Steel Gantry Machine 1500x750x200

[Davek0974]

What cutter are you using, and how accurate do you believe it is?

I normally use single-flute cutters, and my experience is that they tend to cut as if they are slightly smaller than their marked size. For example, I get pretty accurate results from my nominal 6mm cutter if I specify it as 5.7mm in my CAM software.

I spent a lot of time chasing this kind of inaccuracy and whatever tests I did, the machine seemed to be pretty good. However, when I cut a simple test square and measured it, the size was always wrong. Now, I do a quick test with every new cutter to work out its "real" diameter. The problem is that you cannot just measure single-flute cutters. Maybe they are just cheap cutters and no-one else seems to have this problem, but this is the only way I can get accurate, consistent, results.

Looking again at your measurements, though, I'm not sure that this is the case here. If, for example, the cutter were cutting 0.3mm undersize, the internal dimension would also be wrong (29.7mm). Similarly, a scaling error would also see errors in the internal dimension. Maybe it's something silly like using an offset in the CAM settings for the external cut? Or did you forget the finishing cut? Sorry if this sounds rude, but sometimes the answer is a small error that you have made and you only find it when you show it in public! That is something that you might find if you examine the gcode.

I think its normal - I do exactly the same with all my cutters, the 2mm SF carbides i use only give true results if cam'med for 1.96mm so a test is needed if dims are critical.

[Nickhofen]

Sorry if someone else mention it and I didn't notice.
Have you try with another program? Design the same object and cut?

[toomast]

Managed to sort out my problem. Almost too embarrassed to tell but.. as it turns out it is not wise to calibrate machine axis with a dial indicator. At least a machine that has not super accurate ballscrews. Also the testpart that showed inside measures correctly and outside wrong, was coincidence I believe. I re-calibrated my axis with a 300mm caliper. Problem solved.

So anyway here is a little cherry on my strawberry cake:
Material: S335
8000rpm, 350mm/min, DOC 3mm, 2-flute TiAIN 6mm endmill.
Some chatter but nothing major... Shitty workholding. Nice blue chips where flying :)