Bore honing and finishing: how the process works

Bore honing is the finishing process that brings the inside of a cylindrical bore to the required size, geometry and surface finish. It follows drilling, reaming or boring and removes only a few microns of material: enough to correct taper, out-of-roundness and waviness left by previous operations, without heating the part.

How honing works

A tool fitted with abrasive stones or a diamond coating enters the bore and works with two combined movements: rotation and axial stroke. Together they leave the typical cross-hatch pattern, which retains the oil film in cylinders and sliding bores. Coolant flushes away the removed material and cools the cutting zone. On Honitech machines the spindle works on the vertical axis: the part sits on the table, coolant drains by gravity and the bore is gauged in the machine with mechanical or air gauges.

Three ways to hone a bore

  • Multi-stroke expansion honing: the tool strokes many times while expanding to final size. The most flexible process, suited to high stock removal, long bores and plateau finishes. Expansion honing machines.
  • Single-stroke (single-pass) bore finishing: a series of fixed-diameter diamond tools, each slightly larger than the previous one, passes through the bore in one stroke. Short cycle times and highly repeatable size for high volumes. Single-stroke honing machines.
  • Combined honing: expansion roughing and single-stroke finishing on the same machine. Combined honing machines.

What is measured in a honed bore

  • Size tolerance: the final diameter, often within a few microns.
  • Geometry: roundness, cylindricity and straightness of the bore axis.
  • Roughness: chosen according to the function of the bore (sealing, sliding, fit).
  • Plateau: flattened peaks and oil-retaining valleys, required on engine cylinders and hydraulic cylinder tubes.

Choosing the process

Low volumes, long bores or high stock removal call for expansion honing. High volumes of hydraulic valves, spool bodies, injector nozzles or bushings benefit from single-stroke bore finishing. A combined machine avoids a second set-up when both steps are needed. Heavy parts can be honed on VPM moving-workpiece machines, parts that cannot be moved with the HOB portable honing machine.

Honitech machines use Siemens and Beckhoff automation components with in-house software, ball-screw or linear-motor spindles, in-process gauging and robot loading. We also build the expansion honing tools and single-stroke diamond tools. See also honing machines for hydraulic cylinders and the overview of vertical honing machines for bores, or send us your drawing and tolerances.