CNC Tip:

Q & A

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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

Response:

Andrew,

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 reasons.

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.