G20 vs G21: The One Missing Line That Crashes CNC Programs

G20 vs G21 is the inch versus metric switch in G-code, and one missing line at the top of a CNC program is how a 5mm move becomes a 5 inch crash. This Shop Talk breakdown covers what G20 and G21 actually do inside the controller, why CNC modal codes persist between programs, and the 25.4x scale disaster waiting on any machine left in the wrong unit mode. Plain-English fix included. 00:00 The one-line crash 00:25 What G20 and G21 actually do 00:55 Why the default isn't safe 01:30 The 25.4x scale gap, both directions 02:15 The safety-block fix 02:45 Why CAM posts aren't the source of truth 03:15 Pre-cycle units check (Fanuc, Haas, Siemens) 03:50 The offset trap nobody warns you about 04:30 The takeaway + subscribe If this was useful, the G54 work offset video covers why your first part scraps before the toolpath is ever the problem. More shop-floor breakdowns every week on ShopFloorIQ. — — — 🔧 Buyer's guides for makers, written by operators: https://print3dflow.com/guides/ • Best 3D printer for jewelry maker • Best laser engraver for small business • Best CNC mill for hobbyist + production • Best 3D printer for HVAC technician • Father's Day gift guides for tradesmen No generic listicles. Real pros/cons, real price tiers, no fluff.