RE: [CAD_CAM_EDM_DRO] Thoughts on Gcode interpreters and CNC controls....

Doug -

I wonder the same thing. Matt's input on lookahead is what we're talking
about. It seems that most controls like to quote a number in terms of
blocks, but I don't see how that can be a fixed number. I would think it'd
depend on the actual g-code. Like he said, EMC starts looking ahead as soon
as it starts running a program, and can usually get very far ahead, at least
for most programs. Some motion controllers, like IndexerLPT (wish there was
a Windows frontend...) do process the entire program before beginning the
cut. I also hear about other controllers that have a "continuous
contouring" mode (ala Flashcut?), which I believe is a way to allow the
pre-processing, but I think you run into problems this way with feedrate
override, and stopping/holding/starting/backing up/etc. during the motion.
It's got to be tricky to be able to pre-process everything and vary the
feedrate during the motion, unless you work off of a variable frequency
hardware clock or have a few tricks up your sleeve.

Carlos

> Group,
>
> Since there's a fair amount of CNC knowledge in the group I'd like to
> throw out a topic for discussion.
>
> A typical CNC controller operates on Gcode programs in what would be
> called an interpretive mode. As opposed to having an already
> compiled CNC program to work from. In other words it scans through
> the program as it's executing it and forms the motion commands in
> real time. Sometimes it's obvious that this method slows down
> program execution, especially when you're trying to do continuous
> contouring using cutter comp on slower controls. An example would be
> Ahha, I saw a wood router guy trying to contour with cutter comp and
> it was a disaster because of the micro dwells between code blocks.
>
> My question is, why don't controls have an option to pre-scan a
> program and by accessing the tool and part offset tables pre-calulate
> all the cutter paths so that when actually cutting all the controller
> has to do is deal with interrupts and motion control? Or do some
> controls have that ability? I've asked this question to a number of
> sales engineers at trade shows, but was never sure any of them
> understood what I was asking. The nearest I came to getting an
> answer was from a Mazak engineer who thought Mazatrol had something
> like that.
>
> Any thoughts?
>
> Thanks,
>
> Doug