Indexing Headstock Spindle

[digits]

1. 3HP, as I mention above would be a good number. Take a look at the spec on most CNC lathes--that's still light. 7.5 HP is a more likely number.

OK, so that's going to be a pretty chunky main motor and drive - what's a ball park cost for a 7.5HP motor and vector drive?

3. Where's the harm in driving with 2 motors? As I've mentioned, the circuit will be opened, so we're not generating any current. What design do you propose to economically disengage the motor?

Even if the 2nd motor is open circuit, it will still generate a large EMF when spun at turning speeds. You would have to be sure that this can't arc across your disconnection circuit, or reverse bias any semiconductors in the drive if you're disconnecting electronically. What circuit do you have in mind for swapping the motors over, and what happens if you hit e-stop or have a power failure under high-speed/high load operation? If your circuit disconnects a motor while it is still spinning, I would expect really, really bad things to happen to the driver board. Switching a multi-kilowatt inductive load is not trivial!

I'd also imagine that a 7.5HP spindle motor is going to have a shed load of rotational inertia - your indexing motor will have to overcome that if both motors are always physically coupled to the work. If your drive train uses gears, couldn't you just rotate one motor so that it comes out of mesh, or use a mechanical clutch? Similarly, a belt drive could move a motor pulley out of the way and push an idler up against the belt in its place to maintain the same tension?

Just my 2p's worth...

[BobWarfield]

digits, thank you for your thoughts. I don't see anything here that isn't pretty straightforward to deal with.

RE your thoughts disengaging, I have a hard time imagining the gear linkage that isn't full of backlash, which is a no-no for an indexing rig. It's possible a belt and idler arrangement would work better, but even there, we would typically want a toothed belt arrangement for reasons of backlash.

Doesn't sound very practical to me.

Cheers,

BW

[Uguessedit]

I think two motors is feasible. I quick connect clamp engagement makes sense for the servo to prevent feedback and driver death. Trying to run a single large servo where a gearbox is involved brings in the element of a belt drive flexing and a gear box with excessive backlash. I just don’t see how that can be done reliably without a comprehensive overhaul. It could be configured to a 5 minute setup if everything planned accordingly and you’d then have the option for a c axis when needed. My suggestion on another post is to use actual gears for the servo and run a 1:10 ratio with a 400w or even a 750w Chinese servo at a max 3000 rpm which would be more than suitable for machining through holes and various other tasks. I’m not convinced you’d necessarily need a motorcycle brake unless you plan to plunge hard and fast. Even so that wouldn’t be too difficult to implement off the backside of most spindles with a simple clamping adapter for the brake rotor and an externally mounted caliper bracket. I’m considering doing this exact project. Ultimately it’s a small expense is see to integrate it. So what if I have to **** down and swap a couple connectors and engage a servo. I’d leave the main spindle motor intact less the spindle control which could be switched and let the servo rotate that around at low speeds while I proceeded with milling tasks. I’d be curious to know if anyone else has done this since this conversation began. These are discussions I really enjoy. Creativity and ingenuity in implementing a custom application to suit ones needs.

I suppose the first question I need to ask is how much spindle power do you actually need - and better still, if you know it, what sort of torque at what sort of speed? Those homeshopcnc servos are good for 600W continuous (~0.8HP) and can be driven cheaply with a G320.

I'm guessing though that you'd rather have a 3-phase induction motor for your spindle. If you want to simply index without cutting, unless you have very irregular parts , I can't see that you'd need much torque at all for the indexing. So, why not just drive your main spindle with a drive capable of indexing and add an encoder for feedback? If you don't need 3HP to index, couldn't you poweroff your VFD, disconnect it from the motor and then use a cheap AC-servodrive (e.g. Granite Devices) to index at only 100-200W of power?

Two motors can be made to work, but IMHO it is a bad idea to keep them both mechanically connected at the same time - couldn't you just physically disengage the indexing motor from its belt when you change modes?

[Uguessedit]

Perhaps a gear off the back of the spindle and servo mounted to the rear using a brake if desired. Now with a servo directly mounted to the spindle bypassing the gearbox it shouldn’t exceed the rpm if you factor everything correctly. If you have a high speed vfd driven spindle than that may pose complications where you have to disconnect the c axis when not in use. For low speed use I don’t see the spindle motor back feeding through any type of disconnect that could be applied to the spindle control. Simply it may just be easy to have two quick disconnects you swap over from one another and not worry about it. If done with an idler gear you wouldn’t have to remove the servo itself you would just have an engagement lever and quick disconnects to swap out. 30 second changeover?. Maybe implement a safety feature so one can’t be ran while the other is engaged?

OK, so that's going to be a pretty chunky main motor and drive - what's a ball park cost for a 7.5HP motor and vector drive?



Even if the 2nd motor is open circuit, it will still generate a large EMF when spun at turning speeds. You would have to be sure that this can't arc across your disconnection circuit, or reverse bias any semiconductors in the drive if you're disconnecting electronically. What circuit do you have in mind for swapping the motors over, and what happens if you hit e-stop or have a power failure under high-speed/high load operation? If your circuit disconnects a motor while it is still spinning, I would expect really, really bad things to happen to the driver board. Switching a multi-kilowatt inductive load is not trivial!

I'd also imagine that a 7.5HP spindle motor is going to have a shed load of rotational inertia - your indexing motor will have to overcome that if both motors are always physically coupled to the work. If your drive train uses gears, couldn't you just rotate one motor so that it comes out of mesh, or use a mechanical clutch? Similarly, a belt drive could move a motor pulley out of the way and push an idler up against the belt in its place to maintain the same tension?

Just my 2p's worth...

[Uguessedit]

Perhaps a gear off the back of the spindle and servo mounted to the rear using a brake if desired. Now with a servo directly mounted to the spindle bypassing the gearbox it shouldn’t exceed the rpm if you factor everything correctly. If you have a high speed vfd driven spindle than that may pose complications where you have to disconnect the c axis when not in use. For low speed use I don’t see the spindle motor back feeding through any type of disconnect that could be applied to the spindle control. Simply it may just be easy to have two quick disconnects you swap over from one another and not worry about it. If done with an idler gear you wouldn’t have to remove the servo itself you would just have an engagement lever and quick disconnects to swap out. 30 second changeover?. Maybe implement a safety feature so one can’t be ran while the other is engaged?

OK, so that's going to be a pretty chunky main motor and drive - what's a ball park cost for a 7.5HP motor and vector drive?



Even if the 2nd motor is open circuit, it will still generate a large EMF when spun at turning speeds. You would have to be sure that this can't arc across your disconnection circuit, or reverse bias any semiconductors in the drive if you're disconnecting electronically. What circuit do you have in mind for swapping the motors over, and what happens if you hit e-stop or have a power failure under high-speed/high load operation? If your circuit disconnects a motor while it is still spinning, I would expect really, really bad things to happen to the driver board. Switching a multi-kilowatt inductive load is not trivial!

I'd also imagine that a 7.5HP spindle motor is going to have a shed load of rotational inertia - your indexing motor will have to overcome that if both motors are always physically coupled to the work. If your drive train uses gears, couldn't you just rotate one motor so that it comes out of mesh, or use a mechanical clutch? Similarly, a belt drive could move a motor pulley out of the way and push an idler up against the belt in its place to maintain the same tension?

Just my 2p's worth...