Introduction
A design engineer who understands machining methods produces better drawings — ones that are actually machinable, cost-effective, and free from geometry that frustrates the shop floor. This article explains the core machining processes and what they mean for your design decisions.
Turning (Lathe)
Turning uses a rotating workpiece and a stationary cutting tool. It produces cylindrical shapes — shafts, bores, tapers, and threads. Key design considerations:
- All features must be accessible from the end of the bar or the face of a chuck
- Internal corners have a radius — do not design sharp internal corners in bored holes
- Tolerances achievable: ±0.05 mm (standard), ±0.01 mm (precision)
Milling
A rotating cutter removes material from a stationary workpiece. Produces flat surfaces, slots, pockets, and profiled shapes. Key design considerations:
- All pockets have radiused corners — the minimum corner radius equals the cutter radius
- Deep pockets with small radii are expensive — tool deflection increases with depth
- Tolerances: ±0.05–0.1 mm (standard), ±0.01–0.02 mm (precision)
Drilling and Boring
Drilling creates holes; boring refines existing holes to precise diameters. Design considerations:
- Drill entry surfaces should be perpendicular to the drill axis — angled entry causes drift
- Through-holes are cheaper than blind holes; specify minimum depth if blind is required
- Close-tolerance holes (H7 and tighter) require boring or reaming after drilling
Grinding
Grinding removes small amounts of material with an abrasive wheel. Used for high-precision surfaces, hardened parts, and fine surface finish. Typical accuracy: ±0.005–0.01 mm. Use grinding when tight tolerances are needed on hardened steel parts that cannot be cut with standard tooling.
Sheet Metal (Stamping, Bending, Laser Cutting)
- Laser cutting: precise 2D profiles; sharp corners possible; no tooling cost
- Bending (press brake): inside bend radius ≥ material thickness; minimum flange length applies
- Stamping: high volume; tooling cost is significant but per-part cost is low
Accuracy vs. Cost Summary
| Process | Typical Tolerance | Best For |
|---|---|---|
| Turning | ±0.05 mm | Cylindrical parts, shafts |
| Milling | ±0.05–0.1 mm | Flat surfaces, slots, profiles |
| Drilling | ±0.1–0.2 mm (position) | Hole patterns |
| Grinding | ±0.005–0.01 mm | High-precision, hardened parts |
| Laser cutting | ±0.1–0.2 mm | Sheet metal profiles |
Design Rules That Save Money
- Avoid unnecessarily tight tolerances — every tightening of tolerance increases cost
- Design for tool access — if a cutter cannot reach a feature, it cannot be made
- Standardize hole sizes — use common drill sizes rather than odd diameters
- Minimize setups — features that require repositioning the workpiece add cost
FAQ
Q. How do I know if my design is machinable?
A. The key questions are: Can a standard tool reach every feature? Are corner radii larger than the smallest likely tool radius? Is the specified tolerance achievable with the intended process? When in doubt, consult your manufacturing team before releasing drawings.
Q. When should I specify grinding instead of turning or milling?
A. Specify grinding when tolerances are tighter than ±0.01 mm, when surface finish requirements are very high (Ra 0.4 or better), or when the material is hardened steel that cannot be cut with conventional tooling.
Q. What is the most common machining mistake in mechanical design?
A. Designing internal sharp corners in pockets and slots. Every milled corner has a radius equal to at least the cutter radius. If your design requires a true sharp internal corner, specify a relief cut or change the mating part design.
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