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Possible Feed Rate Bug?
« on: February 23, 2018, 11:32:32 PM »
I have been using MACH4 for quite some time now and noticed that the "Current Feed Rate" DRO displays 90% of the commanded feed rate. This potential error has been present in all versions following 3233, industrial also.
I am not sure if this is just a DRO issue or if the machine is actually only running at 90%. In case you ask, yes the feed rate slider is set to 100% and also using the simulator as motion controller to eliminate any plugin issue.

In September of last year I submitted a support ticket regarding this issue with no response. There also doesn't appear to be any other posts pointing out this potential bug.
I don't know for certain but it seems like this could be an issue for anyone out there rigid tapping, assuming this isn't just a DRO issue, causing desync between spindle speed an feed.

Has anybody else noticed this and is it an actual issue? Curious because I have been reluctant to move from version 3233.
Re: Possible Feed Rate Bug?
« Reply #1 on: February 24, 2018, 03:10:11 AM »
Hi,
I have noticed the same issue, I'm running 3481 although if memory serves it was the same in previous builds.

I assumed that Mach could work out a limited number of trajectories that fitted the required move and it picked that trajectory that had the closest
feed rate to that requested. I didn't regard it as a bug but rather as a 'quirk'. Maybe you think differently.

Craig
My wife left with my best friend...
     and I miss him!
Re: Possible Feed Rate Bug?
« Reply #2 on: February 24, 2018, 06:27:12 PM »
I wonder if the program is compensating for tool diameter, trying to figure a cutting speed (tip speed) rather than feed rate (tool speed).

Offline smurph

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Re: Possible Feed Rate Bug?
« Reply #3 on: February 25, 2018, 03:02:53 AM »
The current feed rate is supplied as feedback from the motion controller.  So maybe there is a timing issue? 

Steve
Re: Possible Feed Rate Bug?
« Reply #4 on: February 25, 2018, 08:52:38 AM »
I haven't noticed this on my DSPMC powered mill but I will take a look today and see what I see. It may be controller dependent.

Mike
We never have the time or money to do it right the first time, but we somehow manage to do it twice and then spend the money to get it right.
Re: Possible Feed Rate Bug?
« Reply #5 on: February 25, 2018, 12:32:33 PM »
Thank you for the feedback. I am still leaning toward this being a bug rather than a feature.

joeaverage: That would make sense with CV feed rate turned on and the machine slowed down for a corner. Though with CV on or off, if you MDI G1 X10 F100 the dro will show the machine running at 90 units/min.
                  It also doesnt matter what the feed rate is, it always returns 90%.

roaster: Not likely as that would change the spindle speed and not the feed rate. G96 vs G97. Also my tool table in MACH is blank and all tool comp is handled in CAM software.

smurph: Bug is present on completely stock install of MACH using simulator motion controller with zero third party plugins installed. I would see that as best case scenario so MACH should feedback exactly what was it was told.

totallyrc: Bug is only present on versions of MACH4 after 3233. All previous versions display the correct feed rate. Curious to learn what version you are running.

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Re: Possible Feed Rate Bug?
« Reply #6 on: February 25, 2018, 03:30:24 PM »

smurph: Bug is present on completely stock install of MACH using simulator motion controller with zero third party plugins installed. I would see that as best case scenario so MACH should feedback exactly what was it was told.


The Sim plugin is not a motion controller.  It is not real-time and it uses Windows timers to make it tick.  A 10% variance in timing is quite the norm for Windows.  It doesn't feed back exactly what it is told.  It feeds back the feed rate that it is able to produce.  The Sim plugin is used as an example for people who wish to write motion controller plugins.  Therefore, it is written to use the API in a manner that real motion control plugins would use.  I could make it feed back exactly what it was fed from Mach, but that would not be a very good example.  

So the moral to this story is that Sim is not perfect.  Windows is the "bug".  It relies entirely on Windows and every Windows machine will have different timings depending on what components make it up.  A real motion controller will provide better feed rate feedback.  Also, it is common to have some feed rate variance when CV is in use.  You will see the feed rate drop on corners.  How much depends on the acceleration capabilities of the motors.  But a straight line should report a feed rate VERY close to the desired feed rate.  But do keep in mind that not all motion controllers will report the current feed rate at a frequency that appears instant.  Some could only report at 250 millisecond intervals, for example.  Also, the screen DROs will only update at the screen refresh frequency (stock is 50ms) no matter how often the feed rate is updated.  

Steve
Re: Possible Feed Rate Bug?
« Reply #7 on: February 25, 2018, 05:10:56 PM »
Hi smurph,
thanks for that explanation. I have on a number of ocassions believed that I had found a fault with Mach4
only for its behaviour explained and realise its no fault at all but rather a 'quirk'.

Craig
My wife left with my best friend...
     and I miss him!
Re: Possible Feed Rate Bug?
« Reply #8 on: February 25, 2018, 07:24:09 PM »
My MAchine shows the propper feedrate in jog, MDI and G-code. Didn't look at which version I was running.

Mike
We never have the time or money to do it right the first time, but we somehow manage to do it twice and then spend the money to get it right.
Re: Possible Feed Rate Bug?
« Reply #9 on: February 27, 2018, 06:13:50 PM »
Steve,

Thank you. Turns out that was the problem after all. I first noticed the discrepancy last year when I was designing a new screen set and just assumed, something about an a** out of you an me, the issue would translate to my actual motion controller so I reverted back to an older version of MACH.
I would check new versions as they were released but only with the simulator to see if the issue was resolved. So anyway I tried the latest development version on my machine last night and the current feed rate was correctly displayed.

Thanks for the assistance guys. I hope a I didn't waste too much of your time.

Dustin