The Honing Process
Among a number of different honing processes you will find both internal and external honing. Here we will be concentrating on the “Internal” or Bore Honing process.
Wikipedia: “Honing is an abrasive machining process that produces a precision surface on a metal work-piece by scrubbing an abrasive stone against it along a controlled path. Honing is primarily used to improve the geometric form of a surface, but may also improve the surface texture”.
Described simply honing is a different type of advanced grinding process. Grinding and honing differ in many respects however the overall requirement of both is processes is to produce very good dimensional accuracy(including ovality and taper) and a fine surface finish.
Whereas grinding usually involves an abrasive wheel running at high surface speeds along or around a component under relatively light pressure, Honing uses much slower surface speeds and much higher pressures.
Because of the high speeds of the grinding process you will often get impact shock to the surface often resulting in comparatively poor surface finish.
Honing is generally more suitable for achieving finer surface finishes and good dimensional accuracy especially with bores.
The main advantages of honing over grinding are:
- A finer surface finish
- A random pattern to the surface finish
- An improved roundness in the bore.
A finer surface finish is achieved because the honing process has a much smaller impact on the surface of the component over a longer period. Stock removal is slower (up to 30 or more times slower) and more controlled. Also the bore honing process does not suffer from the impact risks that are present with grinding, mainly as there must be constant contact between the honing stone and the bore of the cylinder throughout the process.
This constant contact is achieved by a tool that holds stones that traverse with a motion that is like a helical motion. That is, it moves up, or down the cylinder as it turns. The tool also presses outwards, spring loaded to tolerate eccentricities in the circular motion. The constant change in the direction of movement means that imperfections on the cutting surface of the tool are not transferred to the surface of the cylinder and so a fine finish is achieved.
A Random Pattern to the Surface Finish
Most honing stones are manufactured as a mixture of aluminium oxide or silicon carbide abrasive grits bonded together by different grades (and strengths) of resin. These are baked in an oven to produce an abrasive block. This block or Honing stone is fitted into a metal holder and assembled in turn into an expanding honing head.
As the grits are arranged randomly in the honing stone and the honing head rotates as it reciprocates up and down the bore a very even but semi random pattern is created. The finish can be almost as fine as you want it to be and has (at a microscopic level) what is often termed a cross hatch type pattern. This type of surface finish has been found to be ideal for holding minuscule amounts of lubricant evenly distributed all over the surface, making it perfect for use in hydraulic cylinder applications. It is also perfect for De-glazing.
Improved cylindrical finish / sphere??? And Taper in the Bore
The expandable honing head assembly ends up as a perfect averaging device. As the head is expanded into the bore and rotated up and down the bore, high spots receive more pressure (and therefore more stock removal) than the low areas. Slowly the high areas are reduced, even to the extent that oval bores will gradually become more round as the smaller part of the bore is honed away in preference to the larger one.
Taper is removed in a similar fashion as the honing head moves up and down the bore. The smaller diameters are opened out in preference to the larger diameters. You end up with a length of bore that is as close to round and parallel as can be achieved, with a beautiful surface finish as well.