safety blocks Re: DIFFERENCE BETWEEN G90 & G91?

Fred, List,

Sorry for the near duplicate of what says below. I read sequentially
through the posts, and often yahoo does not show the "downstream"
thread posts accurately...

Ballendo

--- In CAD_CAM_EDM_DRO@y..., "imserv1" <imserv@v...> wrote:
> --- In CAD_CAM_EDM_DRO@y..., Fitch R. Williams <frwillia@m...>
wrote:
> > On Tue, 22 Jan 2002 00:48:09 -0000, "yethdear0" <yethdear0@y...>
> > wrote:
> >
> > >Most machines "wake" up in G90 mode.
> >
>
> Safety blocks...
>
> >. It seems to reduce
> > the number of surprises I get.
> >
>
> While most machines wake up in G90, if you stop them in mid stream,
> they may think that they are still in incremental, and the next
time
> you start the program. ###$$$*** Kerbango. That's why those safety
> blocks are a good practice. Even though you may only have this
> happen once a year, it's better to never go Kerbango and have a few
> extra lines of code in every program.
>
> If you program multiple tools, it is a good idea to put nearly the
> same set of safety blocks before and after each tool change. It
does
> not improve the sequential safety of the program, but it enables
you
> to safely start in mid program at any tool within the process.
>
> G91 incremental is useful for a situation in which you want to make
a
> number of copies of the same set of cutting moves, say 6 pockets in
a
> 2 x 3 array. You can program an absolute move to each of the 6
> absolute locations and then re-use the same chunk of G-code for
each
> pocket, in incremental mode. Each pocket will be relative to the
> absolute position. You can also use a G92 to reset your Zero point
> at each location, but a mid program start may lose your location (
> not good). Using one set of absolute locations and then relatively
> cutting around each one can be reset and not lose location. Just
> remember the G90 at the beginning of the program, and at the end of
> each relative G91 subroutine call.
>
> Best Regards, Fred Smith- IMService
>
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