The practical points
The guide bushing, incoming diameter range, roundness, ovality, finish, thermal behavior, and finished-part tolerance interact; no single generic stock tolerance fits every setup.
Record actual incoming maximum and minimum diameter across the bar and lot before attributing drift or tight running to nominal size alone.
State diameter tolerance, roundness or ovality method, surface finish, straightness, and lot expectations separately on the RFQ.
Any numerical selection rule must be validated against the machine, guide bushing, operation, material, and customer print before publication or use.
Treat clearance as a system budget
The guide bushing does not see a catalog diameter in isolation. It sees the largest local diameter, cross-sectional form, surface condition, thermal state, cleanliness, and bar alignment presented by the running setup. The finished part also reflects guide condition, tool pressure, operation sequence, and measurement uncertainty.
Questions to answer before specifying a tighter bar
- What finished feature and tolerance are actually influenced by the stock-to-guide relationship?
- What minimum and maximum incoming diameter has the shop successfully run?
- How are roundness or ovality and straightness measured?
- Is the outside diameter left on the part, partially machined, or completely removed?
- What guide type, condition, and adjustment method is used?
- Does heat, lot, finish, lubricant, cleaning, or ambient temperature change the observed behavior?
Record the evidence
For each trial or production heat, tie the following to one record: heat/lot identity, actual diameter maximum and minimum, measurement positions, roundness/ovality result and method, straightness/runout result and method, hardness or condition when relevant, guide setup, process settings, observed symptom, and disposition. That record is the basis for a defensible incoming requirement.
No universal clearance number: Exact Bar does not publish one automatic guide-bushing allowance for every machine, material, diameter, operation, or print. Validate any numerical rule on the actual setup and have the responsible technical authority approve it.
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.