Mazak Nexus - Axis Load Sitting at 30% Idle. Normal?

[Isak Andersson]

Hi. I noticed that the axis load on X and W sits at around 30 % when the machine isn't moving at all. What's even stranger is that the load on X actually goes down when it's cutting in the X direction. How is this possible?

Usually, this wouldn't be a problem but I need a way to monitor the X-axis load during parting-off operations. Monitoring the spindle load, in this case, doesn't really work since it varies too much over time.

The Machine is a QT Nexus 250-II MS.

 

[cnctoolcat]

I noticed that the axis load on X and W sits at around 30 % when the machine isn't moving at all. What's even stranger is that the load on X actually goes down when it's cutting in the X direction. How is this possible?

Higher idle loads are normal for slant-bed CNC lathes, as the X-axis servo motor has to hold the weight of the turret against gravity.

And like you have observed, the load in X will usually drop during cutting, as the cutting forces act as a counterbalance to the weight of the turret, lessoning the amount of current required by the X-axis servo to carry the turret load, and to hold the turret in position while cutting.

As far as monitoring the load in X during cut-off, I would try to get "baseline" X-axis load readings from ideal part-offs, then work from there.

 

[Isak Andersson]

Higher idle loads are normal for slant-bed CNC lathes, as the X-axis servo motor has to hold the weight of the turret against gravity.

Ahh yes, that does make a lot of sense. Still don't know what's causing the W-axis (sub-spindle) load as it's not fighting against gravity. Not really a problem, just a little odd.

 

[Isak Andersson]

As far as monitoring the load in X during cut-off, I would try to get "baseline" X-axis load readings from ideal part-offs, then work from there.

Come to think of it. Wouldn't an overload on X, in this case, mean that the actual cutting forces are less, as it is offsetting less of the weight of the turret? In other words, It shouldn't check for an "overload", it should check for an "underload" because that would actually mean the cutting forces are higher, which is what I'm trying to check for. Can that even be done? Mind-boggling to say the least.

 

[Scruffy887]

Come to think of it. Wouldn't an overload on X, in this case, mean that the actual cutting forces are less, as it is offsetting less of the weight of the turret? In other words, It shouldn't check for an "overload", it should check for an "underload" because that would actually mean the cutting forces are higher, which is what I'm trying to check for. Can that even be done? Mind-boggling to say the least.

Maybe thinking too much about that part.

 

[noname777]

tooooo much thinking.
When X axis is sitting and doing nothing , all you see is amount of work servo amplifier does to hold X axis in stable position. Nothing moves, just fighting gravity, maintaining IN-POSITION tolerance.
When X goes up, servo amp works against gravity.
When X goes down, gravity helps, servo need less power to move the axis.

 

[Isak Andersson]

When X goes down, gravity helps, servo need less power to move the axis.

That's my point. This means that when the insert gets worn cutting forces go up. When cutting forces go up more of the weight of the turret gets offset. When more of the weight gets offset, the X-axis load goes down.

This is what I have observed also. Over time, the X-axis load goes down so not very useful if I'm trying to save my parting-off tool.

 

[Scruffy887]

Can you do the same thing with spindle load?

 

[Kingbob]

Can you do the same thing with spindle load?

That's the value I monitor when parting, I do materials and diameters that create a fair amount of load so I get a pretty good resolution but I can see how on say, small diameter brass you wouldn't load the spindle enough to see much. I don't exactly know the op's need case for x.

 

[EmGo]

Wow. American Tool slantbeds had - wait for it ! - a hydraulic counterbalance. Gravity and turret/slide weight had no effect on X axis.

I will refrain from stating the obvious, since it'll just get your panties in a twist

 

[Isak Andersson]

I don't exactly know the op's need case for x.

I tried monitoring the spindle load (on the main) but it just ramps up too much as it accelerates. It almost hits 100% sometimes and it's not consistent at all. I'd say the actual cutting forces give about 15% spindle load, certainly not 100%.

 

[cwhuffman]

My QTN250MS sits around 25-30% X axis load while it sitting stationary. As others have said, it is gravity pushing against the ball screw and servo motor, causing it to show a load. If the axis is in the cutting direction, gravity is helping to pull the tool through the workpiece so it is understandable why you do not see a spike. When the axis returns going towards home, you will probably see a spike. Also on mine I have noticed that when I power the machine down, the X will sort of "sit" onto the ball screw as gravity forced the weight against the servo motor that is no longer being powered.

 

[noname777]

you should be looking at chips (swarf) formation, as it is the only true indication that tool is used properly and all feeds and other parameters are okay. If you aren't making good chips, you aren't cutting, you are burning

 

[cnctoolcat]

the X will sort of "sit" onto the ball screw as gravity forced the weight against the servo motor that is no longer being powered.

Most CNC lathes (and all Mazaks AFAIK) utilize a small brake that locks the X-axis when the power is off.
Usually located between the X servo motor and ballscrew, this small brake requires 24V DC (for a Mazak, not sure about others) to release, therefore anytime the machine is on, the brake must have power.
The instant you power the machine down, current stops going to the brake and it locks.