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 is supported on both sides. The machine table is designed for both construction forms as either a fixed, round, or NC-table. Particularly for multiple spindle honing machines, manual or automatic loading devices and transfer systems serve the purpose of a flexible feeding, while industrial robots are also used more frequently.
Vertical pipe honing machines as in Figure 5.8d are used for lifting heights up to 4500 mm [3]. In machines designed in flask mold with spindle support, the workpiece looms into an engine pit underneath the honing machine. This underfloor construction keeps the overall height low. Slide times can be shortened by application of slide tables with two uptaking devices.
Heavy honing machines are also installed over an engine pit. The machines, which are often designed in portal construction, serve to machine large, bulky workpieces such as cylinder liners of diesel engines and press cylinders. The automation becomes particularly important for special honing machines, especially multiple spindle machines, where the safe feed of workpieces is significant in addition to the automatic functional process with measuring devices.
Horizontal production honing machines serve, for instance, to machine crankshaft bearing bores and cooling compressors. Smaller workpieces such as planet-toothed wheels are honed together at the same time [3]. The horizontal long-pipe honing machine in Figure 5.8e represents another construction form, which is built today for honing lengths up to 12,000 mm and machining diameters up to 1,000 mm. Long steel pipes, gun pipes, and chills are produced with these machines. In order to develop the machining independent of gravity, the workpiece moves in the opposite direction to the tool. Moreover, a purposeful feed of the cooling lubricant is necessary. High driving powers of approximately 40 kW each for stroking and rotary motion are characteristic for these machines.
With regard to short-stroke honing machines, the opportunities for application range from the machining of small bearings with diameters of 5–10 mm to high-finished and absolutely channel-free rollers of more than 1000 mm in diameter for the production of sheets, films, and qualitative bands.
Automatic short-stroke honing machines in series production are mostly equipped with automatic feed devices, but it is necessary to distinguish between the particularly economic continuous machining and plunge-cut machining. During continuous machining, workpieces are brought into a rotary and feed motion between tilted rollers rotating in the same direction, and transported under the oscillating honing stones [13]. Due to the gradation of grain size of the cutting material bound in the honing stones, significant material removal as well as a high surface quality can be reached. The use of powerful oscillating heads allows continuous speeds of up to 6 m/min; the applications include needle bearings, axes, axles, rollers, and piston pins. An automatic removal of honing stones during the process is required for workpieces with differently shaped elements on the circumference, for example piston rods of telescopic legs. Hereby, workpieces are scanned by sensors and the honing stones are only lowered onto those positions on the workpiece to be machined.
Tapered rollers and gear shafts are machined, for example, with plunge-cut machining, either centerless or between centers. Because the automation of the process is costly, plunge-cut technology is only possible for smaller piece numbers.
Band honing machines (Figure 5.9) are also short-stroke honing machines and are characterized by a number of side-by-side, simultaneously honable machining positions [3]. This honing is achieved by various flexible honing arms, each of which press a honing band that embraces the machining positions and can be tightened. Furthermore, a short stroking motion in the axial direction is overlapped by rotation of the workpieces. Gear, crank, and camshafts can be machined with band honing machines, which are often designed in portal construction.
Figure 5.9.
Band honing machine: (a) two-stage bandfinishing of shafts, (b) two-stage bandfinishing of crank shaft [14]
Figure options
Degrees of Freedom Between Tool and Workpiece
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.
Figure 5.10.
Degrees of freedom between tool and workpiece [15]
Figure options
Feed Systems of Honing Stones
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).
Figure 5.11.
Feed systems of honing stones for long-stroke honing [2]
Figure options
Hydraulic feed systems transform oil pressure into feed force by means of a piston. This feed force is transferred to the honing stones via a pressure bar, double cone, and honing stone support. The stone pressure ps is a parameter for the description of the feed force. It describes the surface pressure between honing stone and workpiece. Recommended values for diamond honing stones for metal machining areps = 1–8 N/mm2 [3].
At a force-activated hydraulic feed, the machining allowance cannot be adjusted directly. The selected honing parameters lead to a constant material removal referred to as the honing time. The machining allowance is regulated by the honing time. In order to produce narrow diameter tolerances, it is necessary to determine the workpiece diameter during the process. The honing process ends when the nominal diameter is reached. Special honing tools can be used in order to detect the actual measure of the bore. For these tools, the quantity of air that escapes from pneumatic jets on the tool circumference serves to define the actual bore diameter. During application of way-activated feed systems, honing stones are fed with defined feed steps in a range of 10–100 micrometers [5]. The feed usually progresses in a number of partial feed steps.
5.4. Honing technology
Honing-in Performance
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.
Figure 5.12.
Honing-in behavior of various honing stone grain sizes [16]