the reversal circuit to slow that so the z drive can keep up, but I can still do a turnaround at the bottom of the hole in 400 milliseconds even at 3000 revs. Now, if the rest of the machine was truly square, I'd have a real mill. But I lack the gages to determine where its off.

Best mod ever for one of those machines.