sizing servos for a cnc lathe

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24 Jun 2022 23:23 #245817 by smc.collins
I have decided to abandon trying to reuse the existing dc servos on my Cincinnati lathe, they don't appear to work like any drive I have any understanding of, there is little to no documentation and they are inefficient DC drives with questionable operating characteristics. 

currently the DC drives on the machine are sized at 

5hp X 

7.5hp Z 

30hp on the spindle. with a max rpm of around 1500 rpm iirc. 

amd I right in thinking I can half size these with AC servos running 2x the speed ?

2.5 hp x 

3.75hp Z 

15hp spindle 

 

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25 Jun 2022 00:01 #245820 by andypugh
Well, you _could_ downsize but one area that DC servos win is low speed "grunt"

I would suggest doing some speculative operations on a feeds and speeds calculator and getting a feel for what 30 spindle HP means.

Your lathe is a monster outside my personal experience.

But: to answer your specific question:

am I right in thinking I can half size these with AC servos running 2x the speed ?


No.

You can halve the _torque_ running 2x the speed with appropriate gear changes, but the power remains the same (that's why we use power in the calcs)

But, what are you making? Do you need to fill a skip with swarf every hour? 30hp will remove about 500kg of metal per day (wild guess, but it is more then 50 and less then 5000)

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25 Jun 2022 00:13 - 25 Jun 2022 00:16 #245821 by smc.collins
I don't need 30hp, and i am ok with reducing power. Option b is replacement dc brushed drives, and obviously they'd need to be correctly sized, so meh. 

I'm open to replacing the drives and keeping the motors, but is the financially sensible ??
Last edit: 25 Jun 2022 00:16 by smc.collins.

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25 Jun 2022 00:17 #245823 by andypugh
I see no reason to change the spindle motor, though. It will only take the power needed. Or if it's truly weird and hard to drive, fit a big conventional indiction motor.

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25 Jun 2022 00:38 - 25 Jun 2022 00:40 #245826 by smc.collins
But can i use a big induction motor like a dc servo ?? I need to be able to orient the spindle, hold it, cut splines etc.  It's got a very difficult to control reliance v*s drive in it now.

Are there any big dc brysged servi controllers out there ?? Outside of this old junk
Last edit: 25 Jun 2022 00:40 by smc.collins.

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25 Jun 2022 00:48 #245827 by andypugh

But can i use a big induction motor like a dc servo ?? I need to be able to orient the spindle, hold it, cut splines etc. 

No. 

You can turn, thread, etc with an induction motor but it won't hold position like a DC servo. In fact a DC servo might be better than a modern AC servo in that role. 

So, why don't we look at a different layer: What about the existing drives is hard to control? You probably don't want to afford equivalent drives. 
 

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25 Jun 2022 01:17 - 25 Jun 2022 01:19 #245828 by smc.collins
The issue is that there is zero documentation, and they operate at non standard voltages. The enable circuits also don't work at all like the schematics call out. To top it off they are using a some kind of bizzare constant current output on one leg with a balancing transformer to control direction, this uses a lot of power at idle for little gain. Also the dc motor on the spindle, that reliance drive doesn't seem to have the capability to do things like splines etc. I'm not really sure it can control a motor at sich low speeds. The documen8 I've found indicate that it isn't a very precise control system.

I'd be ok retrofitting to a newer but used servo drive, i just don't know what to shop for.


The issue os that i am involved with a electric/gas bicycle parts start up, the supplier previous used has gone mia and currently we are milling parts at a huge 10x time penalty. So need to get this lathe operational. I purchased it to help this startup and do some ither dtuff like make obsolete auto parts etc.

I have 60kw of 3 phase power on site, but if i ciuld reduce the power consumption of this lsthe id5 be happy about it
Last edit: 25 Jun 2022 01:19 by smc.collins.

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25 Jun 2022 01:28 #245829 by andypugh
My Spidey-Sense says "Run Away"

Or, at the very least, make it very clear what is "not my fault"

The OEM chose these motors for good reasons, based on what the lathe was expected to do,

You could certainly wind it back, a 3HP lathe can make the same parts as a 30hp lathe. My own lathe is 3hp and it's great,

If it make parts then your "customers" probably won't spot that it makes 4 parts per hour rather than 100.

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25 Jun 2022 01:37 - 25 Jun 2022 03:07 #245830 by smc.collins
The oem chose these motors in 1979 because the industry didn't really have a lot of other proven industrial control solutions.

I put out a email to few suppliers over in chinaville, and i did find a replacement drive.

The lathe belongs nto me BTW, the bike parts startup is just a small business customer I've been serving for a good while now. Ready to grow however.

But I've been trying to find info on dc brushed setvo drives and it's not really out there

Electrokraft i am familiar with. But these Cincinnati servo drives are just, they are weird.

I post the schematics see what you guys think but i suspect the entire machine was designed around these drives

Edit, later units used kollmorgen dc drives, I think this looks like a very affordable way to upgrade and these are welll documented and available in reasonable quantities 
Last edit: 25 Jun 2022 03:07 by smc.collins.

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25 Jun 2022 03:45 #245840 by spumco

But can i use a big induction motor like a dc servo ?? I need to be able to orient the spindle, hold it, cut splines etc.  It's got a very difficult to control reliance v*s drive in it now.

Are there any big dc brysged servi controllers out there ?? Outside of this old junk
 


Are you sure the spindle motor is used for positioning?  Not trying to be insulting - I haven't seen your lathe - but an XZC-axis lathe using the main spindle motor for positioniing is unusual.  Not unheard-of, but odd, unless the spindle motor is an AC or DC servo.

The more common approach is to have an induction motor or PMSM driving the spindle through a belt, and a second positioning motor that gets activated during C-axis moves.  Activation can be a belt & clutch or bull gear on the sindle and a servo that pivots in and engages when needed.

This gives the best of both worlds - spindle motor can get geared properly for a reasonable top speed (4kRPM?), and the 'pony' C-axis servo can have a pretty high reduction for very fine positioning and lots of torque with a smallish motor.

This approach flies out the door on swiss-types or lathes with integral-motor spindles.  But I'd think an old Cincy would have a more standard config.

 

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