Fehlmann Picomax 50 CNC conversion/refurb

[Agathon]

For induction motors, torque remains pretty constant over the rated speed range up to the rated speed, and is a result of the amount of current flowing through the windings. Although frequency controls the speed, as speed drops, the voltage required to drive that current through the windings also drops proportionally I.e. for a 3000rpm 240V motor, peak voltage will be around 120V when running at half speed.
Now because voltage is reduced, so is the power output, so if the above motor was rated at 1KW, at 1500RPM it would only be producing 500W. You could increase current (which some inverters can do to give a low speed torque boost), however you risk overheating the windings due to the extra current.

Above the rated speed/power, current becomes limited. So taking the above example motor, and trying to double it's speed to 6000RPM, you can do so by doubling the frequency, however unless you double the supply voltage, 240V will only be able to force half the current needed to get full torque through the windings, with the result you will only get half the rated torque.
The result is above the rating, the motor becomes power limited. You can have double the speed, but only half the torque, so even though the motor is spinning faster, you still only have 1KW of power.

This is why when fixed speed motors with gearing get replaced, they're often replaced with far bigger motors.
With gearing, say you take a 2Nm 3000RPM (probably about 600W), you gear it down 4:1 and get 8Nm at 750RPM.
To get that same torque at the same speed using direct drive, and still retain 3000RPM, you now need a motor capable of 8Nm and 3000RPM, so you need a motor with 4 times the power. (realistically you'd compromise with some gearing, a bit less torque, and running the motor so higher speeds are above the motor rated speed and into the derated/reduced current area).
That's the reason why most modern CNC machines come with such big spindles. For most end users, it not because they're going to be managing to use upwards of 10KW hogging metal with endmills, it's so they still have enough torque to drive a big facemill at a couple hundred RPM while still taking a decent depth of cut.

Very nicely and clearly explained. I was aware of the frequency voltage relationship, but hadn't really twigged the issue of running higher than the name-plate frequency and the consequent lack of volts. As the video shows, I don't really think that torque is going to be so much of an issue as far as the spindle goes, so I'm fairly hopeful about my plan to use the higher mechanical speed setting most of the time. We shall see.

Having looked into Brains more I understand your previous post and think that it all looks doable even for a novice like myself.

[Agathon]

I've done more fiddling with the table and found that at 2500mm/min the table is repeating position over 200mm for the x and 150mm for the y better than 0.01mm (in fact pretty consistently within 0.003mm) which I am extremely pleased with. I tested fairly extensively with 10 or 20 repeat cycles.

I am still getting the grunting noises from time to time, so I suppose this is down to the latency of the processor generating the pulses?

[Robin Hewitt]

repeating position over 200mm for the x and 150mm for the y better than 0.01mm (in fact pretty consistently within 0.003mm)

3 microns is an interesting number BUT what about the backlash?

[Agathon]

3 microns is an interesting number BUT what about the backlash?

What backlash? This thing is Swiss! I would think that the 3 microns is simply down to vibration in the mechanical loop between the digital dti and the mount. If you're not careful you might provoke another mind numbing video!

[EddyCurrent]

Try the attached spreadsheet (I use LibreOffice if you don't have excel)

Just enter the desired spindle speed in cell A4 and it will calculate the inverter frequency and contactor to energise.

That logic needs put into a Brain using Mach3 DRO 169-Cmd SpindleRPM as the desired frequency.

It uses an inverter frequency up to 100 hz and assumes the manual adjuster is set to 1.5 * motor speed, so for the 1400 speed it's set to 2100 RPM then the inverter will take it to 4200 RPM

[Agathon]

Try the attached spreadsheet (I use LibreOffice if you don't have excel)

Just enter the desired spindle speed in cell A4 and it will calculate the inverter frequency and contactor to energise.

That logic needs put into a Brain using Mach3 DRO 169-Cmd SpindleRPM as the desired frequency.

It uses an inverter frequency up to 100 hz and assumes the manual adjuster is set to 1.5 * motor speed, so for the 1400 speed it's set to 2100 RPM then the inverter will take it to 4200 RPM

Thanks, that's really kind of you. I do have Excel so I'll take a look.

Can anyone tell me what G0 code will make the x axis move +100 back to zero and then +100 repeatedly - say 10 times? I'm looking around for a simple answer but can't seem to find one.

[Agathon]

Ignore that last request I worked out how to do it - probably in a very labourious way but none-the-less it worked.

[Neale]

3 microns is an interesting number BUT what about the backlash?

when I was testing the touchplate setup on my router, I was getting repeatability to within +-0.003mm. Quick back-of-envelope sums showed that this was equivalent to one microstep. Might this be why this magic number is there? Mind you, my backlash is a bit more than that..

[Agathon]

As it happens Neal I've just run some more tests. Prepare to fall asleep! Contrary what the fool in the vid says, the Y does seven cycles!

Despite all the grunting (not mine) the steppers don't appear to be losing steps at 2500mm/min.



Actually, I don't know what the backlash is yet. I would think it was small but I need to set-up a proper test.

[Agathon]

Just measured the back-lash at 0.03mm on each axis. I'm a little surprised by this since I can't push/pull either axis by more than 0.01mm. The motors are coupled to the screws with flexible star-type couplings (see pic) - which are original and in good condition, but I have to say I've always been suspicious of. Now they are robbing me of precious microns!

Seriously though I can live with 0.03mm.