pyotr filipivich wrote:
Something which occurred to me this week.

At work we're required to do a "dry run" after setup [CNC mills], but
before running a "first part" (and if the part is one of the "stack 4
sheets and mill together" sort, the first part is "stack one, shim to fit,
and run just the one"). This can take some time, especially when this is
the first time _you_'ve run the program, and you don't know which way it is
going to twitch. (I recall figuring it would be working to the left, it
worked to the right.)

I rarely do a true dry run anymore, even when making the first part with
a toolpath generated by a custom program. Most of these programs are
based on earlier programs, so I have a pretty good idea how the G-code
will perform. If really in doubt, I might cut the first pass in air
above the real stock, though. I usually run it at a reduced feedrate,
and have my hand hover near the cycle stop button if I think something
looks wrong. At least in my case, the stock is so cheap, and breaking a
$6 carbide cutter is not a major tragedy, either.

Actualy, most of the spectacular goofs have been operator error, not the
fault of the CNC program. For instance, forgetting to zero out the
cutter Z coordinate. So, instead of the cutter approaching the top of
the workpiece while rapid-feeding to the beginning of the cut, the
cutter plunges all the way through the work and then rapid feeds at
60 IPM through the whole depth of the part, while by heart rate hits
200 BPM!
SO, when doing he dry run, "cheat" and put a piece of material in and
do your first part at dry run speeds (which tend, on Robo Mills, to be
about 30% (max) of normal), or dry run & cut an "air piece", and then run
the first piece at normal speeds?

I'm wondering, also, for some of the larger projects, the ones where
high speed production takes an hour or more to run, so 'walking' a dry run
could be an all day adventure in tediousness.

If your CAD/CAM program is reliable, and your part programmers are
reasonably careful, you just shouldn't have to do all this dry run
stuff. If you run the same part for months, the total cost of each
dry run becomes insignificant. But, if you are a mold shop, that would
mean one dry run per part! If your toolpath generation methods are NOT
reliable, don't rely on the dry run, FIX the real problem!

Jon