Manufacturing cost is often influenced long before a part reaches the factory. A designer’s decisions about geometry, material, tolerance, joining, finishing, and documentation can make a component simple to produce or unnecessarily expensive.
Here are seven common design mistakes mechanical engineers should avoid.
1. Using unnecessarily tight tolerances
A tight tolerance is not automatically a high-quality design.
If a component works with a ±0.2 mm tolerance, specifying ±0.02 mm can increase machining, inspection, rejection, and production costs without improving function.
Tolerance should be driven by assembly and performance requirements.
Use general tolerances for non-critical dimensions and reserve tight limits for features where they are genuinely necessary.
2. Choosing difficult or expensive materials
Material selection should consider performance as well as supply and processing.
An exotic alloy may provide excellent properties but can be expensive and difficult to source. A common engineering material may satisfy the same requirement at a lower overall cost.
Always consider availability, standard stock sizes, machining behaviour, heat treatment, finishing, and supplier capability.
3. Designing geometry that is difficult to machine
CNC machines have real physical limitations.
Deep narrow pockets, inaccessible internal corners, unnecessary complex surfaces, and difficult setups can increase machining time and tool cost.
Design internal corners around realistic cutter sizes. Consider how the part will be held and how many setups are required.
A design that can be manufactured in one setup may be significantly easier to produce than an equivalent design requiring several repositioning operations.
4. Ignoring sheet-metal manufacturing rules
Sheet metal parts should be designed around the bending and cutting process.
Very small flanges, sharp internal corners, insufficient bend relief, closely spaced holes, and unrealistic bend radii can create production problems.
Use standard material thicknesses where possible. Confirm bend allowance and tooling assumptions with the fabricator.
Always review the flat pattern before release.
5. Overusing custom components
Custom hardware can increase procurement cost and lead time.
Where practical, use standard bolts, nuts, bearings, washers, profiles, fasteners, and other commercially available components.
Standardization also simplifies maintenance and replacement.
Custom components should have a clear reason for existing.
6. Poor drawing communication
A manufacturing team should not have to interpret what the designer meant.
Missing material specifications, unclear dimensions, incomplete tolerances, inconsistent units, absent surface-finish requirements, and ambiguous revision information can all cause delays.
A good drawing communicates the manufacturing requirement directly.
Include the information needed for fabrication and inspection without filling the drawing with unnecessary notes.
7. Designing without considering assembly
A component can be easy to manufacture but difficult to assemble.
Check tool access, fastener access, insertion direction, service clearance, alignment, and sequence of assembly.
If a bolt cannot be reached with a wrench after two components are installed, the CAD model may still look perfect while the real product becomes frustrating to assemble.
A practical DFM review
Before releasing a design, review material, stock size, machining or fabrication process, tolerance, surface finish, joining method, inspection method, assembly sequence, standard hardware, packaging, and maintenance requirements.
Involve manufacturing early when possible. A ten-minute conversation with a machinist or fabricator can reveal problems that may otherwise require a complete redesign.
Conclusion
Cost-effective mechanical design is not about making every part as cheap as possible. It is about removing unnecessary manufacturing difficulty while protecting function and quality.
Engineers who understand DFM can design parts that are easier to machine, fabricate, inspect, assemble, and maintain. The result is usually a better balance between performance, reliability, lead time, and cost.
The most valuable CAD skill is not creating complicated geometry. It is creating geometry that can be produced reliably in the real world.