Help.....G90 and G91 problem

[turmite]

How do you edit a G90 program when you had to change the starting position??

I have used G91 for 11 yrs and this is my first foray into G90. I have heard everyone tout G90's ease of use and how wonderful it is but I am not finding it that way.

How do you begin a new program? do you have to draw everything in absolute psoition before it can be camed that way??

Mike

[Jennifer]

In BobCAD/CAM you have the option of writing the program in absolute or incremental (in the setup on the CAM side) - nothing changes about the drawing...it seems like this has been the case with other programs I've used, but it's been a long time.

Jen

[turmite]

Hi Jennifer,

I am using V17 Bobcad/cam. I have found a bug that on some of the camming operations what you have drawn is not what you get in the cam. I have used the geometry from code option in the edit function of the cam program. When doing so you can do a translate and lay the toolpath back over the original drawing and they are nowhere the same. This only happens in incremental. Now in incremental as you well know if you get an error in the program it becomes cumlative (sp) if you have no linear encoder feedback, which I don't.

Mike

[HuFlungDung]

Mike,

Absolute is considered safer for just the reason you stated: an error on a single line does not skew the whole program. Actual motor positioning errors however, behave exactly like an incremental error, no matter whether you are programming in G91 or G90.

You've got to watch your post requirements when you switch back and forth between G90 and G91. Arc coordinate format may be different in each mode.

I like absolute (G90) programming also because it can allow for mid program start at any point, with due consideration for what should have happened up to that line, of course . You can also get an idea of how to edit the program at the controller if need be, but an incremental program, well, that is nearly hopeless at the control unless you're just cutting squares or circles.

I tend to reserve incremental for creating subroutine loops, if suitable. It is difficult to switch back and forth in Bobcad. In fact, I doubt that the backplot function can properly deal with any program that has both modes in it. It may be the case that you have to switch the post driver into the proper mode before you even attempt to backplot the code in that mode. I'm uncertain of that, but that is what I would be careful of.

[wms]

Mike,

Can you "adjust " it , the new starting point, at the control?

Do you have the option of setting the work offset to the new location?

Do you have and use work offsets? IE: G54 ,G55, ETC. ?

[turmite]

Hu and wms thanks for the answers.

Hu I know that everyone that uses the absolute mode speaks very highly of it, but please understand for 11 yrs incremental is all I had.

Wms I am also very new to all the Gcodes except G0,G1,G2 and G3. That is all my first controller would do. I have retrofitted my machine with the Mach2 controller and I guess I am having a little bit of a problem consuming all that is on my educational plate right now!

I am trying to figure out how to run my old programs without having to redraw and rewrite them. I have something over 500 so it is no easy matter to undertake.

I don't suppose there is a post that will recompile the old programs to absolute is there??

Mike