CNC milling handles precise faces, pockets, slots, holes, and multi-surface features, but a part that's easy to model isn't always easy to machine. A solid drawing has to account for cutter reach, workholding, tool rigidity, and inspection access. Nailing those details up front usually means a more stable process and a tighter quote.

This CNC milling design guide covers the points Balford evaluates when quoting a custom metal component. The goal isn't to box every part into a simple shape—it's to separate features that actually matter functionally from those that just add setups, special tooling, or wasted spindle time.

1. Start With Function and Datums

Identify the faces, holes, and interfaces that control how the part fits or moves in the final assembly. Those features should set the primary datums on the print. A clear datum structure gives both machining and inspection a common reference and cuts down on 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 clearance to enter, move through, and exit the cut. Deep narrow pockets, hidden undercuts, and enclosed corners often require long-reach or custom tooling. Longer tools lose rigidity, so you'll need to back off feeds to manage deflection and chatter.

Where possible, design for open access from a primary direction. If a feature is only reachable after repositioning the part, note whether its relationship to the first setup is critical. That tells us whether a simple second op, a fixture, or full multi-axis machining is the right call.

3. Use Practical Internal Corner Radii

An end mill leaves a radius in any internal vertical corner. A truly sharp corner means another process—like Wire EDM—or a design change. Bumping up the radius lets us run a larger, stiffer cutter, which typically shortens cycle time and gives you more consistent surface finish.

Don't spec the corner radius exactly equal to the cutter radius. A little extra clearance lets the toolpath sweep through the corner instead of stopping and reversing direction. If your mating part is square, a relief or localized clearance often solves the assembly fit more economically.

4. Review Pocket Depth and Thin Walls Together

Deep pockets increase tool overhang and make chip evacuation tougher. Thin walls can deflect under cutting loads or move after residual stress releases. When both show up in the same part, we're looking at staged roughing, controlled finishing passes, and extra inspection points.

Where the design allows, thicken the wall, shallow the pocket, or add a generous corner radius. For a flexible feature, call out its functional requirement rather than throwing an ultra-tight general tolerance across every surface.

5. Plan Workholding Before Finalizing the Model

The part has to be held without blocking critical surfaces or distorting the material. Plan for temporary stock, clamping pads, or sacrificial tabs—especially on thin plates and irregular profiles. If every face needs machining, expect multiple setups, soft jaws, or a dedicated fixture.

Stable workholding also matters for repeat orders. A defined locating scheme makes it straightforward to hold the same relationship between machined features batch after batch.

6. Apply Tolerances Selectively

Tight tolerances drive tooling, setup, thermal control, and inspection costs. Reserve them for dimensions that affect fit, sealing, motion, or alignment. A general tolerance covers noncritical sizes, and geometric callouts—flatness, position, perpendicularity, runout—often communicate requirements more clearly than a stack of coordinate dimensions.

Surface finish requirements should be functional too. A cosmetic face, a bearing surface, and a gasket interface all have different needs. Flag them individually instead of assigning the finest finish to the whole part.

7. Include Material and Finishing Information

State the material grade, condition, and any traceability requirements. Aluminum, carbon steel, stainless, copper alloys, and engineering plastics all machine differently. Heat treatment and coatings also affect dimensions, surface prep, and the sequence of operations.

If you need anodizing, plating, passivation, or painting, identify masked areas and confirm whether final dimensions apply before or after finishing.

8. When the Drawing Says 0.08: Radius or Surface Finish?

The number 0.08 appears on machined drawings with two different meanings, and they are solved by different processes. Treating them as one question is the most common reason a simple part gets quoted badly.

R0.08 is a corner radius

An internal corner cut by a rotating tool can never be smaller than the tool's own radius. In practice a true internal corner is held at R0.2 as the working floor; below that the tool becomes fragile and the geometry stops being economical.

When the real requirement is that a mating part seats flat, a tighter radius is usually not the answer. The mating corner meets the radius before the two faces touch, and the joint stands off. Cutting a relief at the corner — plunging the tool below the seating face so the corner is cleared — removes that interference while the visible corner stays at R0.2. The part then seats on the flat, which is what the assembly actually needed.

Ra 0.08 is a surface roughness

Ra is the average deviation of the surface profile, measured in micrometres; it is not a radius. CNC turning and milling generally produce 0.8–3.2 µm, with a fine finishing pass reaching around 0.4 µm. A requirement near 0.08 µm is a polishing or grinding operation, quoted separately from the milling sequence rather than obtained by cutting more lightly on the same machine.

State the two requirements separately: put Ra on the surfaces that carry a seal, a bearing or a sliding contact, and put R on the corners that actually touch another component.

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 clearly called out.
Threads, inserts, deburring, and edge-break requirements are fully specified.
Finish, masking, and cosmetic expectations are defined up front.
Prototype and production quantities are included in the scope.

Frequently Asked Questions

Can CNC milling produce sharp internal corners?

Not directly with a standard rotating end mill. The design typically calls for a radius, a relief feature, or a secondary operation like wire EDM.Wire EDM.

Why does workholding affect cost?

Each setup involves locating, clamping, verification, and machining time. A design that can be completed in fewer stable setups is generally easier to control and repeat.

What should be submitted for a quote?

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.

Can CNC milling hold R0.08 on an internal corner?

A rotating cutter cannot produce a corner smaller than its own radius. For a true internal corner we work to R0.2 as the floor. Where the corner exists only to clear a mating part, a relief cut solves the seating problem without a smaller tool.

Can CNC milling reach Ra 0.08?

Not as a routine milling result. Turning and milling land between roughly 0.8 and 3.2 µm, with a fine pass around 0.4 µm. Finishes near 0.08 µm come from polishing or grinding, which are separate operations with their own time and cost.

Planning a machined component? Share your drawing with Balford for a manufacturability review and quotation. and related manufacturing options.

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Need a drawing reviewed for the right process? Send it to our engineering team or browse the case library.

Balford mills these parts on more than 500 machines in a 12,000 m² plant in Zhuji, Zhejiang, with the stamped blanks formed in house on presses from 25 t to 350 t.

CNC Milling Design Guide for Custom Metal Parts