The supplier’s report says the reamed bore is in tolerance. The customer’s inspection says it is not. One side used an air gauge, the other a CMM, or a bore gauge set on a different master, and the sample sits in quarantine while both argue about whose number is right. Often both numbers are honest; they are just measuring different things.
When the supplier and the customer get different numbers for the same reamed bore, check how each side measured before blaming the bore or the reamer.
If the bore really is undersize, see why a PCD reamer produces an undersize hole; for oversize holes from drills, read oversized holes from a new carbide drill; and for the records that travel with the tool itself, check the PCD reamer inspection pack.
An air gauge, a two-point or three-point bore gauge and a CMM each define “diameter” differently, so the same bore can give different results. Air and mechanical gauges compare the bore with a setting master at a few points and heights; a CMM fits a circle or cylinder to probed points and reports a result that depends on the fitting method, point count and filter. When supplier and customer disagree, first check that both measure the same characteristic, at the same heights, with the same size definition, at a stable temperature and against traceable masters. Agree those items in writing before the sample ships, and name one reference method for disputes.
Why the same bore gives different numbers
No reamed bore is an ideal cylinder. It has some ovality or lobing, taper or bellmouth, and surface texture. Each measuring method samples that shape in its own way, so each reports a slightly different “diameter.” The drawing also matters: under ISO 14405-1, a size tolerance with no modifier means a two-point size by default, while modifiers such as GG (least-squares), GX (maximum inscribed) and GN (minimum circumscribed) call for a different, global size. If the drawing and the report do not use the same definition, the numbers are not directly comparable.
What each method reports
Air gauge
An air plug has jets set close to the bore wall, and the gauge reads the change in air flow or back-pressure as the gap changes. It is set against master rings and reads the diameter across the jets at the plug’s measuring height. It is fast, has no contact with the surface, and has good repeatability for production checks. Its reading is influenced by the surface under each jet, it covers a limited range, and it needs clean, dry air and correct masters. A two-jet plug reads a two-point size; rotating it shows ovality, and other jet layouts respond differently to lobing.
Mechanical bore gauge
A dial or digital bore gauge contacts the wall mechanically. Two-point gauges read a local diameter and depend on the operator rocking the gauge to find the true reading. Three-point gauges centre themselves in the bore and respond to lobed forms differently from two-point gauges: a bore with odd-numbered lobing can read nearly constant on a two-point gauge while a three-point gauge shows it. Both need a setting master and a consistent technique.
CMM
A coordinate measuring machine probes points around and along the bore and fits a geometric element to them. The reported diameter depends on the fit method, the number and distribution of points, the filter, the probe size and the alignment. A CMM is the method for position, perpendicularity and relationships to datums, and for form evaluation when enough points are taken, but a diameter from a few points and a least-squares fit is a different characteristic from a two-point size.
Plug gauge
A fixed go/no-go plug checks whether the bore accepts the go member and rejects the no-go member. It gives a pass or fail rather than a value, and the go member is sensitive to form along the bore, so it can reject a bore that point-based methods pass.
What each method suits
The table is a general guide. The right method is the one that matches the drawing’s definition and the function of the bore.
| Characteristic | Air gauge | Bore gauge | CMM |
|---|---|---|---|
| Size in production | Well suited; fast and repeatable | Suited; slower, operator-dependent | Possible; slower |
| Ovality and taper | By rotating and measuring at several heights | By repeated readings | With enough points and sections |
| Lobing | Depends on jet layout | Three-point responds to odd lobing | With dense scanning |
| Position and datums | Not suited | Not suited | Well suited |
| Global sizes (GG, GX, GN) | Not directly | Not directly | Suited, with the stated fit method |
Roundness and cylindricity to a tight tolerance are often measured on a dedicated form instrument; if the drawing controls them closely, agree whether a CMM result is acceptable.
Common causes of supplier–customer mismatch
- Different size definition. One side reports a two-point size, the other a least-squares diameter.
- Different heights. Entry, middle and exit can differ on a reamed bore, especially with bellmouth or taper.
- Temperature. Aluminum expands considerably more than steel masters and gauges, so a part measured warm, or not soaked to the room, reads differently from the same part measured later.
- Masters and calibration. Different setting rings, or rings with different calibration status or uncertainty.
- Part condition. Burrs, chips, coolant film, or measurement while still clamped versus after release.
- CMM programme. Point count, distribution, filter and fit method not stated on the report.
- Gauge capability. A method whose repeatability and reproducibility use up a large part of the tolerance cannot settle a close call.
- Tool measured instead of bore. A reamer measured on a tool presetter shows the tool’s diameter, not the bore it will cut.
Write a measurement agreement before the sample ships
A short written agreement prevents most disputes. It should state:
- the characteristics to be judged and the drawing revision;
- the size definition, including any ISO 14405-1 modifier, and how form and position are evaluated;
- the method for each characteristic and the reference method for disputes;
- measuring heights and orientations along the bore;
- temperature conditions and soak time before measurement;
- setting masters and their calibration;
- for CMM, the point count, distribution, filter and fit method;
- part condition: cleaned, deburred, unclamped;
- sample size and what the report must show.
Put the agreement into the trial terms; the sample validation process shows where it fits. For a reamed bore, also record the tool revision and its measured diameter, so tool and bore data can be traced to each other.
Resolve a mismatch that has already happened
- Exchange the full reports, not only the pass/fail result, and compare definitions, heights, temperature and masters.
- Measure the same serialised parts on both sides, with each side’s method and then with the reference method.
- Check both gauges against the same master, or swap masters.
- If the difference is systematic and explained, agree the correction or the reference method in writing.
- Only when measurement is settled, decide whether the process or the reamer needs to change.
Where the question is whether a reamed bore can replace a honed one, the same discipline applies; see reaming vs honing for precision bores.
Frequently Asked Questions
Why does my air gauge read differently from the customer’s CMM?
They usually report different characteristics. A two-jet air gauge reads a two-point size at its measuring height against a master; a CMM reports a fitted diameter that depends on fit method, points and filter. Check the definition, heights, temperature and masters before assuming either is wrong.
Which method is the most accurate for a reamed bore?
No single method is the most accurate for every characteristic. Air gauging suits repeatable size checks, bore gauges suit flexible checks with good technique, and a CMM suits position and fitted geometry. Choose the method that matches the drawing’s size definition.
What does ISO 14405-1 say about bore size?
ISO 14405-1 sets the default specification for linear size as a two-point size, unless the drawing adds a modifier such as GG, GX or GN for a global size. The report should use the same definition as the drawing.
What should we agree before sample acceptance?
The characteristics, size definition, method and reference method, measuring heights, temperature and soak, masters and calibration, CMM programme details, part condition, sample size and report content.
Next step
Before the next sample ships, write the measurement agreement with your customer, name the reference method, and include it in the trial terms.
Related reading
- Reaming basics: What is reaming in CNC machining
- Accuracy and finish: How PCD reamers control hole accuracy and surface finish
- Tolerance classes: Carbide drill diameter tolerance m7, h7, h8
- Tool inspection: Manufacturing quality evidence
Product
- Custom PCD reamer · Combination drill-reamer · Drawing-based custom cutting tools. Discuss Your Tooling Requirements
Send Your Drawing
Send the part drawing, both measurement reports and the method details for an XRZ engineering review of the reaming step and the acceptance plan. Quick RFQ: Custom cutting tool RFQ.