CNC lathe motors

For a small, non-production, high precision CNC lathe, what sort of spindle drive motor and drive arrangement would be best?

For example, does variable speed matter?
Very slow speed, as in backgear for threading, probably isn't necessary as the XY drives can go over a thread several times, taking small cuts.
For machining operations, would a single speed be enough? Three speeds?
I was going to use an inverter and a three-phase motor, but is that overkill?
As to drives, I think belt drive is probably best, with an encoder if threading is to be done.
I'm looking for thoughts and experience, any comments welcomed.
Ta,
-- Peter Fairbrother
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wrote:

My thoughts would be that variable speed is almost essential. You want to be able to run at the best surface speed for the material, tool and diameter you're working at, that means variable speed. I find it very useful even on the Myford.
Using software such as Mach3 allows you to run at constant surface speed for turning and parting. Whilst it may help most with production rates, it can also be a great help with surface finish.
An inverter and three phase motor is the simplest way of driving a CNC spindle. Gearboxes are not nice unless you use clutches.
Low speed for threading should be totally unnecessary since you want to run the tool at the right speed for the work. Having got the Hardinge HLV working, I now really enjoy threading at up to 1000rpm, that's still too slow for 1/4"-6mm or smaller threads.
Poly V belts are good, helical gears are good in a gearbox.
These thoughts are worth what you paid for them!
Regards Mark Rand RTFM
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Mark Rand wrote:

I had thought of that, but if a lathe only had one speed, how bad would that be?

Hmm, may be a bit overpowered here, using a 0.55kW or 1Hp motor (have the inverter, need to buy a motor) for a lathe with a swing of 4cm / 8 cm diameter.
Talking of buying a 3-phase motor, they come in 1400 and 2800 rpm models, The 2800 rpm ones are smaller, but apart from speed and size (and presumably torque), is there anything different about them which I should know? About using them with an inverter?

Any idea where to buy small poly-V belts? Long ones are easy enough to come by, but short ones seem hard to find.
Got a salvaged washing machine motor and pulleys and polyV belt for something else, the big pulley doesn't have slots for the vees in it, only the small one. The big one is slightly rounded, as in really old factory belt drives, presumably to keep the belt centered.

And a beer, when we meet again :)
-- Peter F

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Peter Fairbrother wrote:

Peter,
I designed an item using the published Contitech data, it was a fairly short belt PJ737/290J EL, and had trouble getting the belt from their main UK supplier who fobbed me off on 3 occasions saying they'd get back to me and didn't, so went back to Contitech and asked for an alternate supplier and IIRC was given the name Stewart Vaughan & Co. Ltd so contacted them and the guy that answered the phone said he thought they stocked those and went of to check and came back shortly after and asked how many I would like. I sent off the order and included a cheque and got the belts a couple of days later.
I can dig out the invoice to verify if required but the contact details are
Stewart Vaughan & Co. Ltd
Power Transmission Belting
Square Root Business Centre
102-116 Windmill Road Croydon Surrey, CR0 2XQ Tel: 020 8665 4224 Fax: 020 8665 4225
The Contitech design data sheet on -poly V belts is quite good and worth downloading, if you can't find it I'll post a link when I dig it up. IIRC regarding the larger pulley being plain if the drivcn is more than 3x or 5x, can't remember, the driving then it can be plain and for that reason my application used the machined outer surface of a VW brake drum as the driven member and I machined the driving pulley myself having machined a single V form tool and indexed it along with the DRO and radiused the peaks manually.

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Yes, inverter and three phase is definitely the way to go. No need for such a big motor, your inverter will probably have a current monitoring setting that can be reduced to suit something more modest.
1400 or 2800 would work fine, depends on your pulley sizes and speed requirements.
Wilf
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Whilst not a true answer to your question, to save yourself a lot of aggro and doubt, I would suggest and recommend a Denford Cyclone lathe if you can find one. Fanuc OT controller, Lathe sits on a bench about 4 foot by 3, very precise, 6 inch chuck, tool changer etc. We have one, ex college with only 120 hours on it when first got it. Has done loads of work since. They come up on Ebay every now and then. For your info, the spindle is belt driven and is variable drive from the Fanuc. Bob
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