Shaft straightness in mm/m states how far a bar departs from a straight reference line, as a rate per meter of length. One millimeter per meter equals 0.001 mm per mm, so a bar at 0.2 mm/m sits about 0.2 mm off the line over 1 m and about 0.1 mm over 500 mm. The number is comparable only when the support points, datum and measuring method are stated.
What does straightness in mm/m mean, and how do I convert it?
It is a slope, not a total allowance: 1 mm of deviation for every 1,000 mm of length. That is identical to 0.001 mm/mm, to 1 µm/mm, and in inch terms to 0.001 in/in or 0.012 in/ft.
The rate becomes an allowance only when a length is fixed:
| Straightness | Over 300 mm | Over 500 mm | Over 1 m |
|---|---|---|---|
| 0.1 mm/m | 0.03 mm | 0.05 mm | 0.1 mm |
| 0.2 mm/m | 0.06 mm | 0.1 mm | 0.2 mm |
| 0.5 mm/m | 0.15 mm | 0.25 mm | 0.5 mm |
| 1 mm/m | 0.3 mm | 0.5 mm | 1 mm |
Two drawings can carry the same mm/m figure and still mean different things. A note reading 0.1 mm/m asks for a rate; a note reading 0.1 mm total asks for an absolute allowance that has to be met over the whole bar, which is a tighter requirement on a 4 m length than on a 1 m length. Say which one you mean.
Keep straightness separate from other geometry callouts. Diameter tolerance, roundness, surface finish and end geometry are specified on their own lines, and straightness does not substitute for any of them.
What straightness limits are typical for each bar condition?
The typical limits follow the manufacturing route: cold-drawn bar is the widest, peeled bar is tighter, and ground bar is tighter again, with fine grinding split into standard and special classes.
| Bar condition | Size tolerance, typical | Straightness, max, typical | Surface, typical |
|---|---|---|---|
| Cold drawn | Standard IT10; IT9 by agreement | 1 mm/m | — |
| Peeled | IT11; IT9 by agreement | 0.6 mm/m | Rt max 50 µm |
| Ground, rough | IT9 or better | 0.5 mm/m | Ra max 5 µm |
| Ground, fine, standard | IT8 | 0.2 mm/m | Ra 1.0 µm |
| Ground, fine, special | IT6 | 0.1 mm/m | Ra 0.6 µm |
These are typical reference values for bright bar; confirm the figures for your order against the applicable standard or the mill data. Out-of-roundness is usually handled on a separate line: for drawn, peeled and ground bar it is typically limited to max 50 % of the diameter tolerance, measured as difference in diameter.
Yixin Precision Machinery (Changzhou) works to a standard straightness of ≈ 0.1 mm/m from Ø25 mm and ≈ 0.2 mm/m below Ø25 mm on its chrome-plated and ground shafting, with the final value fixed on the drawing and order. The plated surface is Ra ≤ 0.4 µm and the ground surface Ra ≤ 0.8 µm, so finish and straightness can be called out together. For linear-bearing work the matching product is the 45 Steel Chrome-Plated Linear Shaft, where straightness is specified alongside the bearing-fit diameter; for hydraulic piston rods and general shafting, the 45 Steel Induction-Hardened Chrome Shaft is the usual route.
How is straightness measured, and why do two readings disagree?
Straightness is read as an indicator value while the bar is rotated in a defined support arrangement, so the reading describes the bar and the setup together. Change the support points and the number changes.
A workable routine, used by both sides, has four parts. The bar is supported between centers or on V-blocks at stated positions. Readings are taken at several stations along the length, not at one point. At each station the bar is rotated slowly and the highest and lowest indicator values are recorded; the difference is the reading for that station. The worst station is reported together with the length basis it was converted to.
| Record | Detail to agree before inspection |
|---|---|
| Support | Between centers, or V-block positions, written on the report |
| Stations | Number and spacing of measuring positions along the bar |
| Instrument | Indicator type and resolution |
| Reading | Highest and lowest value per station; worst station reported |
| Length basis | Value expressed per meter, or as a total over the measured length |
| Surface | Whether readings were taken on the ground bar or the plated surface |
Agreeing this table in the inquiry removes most disputes at receiving inspection, because both parties convert the same readings the same way. Where a formal third-party check is needed, it is arranged on request.
How do I write straightness on a drawing?
Do not write a single number. Write the limit with its unit, the length basis, the support and the reading convention in one note, then state which surface is measured.
A complete callout covers:
- the numeric limit and unit, for example 0.2 mm/m maximum;
- whether the value applies per meter of length or as a total over the bar;
- the support and datum used for acceptance, between centers or on V-blocks at stated positions;
- the instrument and the reading convention, highest-to-lowest per station, worst station governs;
- the surface to be measured, ground or plated;
- the condition of the cut ends, since bars cut to length are supplied with sawn ends.
If your drawing already carries a runout or concentricity callout, state which requirement governs straightness so the two are not read against each other. On bearing-shaft orders, give the diameter tolerance band and the straightness figure together — the fit and the straightness are separate requirements, and neither one implies the other.
What to put in your RFQ
- Product and condition: for example 45 Steel Induction-Hardened Chrome Shaft, 45 Steel Chrome-Plated Linear Shaft, or a ground bar; state whether the surface is to be chrome-plated or ground only.
- Material grade and standard: Grade 45 to GB/T 699-2015, or 40Cr or 42CrMo to GB/T 3077-2015.
- Diameter and tolerance band, written as a range with the direction of tolerance.
- Straightness in mm/m, with the length basis: per meter or total over the bar.
- The measuring method you will use for acceptance, including support positions and reading convention.
- Surface finish, for example Ra ≤ 0.4 µm on chrome or Ra ≤ 0.8 µm on ground bar.
- Chrome layer if plated: 10–50 µm per side, chosen per order.
- Bar length and cut length: full bars of 4 m, 6 m, 7.5 m or 8.1 m, or CNC cut to length from 200 mm with sawn ends.
- Order quantity and whether samples are needed; minimum order is 1 t, and samples of 20–50 kg are available.
Summary
Straightness in mm/m is a slope of 1 mm per 1,000 mm of length, identical to 0.001 mm/mm and to 1 µm/mm. Convert it to an allowance only against a stated length, and always state whether the figure is per meter or total. Typical limits run from 1 mm/m for cold-drawn bar through 0.6 mm/m for peeled and 0.5 mm/m for rough ground, down to 0.2 mm/m for standard fine grinding and 0.1 mm/m for the special fine-ground class. Yixin's standard figures are ≈ 0.1 mm/m from Ø25 mm and ≈ 0.2 mm/m below Ø25 mm, confirmed on the drawing and order. Specify the number, the length basis, the support and the reading convention together, and receiving inspection becomes a comparison rather than an argument.