Choosing between machining and grinding routes

• Hardness: once a part is hardened beyond what cutting tools handle economically, grinding becomes the practical route
• Finish: surfaces that seal, run against a bearing or carry a sliding fit often need a finer finish than turning delivers consistently
• Form: roundness and cylindricity requirements that turning can meet occasionally but not reliably across a batch
• Sequence: any feature that must be correct after heat treatment, because the treatment itself moves the part

If a part is hardened, features that must hold size and form afterwards will generally be finished after treatment, which in practice means grinding. Features that do not need that accuracy can be completed before.

This is why the process route and the heat treatment decision cannot be made separately. Specifying hardness without saying which features must be accurate afterwards leaves the route ambiguous.

A part that goes to grinding needs material left on the ground features, and the amount is a real engineering decision. Too little and distortion during treatment can leave the feature undersize after clean-up. Too much and the grinding operation becomes long and expensive.

Whoever machines the part before treatment needs to know what the grinding allowance should be, which means the route has to be planned as a whole rather than operation by operation.

• Removing bulk material quickly and economically
• Complex profiles, grooves, threads and features a wheel cannot easily reach
• Soft and free-machining materials where the required finish is achievable directly
• Prototype and low-volume work where an extra setup outweighs the accuracy benefit

Internal grinding is more constrained than external: the wheel and quill must fit inside the bore, which limits wheel size and therefore stiffness and reach. Deep or small bores are correspondingly harder.

Where a bore must be both deep and accurate, it is worth discussing early, because the constraint may change the part design or the order of operations rather than just the process.

• The drawing with the features that must be accurate after treatment marked
• Material, its condition on arrival and the target hardness if it is to be treated
• Finish and form requirements on the critical features
• Quantity, which decides whether extra setups are economic
• Whether you are supplying material or the supplier is