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Author Topic: Toolpath Crosshairs Display incorrectly when using Work(fixture) Offsets  (Read 8360 times)

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Hey All,

I am trying to cut 2 different shapes/paths on the same piece of stock using one program and work offsets written into the program. 

G55 =  X10 Y10 Z0
G56 = X10  Y5 Z0

First cut referenced to G55 offset.  Cut and toolpath display are fine.  When machine cuts the cutting path is followed/displayed correctly.

Second cut referenced to G56.  Toolpath displays OK, cut is exactly where it should be (starting @ Y5) However, the cutting path shows it is cutting in the area of the first (G55) offset.  It's weird.  Is this Operator error?  Am I missing some setting/checkbox somewhere?    Or maybe a bug?

Thanks in advance for any replies!

Sid
« Last Edit: April 08, 2008, 09:30:40 AM by sshneider »

Offline jimpinder

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In answer to the question in the heading - yes it is!!!

I don't know how you set up your machine at the beginning of a job, but offsets are calculated from the homing positions of the machine. When you put in G55, Mach3 adds this to 0.0.0. in machine co-ordinates and then calculates everything from there.

Your display - if it is not in machine co-ordinates - displays your program co-ordinates. So if you have written your program starting at X0 Y0 that is shown on the screen as the bottom left hand corner of your program area ( the area of space the program takes up) BUT  you are not actually at X0 Y0 you are at X10 Y10.

On the second half of your program you put in a second offset (G56). Mach3 takes off the old offset and adds the new offset, and runs from X10 Y5 BUT if your program again starts at say X0 Y0,  on the toolpath display it will overwrite the first one.

Although the displays overwrite, the machine working on machine co-ordinates actually cuts at the right place.

As far as I can see, do not regen the toolpath between the halves of the program. It might however do this automatically when it encounters a new offset.  You can use the machine co-ordinates display - but then the co-ordinates will not be the same as those your program is running.

See the Video Tutorial on Co-ordinate systems.

Not me driving the engine - I'm better looking.

Offline Chip

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Hi, Sid

( Second cut referenced to G56.  Toolpath displays OK, cut is exactly where it should be (starting @ Y5) However, the cutting path shows it is cutting in the area of the first (G55) offset.  It's weird.  Is this Operator error?  Am I missing some setting/checkbox somewhere?    Or maybe a bug? )

It's a Mach3 View Screen Issue, Been there a long time.

Thanks, Chip
Thanks fellas!  Any workarounds?  I was thinking maybe about using the M98/M99 commands to call subroutines?  Any thoughts, ideas, Brainstorms?

THANK YOU!
Sid

Offline Chip

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Hi, Sid

It dose the same thing with G52, G92, Offset's. with M98/M99 subroutines.

I think if you write your G-code "parts" in G91 moves you may be able to work around it.

Brian has it on a list for display screen issues.

Thanks, Chip
« Last Edit: April 06, 2008, 07:15:18 PM by Chip »
Cool! thanks chip!  I'll give it a go (and wait patiently for Mr. Barker to work it out)!

Regards,
Sid

Offline jimpinder

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The only way to do it, I think, is having set it going revert to a machine co-ordinates view and regen the toolpath. This should then regen everything from machine co-ords 0.0.0. and then put it on the screen with the offsets.

It should then show the difference between the two different offsets.
Not me driving the engine - I'm better looking.
Jim,

Thanks but, I don't really follow what you are saying.  What do you mean by 'revert to machine coordinates'.

The idea behind this was to have the machine do 3 cuts on the same peice of stock at 3 different offset starting positions without any user input.  If I have to reset or reprogram or edit G-code to re-orient the cut on the stock, it kind of defeats the original purpose.

The machine actually is cutting correctly, it just doesn't display correctly which is mystifying to me. 

Maybe it would help if you loaded the sample G-code and have a look at it on your machine.

Thanks,
Sid

Offline jimpinder

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No - I understand exactly what you are saying - look at the tutorial video on Co-ordinate systems.

You machine runs ALL THE TIME in machine co-ordinates.

Machine Co-ordinates X0 Y0 and Z0 might be at odd places - usually at the bottom left hand corner of table movement - with the Z at rest at 0. This might be a **** position from which to start any work. Your table might be 10" by 10"

When you write a program, it is normal to start the tool at X0 Y0 (with Z0 touching the top of the workpiece). This is easy to imagine, and you write the program. The whole program area might be 5" by 5". AND TO MAKE IT EASY YOU MOUNT THE WORKPIECE IN THE MIDDLE OF THE TABLE.

To reconcile the position of the mill, with the position of the workpiece, you the EITHER jog the mill to the bottom left hand corner of the workpiece, or you use an offset say G54 which takes your mill to the point at which your program starts and you zero the axis. The axis will not zero on machine co-ordinates because they are tied to the home switches, but if you press the machine co-ordinates button and the light goes out you are then on program co-ordinates - and you zero the axis.

If you then look at the toolpath display it will show the crosshairs at X0Y0 at the bottom left hand corner of the workpiece (the centre area round which you wrote your program). The DRO's conveniently show you at 0,0,0 because it knows that we humans are less intelligent, and we like to see what we expect to see. You start the program and the display conveniently follows the toolpath you have written.

THE COMPUTER HOWEVER knows this is a sham !! it knows it's actual position, according to the MACHINE CO-ORDINATES, is 0.0.0. plus the offsets. It displays the toolpath you want to see, but carries on itself on its own (the true) toolpath.

When you come along and put in another offset, it cancels the first and puts in the second, zeros the display and shows you at position 0.0.0.  THE COMPUTER KNOWS THIS IS A SHAM AS WELL - it has just moved to a different location, but for you poor humans, so not to get you upset, it shows that you are at position 0.0.0. because thats what your program says - and it gaily starts to cut, showing you the display it thinks you want to see.

To your eyes, of course, you are seeing two tool displays, one on top of the other - because they both were written starting at the same place - but the COMPUTER IS NOT BOTHERED - it started at the right place according to it's MACHINE CO-ORDINATES and cuts the right path.

I don't know whether that explains it - but if you watch the Co-ordinates video tutorial, I'm sure the two will explain it.



 










« Last Edit: April 07, 2008, 01:11:16 PM by jimpinder »
Not me driving the engine - I'm better looking.

Offline jimpinder

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To carry on from that, therefore - if you want to see the correct tool path display, then press the Machine Co-ordinates button. If this is lit the display you are seeing is machine co-ordinates - the actual path the machine is cutting. It shows you all the table - that may or may not be a disadvantage - it depends how big your table is.

The DRO's however also change to machine co-ordinates - which will bear little resemblance to what you have written, because they also include all the offsets - and make it difficult to check where you are.

The system works fine if you only have one offset, but if you have multiple offsets in the program this will happen.
Not me driving the engine - I'm better looking.