The Part That Didn’t Fit: Tight Tolerance, No Need
A machined part with a hole tolerance of ±0.01 mm. The part was a cover plate on a machine. It just sits there. The customer had specified ±0.01 mm on the drawing. The part was $80 to machine. A ±0.1 mm tolerance would have been $15. This is about tolerance selection, not about the part being defective.
Why tight tolerances cost money
A ±0.01 mm tolerance requires: a CNC grinder, not a mill; a temperature-controlled room; inspection at every step; slower feeds and speeds; more scrap. A ±0.1 mm tolerance can be done on a standard mill in 5 minutes. The cost difference is 5x. The question is: does the feature need ±0.01 mm?
The tolerance decision
| Feature type | Typical tolerance | Why |
|---|---|---|
| Mating hole for a bolt (M6 clearance) | ±0.2 mm | The bolt is loose-fitting |
| Bearing bore (press fit) | ±0.01 mm | The bearing needs precise location |
| Cover plate hole | ±0.2 mm | It just holds a screw |
| Guide shaft bore | ±0.005 mm | The slide runs on it |
| Decorative surface | ±0.5 mm | No function |
What we did
We reviewed the drawing with the customer. The ±0.01 mm hole was a cover plate screw hole. It doesn’t locate anything. We changed the tolerance to ±0.1 mm. The part now machines in 5 minutes instead of 30. Cost dropped from $80 to $15. The function is identical. The part fits the screw the same way.
The GD&T principle
Only datum features (which locate other parts) and functional surfaces (which mate with other parts) need tight tolerances. A cover plate hole is neither. The tolerance should reflect the function, not the designer’s habit. “Always ±0.01” is a habit that costs money. Every tolerance on a drawing should be justified by function.
The tolerance rule: Match the tolerance to the function. A cover plate hole doesn’t need ±0.01. A bearing bore does. Don’t default to tight tolerances because “that’s how we draw.” Every ±0.01 mm on a non-functional feature adds cost. The part that didn’t fit was over-toleranced, not over-sized.
