# Strange issue - RPM's are jumping



## tarmo120 (Mar 19, 2014)

Hi guys.
I haven't been here for a while because of work. But now all of a sudden my lathe has electrical issues.
Just one at the moment.

Whatever speed I use the rpm's are not stable. The lower the spindle speed the more it fluctuates.
This almost feels like the motor is getting low power at one point and then double power the next point.
It appears that only when under a big load this problem does not surface.
The lathe is some asian one bought from Finland.
Make and model is as follows. Nova 290VF x 700G
*http://www.koneita.com/metallisorvit/metallisorvi_nova_290VFx700G.htm*

Now the system is using a potentiometer for controlling the motor speed.
The lathe is built using 220V motor and only 1 phase.
And the power is coming directly from the powerbox or powershield. I am not sure what is correct.
I do have some kind of schematics of the system, but since it was written by the asian plant, I am not sure how accurate it is.
The most annoying thing is that the slowest speed is around 50rpm, and with that speed the rpms fluctuate between 35-80.
I will try to make a short video about this.

Hope someone can help me.


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## Ray C (Mar 19, 2014)

How are you confirming that the RPMs are fluctuating?  Have you put another tachometer on it to double check that the problem is not just the display?

I'm familiar with that basic model but have no idea what factory it was made.  Anyhow, at lower RPMs most tachometers do not show steady readings.  First verify that the RPM is actually fluctuating.


Ray


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## tarmo120 (Mar 19, 2014)

Well there is a digital tachometer that shows the fluctuation. 
And also I can hear and see it on the chuck.
You know when everything is right and the chuck is moving, it has symmetrical reflections.
But in my case I can actually see the chuck slowing down and then speeding up again.
As I said, I will try to make a video.


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## Ray C (Mar 19, 2014)

tarmo120 said:


> Well there is a digital tachometer that shows the fluctuation.
> And also I can hear and see it on the chuck.
> You know when everything is right and the chuck is moving, it has symmetrical reflections.
> But in my case I can actually see the chuck slowing down and then speeding up again.
> As I said, I will try to make a video.




With power off.  Check the belt.  Also check all the side gears and make sure there are no gear keys missing.  Make sure the leadscrew gears are properly engaged.

Taking a video probably won't show the issue since the frame-rate will not accurately depict a rotating object.

Ray


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## dave2176 (Mar 19, 2014)

Is it 220V because that's the standard voltage in the country? Does the hz match? The 1.1kw sounds similar to what's used on some of the larger small mills. Does it convert the voltage to dc to control the speed? It seems I ran across a thread somewhere that was talking about small mills with speed hunting issues. Maybe they were able to adjust the pots on the control board to settle it down? Anyway there was a lot of talk in one portion of the thread that was about the boards and pots. Probably Hoss's G0704 thread on cnczone. I had someone in our word turning club with the same mini lathe as me that had a run away speed problem that the manufacturer replaced the control board to fix it. Sorry I'm not much help here.

Dave


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## tarmo120 (Mar 20, 2014)

The belt is in superb condition, hardly any wear.
The side gears are also like new, hardly any wear.
The power is delivered from the motor by 2 pulleys and one belt.
The side gears are only for feeds and threads.
I am using the lower speed that is documented to run from 50rpm to 950rpm.
And it does when I am looking at the digital tach.

Should I maybe replace the pot with something of better quality/accuracy?


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## Tony Wells (Mar 20, 2014)

If it is accessible, try to get some Deoxit 5 series contact cleaner. Just a small amount on the wiper in the pot can remove possible oxidation and make things settle down. If you have Radio Shack there, I believe they carry it. Definitely try that before replacing the pot. Sometimes when a pot isn't used often, they will oxidize and cause all sorts of weird problems, so it is worth a try before replacing it.


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## COMachinist (Mar 22, 2014)

Hi
Look for a loose shielding or grounding connection. Those inverters put out a lot of broadband electrical noise. Your motor control board maybe picking up noise. If it is not that most likely your motor freq control is shot. 
Good luck
CH


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