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Superhorning - heavy hand with a light touch.

Honing has four basic goals: Generate specified surface roughnesses; create round, straight holes; finish holes to correct size; and leave finishes with good surface integrity. Production honing does this one better by repeating each of these thousands of times with little variation. Add the constant pressure to minimize cost per part and you quickly find that production honing compares in complexity and importance to other major machining processes. For example, certain production honing jobs require producing holes with roundness and straightness tolerances of 0.000 050" tir, generating surface finishes of less than 4 microinches (Ra), removing 0.040" or more stock from the diameter of carbide bores, or roughing our IDs in cast-iron workpieces without smearing graphite inclusions.

Honing isn't new. For more than half a century, holes in pumps, engines, bearings, bushings, and cutters have been finished by the process. The dominant tool used is the vetrified silicon-carbide or aluminum-oxide honing stone. It comes in a range of shapes, sizes, specs, and even colors. Over the years, it performed reliably in a variety of materials and situations.

Because honing is a finishing process, preceding manufacturing steps must allow for its limitations. So, whether a hole is cast or drilled, it usually is necessary to bore, grind, or ream it before honing. Such prehoning steps are necessary because conventional abrasive grains can take only small bites from a work surface.

A new method that meets all honing goals more effectively and efficiently is called superhoning. It takes full advantage of superabrasive diamond and cubic boron nitride (CBN). Stock removal is 2 to 10 times as fast as conventional honing, stone life is 40 to 100 times longer, surface integrity and finish is equal or superior, and performance is more consistent and reliable. Virtually any metal or ceramic can be successfully honed by one or both of the superabrasives.

Romancing the hone

Superhones are much narrower and shorter than conventional hones. The tool consists of a sintered powered-metal binder impregnated with widely spaced diamond or CBN particles, Figure 1. Superhones bonds are consistent performers. On the other hand, conventional bonds, especially vitreous ones, have inconsistent porosity that can introduce variation in product quality.

In a superhone, each crystal takes a big cut because all the force behind the tool is concentrated on just a few cutting points. Because these crystals are so hard and strong, they don't wear or break down as rapidly as conventional abrasives. The tough, abrasion-resistant metal bond as rapid as conventional abrasives. The tough, abrasion-resistant metal bond supports the crystal against tremendous cutting forces. At the same time, the bond allows chips to create their own escape path. This permits superhones to cut faster while lasting longer than conventional stones--a seeming contradiction.

The results are gains in productivity, exceptional part accuracy, and improved process reliability. These benefits often are accompanied by lower abrasive costs per part, and reduced scrap.

Superhones cut rather than plow through the work and chips are easily removed. This lowers heat generated by the tool and horsepower per cubic inch of stock removed.

In designing new or revising old production processes, a manufacturing engineer can eliminate machining steps, going directly from cast or drilled holes to rough and finish honing. This can greatly reduce equipment investment, as well as total production time.

Superhoning, with all its inherent advantages, requires following stringent guidelines, otherwise you're inviting trouble on the shop floor. Here's a brief rundown of what's important.

Design rules

Because virtually all US-made honing machines, and some older European machines, were designed for conventional abrasives, it is difficult (sometimes impossible) to adapt them to superhoning. Usually, each application must be evaluated to determine the feasibility of using superhones. The following general rules will help you decide whether or not your equipment is suitable.

* Peripheral speed should be at least 200 to 250 sfm. For soft materials (mild steel, aluminum, cast iron, etc), it should be greater than 250 sfm.

* Critical stroke speed varies by application, but often is between 200 and 300 ipm.

* It should be possible to reduce pressure by 50 percent compared to conventional honing without causing wide fluctuations in hydraulic control. Pressure should never be arbitrarily manipulated.

* All honing-head elements must fit properly to minimize vibration. Worn tooling isn't acceptable.

* For CBN, straight mineral seal or soluble oil is preferred. Water or water-soluble synthetics sometimes cause severe problems for CBN, although some synthetics work well. For diamond, virtually any kind of coolant works; however, a heavy flow to all parts of the tool surface is important.

* Feed should be automtically and precisely controlled, not hand operated.

A major problem in converting to superhoning is adapting the new cutter to conventional toolholders. To allow for adequate chip clearance and contact pressure between hone and part, superhones should be 1/2 to 1/4 the width of conventional hones they replaced. Sometimes special adapters are required to prevent rotational forces from flinging the smaller superhone out of the holder.

Operation rules

There are occasional conversion failures when superhones are treated as if they are conventional tools. To succeed, be aware of the following:

1. Superabrasives are extremely sensitive to pressure and speed. The honing results in Figure 2 show the typical decline in life as tools are forced to cut faster. Generally, there's a dramatic drop that approximates an inverse square function.

The most important parameters controlling cutting rate and hone life are contact pressure and surface speed. Experience suggests that increased productivity, without excessive tool wear, is gained by increasing spindle speed rather than contact pressure. Further, changes in cutting rate and hone life must be viewed together. Improved tool design will, if successful, generate a new curve above and to the right of the old one.

2. Bond material is almost as important as the abrasive media. Unfortunately, there's no standardized grading for bond hardness, nor even a consensus on what bond is ideal for a given application. Superhoning is in its infancy and, here are few fairly reliable rules to follow when selecting a bond.

Figure 3 shows several starting points. Work materials that allow easy crystal penetration require stronger bonds because greater penetration places more force on the crystal, thereby requiring greater support from the matrix. Brittle, hard, or strong work materials need soft bonds because they break down the abrasive faster, thus requiring faster matrix erosion to expose fresh crystals.

3. Abrasive concentration, always a controversial subject, also is one of the most overrated. Although it's true that the amount of cutting media is important, experience shows that too much abrasive can be s damaging as too little. The important consideration is that abrasive concentration be determined by performance requirements along with all other stone specifications. Work materials that produce long stringly chips usually require low abrasive concentration, and metal-bonded stones usually need much less abrasive than tools with resin bonds (probably because of the much longer cutting life obtained from each crystal in a metal bond).

4. When it's necessary to remove a lot of stock from an irregular bore, while producing a fine finish and good geometry, a two- or three-step process works well. Rough and/or semirough honing removes almost all the stock and creates a hole very close to finish size and shape. This roughed surface is critical for superhone finishing because it helps keep the finisher open and free-cutting. Cutting smooth metal with a finishing superhone often is difficult.

Expected surface finishes from superhoning most common materials (both hard and soft) are presented in Figure 4. The upper limit normally is exceeded only when the hone isn't fully broken in, pressure is too high, speed too low, or the bond too soft. To achieve finishes below the lower limit requires that the stock removal rate be left of the free cutting threshold shown in Figure 2. Of course, part hardness, bond erosion resistance, and a number of other factors determine final finish.

Superhoning brings important improvements to production honing. And it lends itself to emerging manufacturing trends. Flexible manufacturing and CAD/CAM, for example, benefit from predictable performance and longer tool life, and machining centers used for hole finishing can be more productive and generate higher quality parts. Moreover, because superhoning often can obviate ID grinding, reaming, and boring, capital investment and process time can be minimized.

The new superabrasives grades with improved bonds require completely rethinking workpiece finishing. For more information about superhoning, circle E22.
COPYRIGHT 1985 Nelson Publishing
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Taylor, John (English pop musician)
Publication:Tooling & Production
Date:May 1, 1985
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