Testing of honing results

Testing options and display of optimum honing parameters

Information on diagnostics

Honing is a fine machining/manufacturing process for almost all materials. It can be used, for example, on piston sliding surfaces for cylinders in combustion engines. The tool used is known as a honing stone, which rotates and moves along the longitudinal axis. The resulting surface can be recognised by the cross-hatch finish. But how do you measure the results of the honing process on a workpiece, and how do you assess the surface or shape? Are there any optimum honing parameters? This article outlines various testing and evaluation options for assessing the surface quality (surface roughness) and roundness deviation in the cylinder.

Assessment options

Roughness tester

A qualitative, reliable statement about the roughness of the surface can be made using a roughness tester. Through the distribution and definition of the peaks and valleys (topography) in the roughness record, an assessment can be carried out as to the development of the surface during running-in, along with how much oilretaining volume is provided and how high the contact ratio will be. Furthermore, the roughness record provides valuable documentation of the honing quality.

Microscope

The surface structures can also be assessed visually using microscope images. From 200x magnification, the honing angle, profile depth and the exposure of the silicon crystals can be assessed more easily. Unintended dirt deposits or peak folding formations become visible.

In addition to the roughness record and the measurement report, microscope images are a good method of visualising and documenting the quality of the honing process.

Bore measuring device with dial gauge

For a quick assessment of the cylindricity it is sufficient to determine the diameter at several points and thereby gain an idea of the resulting cylinder shape. Basic out-of-roundness in the cylinder can also be determined using the bore measuring device with dial gauge (Subito) using spot-based measurements. The number of measurement points determines the accuracy of the results regarding the shape.

The majority of the cylinder shapes shown below can be determined using the bore measuring device with dial gauge.

Assessment of the surfaces

Measured value		Description
Mean roughness	R_a	Arithmetic average of all profile values of the roughness profile
Individual roughness depth	R_z1	Sum of the height of the largest profile peak and the depth of the largest profile valley of the roughness profile within a sampling length (l_r)
Roughness depth	R_z	Arithmetic average of the individual roughness depths Rz1 of successive sampling lengths
	R_max	Largest individual roughness depth within the entire sampling length

Core roughness depth	R_k	Depth of the roughness core profile
Reduced peak height	R_pk	Average height of the peaks projecting out of the core area
Reduced profile depth	R_vk	Average depth of the valleys projecting out of the core area
Material ratio	M_r1	Smallest material ratio of the roughness core profile
	M_r2	Largest material ratio of the roughness core profile

Legend
01 Profile peak surface
02 Core area
03 Profile valley surface
04 “Peak area”
05 Material ratio curve (Abbott-Firestone curve)
06 “Valley surface”
07 Material ratio

Based on the parameters, in the series production of each engine type the best honing parameters are determined in order to achieve the optimum when it comes to friction, wear and the resulting oil consumption of the surfaces. During reconditioning, more simple and widely applicable machines often result in limits. Despite this, high-quality cylinder surfaces can be created if you pay attention to the important parameters. The table should provide assistance in this regard.

Measured value		Unit	Recommended values Sensing length: 4.8 mm / Sensing tip: 2 μm / 90°
			Petrol / diesel passenger car	Diesel utility vehicle
Arithmetic mean roughness	R_a	μm	0.15 to 0.40	0.30 to 0.50
Reduced peak height	R_pk	μm	0.10 to 0.40	0.20 to 0.60
Core roughness depth	R_k	μm	0.20 to 0.60	0.50 to 1.50
Reduced profile depth	R_vk	μm	0.50 to 1.00	0.50 to 1.50
Smallest material ratio	M_r1	%	4 to 12	4 to 10
Largest material ratio	M_r2	%	75 to 90	80 to 90
Honing angle	α	∠°	25 to 45	40 to 60

Assessment of shapes and geometries

Type of fault		Reason for fault	Remedy
Out-ofroundness	Stage 0: Perfect cylinder	Correct geometry
	Stage 1: Eccentricity	Due to stuck honing head	Check the freedom of movement of the honing head
	Stage 2: Oval cylinder	Caused by deformation and overheating	Reduce the cutting pressure – replace the honing stones if necessary
	Stage 3: Triangular out-ofroundness	Results from distortions from stage 2 and 4	For remedies, please see stage 2 and 4
	Stage 4: Square-shaped faults	Usually caused by distortions resulting from the tightening of the cylinder head bolts	Reduction in distortion by using a torque plate
Trumpet, cone and funnel shapes		Caused by the incorrect stroke position. The stone overrun is too large on the side with the larger diameter	Correct the stroke position – reduce the stone overrun / use shorter honing stones
Barrel shapes		Caused by honing using too little stone overrun/honing stones that are too short	Increase stone overrun/use longer honing stones
Ripples		Caused when extremely short honing stones are used for honing, or when trying to remove narrow points by dwelling on the area with the honing head	Longer honing stones, short strokes for the targeted processing of narrow points