robert wrote:
>> Well - I'd love to see a Fanuc 16/18/21 do as you ask.
>>     
OK, my 1978 vintage Allen-Bradley 7320 had a feature called "jog 
retract".  You could hit cycle stop at any point in a move, press jog 
retract, and then move one axis only as much as you desired.  Normally 
this might be the Z to retract the tool.  You could replace the tool, 
enter a new tool length offset (maybe even diameter, I don't recall), 
and then press cycle start.  It would apply the new offset (if changed), 
return the tool to where the cut left off, and then resume from the 
exact spot where it left off in the middle of the interpolated cut.

You could also hit cycle stop anytime, anywhere, and then hit buttons to 
step forward or backward a block of G-code at a time, then hit cycle 
start to resume from that line.
You could easily get in trouble with this, as you could have interrupted 
it in a G03 move, and then stepped back to a line that had been written 
with an implied G01, and only X and Y axes (no I or J), and it would of 
course complain cryptically that there's no I or J in a G03 move.  So, 
you had to be careful of the modes that were needed for any particular 
block, but it was also very flexible and fast to stop, adjust something 
and restart.

And, this was all in a 1978-vintage control, one of the first real 
Computer NC with a CRT on it.  30 years ago!

Jon

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