Messages

1461

General Mach Discussion / Re: Spindle Speed Troubles

« on: November 12, 2008, 09:10:02 PM »

Ray
I thought of a couple of other things you may want to check. the 12v from your VFD to the board have you checked that the volts are right first thing at start up. Have you got that 12v isolated from the rest of the control power, especially the neutral? If all is good could You have a bit of moisture around the boards giving you a slight short then once things get a bit warm the moisture goes. or maybe you got a bit of dust, Ive seen a 240v power outlet Arc and short from dust settlement on it. Other than that I do not really Know, It must be what you were saying a problem in the board, could have a slight short and as heat increases so does resistance then works fine.

Supply voltage from the VFD is dead-stable.  Entire analog output circuit is totally opto-isolated from everthing else in the system.  It's down to either the SmoothStepper is outputting the wrong step frequency (which I doubt) or the analog output circuit (LM2907) on the BOB is misbehaving.  I just haven't had the opportunity to put the scope on it when it's acting up, to see exactly where the problem is.

Regards,
Ray L.

1462

General Mach Discussion / Re: Mach Conv. Candidate ?

« on: November 12, 2008, 12:56:55 PM »

This pic sent to me from a friend. Calls it a Bridgeport 14x34.
Does anyone recognize the model ?
Is 14x34 the table size or the x/y working envelpoe ?
Might be a good Mach conversion candidate at $700 ?
Thanks,
RC

14x34 is definitely not the work envelope.  The table is barely that long.  The 49" table has about a 34" travel, and that's no 49" table.  But, for $700, looks like a steal.

Regards,
Ray L.

1463

General Mach Discussion / Re: Bugs in Mach 3.42.015

« on: November 12, 2008, 11:02:57 AM »

I had two thoughts about Swarf's issue.

Keep in mind, I am a newbee to CNC and also to MACH 3, so these may not be reasonable ideas . . .

First; my undestanding is that Swarf's button is running some sort of macro. If so, could not the macro reset the machine coords, then MOVE to zero and then reset again, all within the macro? It seems to me that would correct for the minute error in calculating the position and the second reset would be the true zero sought.

Second; is there any practical reason one could not have more than one set of limit swiitches? I do not comprehend the reason to re-zero the machine coordinates at different points, but assumming there is some usefull purpose to that, why could there not be additional limit switched installed for the different set-ups? So you would plug in set A for part A and when you were going to run part B, you just plug in (or switch to) limit switch set B and re-home the mill.

1) You *can't* zero machine coordinates, other than through a homing operation.  That's the whole crux of the issue.

2) You could do that, but it would be a little silly.  That is exactly why we have work offsets.  You tell the controller where machine zero is, and everything else is referenced to that.  You don't *need* multiple machine zeros.

Regards,
Ray L.

1464

General Mach Discussion / Re: Spindle Speed Troubles

« on: November 12, 2008, 10:46:45 AM »

Ray
Just an obscure thought. are you using VFD? If so could it be that your Capacitors in the VFD are starting to get a bit dodgy.
Maybe they are not holding there charge or need a bit to get a charge into them. Have you checked first thing that your speed control unit is sending out the right voltage if so I would look at the VFD itself, and probably the first place would be the Cap's.

I am using a VFD, but it is less than a year old, and work perfectly from the front panel.  The control voltage coming out of the C11 is squirrelly on occasion.  On one occasion, an S command that should have yielded 1V out, instead gave 8V out.  Over a period of 5-10 minutes, it gradually worked it's way down to where it belonged, and worked perfectly the rest of the day.  It just seems to have morning sickness some days.

Regards,
Ray L.

1465

General Mach Discussion / Re: Can someone explain the current draw of these stepper motors, please.

« on: November 12, 2008, 10:38:16 AM »

THX again, Ray

You have a way of using analogies to make things clear(er).

And thx for the link.

I did the retrofit on this little baby mill for a specific project. However, it turned out to be great fun. When that project is completed, I plan to sell the mill complete and go with a little larger mill (X3?) and do another retrofit using servo motors instead of steppers, so an in-depth study of steppers would probably not be benefitial other than as a point of interest.

Here again, I am no expert at this point, but servos seem to have a lot of advantages for not too much more money.

On a small machine like an X3, I don't think servos have any particular advantage.  You won't be able to make use of the potential higher speed, due to the still rather short travels.  For an X3, I think steppers will give you at least the same performance, for less money.  Per Mariss Freimaniss, the owner/designer of all Gecko drives:

Where a stepper looses:

When more than 200W mechanical is needed.
When there isn't a large ratio between feedrate and rapids.
When battery power is used instead of mains AC.

Where a servomotor looses:

When there is a large static load (moving the knee on a Bridgeport).
When the motor chronically turns less than 360 degrees.
When a low RPM load requires an uneconomically large reduction ratio.


Mariss's rule of thumb is, up to 100W, steppers are best.  Above 300W, servos are best.  Between 100W and 200W, either will work.  An X3 will certainly be well under 200W, probably under 100W.

Regards,
Ray L.

1466

General Mach Discussion / Re: Spindle Speed Troubles

« on: November 11, 2008, 11:29:09 PM »

That's completely different from the problem I've had.  M3/M4/M5 always work correctly.  The only problem is occasionally first thing in the AM the speed is off.  Just let it run, and it will settle down to the correct speed.  I'm pretty sure it's a analog flaky in the BOB.

Regards,
Ray L.

1467

General Mach Discussion / Re: Spindle Speed Troubles

« on: November 11, 2008, 06:04:55 PM »

Hi Ray
Just one thing I have noticed with mach, I have also found sometimes if you type in M3 or M4 Or M5 it wont recognise it and comes back with an error. All I do is shut down Mach and restart it and it works fine, Don't Know why. I Haven't tried it yet but I'm going to see if theres a difference in starting Mach First and then powering up the CNC or doing it Vice Verse. Could Be interesting to see.

I've seen more than my share of "quirks" in Mach, but that's a new one by me!

Regards,
Ray L.

1468

General Mach Discussion / Re: Can someone explain the current draw of these stepper motors, please.

« on: November 11, 2008, 05:58:32 PM »

The Gecko is doing nothing more than either connecting the motor winding to the power supply, or not, so I suppose you can think of it as a relay in that respect, albeit a VERY fast one.  Since you're an ME (as I am), think of it as more like a pressure regulator.  When the pressure on the load is too low, it opens a valve, allowing pressure from the compressed air source into the load.  Once the desired pressure is reached, the valve is shut off, and the load is isolated from the compressor.  In this case, current is the analog of pressure, and the Gecko takes the place of the valve, and the pressure sensing diaphragm that controls it.

Your PWM speed controller is a different animal, since you're not running it at 20X the motors rated voltage.  If you put an oscilloscope on the stepper motor winding, you'd see the voltage switching at a high rate between 0V and the supply voltage.  If you looked at the current, you'd see it modulating around the limit current, ramping up to slightly higher than the limit current when the voltage is equal to the supply voltage, and ramping down to slightly below the limit current when the voltage is near 0V.  But, you'd find it's actually connected to the supply a relatively small percentage of the time.  You will NEVER see the voltage applied to the winding for any significant length of time, even when the motor is running with a fairly heavy load.  You PWM controller will be runnig a significant on-time even at low motor speeds.  At max speed, it will be on 100% of the time.  If the stepper winding were connected to the supply for any length of time, the current would very quickly FAR exceed the motors ratings, and the windings would burn out.

This will teach you everything you ever need to know about steppers.  This topic is covered in the section on current limiting:  http://www.cs.uiowa.edu/~jones/step/

Regards,
RayL.

1469

General Mach Discussion / Re: Can someone explain the current draw of these stepper motors, please.

« on: November 11, 2008, 01:28:13 PM »

Hood, the numbers I gave are at the motor's current configuration, which is parallel.

Ray, thanks for the explaination. I do understand the relationships you described.

What was confusing me was that the Gecko drives (203v) require a resistor rated relative to the motor amps without regard for the operating voltage . . that started me thinking I must not understand something about these steppers.

Perhaps the Gecko knows the voltage coming in and does some type of internal adjusment for the load useing the resistor only as a ref.

The fact that one of the three new Gecjo 203v broke after only 2 months also had me fearing I did somehting wrong in my setup. They found a bad component and replaced the drive for me, so all is well now . . plus I now have a spare.


In any event, good to know I have enough juice to drive my rig including the new 4th axis motor.

The Geckos modulate the current to achieve the specified average current.  They don't really know or care what the supply voltage is.  They connect the motor winding to the power supply.  Because of the winding inductance, the current does not instantly increase.  Instead, it starts to increase from zero at a rate determined by the supply voltage and the motor inductance.  When it eventually reaches the limit set by the resistor, the motor winding is disconnected from the supply.  Again, the motor current does not instanly go to zero, but starts ramping down.  When it reaches some lower threshold, which is some fraction of the current limit, the Gecko will re-connect it to the supply.  This cycle repeats for as long as the winding is energized. 

The average current seen by the stepper will be equal to the value set by the current limit resistor.  The average current seen by the supply will be far less, due to the much higher voltage.  The peak current seen by the supply will equal the peak current drawn by the motor, but it's only drawn for a very short period of time, so the average value you read with your DVM is far less.

The whole point of running steppers on a supply voltage 10-20X higher than the rated voltage is the help the current in the windings to build faster, to enable higher speed operation.   Otherwise, at high RPMs, you'd never come close to full rated current, the torque would fall off very sharply.

Regards,
Ray L.

1470

General Mach Discussion / Re: 4th axis ghost move

« on: November 11, 2008, 01:18:10 PM »

Are you using backlash compensation?  That will add moves.

Regards,
Ray L.