Fehlmann Picomax 50 CNC conversion/refurb

[m_c]

Without seeing the videos (my broadband is currently not working, so am limping along on mobile broadband), if you're using a parallel port, you really need to use a charge pump for safety, regardless of what software you're using. As Clive has already said, parallel port pins can act randomly during loading. If the BOB you are using has no charge pump, the easiest option is just not to power up the machine until the computer is fully loaded.

The Clear tecknic servos do seem good, however you have to remember as they need the step/dir signal directly, it adds in a lot of extra potential for interference and losing/missing steps. If I was to use them, I'd be looking at adding differential signal drivers to avoid potential noise issues.
Plus I think they're more closed loop stepper, than what's more commonly referred to as a servo. Good in the fact they're better for direct driving ballscrews, but does mean top speed is limited.

Performance wise, you're more likely to be better of with some good Nema23 motors, and adding a 2:1 drive ratio. Nema 34 really need high voltage drives to get the best performance from them.

[Agathon]

Without seeing the videos (my broadband is currently not working, so am limping along on mobile broadband), if you're using a parallel port, you really need to use a charge pump for safety, regardless of what software you're using. As Clive has already said, parallel port pins can act randomly during loading. If the BOB you are using has no charge pump, the easiest option is just not to power up the machine until the computer is fully loaded.

The Clear tecknic servos do seem good, however you have to remember as they need the step/dir signal directly, it adds in a lot of extra potential for interference and losing/missing steps. If I was to use them, I'd be looking at adding differential signal drivers to avoid potential noise issues.
Plus I think they're more closed loop stepper, than what's more commonly referred to as a servo. Good in the fact they're better for direct driving ballscrews, but does mean top speed is limited.

Performance wise, you're more likely to be better of with some good Nema23 motors, and adding a 2:1 drive ratio. Nema 34 really need high voltage drives to get the best performance from them.

Thanks for the reply. What's a "charge pump"?

Re Clearpath - they are true servos rather than hybrid servo/steppers. I take your point about interference. I might pose the question to Teknic and see what they say - they've been really helpful so far.

There seem to be as many opinions about drives and motors as there are drives and motors. I did canvass this forum and others quite extensively before buying drives and the consensus seemed to be that Leadshine AM882s at 70v was the way to go. Still, the Z axis is fine, and the X&Y drives can be redeployed. If I do make some change to the X and Y drives it will be to servos - not poncing about with these poxy stepper things any more

[EddyCurrent]

Thanks for the reply. What's a "charge pump"?

if the bob does not see a continuous signal from the controlling PC it shuts down.
Also called a 'watchdog timer'

From my bob manual,

The charge pump uses the 12 kHz signal from the parallel port generated by the
CNC software to operate a logic circuit that gives an active low output. Any
piece of machinery that uses powerful motors can be dangerous if controlled
by a computer that can be in an unknown state while being powered up or in
a software crash condition. Using the charge pump circuit to disable power to
motors is a safety device in that it only operates when the software is running
correctly and under user control. The charge pump circuit is also used to
disable the output signals so even if your stepper boards do not have an
enable pin they will be disabled automatically when the charge pump signal
is not present.

[Clive S]

Thanks for the reply. What's a "charge pump"?

Mach3 generates a 12.5Kc/s pulse onto one of the pins of the PP This is used to enable the bob. When Mach3 starts up. It is used in case there is a software problem or e stop situation the pulse disappears and basically switches the bob off.

Re the drives is is usual for us to have a two to one reduction with a belt drive. I use these AM882 on my mill.
Your mill though is in a league of its own.

[Agathon]

Mach3 generates a 12.5Kc/s pulse onto one of the pins of the PP This is used to enable the bob. When Mach3 starts up. It is used in case there is a software problem or e stop situation the pulse disappears and basically switches the bob off.

Re the drives is is usual for us to have a two to one reduction with a belt drive. I use these AM882 on my mill.
Your mill though is in a league of its own.

I'll do some research on charge pumps.

I have been puzzling about using Nema 23s with a reduction drive, but can't see how this will help in my situation. If I wanted to achieve 1000 rpm at the feedscrew the motors would have to run at twice that speed with the inevitable massive drop-off in torque. Even if the torque itself is multiplied by 2:1 (which it won't due to losses in the drive belts etc) the torque of the motor is likely to have dropped way below any mechanical gain. I can see how it helps with gaining torque at lower speeds, but looking at the torque curves of even the lowest induction and rotor inertia steppers it looks like less than a zero-sum game to me since their torque drops down to less than a third of their maximum. By the time any motor gets to having nearly 7000 steps a second they're going to have no torque at all. These topsy-turvy stepper things do my head in!

[Agathon]

How do you get rid of attachments? Realised that the axes on the blue graph don't make any sense.

[m_c]

Thanks for the reply. What's a "charge pump"?

Re Clearpath - they are true servos rather than hybrid servo/steppers. I take your point about interference. I might pose the question to Teknic and see what they say - they've been really helpful so far.

There seem to be as many opinions about drives and motors as there are drives and motors. I did canvass this forum and others quite extensively before buying drives and the consensus seemed to be that Leadshine AM882s at 70v was the way to go. Still, the Z axis is fine, and the X&Y drives can be redeployed. If I do make some change to the X and Y drives it will be to servos - not poncing about with these poxy stepper things any more

I'd like to get my hands on one to see, but the torque curves look more akin to those of a stepper system, than a servo system. Stepper and brushless motors are very similar, it's just stepper motors are synchronous (which is what gives them the detents), while servos are a/non-synchronous, which means you don't lose power/torque overcoming the detents.

Regarding the Nema 23 v 34 argument. Compare them running similar voltages. You'll generally find Nema 34 graphs are using a high voltage driver, while Nema 23 are done using a relatively low voltage driver.
I've just had a quick look to see if I could find a couple graphs to do a comparison, but the Nema 34 graphs I found were mostly using a 110VAC supply (about 155VDC), while the Nema 23 ones were using 30-40VDC supplies. At those low voltage, torque drop of is very noticeable, and crippling Nema 34s with only 70VDC also makes for a very similar torque drop-off. You need voltage to over come the back EMF at speed. Without that voltage, torque at speed is very limited.

[Agathon]

I'd like to get my hands on one to see, but the torque curves look more akin to those of a stepper system, than a servo system. Stepper and brushless motors are very similar, it's just stepper motors are synchronous (which is what gives them the detents), while servos are a/non-synchronous, which means you don't lose power/torque overcoming the detents.

Regarding the Nema 23 v 34 argument. Compare them running similar voltages. You'll generally find Nema 34 graphs are using a high voltage driver, while Nema 23 are done using a relatively low voltage driver.
I've just had a quick look to see if I could find a couple graphs to do a comparison, but the Nema 34 graphs I found were mostly using a 110VAC supply (about 155VDC), while the Nema 23 ones were using 30-40VDC supplies. At those low voltage, torque drop of is very noticeable, and crippling Nema 34s with only 70VDC also makes for a very similar torque drop-off. You need voltage to over come the back EMF at speed. Without that voltage, torque at speed is very limited.

I have to say all this stepper and servo stuff is new to me. I'm very familiar with squirrel cage motors and vfds, but steppers, and now servos are a very steep learning curve. I know what you mean about the Clearpath graphs, but Teknic have stated that they are servos and not steppers. Their torque curves are certainly much more healthy at speed than any stepper graph I've seen (which admittedly isn't that many). Certainly when you compare the torque/speed graph of the Clearpath 2.04 Nm (rms) (CPM-SDSK-3421S-RLN) servo I was thinking of buying to that of the Astrosyn 4.8 Nm (holding torque), there's no competition. Where the Clearpath has no problem producing its nominal rms torque at 1000rpm the Astrosyn has fallen to something in the region of 0.9 Nm at around 3000 steps/s (900rpm).



Just as an aside, if the Astrosyns were geared 2:1 they'd only be producing 0.4Nm

Having looked at the torque curve more carefully I see now that with my table axes running at 2000mm/min that the motors should be producing something in the region of 1.7Nm - which should be fine and indeed seems to be so. I can also see why increasing the speed by 50% causes them to lose steps as they've lost 0.5Nm in torque

[Agathon]

Moving off steppers and on to the spindle motor:

I spent this afternoon setting up the inverter and its control in Mach. I found that by fiddling with the PWM base frequency that I could get 100% on Mach spindle control to correspond to 100hz on the inverter (which previously it would not).

Having sorted this out I turned to pulleys and defined the four motor speeds as pulleys. I then calibrated the spindle speed to a range that I thought would work without me having to move the mechanical variator (Reeves type drive) or even replace it with a single fixed ratio poly-v-belt drive. I reasoned I needed speeds between 150rpm and 4000rpm. However, having run the slowest speed of the motor at the lowest reasonable frequency I quickly concluded that the motor has insufficient torque at this speed.

So I've decided to use be able to use the variator in two positions giving tops speeds of 2000rpm and 4000rpm with plenty of low-down torque in the lower range setting. I'll be mostly machining cast iron and steel so the lower range will be deployed most of the time.

As I understand it Mach3 while it's possible to tell Mach3 the spindle speed it cannot change the pulley setting automatically. It would have been nice if one of the four motor speed control relays could have been linked and operated by this feature of the software, but I guess I'm hoping for too much! In any case I'll have to set the variator manually.

[m_c]

I quickly concluded that the motor has insufficient torque at this speed.

[pedant mode] The motor should have the same torque at any speed provided the VFD is not limiting current. It's the lack of gearing that means there is insufficient torque at the spindle[/pedant mode]

As I understand it Mach3 while it's possible to tell Mach3 the spindle speed it cannot change the pulley setting automatically. It would have been nice if one of the four motor speed control relays could have been linked and operated by this feature of the software, but I guess I'm hoping for too much! In any case I'll have to set the variator manually.

Mach 3 can via Macros. I've never done it, however you can use a Macro that takes the requested spindle speed, and changes gears accordingly. Probably worth having a search for gear change over on the Mach forum.