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or misuse of techniques shown in this web page. We simply publish information
we feel will be of interest to CNC users. In all cases, the reader is totally
responsible for considering the implications, good and bad, of implementing one
or more of the techniques we show.
How wise it is to use wear offsets for different diameters on a lathe?
I have a coworker that always leaves huge offsets instead of editing the
program. Several times I almost crashed a machine because I assumed that tool
will respond to programmed X or Z. Andrew J. Rutkowski
This is a question that comes up on a regular basis in my advanced courses.
My first response is always to do what works best for your company, and the
saying "you can't argue with success" applies. But the key, as you've
found out, is consistency. Everyone must do things in the same way if either
technique is to work.
I assume that the problem is caused by a tool pressure difference from one
diameter to another. Additionally, I'm assuming that as the finishing tool
wears, tool pressure changes again, requiring one or more adjustments during
each tool's life.
First of all, I urge all programmers to program the mean value of each
dimension for all coordinates used in the program. This gives you the best shot
at being able to hold size for all dimensions with one wear offset.
Assuming you're doing this (tolerances are very tight), I always recommend
using secondary offsets to allow sizing for different diameters for two
1) Manipulating programmed dimensions will cause problems the next time the
job is run. If the program is saved at the end of the production run, it will
not contain the mean value for coordinates you modify to hold size during
production. The next time the job is run, the setup person will not know what
coordinate is included in the program. The result will likely be a scrap part.
2) With most controls, you can't change the active program when the machine
is in production. But you can change offsets. This means that adjustments for
holding size in a long production run can be done while the machine is running
if you're using a secondary offset.
Admittedly, using secondary offsets requires that everyone understands how
they work. (For example, if you're using a secondary offset to hold diameters,
the Z value in both offsets must be the same!) There will be but a tiny
deviation in the X registers, representing the tool pressure difference.
Additionally, if you don't get much repeat business and have very small
lots (short runs), the two reasons I give for using offsets over changing
programs are eliminated.
Again, it all boils down to training, consistency, and getting everyone on
the same page.