The stone pressure is chosen relatively low between 10 and 120 N/cm2 and is usually created by compressed air. This way, the material removal is carried out gently without an alteration of the surface structure by contact forces and temperature.
Sinusoidal machining traces result from the rotation of the workpiece and the oscillating motion of the honing stone, which cross at the change of direction during the stroke. Figure 5.4b shows the development of a path curve of a cutting grain relative to the workpiece.
Investigations have shown that a higher productivity as well as better results with regard to roundness and surface quality can be attained by the application of higher circumferential speeds of the workpiece, which are 10–600 m/min.
Ream Honing
The machining of ceramic parts is, in addition to conventional honing, also possible with ream honing. During ream honing, a honing tool adjusted to its final dimensions is used. Therefore, a feed system of honing stones is not needed. The cutting process is usually performed with one to a maximum of three double strokes. Although the stroke speed is lower compared to conventional honing by a factor of 5–10 (vh = 2 – 6 m/min), the values for the circumferential speed are in the range of conventional honing (vu = 30 – 50 m/min) [5] and [6]. The higher form stability of the firmly adjusted tool is the advantage of ream honing. Hence, compared to conventional long-stroke honing, it is now possible to achieve a higher dimensional and form accuracy. Small drillings up to a diameter of 1.5 mm, as well as interrupted drillings and step drillings can be machined with high accuracy. An adjustment of honing stones for the purpose of wear compensation is also possible; it is carried out manually or automatically by a feedback regulation. In order to prevent an overstress of the honing tools ( Figure 5.5), only a few micrometers of material can be removed with one tool during ream honing. Consequently, it is necessary to design ream honing as a multistage process, or to have a conventional honing process precede the ream honing in order to achieve the required material removal.
Figure 5.5.
Tools for ream honing for different application [7]
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5.2. Honing and superfinishing tools
Interior honing tools fill the bore and lean equally over the honing stones at the circumference of the bore (Figure 5.6). The honing stones are fed by a double cone, which converts the axial feed motion of the pressure bar into a radial motion of the honing stone support. These honing stone supports, onto which the honing stones are soldered, adhered, or clamped, are situated in the radial slots of the tool base. Multiple-stone honing tools are available in series for the diameter range of 5–1000 mm.
Figure 5.6.
Structure of a long-stroke internal circular honing tool [2]
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The potential machining diameter of a honing tool is determined by the cone angle as well as the height of honing stone support and the honing stone itself and is only a few millimeters. A conditioned honing tool has a limited operating range, because the outer radius of honing stones is coordinated with the radius of the honed bore. Figure 5.7 shows the schematic description of the feed system and the taking-up of the honing stone onto the oscillating device during short-stroke honing.
Figure 5.7.
Tool and feed system for short-stroke honing [4]
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Structure of Honing Stones
Diamond and CBN-honing stones are used for the honing of ceramic materials. The characterization of honing stones with regard to grain and bond specification is done in the same way as is the characterization of diamond and CBN grinding wheels. The grain sizes of diamond grains are not designated in mesh (in the number of meshes per inch screen length). The grain sizes can be more accurately understood by correlating them to their respective mesh sizes and grit size scatter in diameter. For the designation of the quantity of diamond grains in the honing stone, the diamond quantity is given in a percentage in relation to 4.4 Carat/cm3. Hence, a honing stone of 100% concentration contains 4.4 Carat/cm3, respectively 0.88 g/cm3 diamond. In addition to bronze-bonded honing stones, ceramic and epoxy resin-bonded honing stones are also applied.
Dimensions of Honing Stones
An improvement in form accuracy of the workpiece surface is usually another objective of honing. Hereby, the length and width of honing stones used play a significant role. Whereas long honing stones reduce the cylindricity deviation, wide honing stones can diminish the roundness deviation [8]. Because the chip transportation is more difficult with an increasing width of the honing stones, additional scavenging grooves are also created in the lining during the application of wide honing stones. At a tool overflow of one-third of the length of honing stones, the suitable length of honing stones should be theoretically four-thirds of the height of the honed bore. Under this condition, the total length of honing stones engages during one stroke. Practically however, the length of honing stones is often in a range of one-half to one-fifth of the workpiece height.
Conditioning of Honing Stones
Diamond honing tools are usually profiled by grinding with bakelite-bonded silicon carbide grinding wheels. The following grinding parameters have proved to be favorable [9]:
•
Cutting speed vc: 1 – 3 m/s,
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Grinding wheel: hardness J/K
SiC 400/280 for grain sizes up to D64
SiC 240/100 for grain sizes D76–D126
SiC 80/40 for grain sizes D151–D256
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Wheel dimensions: 500 × 50 mm
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Rotational speed of honing tool vw: ca. 20 min−1
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Feed speed vf: ca. 720 mm/min
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Cooling lubrication: Emulsion, dropwise
After the grinding process the grains and the bond are in one plane, so that grinding of honing stones leads to a minor grain overflow. A sharpening of honing stones is generally necessary prior to application. The honing stone bond is hereby moved back by manual lapping or by using SiC-sharpening stones.
Conditions for Cooling Lubrication
Cooling lubrication in honing has a significant influence on the work result. The cooling lubricant must be matched with the material to be honed and the honing stone specification. The task of the cooling lubricant is, on the one hand, to clean the workpiece and tool by rinsing off chips and broken-out diamond grains. On the other hand, the lubrication prevents the heat development between the two moving surfaces. Honing oils, water-mixed cooling lubricants, or emulsions can be used as cooling lubricants for ceramic machining. However, the effects of additives used during honing of metals cannot be applied to ceramic machining, especially in the case of additives such as chlorine, phosphorus, and sulfur [10] that are activated by a chemical reaction with the metallic material.
The cleaning of the cooling lubricant is significant for the achievement of constant work results. By means of a filter device, it must be controlled such that no abrasive ceramic chips and broken-out diamond grains are flushed back onto the workpiece surface and cause scratches. At the same time, the cooling lubricant circulation components themselves must be protected against abrasive chips by filters [11]. A temperature regulation of the cooling lubricant is imperative because larger quantities of heat are set free during honing of ceramics, particularly during realization of high material removal rates. This way, the viscosity of the cooling lubricant can be kept constant and the work result of the honing process can be reached independent of the machining time.
5.3. Honing and superfinishing machines
The great variety of honed units and parts on the market has led to various construction forms of honing machines. Measuring and controlling tasks in various honing machines, too, became much more complex under the aspects of automation and process security during production of highly accurate bores. Thus, feed of honing stones, measuring and verifying systems, stroking and rotating drives, positioning axes, as well as the tool changer and the tool magazines can be integrated in numerically controlled machines (NC/CNC).
The diverse construction forms can be divided according to the machined workpiece dimensions such as length and diameter of bore, or to the number and position of the main spindles. Machines with a vertical hone spindle are favored in most cases because they are free from influence by gravity of the tool and driving rod, and flexible honing tools and cardanic workpiece uptaking devices can be used [3]. Moreover, the feed of honing oil and chip removal are facilitated. Machines with a horizontal spindle are also used, particularly for extremely small and extremely large workpieces.
Horizontal manual and lifting beam honing machines (Figure 5.8a) are considered to be simple honing machines and are especially applied to the production of small or medium series in workshops [3]. They are characterized by their simple structure and broad application possibilities. A pedal or feed nut is used for the manual feed of the tools.
Figure 5.8.
Long-stroke honing machines [12]: (a) lifting beam honing machine, (b) production honing machine (small-batch), (c) production honing machine (large series), (d) vertical pipe honing machine, (e) horizontal long-pipe honing machine
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Vertical production honing machines in Figure 5.8b and c are offered in C-structure or portal construction, and are generally considered to be typical construction forms of long-stroke honing machines. The degrees of freedom between workpiece and tool, together with the forces active during honing, determine the selection of the simpler and better available C-structure, or of the more stiff portal construction. In the case of the C-structure, the spindle is taken up in a protruding upright, whereas in portal constructions, it is taken up in a case that i