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Messages - joeaverage

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4231
General Mach Discussion / Re: Lathe Spindle Synchronisation
« on: June 24, 2018, 02:03:23 AM »
Hi,
when threading Mach requires an accurate speed, that is not to say it has to be exactly 400 rpm say, but if it slows slightly as the cutting forces act on it to 390 rpm it needs
to know that it is 390 rpm, not 392 or 387 but 390 rpm. The reason is that Mach needs to synchronise the Z axis movement to the rotational speed, any error will result in a thread
with the wrong pitch.

An asynchronous motor will slow under load even if the VFD provides fixed frequency input voltage. If the load is but a small percentage of the spindle ultimate torque
then the speed change will be small, a few percent. Provided your index pulse remains active a small change in speed is not disasterous to your thread, Mach will accomdate small
changes by tweaking the Z axis advance rate.

If there is a large change in speed or it changes speed quite rapidly then Mach struggles and wont cut good threads.

You may have noticed that on the Ports an Pins/Spindle Setup page the opportunity to operate your spindle closed loop. Mach is not a good candidate for closed loop operation
because its so slow to respond but its still better than nothing. The speed is measured by the index pulses, per normal, but if the speed slips below target Mach will increase the PWM output
voltage which will in turn cause your VFD to up the ante. The feedback loop is slow to respond so this works well only with spindles of significant inertia and plenty of torque relative to the
cutting torque. If you expect this arrangement to be able to maintain spindle speed when the cutting forces are as much as 50% of your motors max torque you'll be disappointed.

I recently bought a second hand Allen Bradley AC servo of 1.8kW as a spindle motor for my mill. Its speed, and even more impressive, position accuracy is a real eye opener. Its not that its a great
deal bigger than your induction motor but rather the servo drive/servo combination have a feedback refresh rate of 20kHz. This is broadly true of any feedback control system, assuming the motor has
the torque to drive the maximum expected load it will do so accurately only if the feedback refresh rate is fast enough. For this reason you may have seen large production CNC mills that have
20 plus horse power spindles do a good job of rigid tapping, one) because they have plenty of torque and two) they have very high refresh rate feedback systems to take advantage of that torque.

If it is within your means replacing your spindle motor with an AC servo would be a very strong performer for you when threading. You might be surprised how economical second hand servos and drives
can be and some of the Chinese offerings are very good value for money.

Craig

4232
General Mach Discussion / Re: Z Axis Machine Coordinates Not Changing
« on: June 23, 2018, 10:28:49 PM »
Hi tbj1,
that explanation makes sense, you were looking at the wrong DRO. That does raise the question why the DRO number has been changed for the Plasma screen.

As you may know Mach4 allows for THC control it is in fact enacted by the motion controller rather than Mach. At this time there is only one external motion controller
manufacturer who has devoted the effort to enact THC and provide a custom screen  for its Mach4 controller. Because of the rapidity at which Mach4 software cycles
there may be the opportunity to do THC by scripting within Mach4 with minimal controller support required ie a software solution rather than a realtime hardware solution.

http://www.machsupport.com/forum/index.php/topic,37405.msg255859.html#msg255859

The essential idea is that the Z axis remains just that but in addition you piggyback an out-of-band axis on top of the Z axis to wiggle up and down for THC purposes and not
therefore break the axis coordination that manipulating the Z axis directly would imply. It would of course be possible to wiggle the Z axis as if it were an out-of-band axis but the Z axis
DRO would not follow the THC variations. The actual  (total) position of the Z axis would then be the sum of the Z axis DRO proper and the THC variation, possibly in a second DRO.

My suggestion is that the #86 DRO in the Plasma screen is analogous to the second DRO tracking THC variations as proposed for Mach4.

Craig

4233
Hi ger21,
thanks for your reply.

Quote
but I wouldn't expect to see it for at least a few more years now.
Mach4 does not to my knowledge have a third order planner despite the early discussion. I imagined that such an absolutely basic piece of Mach could not be replaced
without destroying Mach4....but given the modular nature of Mach4 perhaps its not the case.

Quote
but most have more control over the path deviation

I have seen a couple of threads which deal with path deviation verses Constant Velocity settings. To be honest I did not really invest the mental effort to follow it.
I did have occasion to skim read a couple of academic papers on the Net that were concerned with path deviation. The maths was frightening.....but then I have done
that sort of stuff before...its usually a matter of mental application rather than genius or lack thereof.

Do you have any recommendations of software solutions that I might view to see first hand how others have handled path deviation?

Craig

4234
General Mach Discussion / Re: Z Axis Machine Coordinates Not Changing
« on: June 23, 2018, 08:37:43 PM »
Hi,

Quote
The only number that changes when jogging Z is current position.  Machine, Work, and G92 all remain 0.

That seems impossible to me. I think that your copy of Mach3 may be corrupt.

Craig

4235
General Mach Discussion / Re: Lathe Spindle Synchronisation
« on: June 23, 2018, 08:34:08 PM »
Hi leadhead,
I have not experimented with spindle synchronization so my opinions are just that and probably not worth a great deal.....

I have heard that in order for Mach3 to reliably read a spindle index pulse it needs to be of several milliseconds duration. Most lathe threading seems to occur at 500 rpm or less
or 8.33 ms per rev. If you needed a 1 ms pulse for Mach3 to recognize it reliably that would require that your Hall sensor have a magnetic sector of some 30-40 degrees in order
that the pulse be wide enough.

I have read a number of post we users have used an optical sensor rather than a Hall sensor using a CD at the spinning disc with a notch cut in its periphery. Just the perfect
use for the wife's bloody Barry Manillow CD I would have thought!

Craig

4236
Hi ger21,
I understand that Mach3 uses a fairly simple trapezoidal trajectory planner which results in significant 'jerk' at the transition points.
I recall that there was considerable discussion some years ago about introducing a third order planner for Mach4. I did not pay a great deal of attention
at the time.

I have subsequently migrated to Mach4 and like it. There are no extra or unexplained settings that make me think that a third order planner was ever implemented.
Are you at all familiar with the history of Mach4's development...did in fact a third order planner get implemented or if not was there any discussion/reasoning why not?

Craig

4237
General Mach Discussion / Re: Z Axis Machine Coordinates Not Changing
« on: June 23, 2018, 08:14:13 PM »
Hi,
OK that sounds like its operating normally.

When you say that the current position changes when you jog then some if not all of the numbers that make up the current position must also change.

If you look on the Diagnostics page you will see that the current position is:

Current Position = Machine Coordinate - Work Offset - G92 Offset - Tool Offset.

Its common for the G92 Offset to remain zero for the entire Mach session and unless you are actively using tool offsets the Tool Offset will be fixed or zero for the time being.
Thus if the Current Position is changing then either (or both) the Machine coordinate or Work Coordinate MUST be moving also. Does it do so?

Craig

4238
General Mach Discussion / Re: Z Axis Machine Coordinates Not Changing
« on: June 23, 2018, 08:01:25 PM »
Hi,
that sounds unusual.

When the machine homes each axis LED should go from red to green as it is referenced. Does the Z axis LED do so?
The next question is about the Z axis home switch. If you manually flick the switch does the LED for the Z axis (M3Home for example)
show up in the Diagnostics page? When the Z axis is homed the machine should drive until the Z axis switch operates and then back up a little until
it de-operates. Is this what happens? I'm wondering if the home switch does not deactivate then the Z axis may be permanently and repeatedly being
referenced to zero because the home switch is still active.

Craig

4239
Hi Bryanna,
that the first time I've seen a file like that.

I guess its some sort of definitions file....kool.....will look through it with a fine tooth comb.
Many Thanks

Craig

4240
Mach4 General Discussion / Re: My new screenset (and thanks to all)
« on: June 23, 2018, 07:39:16 PM »
Hi,
nice job!

You have taken great advantage of the screen editing functionality built into Mach4. Those features are I believe are, if not unique, certainly much
further developed than any of it competition. As I say...good work!

Craig

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