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Author Topic: Whose Fault Is ThIs?  (Read 23786 times)

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Re: Whose Fault Is ThIs?
« Reply #10 on: November 30, 2009, 04:04:36 PM »
Brian,
    Could you please give us some details about what this bug is and in what verison it appears. I do alot of 3D machining and I have recently been chasing what appears what shows up as lost steps on Z. I've been assuming it was hardware related but maybe not. - Thanks - Tery

Tery,

My understanding is this problem has been in there for a long time, but will only be a serious problem on machines with widely different accelerations on the various axes.  In my case, X and Y accelerations are 25X the Z axis acceleration.  This is why *most* people have no problem, but guys with goofy machines like mine will see it.  It is not respecting the acceleration setting for the "slow" axis.

Regards,
Ray L.
« Last Edit: November 30, 2009, 05:11:58 PM by HimyKabibble »
Regards,
Ray L.
Re: Whose Fault Is ThIs?
« Reply #11 on: November 30, 2009, 05:25:29 PM »
Thanks!!!
Re: Whose Fault Is ThIs?
« Reply #12 on: November 30, 2009, 05:30:57 PM »
Hi,

I have seen the same problem when machining parts with retainer "tabs"
The X & Y accellerations are very different to the Z, as the Z is a driven knee.

If I am feeding x and y at say 300mm per min, the knee tries to do the same if the tabs ramp at 45degs. (Max Z is 250mm/min)
So I have to limit the angle of the tabs so that the Z axis does not "overspeed".

I don't see this as a Mach fault as when the Z moves Mach would need to limit the X & Y feeds to match the maximum acc / vel of the knee.
So I either speed up the knee or fit a Z on the quill....or buy a VMC ;D

ATB
Deker
You can "chop it off" but can't "chop it on"
Re: Whose Fault Is ThIs?
« Reply #13 on: November 30, 2009, 05:44:00 PM »
Hi,

I have seen the same problem when machining parts with retainer "tabs"
The X & Y accellerations are very different to the Z, as the Z is a driven knee.

If I am feeding x and y at say 300mm per min, the knee tries to do the same if the tabs ramp at 45degs. (Max Z is 250mm/min)
So I have to limit the angle of the tabs so that the Z axis does not "overspeed".

I don't see this as a Mach fault as when the Z moves Mach would need to limit the X & Y feeds to match the maximum acc / vel of the knee.
So I either speed up the knee or fit a Z on the quill....or buy a VMC ;D

ATB
Deker

It *is* a Mach fault.  What it is *supposed* to do is always respect the settings (both acceleration and velocity) of *all* axes, and keep the move within the limits of all axes at all times.  That means if X and Y are fast and Z is slow, then X and Y are slowed down so Z does not exceed it's capabilities.  Obviously this is far easier to achieve if all axes have the same capabilities, but that's not the way the real world works.  The operator should not have to worry about keeping within the capabilities of individual axes on complex moves. 

In your example, the correct thing for Mach3 to do is to run X and Y at 300 mm/min, until they get close to the tab.  X and Y should then decelerate so that when they reach the start of the tab, they are down to a speed that allows the Z axis to be moved without it needing to accelerate faster than it's maximum acceleration, or having to exceed it's maximum velocity.  Once past the tab, and the Z axis is again stopped, X and Y should then accelerate back up to 300 mm/min.

Regards,
Ray L.
Regards,
Ray L.
Re: Whose Fault Is ThIs?
« Reply #14 on: November 30, 2009, 06:19:59 PM »
Well , now, then.
Seems to me a while back I put a post in this forum about not driving the Knee for the Z axis. Maybe , just maybe , this is one of the reasons, along with a few more.  ;D

Ed
« Last Edit: November 30, 2009, 06:24:01 PM by edvaness »
Ed VanEss
Re: Whose Fault Is ThIs?
« Reply #15 on: November 30, 2009, 07:52:17 PM »
Well , now, then.
Seems to me a while back I put a post in this forum about not driving the Knee for the Z axis. Maybe , just maybe , this is one of the reasons, along with a few more.  ;D

Ed

Ed,

Look at the bright side - Using the knee as the Z axis has highlighted a bug in Mach, that will now get fixed.  So, using the knee is a good thing!  There are several people piping up who have had this same problem, so I'm far from the only one getting bit.

What's funny about this is, what I'm making right now is my quill drive.  If I'd finished this one project without running into this bug, I NEVER would've seen it, as my quill drive will be at least fast as my X/Y axes!

Regards,
Ray L.
Regards,
Ray L.
Re: Whose Fault Is ThIs?
« Reply #16 on: November 30, 2009, 08:16:33 PM »
Ray ,

Yes , the quill drive will be the quickest and most profitable way to go. Keep the knee servo for tool length offsets.

Ed
Ed VanEss
Re: Whose Fault Is ThIs?
« Reply #17 on: December 01, 2009, 02:36:46 PM »
It's NOT a CV bug!

    I just did an interesting test - I wrote a simple test program that does:

    Short, 2-axis linear move to initial position at 100 IPM
    Short, 3-axis linear move to second position at 5 IPM
    Repeat the above two moves in reverse, ending up back at the start position

    This is run in a loop, and works perfectly every time.

    I then replaced the two 3-axis linear moves with a helical move between the same two points, at the same feedrate

    This faults the Z servo every time.

    I then set the Z acceleration as low as Mach would allow (~0.2), and put an oscilloscope on the Z axis STEP line.  Running the first test, I see a nice acceleration over the first ~0.5 second of the Z move.  Running the second test, the step rate goes instantly from 0 to 10kHz - NO acceleration ramp at all!  But it does appear to do a proper deceleration at the end of the move!

    Note that the behavior is exactly the same, whether I run in CV or ExactStop mode, so it's NOT a CV problem.

Regards,
Ray L.
Regards,
Ray L.

Offline simpson36

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Re: Whose Fault Is ThIs?
« Reply #18 on: December 02, 2009, 07:53:44 AM »
This sounds eerily familiar


If someone is working on this problem, they should also consider that this behavior is present on, or is triggered by the A axis.

Under certain conditions, if the A axis and the X axis (for example) start moving together, MACH ignores acceleration and just slams the X axis from zero to feedrate. I don't recall off hand the circumstances, but I posted some details on this quite some time ago.

Sounds like this may be a symptom of the same bug, so I thought it worth mentioning.
Re: Whose Fault Is ThIs?
« Reply #19 on: December 02, 2009, 10:10:57 AM »
This sounds eerily familiar


If someone is working on this problem, they should also consider that this behavior is present on, or is triggered by the A axis.

Under certain conditions, if the A axis and the X axis (for example) start moving together, MACH ignores acceleration and just slams the X axis from zero to feedrate. I don't recall off hand the circumstances, but I posted some details on this quite some time ago.

Sounds like this may be a symptom of the same bug, so I thought it worth mentioning.

Simpson,

Yes, Brian mentioned that when I spoke to him yesterday.  I believe that problem will also be fixed when he's done.

Regards,
Ray L.
Regards,
Ray L.