What Is Honing?- Definition, Process, and Tools

What Is Honing?

Honing is an abrasive machining process that creates a precision surface on a metal workpiece by scrubbing an abrasive grinding stone or grinding wheel on it under a controlled path. Honing is principally used to improve the geometric shape of a surface but can also improve the surface finish.

Typical applications are finishing cylinders for internal combustion engines, air-bearing spindles, and gears. There are many varieties of hones, although all consist of one or more abrasive stones, which are held under pressure against the surface they are working on.

Other processes similar to honing are lapping and superfinishing.

Definition of Honing

Honing is specifically defined as sharpening some object on a hone or whetstone; the fact you hone the sharp edge does not impact the precise alignment of the cutting machine’s knife.

Honing takes the final size, and produces the finish pattern in the tubing or cylinder bores. Finishing is through expandable abrasive stones of the appropriate grit and grade against the work surface.

The stones are stopped and reciprocated in the part with hone abrasive, using regulated pressure. The combination of rotation and reciprocation creates a cross-hatch pattern on the surface of the machined part.

Why do We Need the Honing Process?

Honing machines are metal abrading tools, and honing with hard tooling or honing stones and perishable abrasive stones for the correction of

• Diameter
• Shape
• Surface finish
• Positional tolerances of bores

The honing process was perfected to provide the ability to improve bore geometry, size control, surface finish, and surface structuring. A honing process that can provide final sizing and create the desired finish pattern on the inner surfaces of tubing or cylinder bores.

Finishing is accomplished by expanding abrasive stones of appropriate grit and grade against the work surface. Stones are both rotated and reciprocated with hone abrasive applied at controlled pressures.

When combining rotation and reciprocation, a cross-hatch pattern is produced on the surface being honed.

Process Mechanics

Due to the similarity of honing stones to grinding wheels, it is tempting to consider honing a low-stock removal grinding operation. However, it is more appropriate to consider honing as a self-truing grinding process.

In grinding, the wheel follows a simple profile. For example, in plunge grinding a shaft, the wheel travels towards the axis of the part, adds stock by grinding it, then travels back.

This means that each slice of the grinding wheel repeatedly touches the same slice of the workpiece, and any inaccuracies in the geometric shape of the grinding wheel will be imparted to the part.

That is to say, the accuracy of the shape of the finished workpiece is bounded by the accuracy of the truing dresser. The accuracy gets worse as the grinding wheel wears, thus, truing needs to take place periodically to reshape the wheel.

In honing, it is an entirely different scenario with respect to the limit on geometric accuracy. Honing uses accurately profiled honing stones, which follow a complex path.

For example, in in-bore honing, the stones move both radially and axially at the same time: the stones are pressed radially outward to get a larger hole while simultaneously oscillating axially.

Because the stones oscillate, each slice of the honing stones are touching a rather large area of the work piece. Therefore, any imperfections in the honing stones profile cannot be transferred to the bore.

Instead, both the bore and the honing stones conform to the average shape of the honing stones’ motion and if you think of bore honing it is a cylinder.

The same averaging effect occurs in all honing operations; both the workpiece and stones are eroded until they are conformed to the average shape of the stones’ contour.

It is the pattern in which the honing stones are worn, because they tend to erode toward a desired geometric form, which would eliminate the need to true them.

As a factor of the averaging effect, the accuracy of a component made from honing is generally accurate beyond the accuracy of the machine tool that made it.

The one thing that differentiates grinding from honing machines is the orientation of the motion of the stone. The other thing that differentiates honing from grinding is the stiffness of the machine. Honing machines have a lot more compliance than a grinder. Grinding is performed to obtain a tight size tolerance.

To achieve this, the grinding wheel must be forced to a precise position relative to the workpiece. Consequently, a grinding machine must be very stiff and move with great precision as the axes of the machine move.

A honing machine is relatively imprecise, and there is no effort to make it exact. Instead of relying on the precision of the machine tool, the honing machine depends on the averaging effect generated by the stone and the workpiece. Compliance is a requisite with a honing machine, and it is required to create the averaging effect.

This brings up an obvious difference between the two is the stone of a grinder is rigidly attached to a slide, and the stone of a honing machine is actuated with pneumatic or hydraulic pressure.

Usually, components that are high precision are ground and honed. Grinding sets the size, honing refines the shape.

The difference between honing and grinding is always the same. Some grinders are complicated in their motions and some are self-truing.

There are also honing machines with in-process gaging to control size. Many through-feed grinding operations rely on the same averaging effect evident in honing.

Honing tools

Honing refers to a process that uses a specialty tool (honing stone) or a hone that develops a precision surface. The hone includes abrasive grains that are bonded by a matrix.

In general, honing grains are irregular in shape and have a diameter of about 10 to 50 micrometers (300 to 1500 mesh grit). Using the smaller grain sizes creates a smoother surface on the workpiece.

Honing tools are used to transmit power from the machine to the workpiece. Honing will correct for taper, size, finish, and straightness for bore, but does not usually correct axial alignment or location.

The honing tool has a spindle nose adaptor, a drive shaft, and a honed body. The hone body has a cone attached to a push rod. See the drawing of a typical hone tool on the following page.

The pushrod, which is located with the drive shaft, is moved up and down in the honed body by a motor or hydraulic cylinder from the actuator in the head. As the cone goes down, it pushes the expansion plates outward, thus expanding the stones.

The stones are used to remove stock material from the work piece. The garter springs are used to keep the stones in the tool and collapse the stones when the tool is withdrawn from the bore.

Hone tools can use super-abrasives or vitrified stones for rough honing, semi-finishing and final honing. Possible only the roughing operation is required for peak honing. Plateau honing (putting a plateau on top) requires fine stones to remove the peaks. Some hone tools have roughers and finishers built into one tool.

Hone Guides

The hone guides are an accessory of the honing tool. They are designed to protect the hone stones when entering or leaving the bore, and also keep the metal of the honing tool from contacting the bore surface.

The guides need to be routinely inspected for wear or pick up of debris. The honing stones are typically collapsed below the size of the guides when withdrawing to protect both the stones and the hone guide bushing.

Hone Guide Bushing

The hone guide bushing is attached to a fixed bracket on the front of the column. The purpose of the hone guide bushing. is to guide hone tool into the center of the bore. The bushing is usually made of hardened steel or carbide.

Carbide is used when the hone guide bushing is also a “collapse bushing”. These are used to force the stones back flush with the tool body when the tooling is too small to use garter springs.

Adjustable Cone

An adjustable cone can be utilized to modify the base of the stones for wear when finishing blind bores to ensure the base is at the proper size. Blind bore over travel illustrated above. Stone cannot over travel past the bottom as in through bores.

This will provide inconsistent wear across the stones and create a taper at the bottom of the part. The issue can be mitigated by the use of an adjustable cone and by using dwell at the bottom.

Single Pass Honing Tools

The honing tool utilizes a diamond abrasive boring bar which is made up of either a variable on demand manufactured soft cast iron sleeve coated permanently with either 100/120 of a natural diamond abrasive or a sintered sleeve that has the abrasive embedded into the body of the sleeve.

The tool is variable on demand to make compensation for sleeve wear with the diamond sleeve. The bar has passage ways that facilitate coolant to flow from the arbor to the grooves in the sleeve. The honing tool is inserted once and withdrawn to facilitate honing.

The self-aligning hone tool is a floating type hone tool, which allows for inspection of bore tolerances and geometric condition of the current bore. The self-aligning hone tool is only for open bores that were generated as a result of a previous machining operation.

A self-aligning hone tool will correct (to some degree) for bore run-out, tapered, and out-of-round conditions. The straight, round, untapered holes that the tool generates in a single downstroke are inconceivable.

The hone guide bracket and bushings are mounted to a fixed bracket that is mounted to the front of the column. The hone guide bracket is responsible for guiding the honing tool into the bore, and the bushing is machined to facilitate coolant to flow to the single-pass hone tool.

Advantages of the honing process

• Less complicated or lower-cost tools.
• Highly accurate.
• It can be used for long or short bores.
• It will maintain the centerline of the original bore.
• It has the ability to finish any material, regardless of hardness.
• The workpiece does not need an external power supply to rotate, so a power chuck faceplate or a fully rotating table is not required. There are no chucking or locating errors to contend with.
• Honing uses a central shaft to drive the hone, so the bending of the shaft will not introduce tapered holes as is done with boring.
• The result is a true round hole with no taper, or high or low spots, provided ample stock was left in previous operations, so the hone can clean all irregularities.
• Honing uses a large contact area at a low speed, whereas grinding or fine boring variables use a small contact area at high speed. A crosshatched pattern emerges using both the rotation and reciprocating occupations of the honing process. This crosshatched pattern is advantageous as it is ideal for holding lubrication. Diameters can be honed to +/- 0.001 to +/- 0.0001 inches, or even closer, depending on the type of stone, while utilizing diamond stones that are similar to diamond grinding wheels.

Limitations Of the Honing Process

• Honing is considered a slow procedure. New machines and stones have substantially reduced hone times.
• Horizontal honing may produce oval holes unless the work is rotated or supported. Even a handful of thin workpieces often results in a slightly oval hole.

Application of Honing Process

• Finishing automobile crankshaft journals
• Finishing round holes
• Finishing hollow cylindrical parts
• Finishing Engine cylinders, bearings, gun barrels, ring gauges, shafts and flange faces, piston pins, automobile crankshaft journals, etc.
• Gear Hobbling

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