6090 Chinese CNC Router for cutting aluminium

[RCaffin]

Sulfuric acid = 'battery acid'.
Hydrochloric acid = brick cleaner
Just have to know the right (apparently harmless) names.

cheers
Roger

[dharmic]

Tried buying 20l of battery acid recently?

Even five years ago when I went through this, no battery shop would let it out the door. Ended up getting a 20l container from a local industrial supplier but it took some hunting down and a bit of fast talking to assure the guy it was for anodising and nothing more nefarious.

[RCaffin]

What are you cutting that stack on? I didn't realise it worked ok to stack sheet like that...
Machine is an Adept Machining Centre - a relatively unknown Australian-made machine. Kinda solid with good accuracy.

Perfectly OK to machine multiple layers as long as you secure them down tightly. I use multiple circuits of the outline to keep the chips small so the cutter does not 'fill up' and break.

Cheers
Roger

[lael]

I asked one of the suppliers about closed loop steppers and they said they had experienced trouble with them and would not recommend using them. Any thoughts? I understood they offered the ability to ensure accuracy with positioning?

[aarggh]

There actually are steppers with enconders as standard on the market, I would think the response curve would be far better with a servo, but I can't see how a closed loop stepper system wouldn't position just as accurately.

cheers Ian

[RCaffin]

Closed loop steppers using conventional stepper motor drivers are really ... well, conventional stepper drivers just don't have that feedback feature.
You see, if the stepper motor fails to make a step, there is nothing the driver can do to recover. The resistance from the load is just too much for the motor+driver.

Somewhere here on CNCZone there is mention of a totally different driver which can use feedback. It's kinda experimental right now. The thing is that it operates the stepper as a multi-phase motor, NOT as a stepper. But I don't think it is a readily-available commercial product yet.

Cheers
Roger

[lael]

Thanks Gents,

Let me see if I understand:

- response curve = ability to make small moves at speed with torque?

- "there is nothing the driver can do to recover. The resistance from the load is just too much for the motor+driver." - I thought the point of closed loop steppers is that the encoder on the stepper provides feedback to the controller, which then still knows 'where' the spindle is located and can adjust? What do you mean by: "resistance from the load is just too much for the motor+driver" - as in - doesn't have the rigidity/force to 'push' into the steel (for arguments sake) and so loses the step instead?

[RCaffin]

so loses the step instead?
Exactly. And no amount of feedback can fix that. Very well known limit to the use of steppers.
Servo drives on the other hand can increase the power to the motor.

Cheers
Roger

[aarggh]

so loses the step instead?
Exactly. And no amount of feedback can fix that. Very well known limit to the use of steppers.
Servo drives on the other hand can increase the power to the motor.

Cheers
Roger

I'll have to defer to you as my understanding of closed loop systems is different to what you described. This site explains it somewhat:

https://phidgets.wordpress.com/2014/...l-isnt-enough/

With the relevant portion as what I've always understood with using encoders whether servo or steppers used:

If an external force were to stop the motor, your encoder would indicate to your program that the motor has stopped moving, and it could increase the duty cycle or activate another system designed to take care of the problem. This type of control is called a closed-loop controller, because the actual output of the system constantly loops back into the calculation that determines the future output. By installing an encoder onto your stepper motor, you can create a similar closed-loop system with all of the same benefits of a stepper motor. If the motor stalls and desynchronizes with the controller, you can restart it. If you miss steps or take too many, they can be accounted for instead of accumulating over time. If your gearbox backlash introduces a few degrees of error after a number of rotations, that error can be eliminated with an encoder.

So I always thought being closed loop whether it's a linear servo or a discrete stepper would be irrevelant, and that anything severe enough to impede or halt movement that can't be re-positioned by the closed loop system, would send the machine into a fault condition, not just lose steps and keep going?

cheers, Ian

[RCaffin]

Hi aarggh

To be sure, there are always exceptions. My statement applies to the very common stepper motor drivers available from a wide range of companies, especially for the hobby market. They do not feature any encoder feedback, because such feedback could not help the driver in any way.
However, there are (at least) two exceptions to this.
* The first exception is when the encoder feedback can tell the driver that steps are being missed, so the driver can flag a fault condition to the host. I don't know of any drivers which have this feature, but I am sure such designs will exist somewhere.
* The second exception is where the driver is not a standard stepper motor driver but treats the motor as a two-phase synchronous motor. Again, there are not many of these around. One could argue that in this case the motor is not really being treated as a conventional stepper. One of these recently featured somewhere on CNCZone I think.

HTH
Cheers
Roger

[lael]

Found an example of what I orginally found - the first time I found them it was on a Korean video... I couldn't understand it - but saw the difference in precision (he was using a gauge to measure return to 0 on the router).



It looks like the driver uses the encoder to keep the stepper in the same place - ElectroCraft : IMDC Closed-Loop Stepper FAQ - these guys refer to it as 'step-loss compensation? (guessing that is what was implemented).

edit: here's another video on the same drive / controller:


and the link to the western version of the same thing: Easy Servo (Closed Loop Stepper) Drives | Leadshine

What is a 'two-phase synchronous motor'?

[RCaffin]

Hi lael

Exactly! And thank you for the URLs. I have to say, these things are less-widely known. Whether they are as powerful and as fast as a full brushed or brushless servo system - I don't think so. On the other hand, they probably have a higher slow-speed torque. Different characteristics, different applications.

What is a 'two-phase synchronous motor'?
It's like a 3-phase VFD system system, but with just 2 phases. It's the third version of control in the ElectroCraft FAQ - which is very good.

Cheers
Roger

[lael]

OK, some more questions I'm after an answer for...

How do I figure out what the max working feed rate and the torque available at that rate - based on the combination of the stepper motors, drivers and drive system (rack and pinion vs ballscrew?)

edit:
- Is rack and pinion or ballscrew better?
- I found a supplier that makes a 6090 with a steel gantry - It should theoretically be more rigid than aluminium?

[RCaffin]

You probably can't, because you have not allowed for the mass of the thing being machined and the reaction forces from the cutter.
I prefer to allow a safety margin here: better a slight loss of efficiency compared to a broken cutter and wrecked job. So I go by experience.

Cheers
Roger

[lael]

any opinion on rack and pinion vs ballscrew? or steel vs aluminium gantry?

[dharmic]

Ballscrew > rack and pinion (accuracy, precision, backlash, strength)
Steel > aluminium (stiffness, resistance to chatter and deflection)

[RCaffin]

Steel > aluminium
Can I add that steel has more mass, which is ALWAYS good?

Cheers
Roger

[lael]

awesome - thanks!
So essentially the mass of the gantry is born by the linear guides (and whatever they are attached to) and the ballscrews/R&P move it back and forth and should not be taking any weight?

[RCaffin]

Right.
Cheers
Roger

[lael]

Thanks! A manufacturer just said that ballscrews don't have enough strength for cutting aluminium. Any ideas why they would say that?