CNC milling can produce precise faces, pockets, slots, holes and multi-surface features, but a part that is easy to model is not always easy to manufacture. A useful drawing must account for cutter access, workholding, tool stiffness and inspection. Making those decisions early usually leads to a more stable process and a clearer quotation.
This CNC milling design guide explains the issues Balford reviews when evaluating a custom metal component. The aim is not to force every part into a simple shape. It is to separate features that carry real functional value from details that add setups, special tools or unnecessary machining time.
1. Start With Function and Datums
Identify the faces, holes and interfaces that control how the component fits or moves in the final assembly. These features should determine the primary datums on the drawing. A clear datum structure gives manufacturing and inspection teams a shared reference and reduces ambiguity between individual dimensions.
- Use functional mounting or mating surfaces as primary references.
- Dimension related hole patterns from the same datum system.
- Avoid chains of dimensions when accumulated variation could affect fit.
- Separate critical characteristics from ordinary profile dimensions.
2. Allow the Cutting Tool to Reach the Feature
A rotating end mill needs space to enter, move and leave the cut. Deep narrow pockets, hidden undercuts and enclosed corners may require long-reach or custom tooling. Longer tools are less rigid, so feeds may need to be reduced to control deflection and vibration.
Whenever possible, provide open access from a principal direction. If a feature can only be reached after the part is turned, explain whether its relationship to the first setup is critical. This helps determine whether a simple second setup, a fixture or multi-axis machining is appropriate.
3. Use Practical Internal Corner Radii
An end mill creates a radius in an internal vertical corner. A perfectly sharp internal corner therefore requires another process, such as Wire EDM, or a design change. Increasing the radius lets the shop use a larger and stiffer cutter, which can improve cycle time and surface consistency.
Do not make the corner radius exactly equal to the intended cutter radius. Some clearance allows the toolpath to move through the corner instead of stopping and changing direction abruptly. If a mating component is square, a relief feature or localized clearance may solve the assembly requirement more efficiently.
4. Review Pocket Depth and Thin Walls Together
Deep pockets increase tool overhang and make chip evacuation more difficult. Thin walls can move under cutting force or after residual stress is released. When both occur in the same part, the process may need staged roughing, controlled finishing passes and additional inspection.
Where the design permits, increase wall thickness, reduce pocket depth or add a generous corner radius. For a flexible feature, specify its functional requirement rather than applying an extremely tight general tolerance to every surface.
5. Plan Workholding Before Finalizing the Model
The part must be held without blocking important surfaces or distorting the material. Include temporary stock, clamping pads or sacrificial tabs when necessary, especially for thin plates and irregular profiles. A component that requires every face to be machined may need multiple setups, soft jaws or a dedicated fixture.
Stable workholding also affects repeat orders. A defined locating strategy makes it easier to reproduce the relationship between machined features across batches.
6. Apply Tolerances Selectively
Tight tolerances affect tooling, setup, thermal control and inspection. They should be reserved for dimensions that influence fit, sealing, motion or alignment. A general tolerance can cover noncritical dimensions, while geometric controls can describe flatness, position, perpendicularity or runout more clearly than many coordinate dimensions.
Surface-finish requirements should also be functional. A cosmetic face, bearing surface and gasket interface have different needs. Mark them individually instead of assigning the finest finish to the entire component.
7. Include Material and Finishing Information
State the material grade, condition and any traceability requirement. Aluminum, carbon steel, stainless steel, copper alloys and engineering plastics behave differently during machining. Heat treatment and coating can also influence dimensions, surface preparation and the order of operations.
If anodizing, plating, passivation or painting is required, identify masked areas and whether the final dimensions apply before or after finishing.
Drawing Package Checklist
- 3D model and controlled 2D drawing use the same revision.
- Material grade and condition are stated.
- Functional datums and critical dimensions are clear.
- Threads, inserts, deburring and edge-break requirements are defined.
- Finish, masking and cosmetic expectations are identified.
- Prototype and production quantities are included.
Frequently Asked Questions
Can CNC milling make sharp internal corners?
Not directly with a standard rotating end mill. The design normally uses a radius, a relief feature or a secondary process such as Wire EDM.
Why does workholding affect price?
Every setup requires locating, clamping, verification and machining time. A design that can be completed in fewer stable setups is usually easier to control and repeat.
What should be sent for a quotation?
Send a 3D model, dimensioned drawing, material, finish, quantity and the features that are functionally critical. Balford can then review the part against its CNC milling service and related manufacturing options.
Planning a machined component? Share your drawing with Balford for a manufacturability review and quotation.