FEEDS AND SPEEDS

For cylindrical grinding: Most cylindrical grinding machines offer a rather wide range of work rotation speeds, table traverse speeds, feed rates, and, sometimes, different wheel speeds as well. The proper selection of these variables, in combination with the choice of the grinding wheel, has a great bearing on the results and the economy of the grinding operation. The conditions which may have to be considered in determining suitable machining data are multiple and may be assigned to one of the following groups: (a) Workpiece characteristics - material, hardness, general configuration and dimensions, rigidity, adaptability for holding, and, of course, the stock allowance to be removed in the grinding operation. (b) Requirements for the ground surface - geometric and dimensional accuracy, surface texture, surface integrity, etc. (c) Economic objectives - grinding performance, optimum cost, desired quality levels, as well as the priorities for any conflicting objectives. (d) Operational conditions - construction and capabilities of the grinding machine, the composition of the grinding wheel, grinding fluid, frequency and methods of truing and dressing, operator's skill, degree of automation, etc. The preceding listing of factors to which machining data will have to be adjusted, although incomplete, indicates why experience, often even experimentation, is so widely relied upon for the selection of machining data in cylindrical grinding. It is obvious that recommendations for machining data can, in most cases, represent an approximation only of the optimum, which may not be attainable without subsequent testing and adjustment. Nevertheless, such recommendations can prove useful in two major respects: 1. To indicate the initial values for setting up a specific grinding operation, even when subsequent refinements of the starting data appear warranted and will be carried out; and 2. To determine approximate values of operational performance, for the purpose of time and cost estimates, and also for manufacturing capacity assessment. To provide starting guidance in the selection of process data for cylindrical grinding the following information will be supplied: (a) A brief list of grinding wheel recommendations for various commonly used work materials (b)Average machining data for basic cylindrical grinding operations under common conditions, established for the grinding of widely used types of work materials (c)A concise review of frequently occurring grinding process variables, and their effects. Grinding Wheel Selection for Cylindrical Grinding The listing of grinding wheel specifications for commonly used materials (on page 80) serves the purpose of general information only. It may be used as a first approach in grinding wheel selection for cylindrical grinding operations, should more pertinent data, as from comparable past operations, not be available. Machining Data Recommendations The data listed in Table 1-7.1 are intended to be used as starting information when establishing process specifications for cylindrical traverse grinding operations. They are predicated on the use of grinding wheels of proper specifications, the operation being carried out on grinding machines of adequate rating and condition, with the required supply of grinding fluid provided. Furthermore, the listed values apply to workpieses with diameters not exceeding about 2 inches (50.8 mm), and are sufficiently rigid to withstand the grinding force without harmful deflection, well-balanced and properly mounted. In cases of deviations from the assumed conditions, modification of the basic data may be required, according to the expected effect of pertinent factors, a few examples of which will be discussed later. Grinding wheel speeds should be maintalned with-in a range of 5000 to 6500 feet per minute (1525 to 1980 meters per mm), unless special conditions prevall or equipment and wheels designed for substantially higher wheel speeds are used. Higher wheel speeds, such as employed in high speed grinding, are predicated on the use of special grinding wheels and on machine design, including reinforced wheel guards, adapted to such operations. The infeed rates listed in Table 1-7.1 refer to the penetration of the grinding wheel into the work material. In the case of cylindrical grinding the diameter of the workpiece will be reduced by twice the amount of wheel advance. It should be noted, ho;vever, that many makes of cylindrical grinding machines have cross-slide handwheels with graduations indicating double the amount of actual wheel advance. thereby expressing the amount of the expected work diameter reduction. In plunge grinding the applicable feed rates per revolution of the part are less than in traverse grinding. The moderating effect on the specific chip removal resulting from the gradual penetration of the wheel into the work material along a helical path, which is taking place during the traverse grinding, is not present in plunge grinding; here the operating face of the wheel is in simultaneous contact with the entire length of the ground section. On the other hand, the infeed during the plunge grinding operation is continuous, as compared with the intermittent feeds at the end of the table strokes in traverse grinding. Consequently, the total machining time might be less in plunge than in traverse grinding, notwithstandmg the reduced feed rates per part revolution. Typical infeed rates for plunge grinding are listed in the chart. quently occurring variations in process conditions, as well as the wheel performance changes which can be expected to result, are reviewed in the following. Changes caused in the action of the wheel will be indicated as trends only, pointing out the sense, but not the magnitudes. Several of the discussed variables have opposing effects and, when concurrently present, may tend to balance out. Other variables can have similarly directed effects which then should be expected to work additively. To avoid duplications, variations in the process and of the participating elements were considered to occur in a single sense, that is, increasing only. Of course, such changes from an assumed base level can occur in the opposite, decreasing sense also, in which case the expected results are the inverse of those indicated above. Being familiar with the directions in which process variables interact with grinding wheel performance, permits the deliberate modification of certain easily controllable variables in the interest of improving the overall operational conditions. Grinding wheel diameter. Increasing the diameter causes the grinding wheel to act harder, because the expanded contact area distributes the stock removal over a larger number of grains, thus reducing the specific force expended on each individual grain. Peripheral speed of the grinding wheel. Increasing the peripheral speed will also cause the grinding wheel to act harder, for reasons similar to those mentioned above; in this case the greater number of grains is brought into action consecutively. Typical Infeed Rates For Plunge Grinding Work Maten.a~ Roughing ~ infeed per Revolution of the Work, in. (mm) Steel, soft 0.0005 (0.013) 0.0002 (0.005) Plain carbon steel, hardened 0.0002 (0.005) 0.000050 (0.00125) Alloy and tool steel, hardened 0.0001(0.0025) 0.000025 (0.0006) Note: In this case too, the resultant reduction of the work diameter will be twice the amount of cross slide movement causing the wheel infeed Process Variables and their Effect on Grinding Wheel Performance Grinding wheel recommendations, unless prepared for specific cases, are predicated on conditions which may be considered as average. Deviations from such basic conditions will generally affect the manner in which grinding wheels actually act. Therefore, fretion to the simultaneous contact resulting from increased wheel diameter. NOTE: In cylindrical grinding, variations of the process conditions occur regularly when the wheel diameter decreases due to usage. To balance the resultant effect, the reduced-diameter wheel should be operated at a higher rpm, when available on the grinder. Two important rules must, however, be observed: (a) In no case must the wheel surface speed in fpm be raised above the peripheral speed of a new wheel when operated at the prescribed rpm. (b) When a new wheel is mounted on the spindle the lower rotational speed of the wheel head must be restored before starting the drive motor. Worhpieee diameter. When greater, it: 1 will cause the wheel to act harder resulting from larger area of contact 2. will reduce the development of heat in the part -assisted by improved heat distribution. Workpieee peripheral speed. An increase causes the wheel to act softer because the specific load on the individual grains is raised. NOTE: Varying the work speed is, perhaps, the most conv~nient and effective way to modify the operational conditions in cylindrical grinding. The implementation of work speed variations is facilitated by the infinitely variable workhead speeds, extending over a wide range, which are available on modern cylindrical grinding machines. Varying the peripheral speed of the workpiece has to be applied, however, within limits, which depend on (a) the potentially adverse effects of higher work speed on the finish, and (b) the development of chatter caused by excessive work speed. Table traverse speed. When increased it will speed up the operation, but can affect the obtained finish adversely and interfere with the required size control. Its effect on the apparent wheel hardness is to make the wheel act softer, for reasons similar to those indicated in the preceding paragraph. NOTE: (a) Applies to traverse grinding, and to some extent, to plunge grinding with a small oscillating movement which can cause the wheel to act softer. (b) Table speed is also a movement whose rate can be varied steplessly on most types of cylindrical grinding machines. Infeed. When its rate is increased, whether by applying larger increments per stroke in traverse grinding, or by increasing the speed of the continual cross-slide advance in plunge grinding, higher stock removal will result, thereby improving the effectiveness of the process. On the other hand, high infeed rates cause greater heat development in the grinding area and increase the pressure on the work, which may result in deflections and impede size and finish control. The effect of increased feed rate on the wheel is to make it act softer. NOTE: In cylindrical grinding, for reasons of productivity, the infeed rate is usually set as high as possible and still avoid interference with the required quality of the process. The specifications of the grinding wheel should be adapted to the selected feed rate.