The honing process provides the final sizing and creates the desired finish pattern on the interior of tubing or cylinder bores. Finishing is accomplished by expanding abrasive stones of suitable grit and grade against the work surface. The stones are rotated and reciprocated in the part with hone abrasive under controlled pressure. Combining rotation and reciprocation produces a cross-hatch pattern in the surface of the part being honed.
Many geometric errors or distortions, such as out-of-roundness, bellmouth, axial straightness, waviness, undersize, barrel, taper, boring marks, reamer chatter, rainbow and misalignment that result from grinding, heat treatment, forming, or other manufacturing processes, can be removed.
Honing can be divided into rough honing, finish honing, microhoning (or superfinishing) and free form honing (or abrasive flow machining). Free form honing is sometimes considered a superfinishing process. Comparison of the grit ranges and surface finish capabilities of the processes can provide some insight in the applicability of the processes for various applications.
HOW HONING WORKS?
The abrasive action of the honing tool removes material from the workpiece’s inside diameter. The tool rotates and expands while the workpiece reciprocates (stroking) back and forth. For example, tolerances of 0.003mm (0.0001´´) round and straight can be achieved in production using special fixturing. To achieve such close tolerances, the workpiece must be allowed to “float” or move in three axes. This movement is the single most important point in achieving the closer tolerances required in industry today.
To correct out-of-squareness, the workpiece must be positioned against a flat face plate that is perpendicular to the machine spindle axis. This, of course, reduces one of the three axes of movement, thus reducing the amount of movement or “float.”
Honing uses small, bonded abrasive stones or super abrasive sticks mounted in a fixture that rotates and reciprocates (strokes) when applied to the surface or bore being finished. On larger surfaces, the hone may reciprocate or oscillate while the workpiece rotates. An operator controls oscillation for manual stroke honing machines. A honing head mounted in a drill press is a basic version of a manual stroke honing machine.
Dedicated commercial manual stroke honing machines are large, rigid machines with more sophisticated features. Power stroke honing machines have a power-driven, automated stroke or reciprocation cycle. The power stroke honing machines can be programmed to provide consistent and reproducible surface finishes, geometries, and lay patterns.
Typically, honing stock removal consists of only a few thousandths of an inch, but larger material removal of up to 0.250 in. is possible with coarse-grit rough honing processes. When higher material removal rates are required, alternative grinding or machining methods can be more efficient to use before honing to reduce honing time and costs. However, this is not always the case. One gear manufacturer found honing to have higher material removal rates compared to ID grinding when the length to diameter ratio of the bore exceeded 2:1.
Honing machines are metal abrading tools and process utilizing hard tooling and perishable abrasives stones for the correction of
- Surface finish
- Positional tolerances of bores
The honing process was developed to allow for perfection of bore geometry, size control, final surface finish, and surface structuring. The honing process provides the final sizing and creates the desired finish pattern on the interior of tubing or cylinder bores. Finishing is accomplished by expanding abrasive stones of suitable grit and grade against the work surface. The stones are rotated and reciprocated in the part with hone abrasive under controlled pressure. Combining rotation and reciprocation produces a cross-hatch pattern in the surface of the part being honed.
Cost-effective machining method for..
- Removing stock
- Generating exacting
- Bore tolerances
- Bore polishing
- Finishing bores of almost any material such a: sprayed coatings, CGI, Ceramics, etc.
Honing Machines Perform Three Operations
- First, it is a stock removal process which takes out ruptured metal and reaches base metal.
- Second, a finish pattern is generated to provide the best possible surface for promoting optimum lubricating conditions.
- Third, honing assures extremely accurate straightness, roundness and size of the cylindrical surface.
The accuracy and surface finish of the hole mainly depends on various process parameter which are described below.
1. RPM of tool:
If the tool speed increases, it increases the metal removal rate and decreases surface roughness.
2. Honing time:
If we increase honing time it will decrease surface roughness at some extent and then starts increases it. It is so selected which gives minimum surface roughness with maximum metal removal rate.
3. Honing stick pressure:
The change in metal removal rate and surface roughness with the change in unit pressure is shown in the graph. The honing stick pressure is so selected as to get minimum roughness with the highest metal removal rate.
4. Stroke Length:
The stroke length should be as enough which can cover the whole working length.
Honing can be classified into two types.
1. Manual Honing:
In this type of honing the hone rotate continuously and the workpiece is moved front and back manually. This process is rarely used now days
2. Machine Honing:
The honing process in which the hone performs combined rotary and reciprocating motion and there is less manual interaction called machine honing. According to the type of machine this process can be classified into two types
A. Horizontal spindle machines.
Horizontal-spindle honing machines, for hand-held work with bores up to 6 in., are among the most widely used. The machine rotates the hone at from 100 to 250 fpm.
The machine operator moves the work back and forth (strokes it) over the rotating hone. The operator must ”float” the work — that is, not press it against the hone or the hole will be slightly oval. Sometimes the workpiece must be rotated.
Horizontal-spindle honing machines are also made with ”power stroking.” In these, the work is held in a self-aligning fixture and the speed and length of the stroke are regulated by controls on the machine.
As a hone is being used, it is expanded by hydraulic or mechanical means until the desired hole diameter is achieved. Various mechanical and electrical devices can be attached to the honing machine to control the rate of expansion, and stop it when final size is reached.
On the simplest hand-held machines, the operator may check the bore size with an air gage, continue honing, and recheck and so on until the size is correct.
B. Vertical spindle machines
Vertical-spindle honing machines are used especially for larger, heavier work. These all have power stroking at speeds from 20 to 120 fpm. The length of the stroke is also machine controlled by stops set up by the operator.
Vertical honing machines are also made with multiple spindles so that several holes may be machined at once, as in automobile cylinders.
Hone body: The hone body is made in several styles using a single stone for small holes, and two to eight stones as sizes get larger. The stones come in a wide variety of sizes and shapes. Frequently, there are hardened metal guides between the stones to help start the hone cutting in a straight line.
Cutting fluid: A fluid must be used with honing. This has several purposes: to clean the small chips from the stones and the workpiece, to cool the work and the hone, and to lubricate the cutting action.
HONES AND HONING TOOLS
Hones and honing tools are abrasive products used to refine the finish of internal bores and external surfaces. There are three basic types: flexible hones, hand hones and honing sticks or tools.
1. Flexible hones have abrasive beads bonded to the ends of bristles. Like other hones and honing tools, they are used to smooth or refine the finish of internal or external surfaces.
2. Hand hones, or diamond hones, are designed for the offhand honing of ground cutting-tool edges.
3. Honing sticks, or honing tools, are used for finishing internal bores. Internal grinding uses smaller-diameter wheels or abrasive products for grinding or finishing the surface of a part’s inner diameter (ID).
Product specifications for hones and honing tools include width, rotary speed, and shank or bore diameter. Hone width is measured in English units such as inches (in), feet (ft), and yards (yd); or in metric units such as millimeters (mm), centimeters (cm), and meters (m). Typically, the rotary speed is expressed as a maximum value. The bore is used to mount or hold the abrasive product on a spindle or mandrel. Bore diameter is the inner diameter of the center mount. Shank diameter is the diameter of the integral shank, pin, shaft, or mandrel on mounted points or wheels. Parameters for hones and honing tools also include measurements such as outer diameter (OD).
Hones and honing tools are available in various mounting styles. Some products are suitable for hand-held use. Others have a central hole or bore for mandrel, arbor, spindle, or shaft mounting. Hones and honing tools that are mounted onto an integral mandrel, pin, arbor or shaft are also available.
Abrasive grain type and grit size are important considerations when selecting hones and honing tools. Choices for abrasive grain include:
- aluminum oxide
- boron carbide
- emery or crocus
- Silicon carbide
- cubic boron nitride (CBM)
- synthetic diamond
- tungsten carbide
Grit size applies to abrasive products with abrasive grains held in a matrix or bonded to a surface. Examples include coated abrasives, dressing sticks, super abrasives, vitrified grinding wheels, and honing stones or grit dressers.
ADVANTAGES AND LIMITATIONS
Honing has developed into a productive manufacturing process, with particular advantages and disadvantages:
The workpiece need not be rotated by power, there are no chucks, faceplates, or rotating tables needed, so there are no chucking or locating errors. The hone is driven from a central shaft, so bending of the shaft cannot cause tapered holes as it does when boring. The result is a truly round hole, with no taper or high or low spots, provided that the previous operations left enough stock so that the hone can clean up all the irregularities.
Honing uses a large contact area at a slow speed compared with grinding or fine boring, which uses a small contact area at high speed. Because of the combined rotating and reciprocating motion used, a crosshatched pattern is created which is excellent for holding lubrication. Diameters with 0.001 to 0.0001 inch and closer accuracies can be honed by using diamond stones similar to diamond wheels.
1. Less complex or low-cost fixtures.
2. It is highly accurate.
3. It can be used for both long and short bores.
4. It maintains the original bore centerline.
5. Any material can be finished regardless of its hardness.
Honing is though of as a slow process. However, new machines and stones have shortened hone times considerably. Horizontal honing may create oval holes unless the work is rotated or supported. If the workpiece is thin, even hand pressure may cause a slightly oval hole.
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