Spindle RPM constant variation than programmed RPM (CNC Turning Lathe)

[creamroll]

Hi. I am using a CNC Turning Lathe with FANUC controls. The spindle rpm is always on a constant offset of 20 rpm increase from whatever the value is programmed for spindle speed. I have checked at multiple rpm values but the difference value remains constant. Also the program uses the actual RPM after applying offset for further calculations ( e.g. if I program the RPM to be 50, the actual value of the spindle and what's shown on the output display will be 70. I've verified with tach. Moreover if the feed is in rev/mm and I take output on display in mm/min, it will have calculated in accordance with 70 rpm instead of programmed 50) What could be the cause of this issue.. is it a parameter issue or a mechanical fault related to encoder or motor drive?

 

[dandrummerman21]

Need more information.

What model of fanuc control? What year machine? What type of spindle drive and motor?

 

[creamroll]

Hi. Thanks for responding! Fanuc model is Oi-TF. Machine is a couple years old but running time is less than 500 hrs. It recently came under my supervision so it's not known if the problem was inherent or not.. but it seems to me that it exists from the start. Spindle is belt driven. Induction Motor with encoder.

Need more information.

What model of fanuc control? What year machine? What type of spindle drive and motor?

 

[Richard King]

vibration most of the time is bad spindle bearings. buy a automotive stethoscope and listen to the bearings by resting the probe on the casting by the front and rear of the headstock. also on the motor housing. you may have to jump the door switch. run the spindle with the brake turned off and shut it off and as it coasts down, that's when you hear the bad bearings.

 

[creamroll]

Hi. Sorry i didn't understand. Is vibration a cause of linear constant difference of RPMs in actual vs programmed values?

vibration most of the time is bad spindle bearings. buy a automotive stethoscope and listen to the bearings by resting the probe on the casting by the front and rear of the headstock. also on the motor housing. you may have to jump the door switch. run the spindle with the brake turned off and shut it off and as it coasts down, that's when you hear the bad bearings.

 

[DouglasJRizzo]

When i worked at Doosan/DN, I had an applications call like this, once. Turned out that the spindle override knob was misaligned with the markings on the panel.

 

[creamroll]

When i worked at Doosan/DN, I had an applications call like this, once. Turned out that the spindle override knob was misaligned with the markings on the panel.

Right!! I considered that.. but the thing is that the override knob is based on a percentage factor while the difference coming here is a constant value of 20 rpm on a wide range of spindle rpm which I've checked. So does a linear zero error align with the possible override knob markings issue? Still i'll have it checked... just in case..

 

[DouglasJRizzo]

Right!! I considered that.. but the thing is that the override knob is based on a percentage factor while the difference coming here is a constant value of 20 rpm on a wide range of spindle rpm which I've checked. So does a linear zero error align with the possible override knob markings issue? Still i'll have it checked... just in case..

There's also a toothed tachometer belt, check it. If the belt is worn, the control may try to "compensate" for it.

 

[dandrummerman21]

So I pulled this from a alpha-i spindle/servo maintenance manual. Not sure what type of drives/motor you've got but you'd want to look in a similar section of the manual for your hardware.



Edit: but I find it way weird that you say the machine knows that it's 20rpm off. As in, you program 100rpm, without using constant surface speed, it displays on the screen and tachs out at 120, and a programmed 1mm per rev feedrate has it going 120mm per minute?

 

[creamroll]

So I pulled this from a alpha-i spindle/servo maintenance manual. Not sure what type of drives/motor you've got but you'd want to look in a similar section of the manual for your hardware.

View attachment 400967

Edit: but I find it way weird that you say the machine knows that it's 20rpm off. As in, you program 100rpm, without using constant surface speed, it displays on the screen and tachs out at 120, and a programmed 1mm per rev feedrate has it going 120mm per minute?

Yes exactly! It's very weird but it's happening and i checked it over a wide range of RPM around 10 - 200.. In all of these an exact 20rpm is added, which is a behavior just like a zero error. But it's intriguing that it is happening in this case without a known cause.
P.S. i think the motor is analog.. is that what u were asking about motor type?

 

[Ox]

I think that Richard King must have posted in the wrong thread?

Interested in finding out your issue tho!


------------------

Think Snow Eh!
Ox

 

[Matt@RFR]

If it threads ok, is it even a problem?

 

[rjwalker1973]

Does the machine use a variable frequency drive and if so is there a chance that it has been adjusted at the unit?

 

[Vancbiker]

Most all spindle drives have an adjustment for speed offset. Pot, or parameter depending on type and age. If the OP would post the make and model of the drive we might be able to provide an answer.

The lathe feeds ok and threads ok because that is controlled by the external spindle encoder.

 

[creamroll]

Most all spindle drives have an adjustment for speed offset. Pot, or parameter depending on type and age. If the OP would post the make and model of the drive we might be able to provide an answer.

The lathe feeds ok and threads ok because that is controlled by the external spindle encoder.

Hi.. The motor is a GSK model: GS3100Y

 

[creamroll]

Most all spindle drives have an adjustment for speed offset. Pot, or parameter depending on type and age. If the OP would post the make and model of the drive we might be able to provide an answer.

The lathe feeds ok and threads ok because that is controlled by the external spindle encoder.

Actually that's the main issue.. threading is not getting done properly.. every pass starts the threading from a different point and as result forms a new helical..

 

[creamroll]

Does the machine use a variable frequency drive and if so is there a chance that it has been adjusted at the unit?

Hi The motor is a GSK GS3000 series model GS3100Y. I've been trying to find the right parameter in it's manual but so far haven't been successful.

 

[creamroll]

If it threads ok, is it even a problem?

It doesn't actually that's the main issue.. threading starts from a new position at every pass.

 

[Ox]

Can we see your threading code?


--------------------

Think Snow Eh!
Ox

 

[Vancbiker]

Actually that's the main issue.. threading is not getting done properly.. every pass starts the threading from a different point and as result forms a new helical..

Which model Fanuc control? Starting a thread at random is most often a bad spindle encoder on a machine with a Fanuc control.