SITE：How to Specify Shaft Straightness on a Drawing: Tolerance Frames, Datums & Acceptance | SHANGDA
Every shaft straightening dispute we have mediated since 2008 began with a drawing note that said “straighten to 0.05” and nothing else. The supplier measured one way, the incoming inspector another, and both readings were honestly obtained. A geometric requirement is only as good as its measurement agreement — written before the order is placed, not negotiated when the crate arrives. This guide covers the four pieces every straightness callout needs, what a per-metre basis and a full-length basis actually promise, what the three common setups sense, and a copy-ready English note plus an acceptance protocol for any purchase order. No external document numbers are quoted; “your applicable GD&T standard” is the only reference this article needs.
By SHANGDA Engineering TeamPublished: September 2026For: design, sourcing & quality engineers
The one rule behind this article: never call out straightness without a value and unit, a stated length basis, an agreed measurement setup and defined gauge positions — with acceptance written as a protocol, not an opinion. If the inspector cannot reconstruct how the supplier measured, the callout is unfinished.
01“Straighten to 0.05” is not a specification
The note appears on thousands of drawings: “shaft to be straightened, 0.05”. Read it as a supplier must. Millimetres or inches? Axis deviation over the whole part, or indicator travel over one metre? Supported between end centres, on V-blocks at the main journals, or on a surface plate? How many sections are checked, and what does gravity sag add to a slim long shaft? The note answers none of it, so every reader fills the gaps with local habits — and habits differ between plants and countries.
The result is predictable. The shop rotates the part on V-blocks, reads total indicator travel at the worst spot and ships. Goods-in inspects between centres over the full length and records a slightly larger number: the support geometry has changed, and the envelope catches gentle bends the local reading ignored. Neither side is lying; both numbers describe slightly different things. A batch hold, re-freight and an engineering concession follow. Tightening the number does not help — a bold 0.02 in the same shorthand creates the same dispute with less room to compromise. The fix is linguistic: make the callout describe the measurement, without bloating the drawing or quoting a rulebook.
02The four pieces a complete callout must carry
A requirement a supplier can execute and an inspector can reproduce contains four pieces; lose one and it silently becomes ambiguous.
Value and unit. A numeric tolerance zone width with the unit explicit, for example 0.05 mm. A unitless number invites an inch/metric reading no email should have to rescue.
Length basis. Whether the width applies over the full length, a defined gauge length, or as a rate per metre. The same 0.05 mm means a different part under each choice.
Measurement setup. How the part is supported and rotated, what instrument reads it, and the reading type — total indicator travel, axis deviation from a scan, or another agreed method.
Gauge positions. Where the readings happen, how many there are, and which value decides acceptance: every point, the worst point, or the worst point plus an end-to-end envelope.
The pieces answer four questions: how much, over what span, measured how, checked where. Designers often assume the setup is obvious from the geometry; a stepped shaft can legitimately rest on three different journal pairs, and each choice moves the reading. The tolerance magnitude is a separate decision, covered from first principles in our straightening tolerance primer. Here the topic is the writing: once the magnitude is chosen, these four pieces commit it to paper.
03Per meter versus over full length: two different promises
The length basis causes more confusion than any other element: both phrasings use millimetres and both are called straightness in casual speech, yet a long shaft can pass one and fail the other.
A per-metre statement, such as 0.10–0.30 mm/m, limits local curvature: inside any one-metre window the axis cannot bow past the zone. It describes how gently the centre line bends, and suits bars and guided elements supported at intervals, where no full-length reference is practical. It does not control accumulated drift: several mild bends can point the same way and walk the ends apart while every single metre passes.
A full-length statement, 0.02–0.05 mm over the entire shaft, does the reverse: it confines the whole axis in one zone, catching the global bow, while saying little about a tight local kink. High-speed shafts between bearings, and anything whose end alignment matters, are normally specified this way. Many drawings need both, on two lines — a local rate against kinks and a full-length envelope against global bow. The trap is one bare number: on a part two metres long, the supplier may read it as a rate and the buyer as an envelope. One extra line erases the argument.
04What each measurement method actually measures
Three setups dominate shaft work. They vary not just in convenience but in the physical quantity they sense, so the callout should name its choice.
V-block rotation with a dial indicator. The shaft rests on two blocks under machined journals and is turned while a gauge rides the surface; the needle sweep at a section is the reported total indicator reading, TIR. It mixes axis deviation with the roundness of the measured and supporting journals, since an out-of-round journal rocks the part. Fast and shop-hardened, it is the production-floor default; its pitfalls are detailed in our TIR measurement guide.
Between centres. Located on machined centre holes and rotated there, the gauge reads relative to the part’s own axis, removing most support error. Centre-hole quality, taper and dirt now enter the reading, and the setup exists only if the drawing provides usable centres.
Laser line scanning. An optical probe samples the profile along the length without rotation and software fits the centre line, reporting deviation at each station. It separates straightness from roundness and handles non-round sections that cannot be rotated for TIR at all, such as rectangular bars and racks — see our rack straightening article. For round shafts it is reserved for long or high-value parts.
None is universally correct, which is exactly why the note must name the setup, support locations and instrument instead of leaving measurement to whichever gauge the inspector owns.
05The tolerance frame and the datum line: what to write where
In the geometric tolerancing language your plant already uses, the requirement appears as a small rectangular feature-control frame attached to the feature: one compartment for the straightness symbol, one for the value with its unit, and further compartments where your applicable GD&T standard prescribes them. Two points matter more than compartment layouts.
First, axis straightness, taken alone, is not referenced to a datum. It controls the form of that feature within its own zone; there is no surface it is measured “from”. Hanging a datum letter on it anyway quietly turns the frame into an alignment or orientation requirement, which may be wanted but should be named as such. Reserve datum letters for bearing bores, mounting faces and the interfaces from which position and runout are genuinely measured.
Second, the datum the floor needs is a measurement datum, written in words. Even a datumless frame needs a note stating where the part sits: support on the two main journals, locate between drawing centres, or support at the positions shown. That line is what makes two plants produce the same number, and any per-metre or partial-length basis goes beside it. A frame without this prose assumes an inspector who has run this part for years; with it, the requirement survives a new supplier and a receiving dock far away.
06Four callout mistakes and the disputes they cause
The bare number. “Straighten 0.05” with no unit, basis, setup or points — the general failure from section 1.
Mixing the bases. The designer means a full-length envelope; the note is read as a per-metre rate. A two-metre shaft can be locally impeccable yet bow enough to bind at assembly. Use two explicit lines whenever both matter.
“As agreed” left blank. The requirement is delegated to a conversation that never happens, or happens verbally between people who later leave. If the value awaits process trials, write the trial into the order: sample quantity, method, and the date the figure is frozen.
Confusing characteristics. Roundness controls the section at one cut; cylindricity controls the surface envelope along and around one diameter; straightness controls the axis. A bent shaft can be perfectly round, and a straight shaft can have lobed sections. “Straight and round and true” specifies nothing measurable; give each failure mode its own line.
A useful habit: read the finished note as an instruction to someone who has never seen the part. If they cannot say what to support on, which gauge to use, where to place it and what number to write, the note is still ambiguous.
07Long shafts: why one number is never enough
A single mid-point reading suits a short, stiff shaft with one crown. As length and slenderness grow, the centre line stops behaving like a single arc. Heat-treatment distortion on a long shaft is usually compound: two or three lobes, an S-shape, a kink near a section change, sometimes a gentle global bow underneath. One gauge cannot see this geometry; a single support can even make the errors partially cancel, so the part “passes” at the check and fails on the assembly stand.
Long parts are therefore accepted at multiple defined stations: gauge two to three sections on short stiff shafts, and define five to eight axial stations on long shafts, bars and lead screws, dimensioned from the same end the supplier uses. Name the same local limit at every station plus a separate end-to-end envelope, so there is no argument about which reading counted. This mirrors the straightening process itself: a closed-loop machine maps the same stations, fits each lobe and corrects it separately, as shown in our working-principle walkthrough. Beyond 12 m, dimension the shaft in sections with a straightness line per section and an assembly note; one zone over the whole length asks for a reference few shops possess.
08A copy-ready technical-requirements template
The clause below is plain English, free of external document numbers and symbols a foreign drawing package may render inconsistently. Paste it into the technical-requirements block and replace the bracketed items; let the feature-control frame carry the symbol. Sentence structure is the deliverable.
Suggested drawing note — straightness and measurement
“Shaft axis straightness shall be within [0.05] mm over the full shaft length, and within [0.2] mm per metre of length, measured at the axial stations dimensioned on this drawing ([3] stations for shafts below one metre, [6] stations for longer shafts). The part shall be supported [between the drawing centres / on V-blocks at the two main journals / at the support positions shown] and measured by [dial indicator reading on rotation / laser axis scan], with the largest indicator travel at any station reported. Gauge positions, support positions, readings at every station and the measurement date shall be recorded on the inspection report supplied with each batch. Acceptance is per the inspection protocol agreed in the purchase order. No straightness requirement in this note shall be read as controlling roundness or cylindricity, which are specified separately.”
Delete whichever basis does not apply rather than leaving a bracket; an either/or requirement is the ambiguity returning. Pick one support method per part number instead of listing all three, since the supplier will choose the most forgiving and the inspector the least. Keep the final sentence. Commercial long stock often needs only the per-metre line; the full-length line earns its place on rotating, guided or aligned shafts.
09Incoming acceptance both sides will actually sign
A clear note settles the measurement; acceptance still needs a sequence, agreed in the order rather than improvised at the dock.
Sealed sample. At order release, both parties measure a small set of parts from the real heat-treated condition with the callout setup, and the readings, supports and gauge locations are signed and filed. It costs a day and detects method drift later.
First-article approval. The first production parts arrive with a full report — every station, both bases where both apply, instrument and calibration reference — and production releases only after countersignature. This is when a gauge sitting a few millimetres off-station costs nothing to fix.
Batch data reports. Each shipment carries readings in the first-article format. On tight or safety-related parts, specify 100% inspection with logged data: every shaft at every agreed station, records retained for your customers’ audits, rather than a sampling ratio nobody re-counts.
Write the hold rules too: what happens after one failed station, who re-measures against which reference, and within how many days. A protocol turns a dispute into an administrative event; none of this is impossible, because machine cells produce these readings automatically as part of the cycle.
10What to attach to the supplier email
Three attachments decide whether the quote and the parts match your intent; a polite email with only the drawing PDF recreates the whole problem.
The drawing, revision-controlled. With the completed note, stations dimensioned on the view, the frame attached to the axis, and centre holes or support journals clearly present. State the revision in the email body; suppliers waste weeks quoting a drawing two revisions behind.
The measurement protocol. One page naming supports, instrument, gauge positions, the two bases where both apply, report format, and the sample and first-article sequence. It turns the note into floor instructions a quality manager can countersign.
Worst-case incoming data. Measured straightness from your poorest heat-treatment lots, not the average. Capacity, supports and cycle are sized for the worst bow the cell will see; typical-only data yields a process that handles most shipments and stalls on the difficult ten percent. Note the heat-treatment condition with the readings.
Ask for written confirmation the requirement can be held with the stated setup, and an offer to run your samples before release. A capable manufacturer answers with a method and a data plan; an incapable one answers with adjectives. Lead time belongs in the same discussion — purpose-built machines normally deliver in 60–120 days — so the acceptance and delivery calendars are planned together.
11A worked example: fixing a disputed drawing the right way
Take a drive shaft about 1.2 m long between its end journals, rotating at speed through two close-tolerance bores. The drawing carried “straighten 0.05”; two shipments passed on goodwill, and the third was held — supplier reading 0.04 at the centre, buyer reading a slightly higher figure between centres over the full length.
The tier was not the problem: for an automotive drive-train shaft the familiar 0.02–0.05 mm family applies, and 0.05 full-length was a legitimate choice — the only place the magnitude needs discussion. The callout became two lines: the axis within 0.05 mm over the full length, and within 0.2 mm per metre against kinks. The frame stayed datumless, while the note named the measurement datum: end journals on V-blocks, gauge on rotation, six dimensioned stations from the flange end.
The order then gained ten jointly measured sample parts, a countersigned first-article report and 100% inspection with logged data per shipment. Lead time stayed inside the normal build window, and the next batches passed goods-in without a held crate. Nothing tightened the requirement; every change made it measurable by both parties. If one of your drawings is living through the same argument, send it over — we will mark the ambiguous lines and return exact wording, with no obligation: request a drawing review, or browse the shaft straightening machine range.
12Frequently asked questions
The tolerance value with its unit, the length basis (full length, per metre, or both on separate lines), the measurement setup (support, instrument and reading type), and the gauge positions with station count and acceptance rule. The frame carries the symbol and a note beneath carries the measurement wording. A bare number such as “straighten to 0.05” leaves every piece to the reader’s imagination and is the origin of most disputes. Alone, it is a form control with no datum: the axis sits inside its own zone. The drawing still needs a written measurement datum — a note saying where the part is supported — so inspection is reproducible. Keep datum letters for requirements genuinely taken from a datum, such as position or runout; adding one to a form frame changes the requirement without renaming it. A per-metre value limits curvature inside any one-metre window and does not control end-to-end drift; mild bends can pass each window while walking the ends apart. A full-length millimetre value confines the complete axis in one zone and catches the global bow, but may miss a tight kink. Rotating shafts commonly need both; bar stock often needs only the rate. Never quote one number without the basis. Not exactly. V-block TIR at a section mixes axis deviation with the roundness of the measured and supporting journals. Between-centres checking reads closer to the part’s own axis, and laser scanning fits the centre line directly and separates form from roundness. The methods answer related but different questions, so the drawing must name the intended one, with supports and instrument. Two to three defined sections for short stiff shafts; five to eight dimensioned axial stations for long shafts, bars and lead screws, because one point cannot resolve compound bends and can let lobes cancel. Use the same end for dimensions on drawing and report, state the local limit at every station plus a full-length envelope, and require all readings on the record. Beyond 12 m, specify section by section. Agree a written protocol in the order: signed sample parts before production, a countersigned first-article report naming every station and instrument, then per-shipment data reports — 100% inspection with logged data on tight parts — plus rules for failed stations, re-measurement and response time. Machine cells produce these readings automatically, so the ask is reporting existing data, not new work, and the dock becomes a comparison against a method both sides signed.What should a straightness callout on a shaft drawing include?
Does a straightness tolerance need a datum reference?
What is the difference between mm and mm/m straightness?
Is TIR on V-blocks the same thing as axis straightness?
How many measurement points should a long shaft acceptance use?
How do I make incoming straightness acceptance fair to both sides?
Send the drawing — let the measurement agreement follow it
SHANGDA has built automatic shaft straightening machines since 2008, all CE certified, for shafts, bars, screws, racks and tubes from Ø5 mm to 600 mm and 100 mm to 12 m. Our cells measure at the stations your drawing names, report full-length and per-metre results separately, and keep 100% inspection with logged data for every batch. Send your drawing and worst-lot readings; our engineers will return copy-ready callout wording and a one-page protocol, proven on your sample shafts.
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