You can cut CNC cost more with a design change than with any negotiation. Most expensive features are not needed for function. They are simply easy to draw and hard to cut.
You can cut CNC cost more with a design change than with any negotiation. Most expensive features are not needed for function. They are simply easy to draw and hard to cut.
Quick answer
The ten mistakes below account for much of the avoidable cost in machined parts: sharp inside corners, deep and narrow pockets, thin walls, tiny deep holes, unnecessary tight tolerances, features needing many set-ups, ignored coating thickness, non-standard hole sizes, decorative fine detail and unusual materials. The numbers here are general guidance for common metals. Confirm limits for your material and size with engineering, since HC-Mold works on 3-, 4- and 5-axis machines with more than 50 materials.
Key takeaways
- A cutter is round, so inside corners have a radius. Draw it.
- Depth is expensive, so keep pockets and holes shallow relative to their width.
- Fewer set-ups mean lower cost and better datums.
- Tolerance and finish should follow function.
The ten mistakes
1. Sharp inside corners
End mills leave a radius in every inside corner. A drawing with sharp corners forces smaller cutters or extra processes. Draw inside radii slightly larger than the cutter, and keep the same radius where possible. As a rule of thumb, a larger radius lets you use a larger, stiffer tool.
2. Deep, narrow pockets
Depth beyond about four times the cutter diameter is slower and risks deflection and poor finish. Widen the pocket, reduce depth or split the part. This is a common rule of thumb for steel and aluminum, not a hard limit.
3. Thin walls and floors
Thin features flex under cutting force and can chatter or move after machining. As general guidance, metal walls much thinner than about 0.8 mm and plastic walls thinner than about 1.5 mm are hard to hold. Use ribs or thicker sections where you can.
4. Small, deep holes
Drills much longer than about ten times their diameter are slow and can wander. Use through-holes when possible, and avoid blind holes with flat bottoms and tight tolerances.
5. Blanket tight tolerances
Tolerance every feature to ±0.01 mm and the cost multiplies. Use a general class such as ISO 2768-m and tighten only functional features. See tolerances explained.
6. Features on many faces
Each new orientation is another set-up. Group features on fewer faces where design allows. For parts that really need access from many sides, 5-axis machining can reduce set-ups: see 5-axis CNC.
7. Ignoring coating thickness
Anodizing, plating and paint add thickness, which changes fits. Say whether dimensions are before or after finishing. For masked areas and rack marks see CNC aluminum and anodizing.
8. Non-standard hole and thread sizes
Standard drill and tap sizes are faster and cheaper than custom ones. Avoid very deep threads, because most of the holding strength is in the first few turns.
9. Fine decorative detail
Text, logos and engraved patterns add time. Use larger text, shallow depth and a font that can be cut. For marking only, consider laser marking, which HC-Mold offers as a finishing option.
10. Unusual or oversized stock
Material that is hard to source or that needs a large block for a small part adds cost and lead time. Choose common alloys and sizes where function allows.
Design checklist
| Check | Question |
|---|---|
| Corners | Do all inside corners have a radius that matches a standard cutter? |
| Depth | Are pockets and holes shallow relative to width? |
| Walls | Are thin walls and floors really needed? |
| Set-ups | Can features be reached from fewer sides? |
| Tolerance | Are tight tolerances limited to functional features? |
| Finish | Is the surface finish only where needed? Is coating thickness handled? |
| Material | Is the material common and available? |
| Quantity | Does the batch size share set-up cost? |
Worked example (illustrative, not a customer project)
An aluminum bracket is drawn with sharp inside corners, a 3 mm wide pocket 20 mm deep, five different hole sizes, ±0.01 mm on every dimension and features on all six faces. A redesign could:
- Add 2 mm inside radii and widen the pocket
- Reduce the pocket depth or split the part
- Standardize hole sizes to common drills
- Keep ±0.01 mm only on the two mounting bores and use ISO 2768-m elsewhere
- Move features to three faces so the part is machined in fewer set-ups
The part keeps its function, while machine time, set-ups and inspection all fall.
Material notes
| Material | Design note (general guidance) |
|---|---|
| Aluminum | Machines quickly; watch thin walls and burrs; allow for anodizing thickness |
| Stainless steel | Slower and work hardens; keep radii generous and avoid unnecessary deep features |
| Titanium | Slow and tool-intensive; limit features and tolerances to what is needed |
| Brass | Machines easily; good for small precision parts |
| Engineering plastics such as PEEK | Heat and stress sensitive; avoid thin sections and keep tolerance expectations realistic |
Prototype first when the risk is high
Check fit and function with a CNC prototype before committing volume or tooling. HC-Mold's CNC prototypes take 3 to 5 days, and 1 piece is welcome. CNC machining has no minimum order, with a lead time of 5 to 10 days depending on the part. See how the price builds up in the CNC cost guide.
What to send for feedback
STEP/IGES and a PDF drawing with critical dimensions, material, quantity and finish. HC-Mold accepts STEP, IGES, SolidWorks, AutoCAD, STL and PDF with dimensions, and replies within 12 hours. Use the CNC RFQ checklist for the full package.
FAQ
What is the most common CNC design mistake?
Sharp inside corners and blanket tight tolerances. Both force slower cutting and higher cost with no functional benefit.
How deep can a CNC pocket be?
As a rule of thumb, depth beyond about four times the cutter diameter is slower and riskier. Confirm limits for your material with engineering.
Should I specify tolerances on every dimension?
No. Use a general tolerance for the drawing and tighten only the features that control fit or function.
Does anodizing change my dimensions?
Yes. Coatings add thickness, so state whether dimensions apply before or after finishing.
Can I get design feedback before ordering?
Yes. Send your files and engineers will mark cost drivers and suggest changes before quoting.
Get CNC feedback before you order
Send your STEP file and drawing. Engineers mark the features that drive cost and suggest changes before quoting. We reply within 12 hours.
Talk to HC-Mold engineering
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