The cutting motion during long-stroke internal circular honing is created by a stroke and rotary motion of the tool (Figure 5.2).

The position of stroking reversal points, also described as honing stone overflow, has a significant influence on the cylindricity of the honed surface. In order to achieve cylindrical shapes of bore, the overrun length should be one third of the honing stone length on the upper and lower stroking reversal points during machining of both metallic and ceramic materials [3].

During short-stroke honing, a close-grained honing stone with an encircling angle of 60°–70° is pressed on the circulating workpiece, and thus set into oscillation parallel to the machining surface (Figure 5.4a). The stroke length of the honing stone is 1–6 mm at 1000 to 3000 oscillations/min, which are produced by compressed air or electromechanical means [4].

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 (v_h = 2 – 6 m/min), the values for the circumferential speed are in the range of conventional honing (v_u = 30 – 50 m/min) [5,6]. 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.

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.

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.

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]:

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 degrees of freedom between tool and workpiece are decisive for the opportunities of correction of form and position during the honing process; they are determined by the positioning of tool and workpiece. Potential forms of position are the double cardanic, cardanic, swimming, or solid position. A restriction in the degrees of freedom leads, starting with an improvement in roundness and cylindricity by the honing process, to an improvement in the right-angularity of a bore. Moreover, it results in an increase in the accuracy of position of a bore at further restriction (Figure 5.10). However, a high stiffness of the system is required.

The radial feed of honing stones against the workpiece serves on the one hand to adjust the machining allowance of the material and, on the other, to compensate the wear of honing stones. It is important to distinguish between force-activated hydraulic and way-activated mechanical feed systems for long-stroke honing (Figure 5.11).

During the cutting of ceramics, constant cutting conditions are often impossible to achieve. The honing process is characterized by a honing-in behavior of the honing stones. The initially sharp cutting grains blunt with increasing machining time, which consequently leads to a decreasing material removal rate. Depending on the machined material and the implemented honing stone specification, constant cutting conditions appear after a certain time; the honing stones operate in the self-sharpening range, or they lose their cutting ability until no further removal can be reached and the stones must be sharpened. Figure 5.12 shows the honing-in behavior during machining of SiSiC. Although close-grained honing stones D3 and D7 show a constant performance after a short honing-in time, the specific material removal rate at grain size D15 goes back to zero in the first 500 s. Yet, only a relatively low specific material removal rate can be reached with close-grained honing stones, which is not sufficient for economic machining.

As can be seen in Figure 5.13, the specific material removal rate related to the surface depends on the size of the diamond grain and of the stone pressure during machining of A1₂O₃ and ZrO₂. While diamond grain size D3 does not allow efficient material removal during the machining of the chosen Al₂O₃-material, all diamond grain sizes above D3 can be implemented. However, the application of diamond grain sizes exceeding D10 leads to significant heating of the tool and of the workpiece. In order to avoid damages to the tool and workpiece, the stone pressure is not increased above 2 N/mm². Here the specific material removal rate increases with increasing stone pressure because the normal force that presses the single diamond grain onto the material increases. The influence of the diamond grain size on the material removal rate, however, does not support such a general statement. At diamond grain sizes below D64, the specific material removal rate increases according to the size of the grain. Especially during the transition from the diamond grain size D10 to D35, a considerable increase occurs in the material volume removed per time unit. The transition from diamond grain size D64 to grain size D126 leads to a reversion of this tendency and thus to a decrease in the material removal rate. This decrease can be explained by a decrease in the number of active diamond grains, while the size of the diamond grains increases. The reduction of the number of grains is connected to an increase in the normal force at each diamond grain. If the increase in the normal force at each grain leads to an underproportional increase in the material volume removed from each grain, a decrease in the material removal rate results.

During short-stroke honing of the materials SiC, Si₃N₄, and TiN (Figure 5.16), an increase in the wear of honing stones becomes evident with increasing diamond concentration. This increase can be explained by a reduced strength of the honing layer bond, whose mass is diminished at increasing diamond concentrations, and by a bigger number of free, chipped-off grains that intensify the abrasive wear of the tool. Using low diamond concentrations, the roughness of workpiece surfaces can be reduced. For long-stroke honing, no examinations are known.

Figure 5.17 demonstrates the increase in material removal with increasing cutting speed. A longer cutting path is covered by the single grain at the same time with increasing cutting speed. It proves that the decrease in material removal for honing of metallic materials takes place above v_c = 60 m/min for the investigated honing stone and material combination, due to hydrodynamic effects that occur once a certain cutting speed has been exceeded [19].

Indentation hardness tests and scratch tests with different diamond geometries, as well as machining tests, revealed a clear relation of the working result to machining conditions and properties of the ceramic material. Particularly the grain size of the ceramic material had a significant influence during tests with A1₂O₃ and different diamond grain sizes.

Due to the higher normal forces on the individual cutting grain, Figure 5.20 demonstrates that intercrystalline grain chippings are predominant for honing A1₂O₃ with the bigger grain sizes D35 and D64. The low normal force active for diamond grain size D10 leads to intercrystalline grain chippings only at low alumina grain sizes. Bigger alumina grain sizes cannot be removed during the active lower stress; instead, a smoothing of the surface is caused here by microplastic deformations. Therefore, roughness values decrease under usage of diamond grain sizes D3 and D10 with increasing alumina oxides grain size, while at the same time, the specific material removal rate related to the surface diminishes. For diamond grain sizes D35, D64, and D126, microplastic deformations emerge as a result of the increasing normal force of the single grain, which leave traces along the direction of motion of single grains. Simultaneously, intercrystalline grain chippings also occur at bigger A1₂O₃ grain sizes. These chippings cause an increase in the roughness with the Al₂O₃ grain size because bigger, chipped-off grains leave bigger flaws.

It is evident that, for the economic honing of metals and ceramics, the demands for a high specific material removal rate as well as high surface quality are not met with a single honing stone specification. The realization of the honing process as a multistage process is necessary, because for ceramics, the decrease in stone pressure does not lead to a significant improvement in the surface quality. Although necessary material removal is achieved in the first stage, the required surface quality can be produced in the following stages. The finish machining of circulation pump bearings made of aluminum oxide is one example of such a multistage process (Figure 5.21).

Technical components with coplanar functional surfaces and high demands on evenness can be found in various technical applications, including control wheels, seal disks, and bearing rings. The components may vary regarding their geometry, material, or application. Manufacturing technologies that allow simultaneous processing of both functional surfaces, such as double face grinding, are most suitable for machining such workpieces.

The most commonly implemented process in the industrial field for double face machining of ceramic workpieces is double face grinding with planetary kinematics. This machining process represents a logical improvement of the lapping technology. For processing, the workpieces are placed in circular outward-teethed workpiece holders, so-called rotor discs. These discs are placed between two grinding wheels, held in place by a fixed outer pin ring and driven by a rotating inner pin ring. The circular motion of both tools combined with the rotation of the inner pin ring creates a relative velocity between tools and workpieces analogous to the kinematics of epicyclical gears. In comparison to other processes, for example longitudinal face grinding or plunge face grinding, double face grinding with planetary kinematics offers a lot of advantages. The process allows tension-free mounting as well as a constant stress over the entire functional surface of the workpieces. A schematic setup for double face grinding with planetary kinematics is depicted in Figure 5.23.

In addition to the increase in profitability achievable by raising the cutting speed v_c, which was described in the previous section, machining of ceramic workpieces using double face grinding with planetary kinematics can be further improved. Current and future research projects focus on the qualification and design of the process in high-speed applications as well as analysis of the adequacy of this technology when machining advanced high-performance materials. For example, the profile wear, which is dependent on the radius of the grinding wheel due to the special kinematics of the process, must be analyzed; and solutions must be found to counteract this wear phenomenon. Furthermore, the influences of the dynamic properties of the machine on the work result are also an important subject of research and must be analyzed comprehensively. Based on these analyses, the process of double face grinding with planetary kinematics can be optimized. With an improved machining process, the efficiency of machining ceramic workpieces can be increased.