vfd on a lathe quetion

i have a carrol jamieson 16 swing. i was thinking of adding a vfd to controll the speed. what is the max i can turn rpm wise. i think the highest it turns
now is 510 rpm. i would like to turn small stuff as well as large item. this is a gear head machine head is filled with oil i believe it has taper roller bearing.
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Asp3211968 wrote:

I have a 15" Sheldon R15-6 lathe. It was designed as a general-purpose toolroom lathe, and so has higher speeds than many 15" lathes. The standard model does 1250, the model with the optional 2-speed motor does 2500. Note they specify a different headstock lube for the high-speed model. If you have tapered roller bearings, there may be an upper limit on how fast you can spin it, depending on the size of the bearings. On mine, the main spindle bearings are over 4" diameter, though, so that is fairly big. The high speed oil is Mobil Velocite #6, which looks about as thin as water! There is some confusion on the stadard headstock lube, they specify Mobil Vactra oil Heavy, which is a way oil, but the bearings overheated dramatically with that oil. It may be they meant to specify Mobil Velocite oil heavy, which is about 20 times less viscous.
Anyway, depending on the way the motor-spindle drive is accomplished, there may be easy ways of getting more RPM. If there is a belt-drive anywhere between the motor and headstock, a change of pulleys might allow you to get a higher range of speeds. You can run AC induction motors above 60 Hz to get more RPM. It seems many people think it is pretty safe to take 1725 RPM motors up to 3450 RPM, and in fact some claim motor manufacturers have told them the rotors on 1725 and 3450 RPM motors are identical.
Some newer CNC turning centers run up to 3000 RPM or so on machines with pretty large spindles. I'm not sure it is necessary to get that fast, however. I haven't had my Sheldon over 1500 RPM yet. I so very rarely used the high speed range on my 12" Atlas lathe (mostly because the whole lathe started shaking) so I think maybe 1020 RPM might be enough for your purposes. Be aware that crashing the toolpost into chuck jaws at 1000+ RPM might total the lathe!
Jon
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No, but it sure makes a mess!
I've seen the results when the wrong program got loaded into an Okuma LB2. The spindle was run up to about 3K rpm when the boring bar went in for a deep dive.
End result, a one inch steel boring bar bent like a pretzel, and the machine was down for a few days because the turret had to be re-aligned.
But there was no lasting damage. To the machine.
Jim
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    The critical thing is what is the motor RPM? If it is 1800 RPM, (or slightly slower for slip), you can probably double it safely with a VFD. However, if it is 3600 RPM (or slightly slower for slip), I would not suggest trying to double the speed.
    If the motor runs at around 1200 RPM, you might be able to get away with tripling the motor speed.
    The basic principle here is that the motor manufacturers often use the same rotor in various motors, selecting the speed by the windings. This means that the rotor will take the equivalent speed of a two-pole winding -- e.g. 3600 RPM (with no slip). Slip will always lower the speed somewhat, except perhaps with a hysteresis synchronous motor, which is unlikely to be used to power machine tools. It is more often used to set the speeds of audio tape capstans, or turntables, or other things where the speed must be held constant, and there is little torque required.

    As has already been mentioned, you may well have to change the gearhead and spindle lubricant for the higher speed.
    Another thing, which has *not* yet been mentioned (at least in the articles which I have seen), is that each lathe chuck has its own maximum safe RPM. Either move to a smaller chuck for the higher speeds, use a collet for the higher speeds, or at least check with the manufacturer of the chuck to see whether it is safe to use at the higher speed.
    The top safe speed from a VFD (in terms of damage to the motor rotor) will probably give you a top spindle speed of twice what you already have, or 1020 RPM.
    Good Luck,         DoN.
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<snip horrifying story...>
Even if the chucks don't actually blow up, they also lose their holding grip on the stock.
The okuma I used to run had a chart that showed spindle speed, and hydraulic chuck pressure. And there was a definite 'no fly zone' outlined in red....
Jim
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i have to check the motor, but i was planning on replacing the motor. this machine has the original motor and i do not now if it will handle rpm changes. the chuck that is on it is 10" i could go to a 6" for the small stuff and back to the large for bigger items.
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    I *think* that a 10" chuck will handle 1000-1200 RPM or so, with no problems -- but going to a smaller chuck is probably a good idea anyway.
    Good Luck,         DoN.
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