Had a talk with a old school machinist that made me rethink my tool offsets

I was setting up a new job on a Haas VF-2 last Thursday and this guy Bob who's been running CNCs since the 80s walked over. He watched me punch in my offsets and told me I was doing it all backwards, that I should be setting my tool length offsets off the table instead of the part top. I argued with him for like 10 minutes but he showed me a test cut and the repeatability was WAY better. Has anybody else tried doing offsets from the table surface instead of the part?

2 comments

2 Comments

the_margaret16d ago

Doesn't it mess with your offsets when you switch vises or clamp different parts? I set mine off the part because the table height changes with different fixtures.

ross.kevin15d ago

Bob's been around long enough to know what works, that's for sure. I used to set everything off the part top too, but after getting burned by inconsistent part heights from casting variations I switched to the table. That test cut he showed you sounds like the kind of proof that shuts down arguments quick. @the_margaret brings up a good point about fixture changes though. I deal with that by keeping a dedicated reference tool that I touch off on the table once. Then all my relative lengths stay the same no matter what fixture I swap in, as long as I update the work offset for the new part location. It took me a few fiddly setups to get the habit down but now it feels like second nature. The real question is how do you handle tool breakage detection with table offsets? My machine freaks out if I try to probe a tool that's already touching the part, but with table offsets I can touch off air without worrying about smashing something.