Tips for Designing Parts for Machining

Key Takeaways

• Keep geometries simple and easy to machine.
• Use fillets instead of sharp corners for smooth tool access.
• Standardise threads, bores, and tolerances where possible.
• Select materials that balance strength with machinability.
• Design with tool access, setup, and clamping in mind.
• Avoid thin walls or tall features that risk vibration.
• Consider alternative processes like welding or fabrication when machining isn’t ideal.
• Support inspection and measurement with clear design features.

1. Why Machining Design Matters in Cumbria

Designing parts for machining is not just about technical drawings. In areas like Cumbria, where industries rely heavily on plant machinery repairs and custom fabrication, the design stage directly affects cost, turnaround, and quality. Poorly designed parts can lead to wasted material, longer machining times, and repairs that don’t last. From my own experience in North-West UK workshops, a small design oversight—like specifying an unnecessary tight tolerance—can add hours of work and significant costs.

2. Simplify Geometry for Easier Machining

When it comes to machining, simpler is always better. Complex undercuts and unusual curves often require multi-axis CNC machines or custom tooling, which increases costs. Flat surfaces and smooth arcs are far easier to program and machine.  The difference in price between that and a standard arc was over 40%. Keeping it simple ensures your design works both technically and economically.

3. Optimising Internal Features with Fillets

Sharp corners are a machinist’s worst enemy. Tools naturally cut in curves, not straight angles. Adding a 3 mm fillet or larger allows standard cutters to move efficiently without leaving stress points. For example, in mechanical engineering projects, we often recommend designing cavities with depths no more than 3–4 times their width. This reduces tool deflection and keeps the part stable during production.

4. Choosing the Right Materials

Material choice can make or break a machining job. Metals like aluminium are easier to cut for complex shapes, while stainless steel demands slower feeds and more expensive tooling. Plastics require different feeds and speeds altogether. For clients in Cumbria, I always suggest starting with what the part needs to withstand wear, pressure, or heat and then narrowing down to a machinable material. This saves both time and rework later on.

5. Standardisation Saves Time and Money

Whenever possible, use standard parts such as common thread sizes and bore diameters. Non-standard threads require specialised taps and cutters that many local shops may not stock. At Cumbria Access & Engineering, we encourage clients to standardise because it not only reduces costs but also ensures replacement parts can be sourced quickly in the future.

6. Applying Tolerances with Care

It’s tempting to specify tight tolerances everywhere, but this approach often leads to unnecessary expense. For instance, a ±0.01 mm tolerance is only needed for high-precision fits, not for a bolt hole that only needs clearance Only apply tight tolerances where they matter such as press-fit shafts or bearing seats.