G90 to G91 conversion

[Brouwer]

Hello, I'm new to this forum. I use a small router to cut sheet metal /polystyrene parts for models as a hobby.

I've been manually entering Gcode into my programmes but am getting tired of the time taken to do the maths (simple but time consuming) for curves , especially on sloping lines etc.

The are a number of DXF to Gcode converters about (free ones too, I found DXF2gcode good). But it/they do not convert G90 to G91 incremental. To do assemblies of parts on sheets G91 is essential especially of multiples are also wanted.

Does anyone know of a simple absolute to incremental converter?

Even some of the "commercial" programmes which have all the bells and whistles to post process and drive machines do not convert to incremental! Strange.
All I need is a simple G90 to G91, nothing else! :-)


John Brouwer

[HuFlungDung]

What you ask for is usually a setting that can be made in the post processor: absolute or incremental output. There is a likelihood that these two modes cannot be mixed together in a single posting from one group of operations.

But you could post individual incremental programs for each part, then cut and paste them together as subroutines or sub programs called from a main program that is written using absolute mode code.

In this system, your main program might consist of a regular point grid of some sort, representing the moves to one corner of a rectangular boundary. The grid would be your own design representing efficient use of the sheet stock.

So then the main program would be something like:
G90 G00 X0 Y0 Z1.
M98 P1000 (find and execute program O1000 once)
G90 G00 X3.0 Y0. Z1. (move to second part datum on the grid)
M98 P1000

and the subprogram:
O1000
G91
incremental code, bla, bla
G90
M99 (return to main)

There are more tricks, too, like using workshift G52 to rename the datum point, and then actually run absolute mode code over and over again. The G52 is a local workshift and may not be available on all controllers.

Least desireable would be the old G92 coordinate system shift. This is a bit trickier to control because it cannot be cancelled without some effort if things go awry. I won't go into detail, but you can search the forums for discussions on G52 and G92.

[Brouwer]

Thanks for that.

After a couple of days searching before I posted , I found Lazycam not long after :-) This can directly import my R12DXF (some others won't) and reset the first xy to zero so giving me a sort of incremental for the rest. The first xy for Lazycam is the bottom left of a bounding box/envelope so is not necessarily the actual entry point but it is close enough meantime. If I could just choose the point to reset.....

Otherwise I do as you suggested. I manually rest individual component absolute placement points then relative from there till the next item etc.
Very easy to move stuff around like that.

John Brouwer

[Brouwer]

Hmm, bit of a brain explosion there! Of course that does not work!
Back to searching..

John Brouwer

[ger21]

Download a free CAD program, open your .dxf, and copy and paste your parts. Then create the G90 code for all the parts? Not sure why you need to use G91 to cut multiple parts.

[Brouwer]

I find that I can move parts around easier and if I have a sheet that I want to make additions to or parts that need to be transferred to another sheet I can just copy and paste then change the absolute start point.
In any case I have not yet found a DXF to Gcode converter that I like :-)
I also find that for most parts the direct entry Gcode for me is faster, yes!


John Brouwer