GH1440 Lathe Bed Conversion

[Valfar]

What you guys are saying makes perfect sense to me, but here's one other thing I cannot understand without actually testing a stepper.

Say a (standard 1.8 degrees/200 steps) stepper motor is rated at 10Nm holding torque (never mind the micro stepping), and we apply a tangent force. With no rotation, just holding the spindle in place, what does this actually mean in practice?

Will the 10Nm stepper keep the spindle dead locked until the 10Nm value is reached, and then it starts skipping in the direction of the force?

Ok, say not until 10Nm value is reached, but will it keep the spindle dead locked until at least 5Nm is reached and than it start's vibrating within the 1.7 degrees interval, and when it reaches 1.8 degrees (10Nm torque) value, it skips in the direction of the force?

Or will it vibrate anyway, no matter the applied force? So for 1Nm it'll vibrate 0.2 degrees, for 2Nm -> 0.4 degrees and so on (is it a linear or logarithmic graph?). If yes, I can understand why a brake is needed in this case.

I hope it makes sense.

Cheers!

[hanermo2]

Depends.
At the full-step point, it will hold at 10 Nm, until the torque is over 10 Nm, and then it will move one step.
Because the holding torque is very much stronger at full-steps only.

So at less than 10 Nm stress, at the full-step position, it will not rotate at all.

At 1/10 microsteps the torque is == 10% iirc.

So, if the stop point is at 9 microsteps off a full step, it will "bend" or comply until it gets to the full step point.
It does not lose steps, but acts as a spring.
When torque is removed, it goes back to the position it was meant to be in.

Servos are different.
A servo "knows" where it is supposed to be, ie what the encoder count must be to stop.
If I rotate the chuck the servo drive led shows how many counts off it is.
It tries to use peak torque, 3x of rated torque, to get back to the position it needs to be in.

So, the 2.5 kW servo, at 10Nm torque and 30 Nm peak torque, tries to use 30 Nm torque x 1:3 belt drive = 90 Nm to get back to the position it "wants" to be in.
The peak torque applies for upto 3 secs, and then goes to sustained rated torque, 10 Nm in my case (2.5 kW).

The effect is very obvious and intuitive.
You can see on the servo drive led, the error by encoder count, in real time.
If you use so much force that you overpower the servo max-error setting, D iirc, it faults.

This takes less than 1 ms, or 0.001 secs.
Typical servo loops are 12 kHz, or 0.12 ms.

Mostly, the spindle can be off by == 0.1 mm at outer edge of 12" chuck before fault.
With very very loose (poor) servo tuning (factory default).

Servos do not have "microsteps" and cannot loose steps.
Servos position perfectly at rest, and then lock (modern ac servos).
When in use, they lag a bit, and this depends on tuning.
The lag is shown on the led at the drive.

The faster the servo runs (or more acceleration), the more lag it has.

[Valfar]

It makes sense, what I understand is when a 200 steps stepper motor is used for the 4th axis, it stays dead locked until the rated torque is reached. So basically I could machine a stock on 200 faces with the ultimate precision (which is a 3 to 6 microns variations as stated by Neale - more than enough for me, as I'm not planing to machine air bearing for instance) dictated by the belt (which is the weakest link in the chain). The stepper will never act like a spring unless micro stepping comes into the equation. To me it sounds like a good option to drive the C axis.

Regarding your particular project and the servo motors, I understand your not happy with how it performs when the spindle takes the C axis role. Is that because the servos tend to act like a spring all the time, no mater the position, due to the lack of steps?

Also, there are these hybrid servo (servo stepper) motors. Do they also come with 200 steps + microsteps + all the other benefits of a servo motor? Or are they just as described by you, and 'hybrid' is just marketing? It's a bit confusing for me. If just marketing, I understand a 200 steps stepper would still be the best option for the C axis, provided there is no brake involved, and provided one must never ever go beyond the rated torque? Also, I'm sure there should be some electrics which could shut down everything in case a step is skipped (but this is not important right now).

I want to connect 2 motors to the headstock spindle. One for turning and one for the C axis. For turning I've got a 2.2kW AC. I'll invest in some 3kW VFD and make an indexer (not sure yet how). But at least I know the direction I should investigate. Now I'm looking for a good compromise for the C axis. The machinery will act either as a lathe, or as a 4 axis milling machine, but NOT at the same time. The AC motor will be permanently engaged (but powered off while in milling mode). I need to find a good way to disconnect the C axis motor while in lathe mode (even if I have to manually disengage the belt), so there would't be any backlash involved. The lathe and the mill would share the X Y axis motors. A BT30 mill would be fitted on the Z axis, to take care of the milling operations. I hope it makes sense.

Please don't hesitate to write down anything you think it may help (or not :) ).

Cheers!

[Valfar]

Servos position perfectly at rest, and then lock (modern ac servos).

Do these modern servos you've mentioned come with an integrated brake of some sort? Similar to what m_c has been suggesting, but integrated?

[komatias]

Do these modern servos you've mentioned come with an integrated brake of some sort? Similar to what m_c has been suggesting, but integrated?

Some do. I have one on the Z axis of my milling machine. Requires a 24V supply to release it.

[Valfar]

Hmm... do you think these servos with integrated brakes would be more suitable for a C axis than a standard stepper? Can these brakes operate fast enough when machining a sphere for instance, with a ball nose end mill?

[hanermo2]

Any brushless ac servo will be vastly better than any similar sized stepper.
A break will work with a stepper, and any servo, just fine.
Numerous examples.

All my servos (9+) and suppliers (3) offered brakes as options.
A break signal is a std output from the servo connector.

The stepper torque depends on where it was stopped.
E. 3Nm stepper, 200 steps, 10 ustep driver (M542 ( gecko 251)).
== same size Nema 23 1.3 Nm / 3000 rpm / 3.9 Nm peak torque servo.

Your position at-rest vs twist depends on how much torque You have, but in terms of accuracy at 1/10 microsteps you only get the 0.3 Nm from the stepper.

The servo positions to 5.000 points (+/- 0-1-2 counts depending. Often zero error).
In each of 5.000 points you have 3 x rated torque, so a 1.3 Nm servo has 3x 1.3 Nm = 3.9 Nm to bring it back to the desired position.

So the servo has about 13 times more torque best case, and 6 times more torque for positioning typical case.

Real world.
The servo positions to 2000 positions and the stepper to 400 positions.

Servo is 5 times more accurate.

Real world.
Servo accelerates to 3000 rpm in 0.1 - 0.2 secs, real world.
Stepper in 0.5 secs to 600-1200 rpm.
Servo is 5-25 times faster in acceleration.
Depending on application and how you measure.

Real world.
Servo runs 5-2.5 times faster top speed.
Depending on application.

Hmm... do you think these servos with integrated brakes would be more suitable for a C axis than a standard stepper? Can these brakes operate fast enough when machining a sphere for instance, with a ball nose end mill?

[m_c]

I've been busy for a few days, so have not had a chance to reply.

Regarding the cutting forces, I'm not sure if tangent was the correct term (it's been a few years since I've had to use such terms!), but imagine you have a bit round bar mounted on the 4th axis, and you want to machine a flat across the top of the bar using a vertical cutter. At the point where the cutter is directly over the centreline and at maximum cutting depth, is where you're going to get maximum torque working against the 4th axis.

You can calculate things to a reasonable accuracy, if you know the angles involved, and how the cutter torque will be getting applied to the workpiece/4th axis.


Regarding brakes, they're generally used for where you need to lock an axis in use (i.e. where you need to move to a set position and lock solidly), or when powered off (i.e. to stop a vertical axis dropping when the system is powered off).
You could potentially use one on a 4th axis, but you would have to generate code that continually unlocks, moves, then locks the motor in between machining. Otherwise you still need sufficient torque from the motor to hold things steady against the cutting forces.


Now Steppers and servos.
What you have to bear in mind, is a stepper is essentially a form of brushless servo motor. Using a suitable encoder and servo driver, you can run a stepper motor as a servo motor.
The big draw back though, is due to the internal design of a stepper motor, you get magnetic detents as the slotted rotor aligns with the permanent magnets, which affects performance compared with a properly designed brushless servo motor, which will have hardly noticeable detents.

Steppers don't lock solid. There is an air gap between the rotor and coil, so you're relying on a magnetic field to hold the rotor, which means there is a bit 'spring' to even the full step position.
As Hanermo has mentioned, microstepping reduces holding torque. The worst point is at the halfstep point, as you have two coils 50% energised, which theoretically puts the rotor exactly between the motors natural detent point, meaning the motor itself is trying to push/pull the rotor to the nearest detent point.

Where servos have the advantage, is the lack of magnetic detent improves performance, and you have an encoder. Servos are rarely perfectly held on position. They will normally always be dithering at least an encoder count or two, especially if they're subjected to any kind of varying load. Under normal use, even with perfect tuning, they will always be out a few counts, however compared with a stepper motor, servos should produce near continuous torque at any point in their rotation, and produce near constant torque over their entire rated speed range.

However, you don't really need to know any of this. If you design the system around rated torques (in the case of steppers, look at the torque/speed graph, to get the torque at the maximum speed you think you'll be machining), then you shouldn't have any problems.

Don't rely on servo peak torques, as they're more to allow for rapid acceleration. If you exceed the rated torque, depending on the motor/driver, the driver will shut down after a set time (my drivers calculate how much energy has been put in the motor, and use an algorithm to calculate if the motor has overheated), the motor may have a thermal switch to shut things down, or worst case scenario with no overheat detection system, you end up with a very hot paperweight.