GH1440 Lathe Bed Conversion

[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.

[hanermo2]

1. A slight correction.
Old-tech type servos, dc brushed servos like geckodrive 320, of which I have 7, used to dither.
The geckos are not in use anymore.

New ac brushless servos find the position commanded, and lock.
Zero dither.
They are "live" about 0.5 secs, and then lock solid.

I have 2 brands, about 15 total in use and in stock, all are the same.
From 400W / 60V, 750W/220, 2.5 kW/220V.


The bigger servos mostly have a led display, you can configure, as std it shows the error count.
So you can see the error on the led, which always goes to zero, and then the servo locks.

The moral:
A 1.3 Nm (400W) servo is the same size as a Nema 23 stepper.

A full set costs == 290€ EU 22% VAT.
A Nema 23 new stepper set == 40 + 50 + cables ==100 €.
A servo is about 200€ more / cheap small nema23 stepper, per axis.

But a Nema 34 stepper system, high voltage, with a 150€ driver, is about 250€ all-in.
The modern ac servo system is vastly better.

So the steppers are very cheap, pretty accurate, but have low dynamic range.
This means stepper systems are either accurate, fast, powerful, but not all 3.

Vast numbers of excellent routers have been made with steppers.
Including mechmate-sized 10k$ systems for workshop use.

I am not a servo-zealot by any means.
For one, you must have hw limit switches, imo.
Unlike with steppers.

Likewise, almost all lathe conversions I see are with steppers direct coupled.
This is a terrible idea. Imo. Ime.
And I tried 2000 hours.

1.
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.

[m_c]

1. A slight correction.
Old-tech type servos, dc brushed servos like geckodrive 320, of which I have 7, used to dither.
The geckos are not in use anymore.

New ac brushless servos find the position commanded, and lock.
Zero dither.
They are "live" about 0.5 secs, and then lock solid.

Unless you happen to have a constant or no load, a servo has to dither. The drive only knows how much power the motor needs to maintain position, by the motor moving off position.
Modern drives are more accurate and will dither less, but they still need to dither to obtain position. The drive display may tell you it's exactly on position, but an oscilloscope on the encoder will likely tell you otherwise.

I have 2 brands, about 15 total in use and in stock, all are the same.
From 400W / 60V, 750W/220, 2.5 kW/220V.

Who does a 2.5kW 220V servo and drive?
And is that 2.5Kw continuous or peak?

[hanermo2]

Em...
I did mean when the servo is in-position, ie stopped.
At that point there is no dither.

Of course, if you try to move it off-position, it then activates and tries to get back into position.

I think all manufacturers make servo drives in to multiple kW or more.

Mine is 2.5 kW continuous.
Much bigger ones are available for not much more money.

30 Nm peak, 10 Nm cont.
Belt drive at 1:3, with HTD8 profile belts, 30 mm wide, taperlock pulleys.
So 90 Nm at the spindle.

The motor mount is a frame, from 30x40mm tool steel bars, and the motor is hard mounted to a steel plate 2 cm thick.
Motor is on top of HS, so heat does not distort spindle (grow it).


I use step/dir with a csmio-ip-s controller
+, enc, mpg, extra io addons

Unless you happen to have a constant or no load, a servo has to dither. The drive only knows how much power the motor needs to maintain position, by the motor moving off position.
Modern drives are more accurate and will dither less, but they still need to dither to obtain position. The drive display may tell you it's exactly on position, but an oscilloscope on the encoder will likely tell you otherwise.

Who does a 2.5kW 220V servo and drive?
And is that 2.5Kw continuous or peak?