The practical points
Straightness affects rotation, guide-bushing behavior, and remnant handling.
Roundness describes how closely a cross-section approaches a true circle.
Ovality is diameter variation measured across the same cross-section.
Four characteristics buyers should not combine
| Characteristic | What it describes | Why it matters |
|---|---|---|
| Diameter | Size measured across the bar at a stated location and orientation. | Clearance, collet grip, guide-bushing fit, and stock allowance. |
| Roundness | How closely one cross-section approaches a true circle. | A nominally correct bar can still present local high and low points. |
| Ovality | Diameter variation across directions at the same cross-section. | Rotation can translate cross-sectional variation into guide or part behavior. |
| Straightness / indicated runout | Departure of the bar axis from a straight reference; the reported result depends on support and method. | Loading, rotation, vibration, guide pressure, and unattended stability. |
A repeatable incoming inspection sequence
- Verify grade, condition, finish, heat/lot identity, and the accepted purchase description before measuring.
- Let material and instruments reach a suitable inspection condition; record the method and instrument.
- Measure diameter in more than one orientation and at more than one location along representative bars.
- Record maximum and minimum readings instead of only an average.
- Evaluate straightness or indicated runout with the support spacing and method stated; a result without a method is difficult to reproduce.
- Keep readings tied to the heat, lot, bundle, and bar sampled.
From shop symptom to purchasing requirement
| Observed symptom | Check before blaming the alloy | Possible RFQ field |
|---|---|---|
| Guide bushing runs tight | Maximum diameter, ovality, surface condition, temperature, debris, and guide setup. | Incoming diameter range, measurement method, finish, cleaning, and lot expectation. |
| Part diameter changes with rotation | Cross-sectional variation, guide clearance, measurement repeatability, and operation sequence. | Roundness/ovality method and maximum accepted variation if the print/process requires it. |
| Bar vibration or feeder alarm | Straightness/runout, support positions, bar length, spindle liner, speed, and damage in transit. | Straightness method, cut length, packaging, and handling requirement. |
| One heat behaves differently | Heat identity, diameter range, hardness/condition, finish, and process settings. | Heat segregation, MTR request, condition/hardness, and repeat-source review. |
Use the incoming round-bar inspection checklist or build the guide-bushing tolerance budget.
Put it on the purchase description
If diameter tolerance, straightness, roundness, surface finish, hardness, chemistry, origin, or documentation can affect acceptance or machining performance, state it explicitly. A familiar grade name is not a complete purchasing specification.
Reviewed by the Exact Bar technical sourcing team. Last reviewed August 17, 2026. The review checks primary sources, separates grade-specific claims from general guidance, and identifies what still requires machine, print, source, or lot validation. This is procurement guidance—not permission to substitute material or bypass the drawing, purchase order, accepted quote, order acknowledgement, incorporated specifications, or machine-specific validation.