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View Full Version : Increasing top speed of Sherline lathe?



rklopp
04-07-2013, 01:48 AM
Is there any easy way to increase the top speed of a Sherline from 2,800 RPM to, say 3,300 RPM? I have a shaft vibration test I want to run. The test fixture is driven by a Sherline lathe headstock. The fixture would work better with a top speed of 3,200-3,300 RPM. I recognize I can turn up new sheaves (using one of my bigger lathes), but I am hoping for something more off-the-shelf. What happens if I give it 10% higher AC voltage via a Variac?

darryl
04-07-2013, 03:24 AM
2800 rpm- that's it? I would have thought those small lathes would have a much higher maximum- hmm. Anyway, doesn't the Sherline have an ac induction motor? You would have to increase the HZ for it to turn faster, so a VFD might work with it.

Increasing the voltage using a variac is going to overload it with current, and won't increase the rpm at all.

If it has a brush motor, then it's a different ballpark- an increase in voltage will make it turn faster. For the difference you're looking for, it won't hurt it at all. That's assuming the brush motor is a universal one. But I think the newer ones use a dc motor with speed control, so there is a possibility that a simple adjustment will increase your top speed. I don't know what's in the control package, but often there's a means of providing a high limit, whether by an adjustment to a PC board mounted pot, or by changing the value of a resistor.

Would need to know what model you have.

The Artful Bodger
04-07-2013, 04:47 AM
Send an email to Sherline, they answered my question in about 24 hours.

DICKEYBIRD
04-07-2013, 09:00 AM
Don't most all Sherlines have a brushed DC motor/KB motor controller? If so, you can adjust the max speed pot as Darryl suggested and get over 3000 RPM. I haven't tried 3300 on my Sherline mill but I have done over 3000. Worth a try

From the KB manual:

"Maximum Speed Trimpot (MAX)

The MAX Trimpot sets the maximum speed of the motor
when the Main Speed Potentiometer is set fully clockwise.
The MAX Trimpot is factory set to 100 % of base motor
speed. To increase the maximum speed, rotate the MAX
Trimpot clockwise. To decrease the maximum speed,
rotate the MAX Trimpot counterclockwise. See Figure 5.
Note: Do not attempt to adjust the MAX speed above the rated
motor RPM, since unstable motor operation may occur.
For moderate changes in the MAX speed, there will be a
slight effect on the MIN speed setting when the MIN
speed is set at zero. There may be significant variation in
the MIN speed setting if the MIN speed is at a higher than
zero setting."

Set @ >3K, my motor pulsates a bit as it gets close to top speed but stabilizes pretty quickly.

vincemulhollon
04-07-2013, 09:20 AM
I notice my sherline headstock gets warm, almost hot, after an hour or two of CNC work. So hopefully your test at a higher speed is somewhat faster. If its cool to the touch at 2800 then 3300 would likely be OK. Pretty sure it'll be warmer at 3300 than 2800, donno if it'll matter.

Weston Bye
04-07-2013, 10:41 AM
Sherline sells a 10krpm headstock. It is equipped with different pulleys to get that speed and the spindle bearing preload is slightly relaxed. Mine runs cool even at max speed. Now the motor... after an hour of continuous operation has me considering some sort of forced cooling.

darryl
04-07-2013, 07:13 PM
3300 rpm is pretty slow. Most treadmill motors go to 5000 easily, and there are several common motors that go to 20,000 rpm plus- no rocket science here.

On the lathe though, I wonder if the motor is turning much faster than the spindle. I haven't looked at the pulley diameters, but I can easily imagine there being a 2-1, or 3-1 step-down ratio. That still puts the motor rpm in a very safe and achievable range, and 3300 rpm on the spindle would be a sleeper as well. Some of the bearings used on much larger machines have ratings of 5000 rpm and more with grease, and higher with oil.

There may be some effects with the drive circuitry as you increase the rpm, but most likely the circuitry has not been optimized as much as it could be, since normal operating speeds don't show up any problems. There are lots of circuits that work well driving motors over wide speed ranges. I think your biggest limitation is going to be the rpm achievable with the full supply voltage across the motor- you won't be able to increase the speed beyond that unless you increase the supply voltage.

As suggested, there may be an issue between spindle bearing pre-load, rpm, and heat build-up. Still, 3300 rpm is pretty slow. If I recall correctly, the Unimat spindle can be run up around 9000 rpm, maybe more.

rklopp
04-07-2013, 08:34 PM
Thanks, guys. I will first open the circuit box and see if there is a max speed pot that I can adjust. HSM is such a great resource.

rklopp
04-08-2013, 02:02 PM
Adjusting the Max Speed trimpot did the trick. Just as Dickeybird said, the speed is a little unstable and hunts around. This occurs not at top speed, but just below. A sheave change or buying a 10k-RPM headstock is the proper long-term solution. I'm not sure the current application warrants it. Thanks!!!


Don't most all Sherlines have a brushed DC motor/KB motor controller? If so, you can adjust the max speed pot as Darryl suggested and get over 3000 RPM. I haven't tried 3300 on my Sherline mill but I have done over 3000. Worth a try

From the KB manual:

"Maximum Speed Trimpot (MAX)

The MAX Trimpot sets the maximum speed of the motor
when the Main Speed Potentiometer is set fully clockwise.
The MAX Trimpot is factory set to 100 % of base motor
speed. To increase the maximum speed, rotate the MAX
Trimpot clockwise. To decrease the maximum speed,
rotate the MAX Trimpot counterclockwise. See Figure 5.
Note: Do not attempt to adjust the MAX speed above the rated
motor RPM, since unstable motor operation may occur.
For moderate changes in the MAX speed, there will be a
slight effect on the MIN speed setting when the MIN
speed is set at zero. There may be significant variation in
the MIN speed setting if the MIN speed is at a higher than
zero setting."

Set @ >3K, my motor pulsates a bit as it gets close to top speed but stabilizes pretty quickly.

DICKEYBIRD
04-08-2013, 02:31 PM
Great, glad it helped. Hopefully it's a temporary fix & doesn't need to wail away for hours? Dunno if it'd hurt anything long term or not...KB's are pretty tough.:)

rklopp
04-08-2013, 08:13 PM
I found the speed is dependent on the wall socket voltage, because the speed was lower when I moved to a different building. Running the power through a Variac fixed that problem. (The Variac can boost the voltage about 10% above incoming.)