[phil123456]

Hello

I use a chinese cnc3018 and bCNC as software

even if I lower the feed, the bits breaks off after 1 second of milling; I scan the area first, so I dont understand why it breaks

it's just the tiny bit at the edge, so I redo the scan and then the milling but I loose in precision

how can I solve this ?

thanks

[ger21]

what are you cutting, and what bits are you using?
Feedrate, rpm, depth of cut?

[ZASto]

As ger21 wrote, check your depth of cut and feed rate.
From your writing I'm guessing that you are mechanically etching PCB. With tiny engraving bits you cannot mill too fast. And bCNC is only a front end for Grbl, arduino based controller.
Have you calibrated Grbl, ie checked settings?
I've tested bCNC/Grbl combo against Mach3, on same computer, and I'm very pleased with Grbl performances. Using it mostly when laser engraving with 2.5W blue laser (Grbl has 'laser mode' which I do like a lot).