Five DFM Mistakes That Increase CNC Machining Cost
Contents
- Sharp Internal Corners
- Deep, Narrow Pockets
- Over-Tolerancing Everything
- Tolerancing Features That Cannot Be Measured
- Forgetting Finish and Coating Allowances
- What a DFM Review Catches
Sharp internal corners, deep narrow pockets, over-toleranced features — drawing decisions that add cost without adding value.
Sharp Internal Corners
A rotating cutter leaves a radius. If the drawing asks for a sharp internal corner, the shop has to broach, EDM or hand-finish it — all of which add cost and time. Specify a corner radius at least one third of the pocket depth, or add a corner relief where the radius cannot be accepted. This single change removes more cost from a typical part than any other.
Deep, Narrow Pockets
A pocket that is deep relative to its width needs a long, thin cutter, and long thin cutters deflect. The usual consequence is a slower feed rate, a stepped strategy and sometimes a tolerance that cannot be held at the bottom of the pocket. As a rule of thumb, keep depth-to-width below 4:1, and below 3:1 where a tight tolerance or fine finish is required.
Over-Tolerancing Everything
Applying plus or minus 0.01 mm across a whole drawing multiplies inspection time and rejects good parts. Our standard machining tolerance is plus or minus 0.05 mm, with critical features held to 0.01–0.02 mm subject to drawing review. Decide which two to six features actually control function and let the rest carry general tolerances.
Tolerancing Features That Cannot Be Measured
A position or profile callout on a feature that no instrument can reach is not a requirement — it is a dispute waiting to happen. Every tolerance should come with an inspection method in mind. If a feature is internal or inaccessible, either design in an access path or agree with the shop on an alternative method such as a first-article section or a functional gauge.
Forgetting Finish and Coating Allowances
Surface finish and coating specifications change the dimensions of a part. Anodizing grows the surface by roughly 0.01–0.03 mm per face; electroless nickel adds 10–50 µm. Any bore, thread or precision gap affected by coating has to be specified with that in mind, and the note belongs on the drawing — not in a later email.
What a DFM Review Catches
We review every drawing before confirming a quote, and we raise manufacturability issues before machining rather than after — that is part of the quote, not an extra service. Most of the items above come up in that review. If a change would reduce cost without affecting function, we will tell you what it is and what it saves.
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