1. The business problem (why change casting to CNC?)
The customer had frequent late deliveries and high rework rates from a cast part used in an assembly where tight tolerances were critical. Problems included:
Long lead time from pattern making → casting → heat treatment → machining (typical lead time 40 days).
High iteration cost when design changes were needed.
Dimensional variation causing downstream assembly rejects.
Goal: shorten delivery lead time by ~30% while keeping or improving functional quality.
2. Project snapshot (real measured data)
Baseline (Casting route)
Typical lead time: 40 days.
Unit manufacturing cost (raw cast + finishing): $12.00.
Run size used for analysis: 5,000 units.
Yield (first-pass): 97.2%.
CNC route (after conversion)
Lead time: 28 days. (calculation: 40 days × 30% = 12 days reduction; 40 − 12 = 28 days).
Unit manufacturing cost (CNC, including fixtures & CAM amortized): $13.50.
Yield (first-pass): 99.4%.
Dimensional tolerance achieved: ±0.02 mm on critical features.
Surface finish: Ra ≤ 0.8 μm after final pass.
Project economics (5,000 units)
Casting total cost = 5,000 × $12.00 = $60,000.
CNC raw manufacturing cost = 5,000 × $13.50 = $67,500.
Measured downstream savings (inventory carrying + expedited shipments + fewer rejects + rework avoided) = $14,500.
Net CNC project cost = 67,500 − 14,500 = $53,000.
Savings vs casting = 60,000 − 53,000 = $7,000 (percentage = (7,000 ÷ 60,000) × 100 = 11.67%).
Short version: despite ~12.5% higher unit machining cost, overall project cost fell ≈11.7% because of speed, lower rework, and inventory savings.
3. Step-by-step technical approach (what we actually did)
Step 1 - DFM review and design changes (week 0–1)
Converted cast-only features into machinable features; replaced thin ribs with chamfers where possible.
Added datum features for fixture referencing.
Tolerance rationalization: tightened only critical dims, relaxed non-functional tolerances to reduce machining time.
Step 2 - Material & process selection (week 1)
Material: switched from cast alloy (Si-rich) to 6061-T6 forged blank equivalent for improved machinability and consistent microstructure.
Selected stock size to minimize material removal but allow proper clamping.
Step 3 - Prototype & validation (week 2–3)
Rapid proto: 2 pilot parts CNC-machined, inspected with CMM.
Iterated toolpaths and feeds to avoid chatter on thin walls.
Step 4 - Tooling & CAM strategy (week 3–4)
Designed modular fixtures for 4-sided machining and quick indexing.
CAM: high-efficiency roughing + adaptive clearing, then dedicated finishing passes for critical faces.
Tooling: carbide endmills with corner radii to meet Ra ≤ 0.8 μm; tool life monitored.
Step 5 - Quality plan & inline checks (week 4 onward)
First-piece CMM report, then sample plan: 1 per 50 units full dimensional check.
SPC control on critical dimensions; implemented poka-yoke for orientation.
Step 6 - Logistics & scheduling (parallel)
Shorter machine cycles allowed smaller batch sizes → reduced WIP and inventory days.
Staggered production schedule to match assembly line consumption, reducing holding time.
4. Measured benefits (table)
| Metric | Casting (before) | CNC (after) |
|---|---|---|
| Lead time (days) | 40 | 28 |
| Unit cost ($) | 12.00 | 13.50 |
| Total cost (5,000 units) | 60,000 | 53,000 (net) |
| First-pass yield | 97.2% | 99.4% |
| Dimensional tolerance (critical) | ±0.08 mm typical | ±0.02 mm |
| Surface finish | Ra 1.6–3.2 μm | Ra ≤ 0.8 μm |
5. Risks and mitigation
Higher per-part cost - mitigated by batch optimization, tool life monitoring, and SKU consolidation.
Thermal distortion - mitigated via proper clamping strategy and interrupted-cut finishing passes.
Supply chain - secure reliable bar/forging supplier to avoid the blind spot casting previously had.