Most rotational parts end their journey on a lathe. There is, however, a limit beyond which turning — even careful turning — is no longer enough: bearing seats, guide bushes, hardened surfaces. That is where grinding begins. In this post we show when a bore or shaft genuinely needs grinding, what accuracies are achievable and how to design the part so you pay for accuracy only where it is needed.
Where turning ends and grinding begins
The approximate boundaries of the two technologies:
| Parameter | Finish turning | Grinding |
|---|---|---|
| Dimensional tolerance | roughly to IT7 | roughly IT5–IT6 |
| Roughness Ra | approx. 0.8–1.6 | approx. 0.2–0.8, less with fine grinding |
| Form accuracy (roundness, cylindricity) | limited by cutting forces and clamping | markedly higher |
| Hardened material | uneconomic or impossible | natural application |
Treat the values as indicative — they depend on diameter, slenderness, material and clamping. The practical takeaway: if the drawing calls for an IT6 fit or Ra 0.4 on a working surface, expect a grinding operation — and its cost.
Grinding shafts — fits and hardened surfaces
Typical reasons to grind external surfaces:
- bearing journals — fits require a tight diameter tolerance and good roundness; journal runout transfers directly into bearing life,
- sealing surfaces — lip seals run on a smooth surface; excessive roughness destroys the seal,
- guides and sliding surfaces — low roughness reduces friction and wear,
- shafts after hardening — heat treatment adds hardness but distorts the part; grinding restores dimension, straightness and geometry.
The practical sequence for a hardened shaft: turn with an allowance, harden, finish-grind the functional surfaces.
Grinding bores — the harder side of the fit
A bore is inherently harder to machine accurately than a shaft: the tool works in a confined space with less rigidity and worse heat removal. That is exactly why bearing seats, guide bushes and fitted bores in hardened parts are the classic applications of internal grinding. Practical notes:
- the deeper and more slender the bore, the harder the accuracy — if the function allows, place the fit near the face of the part,
- a through bore grinds more easily than a blind one — the bottom of a blind bore needs a relief groove for the wheel,
- assign tolerance and roughness to the specific bore surface, not to the whole part.
Designing without overpaying
Rule number one: accuracy is bought for surfaces, not for parts. A few practical rules:
- Reserve grinding for functional surfaces — fits, seals, guides. The rest of the part can stay as turned.
- Provide a grinding allowance and relief grooves at shoulders and blind bores.
- Do not tighten tolerances "just in case" — see how much accuracy costs.
- For series of ground parts, a trial batch with an inspection report is particularly valuable — dimensions in grades IT5–IT6 need confirming by measurement, not by assurance.
When grinding enters the picture at all and what its variants are, we cover in precision grinding — when it is necessary.
Summary
Grinding bores and shafts is the technology of the last few microns — it steps in where fit, roughness or material hardness exceed what turning can do. A well-designed part is ground only where it works — and that is the main cost lever. We provide precision grinding of shafts and bores, including series with inspection reports — if you have a part with fits, send the drawing for a quote.
FAQ
When does a bore require grinding instead of turning or reaming?
When the fit requires a tight diameter tolerance and low roughness — typically bearing seats and guide bushes — and always when the bore is in a hardened surface. A practical boundary is grade IT7 and Ra below 0.8.
What does grinding realistically add over finish turning?
Tighter dimensional and form tolerances (roundness, cylindricity), lower surface roughness and the ability to machine hardened materials. Good finish turning ends around grade IT7 and Ra 0.8–1.6 — grinding starts where turning ends.
Does grinding always require heat treatment of the part?
No — soft parts are also ground when roughness or form accuracy is decisive. The sequence hardening-then-grinding is typical though, because heat treatment distorts the part and grinding restores dimension and geometry.
How much does grinding raise the part cost?
It is an extra operation with its own setup and slower material removal, so the cost rises noticeably — which is why only the surfaces that require it are ground. A part with one ground diameter costs a little more; a part ground everywhere can cost a multiple.
How do you mark a surface to be ground on the drawing?
With the dimensional tolerance (for example an H7/g6 fit), the required Ra assigned to the specific surface and — if the function demands it — form and position tolerances such as roundness or coaxiality. Without those callouts the manufacturer does not know the surface is meant to be ground.
Related topics
Precision grinding — when it is necessary and what it really delivers
When is accurate turning enough, and when must the part go to the grinder? IT grades, Ra surface roughness, machining after hardening and the situations where grinding is a needless cost.
Read the articleTolerances and surface roughness in CNC turning — what really costs
Which IT grades and what surface roughness a CNC lathe realistically achieves, when grinding becomes necessary, and how to tolerance a shaft drawing so you do not overpay.
Read the articleTolerances in CNC machining — how much does accuracy cost?
Why tight tolerances raise the cost of a CNC part and how to specify accuracy so you pay only for the critical dimensions — general tolerances, fits, IT grades and surface roughness.
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