Impact of Good Design on CNC Cost (2026 Yuqing Data)
|
Design Choice |
Cost Impact |
Notes |
|
Avoid undercuts |
-20% to -40% |
Undercuts = 5-axis or EDM |
|
Generous internal radii |
-10% to -30% |
Small radii = small tools = slow |
|
Wall thickness ≥ tool diameter |
-15% to -25% |
Thin walls vibrate, scrap |
|
Specify finish only where needed |
-5% to -10% |
Surface finish is per-feature |
|
Datum scheme consistency |
-10% to -20% |
Reduces fixturing complexity |
|
Avoid deep pockets (4× width) |
-25% to -50% |
Long tools flex, slow feeds |
|
Use standard stock sizes |
-10% to -15% |
Custom billets cost more |
|
Total typical savings |
-30% to -50% |
Compounding effect |
35 Design Tips for CNC Machining
Material Selection (Tips 1–5)
Default to aluminum 6061-T6 unless your application demands something else. It's the easiest to machine, most available, and cheapest.
Use aluminum 7075-T6 when you need higher strength (especially aerospace). 7075 machines similarly to 6061 but is 30% stronger.
Choose brass C360 for tight-tolerance parts — its free-machining properties allow tighter tolerances and faster cycle times.
Avoid titanium unless required — it's 4× the cycle time vs aluminum. If your design is "weight-critical," consider 7075 aluminum or carbon fiber composites first.
Specify material condition — T6, T651, H102, etc. Pre-hardened materials machine differently than annealed.
Wall Thickness and Geometry (Tips 6–10)
Wall thickness ≥ 1 mm for parts under 50 mm, ≥ 3 mm for parts 100–300 mm, ≥ 5 mm for parts >300 mm.
Avoid thin, tall walls — they vibrate during cutting and chip. Add fillets at the base of walls for stiffness.
Internal corner radius ≥ 1/3 of cavity depth — this lets you use a standard end mill instead of a custom form tool.
Minimum internal radius = 1 mm for general machining. Smaller radii need micro-tools (slow, fragile, expensive).
Maximum depth-to-width ratio = 4:1 for pockets — beyond this, tool deflection causes taper and poor finish.
Undercuts and Features (Tips 11–15)
Avoid undercuts when possible — they require 5-axis machining, EDM, or special tooling (jumping $5–$50 per feature).
If you must have an undercut, specify the width and depth so the machinist can choose the right process (T-slot cutter, dovetail cutter, or EDM).
Threads should be ≥ 1.5× the major diameter deep — shallower threads are weak and hard to inspect.
Hole depth ≤ 4× diameter for standard drilling — deeper holes need gun drills or EDM ($5–$20 extra per hole).
Counterbores and chamfers — specify angle (45°, 60°) and size. Don't assume a "small chamfer" — give a dimension.
Tolerances (Tips 16–22)
Default to ISO 2768 medium (±0.1 mm for decimals, ±0.2 mm for lengths under 400 mm). Use this for non-critical features.
Reserve ±0.05 mm or tighter for functional interfaces — bearing seats, sealing surfaces, mating parts.
Don't mix metric and imperial units on the same drawing — it creates confusion and conversion errors.
GD&T over +/- tolerances for complex 3D relationships — datum scheme A-B-C should be unambiguous.
Specify datum features explicitly — every critical tolerance should reference a primary, secondary, and tertiary datum.
Avoid chained tolerances — they accumulate. Use datum reference instead.
Bonus tolerance (.X) — for parts under 10 mm, single-digit micron tolerances are unrealistic; default to 2 decimals (e.g., 12.5 mm, not 12.523 mm).
Surface Finish (Tips 23–26)
Specify surface finish only where functionally required — every "Ra 0.8" or "Ra 0.4" callout adds cost. Default unspecified surfaces to Ra 3.2.
Cosmetic surfaces don't need tight finish — they just need to look good. Ra 1.6 is usually enough.
Sealing surfaces need Ra 0.4–0.8 — too rough, and they leak; too smooth, and they gall.
Bearing seats need Ra 0.4–0.8 with specific lay direction (cross-hatched is usually best).
Draft and Machining Access (Tips 27–30)
No draft angle is required for CNC — unlike injection molding or casting, CNC cuts vertically. Don't add draft unless functionally needed.
Provide tool access — features inside deep pockets need clearance for the cutting tool to enter and exit.
5-sided access for complex parts — if possible, design so 5 of 6 faces are accessible without flipping.
Use fillets instead of sharp corners — sharp internal corners require micro-tools; fillets allow standard tools.
Drawings and Documentation (Tips 31–35)
Provide a 3D model (STEP preferred) — eliminates ambiguity from 2D interpretation.
Submit the 2D drawing alongside the 3D model — the drawing shows tolerances, datums, and notes that don't fit in 3D.
Add a notes block — material spec, finish requirements, inspection requirements, regulatory standards (AS9100, IATF 16949, FDA, etc.).
Mark critical-to-quality (CTQ) features clearly — with symbols, balloons, or callouts — so the machinist knows where to focus inspection.
Include a revision history — even a simple "Rev A — initial release" helps. Engineers who don't version their drawings lose track of which is current.
Real Cost Impact Examples
Example 1: Tolerance over-spec
A buyer submitted an aluminum bracket with all dimensions marked ±0.01 mm. Our quote: $85/piece.
We asked: "Which dimensions are functionally critical?" — only 2 of the 15 dimensions needed that tolerance. The other 13 were ±0.1 mm acceptable.
Revised quote: $52/piece — 39% lower. Same functional part.
Example 2: Internal radius too small
A buyer specified an internal corner radius of 0.3 mm on a 50 mm deep pocket. Our quote: $140/piece (required a custom micro-tool with multiple passes).
We suggested: increase to 1.5 mm radius (still functional, just more material at the corner).
Revised quote: $72/piece — 49% lower. Same part geometry, just less aggressive internal corner.
Example 3: Deep pocket
A buyer submitted a part with a 40 mm deep, 8 mm wide pocket. Our quote: $310/piece (required a 4 mm diameter, 160 mm long end mill on a 5-axis machine to reach).
We suggested: split into two pockets from opposite sides, each 20 mm deep.
Revised quote: $145/piece — 53% lower. Same functional depth, much easier machining.
Frequently Asked Questions
What is the minimum wall thickness for CNC machining?
For small parts (<50 mm), 1 mm is the practical minimum. For medium parts (50–150 mm), 1.5–2 mm. For large parts (>150 mm), 3 mm+. Thinner walls risk vibration, deflection, and chipping during machining.
What is the smallest internal corner radius I can specify?
1 mm is the practical minimum for general CNC machining. Below 1 mm requires micro-tools (0.5 mm or smaller end mills) which are fragile, slow, and expensive. For radii below 0.5 mm, EDM is the better process.
Can CNC machining do undercuts?
Yes — but they add cost. Simple undercuts (slots, T-slots) can be done with form tools or 5-axis machining. Complex internal undercuts require EDM. Avoid undercuts when possible to minimize cost.
What is the deepest hole CNC can drill?
Standard drilling: 4× diameter depth (e.g., 8 mm hole can be 32 mm deep). For deeper holes, gun drilling extends to 100× diameter. Beyond that, EDM is required.
What tolerance should I specify on my drawing?
Default to ISO 2768 medium for non-critical dimensions (±0.1 mm for decimals under 400 mm). Use tighter tolerances (±0.05 mm or ±0.025 mm) only for functional interfaces. Avoid specifying tolerances tighter than the process can hold — it adds cost without adding value.
What is the cheapest material for CNC machining?
Aluminum 6061-T6 is almost always the cheapest. Brass C360 has faster cycle times (free-machining) but higher material cost — sometimes cheaper overall for tight tolerance parts. Plastic (Delrin, HDPE) is also very economical for prototypes.
Do I need to add draft angle for CNC parts?
No. CNC cuts vertically, unlike injection molding or casting. Draft angles are unnecessary and waste material. Specify 0° draft unless functionally required.
Can you machine parts from a 3D print?
Yes, but rarely advisable. 3D printed substrates (especially metal) have porosity and residual stress that causes chatter, poor surface finish, and accelerated tool wear. For prototype metal parts, CNC from billet is faster and cheaper than 3D printing + finish machining.
Do you provide DFM feedback?
Yes — free of charge. Send us your STEP file and drawing, and we'll provide DFM (design for manufacturability) feedback within 24 hours. We typically identify 2–5 cost reduction or quality improvement opportunities per design. Most customers save 15–30% after implementing our DFM suggestions.
