If your last quote came back higher than expected, check the drawing before you shop for a new supplier. One of the most common CNC machining cost drivers is not material, complexity, or lead time, it is CNC tolerance specifications applied to features that do not need them. It is called over-tolerancing, and it is more common than most teams realize.
What Over-Tolerancing Is, and Why It Happens
Over-tolerancing means specifying tighter CNC tolerances than the part’s function actually requires. It is not sloppy engineering, it is usually the result of two habits that feel responsible at the drawing stage.
The first is defaulting to tight tolerances. When you are not sure which features are critical, calling everything to ±0.001″ feels safe. The second is blanket specs: one tolerance callout applied to the entire drawing for consistency. Both approaches have good intent. Both inevitably inflate your overall CNC machining cost.
Why CNC Tolerance Specifications Drive Up Machining Cost
This is not abstract, the cost mechanism is direct and specific.
- Cutting speed drops. Holding a tight CNC tolerance requires slower material removal to avoid chatter and deflection. Slower cuts mean more passes, more time, higher cost.
- Setup is more demanding. Tight-tolerance features require more precise fixturing and dial-in before the first cut. That time is billable whether or not chips are flying.
- Inspection multiplies. A feature at ±0.001″ needs verification steps that a feature at ±0.015″ does not. On short runs, inspection time per part adds up fast.
- Specialized tooling may be required. Certain tight-tolerance features, precision bores, close-fit slots, need tooling outside a standard setup. That means added cost and sometimes added lead time.
Now apply this to a drawing where every feature carries the same tight spec, including the bolt holes that just need to pass a fastener, the clearance pockets, the non-mating faces. Every feature gets machined with the same caution, the same inspection, the same setup rigor. The quote reflects all of it.
What a Good Supplier Does Here
Tolerance review should happen before the order. A qualified CNC supplier flags over-tolerancing at the quoting stage as DFM feedback, not as a design critique, but as a cost-saving conversation. If a supplier quotes the drawing without reviewing it, your final CNC machining cost reflects exactly what you specified, including every CNC tolerance that was tighter than necessary.
Up Next
Over-tolerancing is the problem. The next post covers the fix in detail: which features actually need tight tolerances, which ones do not, and a simple test you can apply at the drawing stage. Stay tuned.
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