Honing and plateau honing

Honing is the process that finishes a bore to size, form and texture at the same time. A tool with abrasive stones turns in the bore while it strokes up and down, and the stones are pressed outward as material comes off. The result is a bore with accurate size and form — KADIA quotes dimensional and geometrical tolerances below 1 µm for precision bores — and a crosshatch texture whose angle and depth can be set on purpose.

Kinematics and the crosshatch angle

Three motions run together: the tool rotates, it oscillates along the bore axis, and the stones are fed radially outward. KADIA, Sunnen and the academic literature describe the process the same way.

Every abrasive grain therefore moves along a helix. On the upstroke and on the downstroke the helices run in opposite directions, and their scratches cross. Unrolled, the wall shows two families of lines at an angle α/2 to the circumferential direction, with

$$\tan\frac{\alpha}{2} = \frac{v_a}{v_c}$$

where va is the axial (stroke) speed and vc the peripheral speed, vc = π · D · n / 1000 in m/min with D in mm and n in rpm. α is the included angle between the two families of scratches.

Bore diameter D (mm)Spindle speed n (rpm)Stroke (axial) speed va (m/min)

20

Target included angle

50

°

Peripheral speed vc

—

Crosshatch angle α (included)

—

Angle to the horizontal α/2

—

va needed for the target angle

—

α

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Drag to turnUse the view buttons to turn

Whole boreWall

Crosshatch angle

50°

A faster stroke relative to the rotation gives a steeper (larger) angle. Near the ends of the stroke, where the tool stops and reverses, the angle becomes slightly smaller, as Sunnen notes.

Typical values for engine bores cluster around 40 to 60°: MS Motorservice gives 25–45° for passenger cars and 40–60° for diesel utility vehicles, Sunnen calls 45–60° the average in practice, and the Zynp liner patent specifies 40–60°. We found no maker’s value specific to hydraulic cylinders.

What honing corrects — and what it does not

Honing corrects:

• Size and roundness, including waviness;
• Taper, barrel and bell mouth shapes;
• Straightness along the bore;
• the surface left by boring or reaming.

Its result depends on the stroke. Sunnen and Precision Surfacing both recommend that the stones overrun each end of the bore by about one third of the stone length. Too much overrun makes a bell mouth; too little leaves the ends tight or the bore barrel-shaped. For short bores, Sunnen recommends a stone length of about two thirds of the bore length.

• Too little overrun

  →ends stay tight, barrel

• About ⅓ of the stone length

  →straight bore

• Too much overrun

  →bell mouth

stone at the upper reversallower reversalresulting bore (exaggerated)o: overrun · L: stone length

Honing with a floating tool does not correct the position or the axis of the bore: the tool follows the existing bore. Sunnen notes that rigid multi-stone tools can correct hole location, which is the exception.

• Taper

  ✓corrected

• Barrel

  ✓corrected

• Bell mouth

  ✓corrected

• Straightness

  ✓corrected

• Out-of-round

  ✓corrected

• Waviness (lobing)

  ✓corrected

• Position and axis

  ✕not with a floating tool

• bore as found (exaggerated)

  target bore

  stock removed by honing

  Sunnen: rigid multi-stone tools can correct hole location.

Abrasives and bonds

Stones are bonded vitrified (ceramic), metal or resin; electroplated diamond and CBN are used for single-pass tools. Diamond and CBN grits are designated by grain size in micrometres, for example D107 or D25. Sunnen notes that a harder work material gives a finer finish with the same grit, and that higher stone pressure gives a rougher one.

Honing is done with plenty of fluid, mostly mineral or synthetic honing oil; water-based fluids are used less often. The fluid cools, flushes the chips and keeps the stones from loading up.

Plateau honing

A bore honed in one stage has a profile of sharp peaks and valleys, and the peaks wear off during running-in. Plateau honing removes them in the factory instead:

1. Base (rough) honing establishes size, form and the depth of the grooves.
2. Plateau honing with fine stones or brushes cuts the peaks down to flat plateaus and leaves the deep valleys. It takes only a few strokes; Yousfi and co-authors observed the plateau forming in the first ten.

The plateaus carry the piston rings like a run-in surface; the valleys retain oil. The aims named in the literature are fast running-in, low friction and low oil consumption. The surface is specified with the Rk parameters; see Rk, Rpk, Rvk.

Three independent sources give similar targets for engine bores:

¹ The patent’s own table gives 1.2–2.8 µm. Measuring conditions differ: MS Motorservice measures over 4.8 mm with a 2 µm, 90° stylus; the patent uses a 2.5 mm cut-off over 12.5 mm with a 5 µm stylus. The values are not interchangeable without the conditions.

MS Motorservice, passenger carZynp patentGruszka et al., “today”

Hydraulic cylinder tubes: honing or skiving and roller burnishing

Cylinder tubes for hydraulics are usually cold-drawn tubes of E355 (the former St 52) to EN 10305-1 (seamless) or EN 10305-2 (welded). Their bore is finished in one of two ways:

• bore tolerance H8
• Ra ≤ 0.3–0.4 µm
• abrasive: removes material, corrects form

• bore tolerance H8 to H11 (Tenaris)
• Ra ≤ 0.30 µm (Tenaris); 0.05–0.20 µm (Sunnen)
• cuts first, then smooths by plastic deformation

ECOROLL states that skiving and roller burnishing has almost completely replaced honing for cylinder tubes, and Sunnen says it can be up to 70 % faster than honing; both make tools and machines for the process. Tenaris gives a local straightness of 1 mm per metre, which Marcegaglia (1:1000) and other makers repeat.