Precision grinding — when it is necessary and what it really delivers

On many drawings, grinding appears "out of momentum" — because that is how it was in the previous project, because the surface should look decent, because the designer preferred to play it safe. The problem is that precision grinding is an additional operation, an additional setup and an additional cost on every piece. Sometimes it is absolutely necessary. And sometimes accurate turning does the job cheaper and faster.

From this post you will learn what grinding really delivers after turning, milling and hardening, where it is the standard (bearing journals, guideways, H7/g6 fits), how it compares with accurate turning in numbers — and when it is simply worth striking off the drawing.

What precision grinding adds after turning and milling

Grinding is an abrasive process: instead of a single cutting edge, thousands of grains of the grinding wheel do the work, and the stock removed is in the order of hundredths of a millimetre. Three concrete effects follow from this:

• a tighter dimensional tolerance — grinding gets down to grades IT5-IT6, while accurate turning typically ends at IT7-IT8,
• lower surface roughness — Ra 0.2-0.4 is a typical grinding result, versus Ra 0.8-1.6 after finish turning,
• better geometry — roundness, cylindricity and runout relative to the datums improve, because the part is machined with small forces, from a fixed datum (centres, chuck), without the deflections typical of edge cutting.

In workshop practice, the process looks like this: turning shapes the part with stock left on the functional surfaces, and grinding brings those surfaces to final dimension, geometry and finish. Selected surfaces are ground, not the whole part — and that is the key to sensible costs.

Grinding itself comes in several variants: cylindrical grinding between centres, internal grinding, surface grinding and centreless grinding for highly repeatable parts. From the buyer's point of view, the differences come down to which surfaces can be machined and which datums the process needs — the rest is chosen by the process engineer.

If you want to see what turning alone achieves before you add an abrasive operation, read the post tolerances and surface roughness in CNC turning.

Grinding after hardening — where the cutting edge ends

The second, often more important reason for grinding is heat treatment. Hardening raises hardness, but it has two side effects: the part distorts (dimensions and geometry "drift"), and a material at around 55-62 HRC stops being practical for conventional cutting. An attempt to "finish off" a hardened part with an ordinary tool usually ends with chipped inserts and a surface out of tolerance.

That is why the typical sequence for hardened parts looks like this:

1. Rough and shaping turning or milling, with stock left on the functional surfaces.
2. Heat treatment: hardening and tempering to the required hardness.
3. Grinding the functional surfaces to final size — removing the distortion, restoring dimension and geometry, achieving the target surface roughness.

Without that last step, a hardened shaft with a bearing fit simply does not hold the drawing. It is also the standard route when reproducing worn machine parts — more on that in the post recondition or remanufacture a part.

Typical applications: where grinding is the standard

A few families of surfaces where precision grinding is the norm, not a whim:

• bearing journals — bearing manufacturers require diameter tolerances, runout and surface roughness that turning usually cannot guarantee in precision assemblies,
• H7/g6 fits and tighter — rotational and transition fits on surfaces that mate in sliding contact,
• guideways and datum faces — the flatness and straightness of machine guideways require surface grinding,
• sealed surfaces — the running areas of radial shaft seals and other seals require low, controlled roughness with no toolmarks from cutting,
• tools and hardened components — dies, punches, arbors, shafts after heat treatment.

The common denominator: everywhere here the tolerance, geometry or roughness follows directly from the function — bearing, seal, guiding. If your surface has none of these functions, you are a candidate for savings. It is also worth remembering that the requirements cascade: since the journal is ground, its runout is measured against datums that must also be made accurately — which is why ground surfaces are planned as a coherent system, not as isolated items on the drawing.

Accurate turning or grinding — a numerical comparison

The values below are typical textbook data for stable processes — not limits of capability nor a declaration of any specific machine's parameters. They serve for a preliminary decision at the drawing stage.

The practical conclusion: if the drawing requires IT7, Ra 1.6 and the material is not hardened — accurate turning is usually enough. Grinding comes into play from IT6 downwards, at Ra 0.4 and better, and always after hardening of the functional surfaces.

There is also a third route: hard turning with CBN inserts, which replaces grinding for some hardened parts. It mainly makes sense for short runs and simple journals — the choice is decided by the cost calculation of the specific part, not by a general rule.

How to prepare the drawing and the part for grinding

If grinding is needed, a few design decisions make the process easier and lower the cost of the operation:

• leave a machining allowance — surfaces intended for grinding are turned with stock (indicatively in the order of 0.2-0.4 mm on the diameter, depending on the part and the heat treatment), so the drawing must clearly indicate which surfaces are to be ground,
• provide for wheel run-out clearance — relief undercuts at shoulders allow the ground surface to be finished right up to the face of the step without wheel collision,
• settle the datums — slender shafts are ground between centres, so centre holes should be either permitted on the drawing or explicitly forbidden if the design does not allow them,
• specify geometric requirements relative to datums — runout of journals relative to a common axis only makes sense when the drawing defines the measurement datums,
• describe the sequence relative to heat treatment — hardening after grinding destroys the effect of the operation, so the sequence must be written down, not assumed.

These decisions are best made at the documentation stage, because correcting them during production means additional consultations, downtime and the risk of scrap.

When grinding makes NO sense

Symmetrically — the situations in which grinding is cost without function:

• free and non-contacting surfaces — nothing mates with them, so Ra 3.2 after turning is fine,
• "a nice look" — aesthetics can be handled by cheaper finishes; grinding is a tool for accuracy, not decoration,
• tolerances copied in as a precaution — a blanket tightening of tolerances across the whole part drags abrasive operations onto surfaces that do not need them; how much that costs, we calculated in the post how much accuracy costs,
• soft parts with loose fits — a spacer sleeve or a washer does not need IT6.

Good drawing practice: mark grinding only on specific surfaces (journal, seat, datum face) instead of a general note "grind". The manufacturer immediately sees where to leave stock — and you do not pay for abrading metal where nobody will ever notice it. This one decision on the drawing can noticeably lower the part price — indicatively, the more so the larger the share of abrasive operations in the whole process.

Summary

Precision grinding is not "better turning" but a separate tool for three jobs: grades IT5-IT6, surface roughness of Ra 0.4 and better, and machining surfaces after hardening. Where the function demands it — bearings, guideways, H7/g6 fits — it is necessary. Where it does not, it is just another line on the invoice.

Do you have a drawing of a shaft or a hardened part and are not sure which surfaces actually need grinding? Send the documentation via the contact form — we will analyse the requirements surface by surface and return a quote within 48 hours.