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Avoiding These Mistakes Can Save Time, Money, and Headaches
What are CNC design mistakes, and why do they matter?
What are CNC design mistakes, and why do they matter?
When we talk about "CNC design mistakes," we’re not just referring to errors that cause a part to fail — we mean the seemingly harmless design decisions that quietly drive up manufacturing cost, increase lead time, or force a redesign mid-production.
From tolerances tighter than necessary to awkward geometries requiring multiple setups, these issues often arise not from bad intentions but from a disconnect between CAD design and real-world machining capabilities.
If you're designing for CNC but don’t understand how your part is machined, you may end up paying significantly more — without gaining any functional benefit.
Why do small design decisions lead to big cost differences?
Why do small design decisions lead to big cost differences?
In CNC machining, cost is tied directly to setup time, machining time, tool changes, and scrap rate. When a part requires special tools, tight corner radii, or extreme depth-to-diameter ratios, it demands more time and attention from the machinist.
Some classic examples:
A fillet that requires a 0.5 mm endmill will need a slow toolpath and multiple passes.
A deep pocket without relief corners may require expensive EDM rather than simple 3-axis milling.
An unnecessarily tight ±0.01 mm tolerance across all surfaces leads to increased inspection time — and potentially, rework.
Individually, each factor may seem minor. But combined across multiple parts or batches, the impact can be dramatic.
When do these issues usually appear in the design process?
When do these issues usually appear in the design process?
Most costly design flaws happen during the early CAD stage — especially when engineers design in isolation, without consulting the shop floor.
Common triggers include:
Rushing to prototype without DFM (Design for Manufacturing) checks
Copying geometry from another part with different manufacturing constraints
Over-optimizing for aesthetics instead of function
By the time these issues reach manufacturing, rework becomes expensive, both in dollars and in time lost.
How to identify and avoid these design mistakes?
How to identify and avoid these design mistakes?
It starts with mindset: design with the machinist, not for the machinist. Here are practical strategies:
Talk to your CNC supplier early.
Share initial drawings or even rough sketches to get feasibility feedback.Tolerances: Only where needed.
Don’t apply ±0.01 mm across the board. Use looser tolerances for non-mating features.Use standard tools and radii.
Design internal corners with ≥2 mm fillets to allow for standard endmills.Avoid deep holes or thin walls.
High aspect ratios (depth vs diameter) are difficult to machine and tend to chatter or deform.Limit setups.
Parts requiring 5 sides of machining or rotation across multiple axes significantly increase cost.Simulate toolpaths.
If possible, run a CAM simulation or ask your supplier to provide a machining analysis.
CNC Design: Precision vs. Practicality
CNC Design: Precision vs. Practicality
It’s tempting to chase perfection — tight tolerances, sharp features, and mirror finishes. But in many cases, designs that are technically perfect may be practically excessive.
Ask yourself: Is this detail serving function, or is it just habit?
Author's Note
Author's Note
Written by the technical content team at EKINSUN Precision Manufacturing, this article is based on over 10 years of experience helping startups, engineers, and product designers bring their concepts into production. We combine in-house CNC knowledge with practical manufacturing insights to help you make smarter design choices — not just cheaper, but better.
Final Tip
Final Tip
Next time you send out a CNC quote, ask your machinist:
"Is there anything in this design that makes your life harder?"
That single question can save you hours — and hundreds of dollars.
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