The Decision That Quietly Kills Your Product Margin
You have a new part to manufacture. The drawing is ready, the design is approved and your team needs to place an order. Someone asks: should this be CNC machined, die cast or made from sheet metal? If your answer is based on what you used last time, what your current supplier can do or which process feels familiar—you are leaving money on the table. In some cases, you are adding weeks to your lead time and building a cost structure that will not survive volume scaling.
At EMUSKI, we see consistently that process misalignment is one of the top three avoidable cost drivers in electromechanical product programmes.
What Each Process Is Actually Built For
CNC Machining: Built for Precision and Flexibility
CNC machining is a subtractive process—a computer-controlled cutting tool progressively removes material from a solid block until the desired geometry is achieved. The result is a part that can hold tolerances to ±0.025mm as a standard expectation, with precision machining capable of going significantly tighter.
What CNC machining excels at: solid or near-solid three-dimensional geometries, tight tolerances on critical features, complex internal contours, parts with sharp internal corners or undercuts, rapid iteration without tooling investment, and wide material compatibility across metals, plastics and composites.
What it struggles with: producing thin-walled hollow structures efficiently, competing on per-unit cost at high volumes, and managing material waste on complex geometries where large amounts of stock must be removed.
Die Casting: Built for Volume and Complex 3D Form
Die casting is a formative process. Molten metal—typically aluminium, zinc or magnesium alloy—is injected under high pressure into a hardened steel mold. The metal solidifies rapidly, the die opens and the part is ejected. Once the die is validated and the process is dialled in, thousands of identical parts can be produced per shift.
What die casting excels at: complex 3D shapes with thin walls and integrated internal features, high-volume production where tooling cost amortises quickly, dimensional consistency across large production runs, and parts where assembly consolidation is a design goal.
What it struggles with: tight initial tolerances without secondary machining, sharp internal corners and undercuts without complex tooling, and any situation where the design is still changing.
Sheet Metal Fabrication: Built for Large Structures and Enclosures
Sheet metal fabrication starts with a flat sheet of metal transformed through cutting (laser, waterjet, plasma or punch), forming (bending, stamping, rolling) and joining (welding, fasteners, adhesives) into a structural part, enclosure, bracket or chassis.
What sheet metal excels at: large enclosures and housings, structural frames and chassis, brackets and mounting hardware, parts requiring multiple variants, and cost-sensitive structural components at medium to high volume.
What it struggles with: tight tolerances and precision-critical features, solid three-dimensional forms, and geometry requiring integrated bosses, ribs and complex internal structures.
Process Comparison: Engineering Parameters at a Glance
- Ideal volume range: CNC 1–5,000 units | Die casting 5,000–500,000+ units | Sheet metal 500–500,000+ units
- Tooling investment: CNC none to minimal | Die casting high (₹3L–₹25L+ depending on complexity) | Sheet metal low to moderate
- Tooling lead time: CNC none | Die casting 12–20+ weeks | Sheet metal 1–4 weeks
- Standard tolerance: CNC ±0.025mm | Die casting ±0.1mm per 25mm | Sheet metal ±0.1mm–±0.5mm
- Precision capability: CNC ±0.005mm or tighter | Die casting ±0.05mm with post-machining | Sheet metal not suited for precision features
- Material flexibility: CNC widest—metals, plastics, composites, ceramics | Die casting limited to Al, Zn, Mg alloys | Sheet metal primarily metals
- Per-unit cost at low volume: CNC moderate | Die casting high (tooling not amortised) | Sheet metal low to moderate
- Per-unit cost at high volume: CNC high (machining time scales linearly) | Die casting low (tooling fully amortised) | Sheet metal low to moderate
- Design iteration speed: CNC fast (update CAD, re-run) | Die casting slow (tooling modification required) | Sheet metal moderate
- Structural strength: CNC highest (wrought material properties) | Die casting good (slight porosity risk) | Sheet metal good (weld joints are potential weak points)
Volume Is the Dominant Variable: Understanding the Cost Crossover Points
Under 500 Units: CNC Machining Almost Always Wins
At very low volumes, die casting tooling simply cannot be justified. A die cast mold for a moderately complex aluminium part in India typically costs between ₹3 lakh and ₹15 lakh. At 500 units, that tooling cost alone adds ₹600 to ₹3,000 per piece before any machining or finishing. CNC machining with no tooling cost almost always delivers better total cost at this volume.
500 to 5,000 Units: The Grey Zone Where Most OEMs Get It Wrong
Die casting starts to look attractive on a per-unit basis but the tooling cost has not fully amortised. CNC machining starts to look expensive as cumulative machining hours add up. The correct approach in this range is a total cost model, not a unit price comparison—accounting for tooling amortisation, secondary operations, yield rates, lead time risk and volume variability. Investment casting is also worth evaluating for certain geometries in this range.
5,000 to 20,000 Units: Die Casting Becomes the Clear Cost Leader
Above 5,000 units annually with a stable design, die casting typically delivers the lowest total cost for aluminium, zinc and magnesium parts. The tooling cost amortises over a large enough run, per-unit cost drops significantly compared to machined parts and production throughput supports demand without large inventory builds.
Above 20,000 Units: Die Casting and Sheet Metal Dominate
At high volumes, both die casting and sheet metal deliver per-unit costs that CNC machining cannot approach. The choice between them comes down entirely to part geometry—complex 3D forms with integrated features go to die casting; large structural enclosures and brackets go to sheet metal.
Geometry Rules: What Your Part Shape Is Telling You
Choose CNC Machining When Your Part Has:
- Thick solid walls with complex profiled features requiring material removal from a block
- Sharp internal corners with tolerances—die casting cannot produce sharp internal corners
- Undercuts that cannot be accommodated with simple side-action tooling
- Threaded features in small sizes better achieved through CNC post-machining
- Tolerance requirements tighter than ±0.05mm on critical features
Choose Die Casting When Your Part Has:
- Complex three-dimensional external form with thin walls and integrated internal cavities
- Multiple small ribs, bosses and wall features that would be extremely time-consuming to machine
- A geometry that consolidates what would otherwise be a multi-piece assembly
- Large flat areas with functional three-dimensional features integrated in
Choose Sheet Metal When Your Part Has:
- A fundamentally flat or bent profile—enclosures, boxes, trays, panels, brackets and frames
- Large surface area relative to thickness
- Multiple variants with the same basic structure but different dimensions
- A requirement for large physical size beyond typical machine bed limits
Tolerance and Precision: Where the Three Processes Separate
CNC machining holds ±0.025mm as a standard production tolerance. For precision applications in medical, aerospace and instrumentation, tolerances tighter than ±0.010mm are achievable. Die casting in aluminium holds approximately ±0.1mm to ±0.15mm per 25mm without secondary machining. For precision features—mating surfaces, bearing bores, critical clearance holes—CNC post-machining after casting is the standard approach. Sheet metal bending introduces variation based on material springback, bend radius and tool wear; for assembled structures, tolerance stack-up across multiple bend and weld operations requires careful GD&T management.
The practical implication: if your part has a mix of tolerance requirements, the correct answer is often a hybrid process strategy—cast or fabricate the basic form, then CNC post-machine the critical features. This hybrid approach is standard practice in automotive, industrial equipment and consumer electronics manufacturing.
The Four Most Common and Expensive Process Mistakes OEMs Make
Mistake One: Staying in CNC Machining Past the Volume Crossover
A product launches in prototype quantities on CNC machining—which is correct. Volumes scale, but the programme continues to source CNC-machined parts because the supplier relationship is established and the tooling investment for die casting feels risky. The result is a per-unit manufacturing cost two to three times higher than necessary at mature volume. Across 10,000 to 50,000 units per year, this cost penalty runs into crores annually.
Mistake Two: Investing in Die Casting Tooling Before Design Freeze
Die casting tooling is expensive and slow to modify. When OEMs commit to tooling before the design is stable, they face expensive mold modifications or scrap tooling when engineering changes occur—which they almost always do. Use CNC machining through design validation and commit to tooling only when the design is frozen and volumes are confirmed.
Mistake Three: Applying Die Casting Design Logic to Sheet Metal Parts
Dense bosses, thick integrated ribs and three-dimensional features have no place in a sheet metal design. When these appear in a sheet metal drawing, the fabricator is forced into expensive workarounds—welded-in inserts, additional fabrication steps, or weld distortion requiring straightening. A proper DFM review before releasing drawings to sheet metal suppliers catches these issues before they become your cost.
Mistake Four: Comparing Only Unit Price Across Processes
Unit price comparison without total cost modelling is the most common cause of process misalignment. A die-cast part at ₹450 per unit looks cheaper than a CNC-machined part at ₹820 per unit—until you add tooling amortisation at your actual volume, secondary machining cost, longer lead time carrying cost and yield loss. At low-to-medium volumes, the total cost comparison can reverse entirely.
The EMUSKI Process Selection Framework
Step One: What is the annual volume and is demand stable? Below 1,000 units with variable demand, CNC machining is almost always right. Above 10,000 units with stable demand, die casting or sheet metal are right. In between, model total cost.
Step Two: Is the design frozen? If no, stay in CNC regardless of volume. Tooling investment before design freeze is almost always more expensive than continued CNC machining.
Step Three: What does the geometry demand? Apply the geometry rules above. If the geometry demands CNC, volume does not change that answer.
Step Four: What are the tolerance requirements? Identify which features are tolerance-critical and plan for CNC machining of those features regardless of base process.
Step Five: What are the material requirements? If the material restricts process options, process selection narrows accordingly.
Step Six: What is the total cost at projected volume? Run the tooling amortisation, per-unit cost, secondary operations and lead time carrying cost comparison across viable process options. Select the process with the lowest total cost meeting engineering requirements—not the lowest unit price.
Step Seven: What is the supply chain risk? For critical programme components, single-source risk, geopolitical exposure and supplier qualification depth factor into the final decision.
