Precision boring as a subcontracted operation: understanding H6 and H7 tolerances to select the right supplier

Outsourced precision boring has a direct bearing on the functional quality of an assembly: a deviation of just a few microns on an internal diameter can make a guidance fit impossible or cause excessive play in service. Understanding H6 and H7 tolerances, identifying the processes capable of holding them, and selecting the right subcontractor are three inseparable skills for any methods engineer or purchasing manager in industrial machining.

What is precision boring and why outsource it?

Boring is the machining operation used to produce or finish a cylindrical internal bore to an exact size. The goal is to achieve a tight dimensional tolerance, good circularity and a controlled Ra surface finish simultaneously. These three parameters determine the nature of the fit: sliding, running, interference or shrink.

Outsourcing becomes necessary when the customer's own workshop lacks suitable machines or metrology equipment, when production volumes do not justify investment in dedicated tooling, or when the materials involved require specific expertise. A specialist precision machining supplier amortises its equipment across a wider range of part numbers, which reduces the unit cost while maintaining process capability.

The ISO tolerance system: understanding IT grades and H6/H7 fits

ISO 286 defines a tolerance system based on two parameters: the position of the tolerance zone (position letter) and the width of that zone (IT grade). For bores, the letter H means the lower deviation is zero — the minimum size corresponds exactly to the nominal diameter. The entire tolerance is therefore positive, which is characteristic of hole-basis fits.

Tolerance zone width in microns

The IT (International Tolerance) grade determines the zone width. For a 50 mm diameter:

• IT6: width of 16 µm — an H6 bore on ⌀50 mm has a zone of +0/+16 µm;
• IT7: width of 25 µm — an H7 bore on ⌀50 mm has a zone of +0/+25 µm.

The difference between the two grades seems small in absolute terms, but it is decisive in terms of process and capability. An H6 bore requires machine repeatability and thermal stability that only high-specification equipment and a fully controlled process can deliver. The target Cpk is generally above 1.33, meaning the process spread must remain within 16 µm with sufficient margin for drift.

Typical use cases

H7 covers the majority of common sliding fits: ball bearings mounted in H7/k6, guide bushings, pneumatic cylinders. H6 is required for precision guidance applications (spindles, grinding machine slideways), interference press fits, and high-speed angular contact bearing seats.

Boring processes used in the workshop: reamers, single-point boring, and machining centre boring

Two suppliers may both claim to hold H7, yet use very different processes that produce notable differences in repeatability, cost and lead time.

Machine reaming

A machine reamer is a multi-flute tool that removes material evenly along the full length of the bore. It delivers good repeatability in production runs, Ra values below 1.6 µm and stable cylindrical geometry. Its limitation is that the diameter is fixed, which requires holding a stock of reamers and complicates non-standard sizes.

Adjustable boring bar

An adjustable single-point boring bar makes it possible to hit an exact size regardless of diameter, including non-standard dimensions. Mounted on a 5-axis machining centre or a lathe, it allows fine boring with precise part location. Flexibility is maximised, but resetting for each change of size requires significant setup time and operator expertise.

Turn-boring on a CNC lathe

A CNC lathe allows external turning and internal boring to be combined in a single setup, eliminating repositioning errors. For rotational parts, this is often the most economical and accurate process. Capability depends largely on the rigidity of the boring tool and the overhang: a length-to-diameter ratio above 4 can degrade the cylindricity achieved.

Which materials and geometries make it harder to achieve H6–H7 tolerances?

Standard engineering steel (42CrMo4, C45) behaves predictably. Aluminium alloys, widely used in aerospace and automotive applications, have a coefficient of thermal expansion two to three times greater than steel: a 10 °C temperature variation on a ⌀50 mm bore produces a dimensional shift of around 12 µm — almost the entire IT6 zone. The supplier must work in a temperature-controlled room or compensate for thermal drift.

Engineering plastics (PEEK, Delrin), grey cast iron and austenitic stainless steels each present specific difficulties: springback, sensitivity to heat generation, inconsistent hardness. Complex geometries — deep blind bores, eccentric bores, internal tapered forms — require dedicated tooling and careful programming on a machining centre.

Metrology and dimensional inspection: how to verify that an H6 tolerance is being held

Inspecting a precision bore goes beyond a single-point measurement. Three levels of verification coexist on the shop floor and in the metrology room.

A plain plug gauge (go/no-go) is the fastest method: it confirms that the bore falls within the tolerance zone without giving a precise value. It is suited to 100% inspection in series production.

A bore gauge (dial bore gauge) measures the internal diameter by comparison with a reference ring. It gives a numerical value and enables statistical process monitoring (control charts, Cpk calculation). This is the standard tool for in-process surveillance.

A coordinate measuring machine (CMM) is essential for complex parts: it measures circularity, cylindricity, coaxiality or perpendicularity of the bore relative to other functional features. It is indispensable in aerospace and medical specifications.

A reliable supplier provides an inspection report showing measured values and their associated measurement uncertainty — not just a pass/fail validation.

Technical criteria for assessing a supplier's precision boring capability

Before releasing a batch of parts, several points are worth verifying.

• Declared vs demonstrated machine capability: request Cp and Cpk indices based on similar reference parts, not just commercial data sheets.
• In-house metrology equipment: the presence of a CMM and a temperature-controlled room is a prerequisite for reliable H6 inspection.
• Calibration traceability: instruments must be traceable to national standards with current calibration certificates.
• First article management: a First Article Inspection (FAI) procedure validates the process before series production is launched.
• Material experience: a supplier who works with aerospace aluminium every day is not necessarily the best fit for medical-grade stainless steel, and vice versa.

Industrial clusters such as Lyon, Saint-Étienne and Annecy are home to subcontractors equipped with recent precision machining technology; other areas such as Oyonnax and Cluses have a strong tradition in fine mechanics inherited from screw machining and plastics processing. Geographic location affects logistics lead times and sometimes the quality culture of the local supply base.

Preparing the technical specification: essential information to pass on to the subcontractor

An incomplete specification is the primary source of non-conformances and costly back-and-forth. The minimum information to supply is as follows:

Geometric and dimensional data

• Nominal diameter and full tolerance (ISO class or explicit values in µm);
• Bore length, blind or through;
• Associated geometric tolerances: circularity, cylindricity, coaxiality, perpendicularity;
• Target surface finish: Ra and, where applicable, Rz.

Material and treatment data

• Exact material designation and standardised grade;
• Condition of the blank on delivery (raw, pre-machined, hardened);
• Heat or surface treatments planned after boring (which may alter the finished size).

Functional data

• Nature of the intended fit and mating part (shaft, bearing, bushing…);
• Required functional clearance or interference;
• Assembly conditions (hand fitting, press fitting, cold assembly).

This information allows the supplier to select the appropriate process — machine reamer, boring bar on a machining centre, or turn-boring — and to design the inspection plan accordingly. A complete specification reduces deviation requests and protects delivery schedules.