Tech Talk
What is the preparation process for the “Grinding Wheel”?
A grinding wheel is seldom ready for grinding once it has been mounted on the spindle of the machine. Of course, plated wheels are the exception. Careful preparation of the grinding wheel periphery has to be carried out in order to achieve the correct form, concentricity, and cutting action for the desired workpiece surface. The preparation involves two important steps – truing and dressing.
Truing a grinding wheel almost speaks for itself. It is the method by which a grinding wheel is mounted on a grinding spindle so that both the radial and axial run out is minimized or eliminated. Certain grinding wheels are never dressed (e.g., single layer, plated, superabrasive wheels). These wheels can be trued only by accurate mounting. Superabrasive wheels in both resin and vitrified bonds are also mounted carefully to minimize their run out. These wheels have a peripherally thin layer of very expensive abrasive. Minimizing the run out prior to truing and dressing is driven by the abrasive cost and not necessarily the time to perform a lengthy truing operation.
Vitrified bonded wheels, be they conventional or superabrasive, are generally trued and dressed in one operation. This nomenclature has caused much confusion in the industry, as the combined truing and dressing of vitrified wheels is generally referred to as “dressing”. The combined truing and dressing operation begins by mounting the grinding wheel within reasonable concentricity and then dressing the wheel periphery to assure exact concentricity, while at the same time imparting a level of peripheral sharpness. Dressing the grinding wheel can be carried out using a number of different methods, depending on the desired results from the grinding process. The combined truing and dressing operation not only conditions the wheel periphery, but also defines a form on the wheel periphery. The form may be a simple flat form for plain surface grinding. It may be a very complex form, requiring extremely tight profile tolerances, less than 0.005 mm (0.0002 in). Remember, dressing the form onto a vitrified grinding wheel trues and dresses the grinding wheel at the same time.
Truing and dressing become two distinctly different operations when using resin-bonded superabrasive wheels. One popular truing method is carried out using a brake dresser. A brake dresser is a rotary device which is limited to a slower peripheral running speed than the grinding wheel. An aluminum oxide or silicon carbide wheel, generally a 46 to 60 grit J to M hardness, is mounted on the brake dresser, brought in contact with the grinding wheel with a down-feed in the order of 0.02 to 0.04 mm (0.001 to 0.0015 in), and traversed at 0.5 to 1.5 m/min (20 to 60 in/min). It is important to engage the feed with the brake dresser turning. Otherwise, a flat can develop on the dressing wheel, inducing a vibration which will adversely affect the preparation of the wheel periphery. The brake truing operation results in the removal of material from both the brake dressing wheel and the resin-bonded grinding wheel. An allowance has to be made for the radial wear on both wheel, usually in the range of 10:1 to 20:1 (the majority of the wear taking place on the brake dressing wheel), depending on the size and grade of the wheel used on the brake dresser. This means that the final wheel diameter has to be determined by touching off the workpiece or in-process sensing. This poses a particular disadvantage in CNC or automatic operations, where knowledge of the exact wheel dimensions is an inherent part of the grinding cycle. In addition, truing may be carried out using a metal-bonded impregnated diamond nib to traverse across the wheel periphery, typically FEPA D126 and D301 (60/120 U.S. mesh). Wear on the diamond nib is generally minimal per dress cycle and provides an alternative for CNC and automatic operations. A single-point diamond must never be used to true a resin-bonded, superabrasive grinding wheel, as the diamond wears rapidly, generates an intense localized heat at the diamond tip, and causes thermal as well as mechanical destruction of the bonding layer.
Another and more efficient method of truing a resin-bonded wheel is to mount the resin-bonded wheel on a cylindrical grinding machine and use an aluminum oxide or silicon carbide grinding wheel to grind the resin-bonded wheel true. The aluminum oxide or silicon carbide grinding wheel should be run at 20 to 30 ms-1 (4000 to 6000 sfm) and the resin bonded grinding wheel at 0.5 to 2 ms-1 (100 to 400 sfm). The grain size of the dressing wheel should be between 15 to 30 percent larger than the grain of the wheel being trued. The grinding wheel may be very quickly trued by this method, but there is a disadvantage. This truing operation is not carried out on the grinding machine it is intended to be used on. Any error in mounting concentricity will therefore show up in the grinding wheel.
It is very important to recognize that these truing operations true the wheel only, causing the periphery of the wheel to be closed and not suitable for grinding. The wheel has to be subsequently dressed to prepare the periphery for grinding.
Dressing a resin-bonded wheel entails opening the bond to expose the grain by using an aluminum oxide stick to “stick” the wheel. The grain size of the stick should be FEPA D64 (230 to 270 U.S. Mesh) or finer and typically F to J hardness. Using hand pressure, the stick is pushed against the wheel periphery as it rotates at operating speed. “Sticking” is best performed wet, as it will keep the dust down but moreover it helps to develop a slurry in the nip between the stick and the wheel periphery, and enhances the erosion of the resin-bond from around the superabrasive grain. The question arises now: when is the “sticking” operation complete? Most publications say that when the stick begins to wear rapidly, under only slight hand pressure, the dressing process is complete. Unfortunately, it appears that in practice the stick wears rapidly all the time, and proper sticking of a large diameter wheel may require a number of sticks. The grinding wheel periphery will be very smooth to the touch after truing. After sufficient dressing it will feel rough and aggressive, an indication that successful dressing has taken place. “Sticking” is a very arbitrary operation and does not lend itself to CNC or automated grinding operations, particularly when the grinding wheel is wide and narrow sticks are being used. It is important to manually “stick” by first feeding the stick into the center of the wheel and moving slowly across to the left-hand edge, and then feeding into the center once again and moving across to the right. Needless to say, an operator does not particularly enjoy what is a rather dangerous task to perform for wheels 250 mm (10 in) and larger. Semiautomatic “sticking” devices have been manufactured, but they tend to fail in the grinding environment, being particularly prone to the jamming of the sticks in the mechanism.
Truing and dressing together can be more easily accomplished by machining a mild steel block on a surface grinder or using a mild steel roll on a cylindrical grinding machine with an infeed 0.01 to 0.02 mm (0.0004 to 0.0008 in). The swarf generated when machining mild steel is sufficiently aggressive to erode the resin matrix in front of the grain, as well as loosen and pull eccentric grains from the periphery and true the wheel. This is a particularly good method for truing and dressing, as there is clear evidence that the grinding wheel is ready for use once the chatter marks of eccentricity disappear. The very aggressive nature of the steel swarf, however, tends to damage the edge of the wheel by excessive chipping. Softer resin-bonds can also be damaged to a considerable depth by the penetration of the invasive mild steel swarf.
A novel, yet somewhat expensive, sysetm called a “Roll-2-Dresser” both trues and dresses a resin-bonded wheel more accurately than any of the previously mentioned systems. The Roll-2-Dresser consists of two rotating steel cylinders, which are driven in the same direction, but at slightly different speeds. The grinding wheel is brought into contact with the cylinders and moved axially across their periphery. The grinding wheel is fed into the rotating rollers with the objective of machining them to generate an aggressive swarf. The axial motion results in a very flat wheel periphery with exceptionally good edge retention.
