Machine Design #81: Center Holes on Shafts — A Small Feature That Decides Rework Capability
The shaft center hole must be readable, manufacturable, inspectable, maintainable, and safe when the boundary changes.
Start with the function
Write the input condition, expected result, acceptance limit, failure symptom, and measurement method before choosing a view, symbol, feature, or note. Do not hide unknowns behind a large factor; assign an owner and a test.
Core checks
- Purpose as a turning or grinding datum: decide whether the centre is a turning and grinding datum for the life of the shaft, or only for the first operation.
- Center type, size, depth, and edge condition: state the centre type, size, depth and edge condition on the drawing; do not leave it to the operator's habit.
- Concentricity and relation to finished axis: tie the centre to the finished axis, because a centre that is off shifts every diameter turned between centres.
- Protection from damage, chips, and corrosion: protect the centre from knocks, chips and corrosion, especially where the shaft end is handled or stored on that face.
- Inspection and rework access: check that the centre can be reached for inspection and for regrinding without dismantling the assembly.
- Drawing note and supplier evidence: write the note the supplier needs, and keep the evidence that the centre was made and checked as specified.
Failure modes
| Failure mode | Symptom | Verification |
|---|
| Ambiguous meaning | Wrong process or inspection | Review standard and evidence |
| Omitted interface | Assembly or maintenance error | Walk the real route |
| Documents out of sync | Correct name, wrong revision | Baseline all references |
A centre hole is a datum that lives for the whole life of the shaft
A precision shaft is rarely finished in one setup. The usual route is rough turning, heat treatment, then finish grinding — and between those steps the shaft has to be re-located on its original axis. The centre holes at each end are what hold that axis.
If the centre holes are removed after turning, the grinding operation has to be indicated from a machined cylindrical surface. The error of that surface then adds into every dimension made afterwards, and concentricity between shaft steps degrades with nobody able to name the cause. How error accumulates across operations is covered in Machine Design #97 — Functional tolerance allocation.
Three centre-hole types and why each is chosen
The centre-hole standard uses a 60-degree cone as the contact surface with the centre, and distinguishes three main types:
| Type | Geometry | Choose when |
|---|
| Type A | Pilot cylinder plus the 60-degree cone, no protecting chamfer | The shaft will not be re-located often; the end will be cut off or machined away later |
| Type B | As type A plus a protecting chamfer at the mouth | The shaft is re-located after heat treatment, after transport, or during future repair |
| Type R | The contact surface is a radius instead of a straight cone | Long shafts, where the effect of angular misalignment between centre and hole must be reduced |
The protecting chamfer of type B is a small detail that pays for itself: it keeps the mouth of the hole from being knocked and stops burrs being pushed into the working cone. A centre hole with a damaged mouth makes the shaft run out as soon as it is mounted, and the operator will usually blame the machine or the centre instead.
Protecting and restoring the centre hole between operations
- Impact during transport is the most common cause of damage. Long shafts stacked together,
the two ends touching, is enough. The requirement for end protection during packing belongs in the documentation, not in a verbal instruction.
- After heat treatment the centre-hole surface carries oxide and may have moved slightly.
Standard practice is to recondition the centre hole before grinding, and that step has to be in the process sheet because it directly affects the runout measured afterwards.
- Rust during storage also corrupts the datum. Shafts held in stock need corrosion protection
at both ends.
Because the centre holes are the mounting datum for the runout check, they also decide the acceptance figure — see Machine Design #100 — Rotary shaft runout.
Three states that must be stated, so the drawing does not stay silent
Shaft drawings often say nothing about centre holes, and that silence is exactly where the acceptance argument starts. There are only three possibilities, and each needs one sentence:
- Centre holes must be retained — where the shaft will be reground, repaired, or checked
for runout between centres during its life. State the type per the standard.
- Centre holes may be retained — the convenient default, used where they do no functional
harm.
- Centre holes must not be present — where the shaft end is a working face or a sealing
face, or where the equipment serves food, medical or cleanroom applications, in which a small cavity that traps soil and resists cleaning is unacceptable. In that case the drawing must also say how the shaft is to be re-located in the later operations.
Dead centre or live centre: choose by speed and load
A centre hole only works with a matching centre. The two common types behave quite differently.
| Dead centre | Live centre |
|---|
| How it works | Stationary; the shaft slides on it | Rotates with the shaft on internal bearings |
| Rotational accuracy | Highest, no bearing clearance | Depends on its internal bearings, adds a small error |
| Speed | Limited, generates frictional heat | Runs at high speed |
| Lubrication | Required, and has to be watched | Not needed at the contact face |
| Suits | Finish grinding, moderate-speed finish turning, parts needing the lowest runout | Rough turning, high-speed turning, heavy loads |
The point to remember: a dead centre is more accurate but generates heat. As the shaft heats it grows longer, presses harder on the centre and generates more heat — a loop that burns the centre hole if the initial force is too high or the lubrication is insufficient.
Tailstock force: too light and too heavy both fail
This parameter is rarely written down yet it directly affects the result.
- Too light: the shaft chatters during cutting, the surface shows wave marks, and the diameter
varies along the length.
- Too heavy: the shaft bows elastically and turns to a barrel or an hourglass shape; with a dead
centre it also generates heat and damages the centre hole.
On long shafts the force has to be rechecked once the shaft has warmed up, because axial growth raises it without anyone touching the handwheel. That is why some machines use a tailstock with thermal compensation.
Long shafts: steady rests and the deflection problem
When the length-to-diameter ratio is large, the shaft deflects under its own weight and under the cutting force. Two intermediate supports are common:
- A fixed steady rest mounted on the bed, supporting at one position and allowing the portion
beyond it to be machined.
- A travelling steady rest mounted on the carriage, following the tool and supporting right at
the cutting zone.
Both bear on the shaft surface, so that surface has to be round enough and smooth enough before they touch it — usually a band is turned first as a bearing area. That belongs on the drawing where the function allows, because it affects whether the part can be machined at all.
Measure the way you machined
The rule that closes the loop: if a shaft is machined between centres, it must also be measured between centres. Chucking it for a quick check feeds the chuck's own error into the result and creates a dispute in which both parties are right in their own terms.
MINATA release checklist
- [ ] Function, boundary, and failure symptom are written.
- [ ] Interfaces, ownership, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, assembly, inspection, and maintenance were tried.
- [ ] Revision, supplier, and configuration records agree.
Good engineering is a chain of explicit assumptions that survives manufacture, operation, maintenance, and change. For shaft center hole, evidence that the result continues to work is the MINATA standard.
Frequently asked questions
Why not simply remove the centre holes for a cleaner part?
Because losing them means losing the ability to re-locate on the original axis. Every operation after heat treatment, every regrind, and every runout measurement then has to rely on a less accurate datum and inherits extra error.
What is the difference between type A and type B?
Type B adds a protecting chamfer at the mouth, which keeps the working cone from being knocked and stops burrs being pushed into it. Shafts that are re-located several times or that are transported should use type B.
Does anything have to be done to the centre hole after heat treatment?
Yes. The centre hole must be reconditioned before grinding, because oxide and slight distortion corrupt the rotational datum. This step belongs in the process sheet, not in the memory of the operator.
How are centre holes handled for a shaft used in food equipment?
They usually have to be removed, because the cavity traps residue and is hard to clean. The drawing must then state that no centre hole is permitted and describe how the shaft is to be re-located during later machining, so the shop is not left stuck.
How should shaft runout be measured?
Between centres, which separates the geometric error of the shaft from the error of the fixture. That requires the centre holes to be intact and reconditioned after heat treatment; otherwise the measured figure describes the state of the centre holes more than the state of the shaft.
Frequently asked questions, continued
Which is more accurate, a dead centre or a live centre?
The dead centre, because it has no internal bearing clearance. In exchange it generates frictional heat, so it needs lubrication and a controlled force. A live centre handles high speed and heavy load but adds a small error from its own bearings.
Why does a turned shaft come out barrelled or waisted?
Usually the tailstock force and deflection. Too much force bows the shaft elastically; a long shaft with no intermediate support deflects under the cutting force. Both give a non-straight profile even though the tool path is correct.
Does the tailstock force need rechecking while the machine runs?
On long shafts, yes. The shaft heats and grows, raising the force without anyone touching the handwheel. That is why some machines use a tailstock with thermal compensation.
Where does a steady rest bear on the shaft?
On a band turned beforehand to be round enough and smooth enough. Bearing on a rough or scaled surface scores the shaft and gives results that do not repeat. Where the function allows, state that bearing band on the drawing.
Is chucking the part for a quick measurement acceptable?
Only for in-process checks. For acceptance, measure using the same setup as the machining — between centres — otherwise the chuck's own error is inside the result and both parties argue while each is right in their own terms.
Conclusion
Center Holes on Shafts is not paperwork done to make a file look tidy. It is how intent becomes a result that can be manufactured, assembled and measured repeatedly. A good drawing does not need the designer standing beside it to explain it; the structure of the information has to do that work.
View all MINATA technical articles