The Hole Story

Making holes is probably the most common job in the workshop today but it is how to produce them to high degrees of accuracy and surface finish. For many companies the answer is simple - HONING

The honing process is quite ancient in its origins, but the techniques and abrasive elements have developed so substantially that today honing is a fast stock removal process comparable with, and in many cases, more cost effective than internal grinding and capable of producing components of great accuracy. Honing has proved to be highly economical in countless applications in virtually all branches of engineering, being suited to all volumes of production from one offs to thousands off and to most metals and many non metallic materials used in engineering today.

Honing is often compared with internal grinding, as the objectives of both processes are to achieve dimensional accuracy and specified surface finish, although the two processes differ in many respects. Grinding usually involves an abrasive wheel moving at relatively high speeds and imparting a high impact shock to the Workpiece, normally along a line contact.

So what is honing?

It is a process whereby a cutting medium, carried by the honing tool is expanded, rotated and reciprocated against the work surface at relatively low speeds and contact pressures compared to grinding, and is accomplished by one or more honing stones traversing with a helical motion within the Workpiece, creating an abrading shearing action against the work surface.

The process is concerned essentially with the finishing of bores after drilling, reaming etc., although it is not necessary for all bores to be machined prior to honing; many tungsten carbide and ceramic components are honed straight from the sintered condition and steel tubes are often honed in the condition as received from the rolling mill.

A requirement of the process is for either the honing tool or the workpiece to be fully floating thus axial alignment or concentricity cannot be generated by honing although these conditions will be maintained in a bore that has been correctly positioned by the previous machining operation. For this reason, honing is often done in the early stages of component manufacture to produce a datum location for subsequent machining operations.

The surface speed of the abrasives in honing being 15-76 metre/min is much slower than grinding, which may be from 762-2900 metres/min. This means that unlike a grinding machine, a honing machine requires no heavy duty castings and specialised bearing systems to locate the work spindle and this is reflected in the price of the basic machine.

Because the honing stones remain in contact with the work surface at all times there is no high impact shock as in grinding, and the large contact area of the honing stone allows any heat generated by the honing process to be dissipated rapidly.

All the parameters of the honing process - rotation and reciprocation speeds, stone pressure, honing fluid, abrasive type, grit size and bond hardness, are all variable which enables the machine setter to achieve optimum performance with economy in any given situation. Out of round bores, tapered bores, bores with key ways and ports present no great problem for the honing process and bowed bores, tandem bores and bores of dissimilar materials can be successfully honed. Honing prior to heat treatment will lessen the degree of distortion caused by the heat treatment process.

Bores to be honed, may vary from 1.0 - 1000mm in diameter and up to 10 metres or more in length and several different types of honing machines are manufactured to accommodate this range.

The honing stones are manufactured from aluminium oxide or silicon carbide abrasive grits which is mixed with a bonding agent and then baked to form an abrasive block from which the required size of honing stone is cut. Changing the grit size and bond hardness of the abrasive block produces the wide range of stones used in the honing process.

Continued developments of super abrasives have led to a marked increase in the use of diamond and cubic boron nitrate (CBN) as a cutting medium. These fast cutting, hard wearing stones are use in conjunction with automatic honing machines incorporating in-process size control to allow accuracy and high-speed production with a minimum of operator involvement.

Most materials from copper to carbide, graphite to glass, powdered metals to polycarbonates, not to mention steels and non ferrous materials can be honed and the relatively low cost of the honing machine and its associated tooling makes the honing process an attractive alternative to the more conventional methods of bore finishing.