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Author Topic: real meaning of G28 ?  (Read 23141 times)

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Offline Graham Waterworth

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Re: real meaning of G28 ?
« Reply #10 on: March 18, 2010, 04:00:35 PM »
If Mach has no known home position it can not work to absolute values so it reverts to incremental values in G53

Without engineers the world stops
Re: real meaning of G28 ?
« Reply #11 on: March 18, 2010, 04:17:21 PM »

Now G28.1 gives me a headache.
      Assuming this is all in Mach. coords.
Using the Summary of GCodes in Mach
current position = X10 Y10 (mach. coords)
MDI G28.1 X1 Y1   I would expect the axis's to rapid simultaneously to machine coord x1y1, then commence the normal referencing routine per config. but it doesn't.
Both axis's make a simultaneous INCREMENTAL move in the POS direction of 1 to X11 and Y11 and then the normal referencing routine begins to seek the switches in the proper NEG direction.

Been banging my head on this for 2 weeks ..off and on. lol

The incremental preliminary motion of G28.1 is easy to explain. You are probably in G91 mode (if not, i.e. in G90 mode, then see next absatz).

And please note, the preliminary position in G28 and G28.1 is actually in work+tool coordinates.  (if you have been in G90 mode, then your work [+tool] offset should happen to be (-10,-10,*)  (+10,+10,*) ).

No matter what the NIST-compliant documents say. I just simulated it in Mach3 on my laptop.


The G-Code generator (Mach3-Postprocessor) we are using seems to take Mach3 into account. It clears to home with two lines, and note G91 mode for both lines.
Code: [Select]
N990 G91 G28 Z0.
N992 G28 X0. Y0.
« Last Edit: March 18, 2010, 04:33:16 PM by PeterF »
Re: real meaning of G28 ?
« Reply #12 on: March 18, 2010, 04:46:34 PM »
Thanks guys, please excuse the interruption Pete.
I'm beginning to get a grip on it.
I see now how to make it work, just not exactly as I expected.
I have had very little exposure to CNC in general.
Thanks again.
Re: real meaning of G28 ?
« Reply #13 on: March 18, 2010, 05:06:41 PM »
No reason to excuse. Your question was welcome.

I just noticed the release of Mach3 version 3.042.038 and will install it tomorrow in the shop. Actually the spindle (unrelated to this thread) is in the "fixed" list.

Re: real meaning of G28 ?
« Reply #14 on: October 11, 2018, 06:03:25 AM »
What specifically does the G28  Z input in SoftLimits setup menu?   

If i use G28 in mach3 POST, my 3040 that i ZERO over the origin on my workpiece,,,, at start Z goes up to zero and then to XY to carve position, all good to avoid clamps,,, if i know i have clearance at want Z to stop at ie 20mm over the stock to save time, where should i write it, in Fusion360 POST or uve Mach3?
My Z is 0 at top and -60 at bottom..
« Last Edit: October 11, 2018, 06:06:22 AM by TommyF »

Offline RICH

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Re: real meaning of G28 ?
« Reply #15 on: October 11, 2018, 08:37:01 AM »
What specifically does the G28  Z input in SoftLimits setup menu?  

 I assume you are refering to the G28 home location coordinates on the bottom of  the flyout screen
( Motor Home/SoftLimits)  via Config>Homing Limits. There is no information in the Using Mach3Mill manual
for that part of the screen.

G28 home location information is given in section of  Mach3 CNC Controller Software Installation and Configuration
manual. It states:
The G28 coordinates define the position in absolute coordinates to which the axes will move when a G28 is executed. They are interpreted in the current units ( G20 / G21 ) and are not automaticaly adjusted if the units system is changed.

Now just some comments:
- I personaly have never used that part of the flyout.

- The thread topic is Re; real meaning of G28.
   G28 is a G code commmand, defined by Mach3 in the manual 10.7.10 ( see also G28.1 , G30, G53). I have seen many
   comments / questions over time about it's use.  There are threads on the subject. One should have an understanding
   of  G28, G28.1, G30 in light of referenced / unreferenced machine/ type of machine, G54 / G90 / G91 active or inactive, and
   what  happens using  the commands and screen buttons, may as well throw in axis feedrate.
- There is a lot to gained by playing around with G28 relative to differant states of the machine...........one can learn a lot!