Tech Talk
What is "Single Point Dressing"?
The most common method for dressing a vitrified grinding wheel is to use a mounted diamond point. The mounted point may have only a single diamond or have multi-pointed diamonds. The diamond can be mounted on the table of the grinding machine or be part of a sophisticated system called a pantograph or tracing attachment above the grinding wheelhead. With CNC control, these complex mechanical devices are unnecessary, as the tracing of the diamond path is easily and more accurately accomplished using the computer control.
Single point diamond dressing is carried out using a single point diamond, set at an angle to the grinding wheel. A single point diamond cuts into the periphery of the grinding wheel much like a lathe cutting tool turns a piece of bar stock and requires clearance angles and attack angles in order to perform its function efficiently. If the diamond were stood upright to dress the grinding wheel, it would wear very quickly. If the set angle of the diamond were equal to the facet angle on the diamond point, it would generate a great deal of heat. The heat would not only destroy the mount, but the thermal growth would upset the accuracy of the traced form as well.
There are many types of mounted point dressers. The single-point is a special case and must be treated with care. The diamond must be rotated during its life to give even wear around the point. The diamond must not be abused, hit, dropped, or allowed to heat excessively, as the point may chip or become distorted in its mount and upset the accuracy of the system. Flood cutting fluid must be used at all times when dressing.
Multi-point diamonds and impregnated points have more than one active diamond point in contact with the grinding wheel periphery. They are far more robust and can take a certain amount of absue. Multi point systems are often used in very rough grinding operations where precision is not of major importance. Multi point systems are less accurate than single points and cannot achieve the fine detail of precise and delicate forms.
Another type of diamond dressing tool is available called a "fliese" tool which has shown extended life and better performance over a single point diamond. "Fliese" is a German word for tile, the name given to this type of tool by Ernst Winter and Company, who claim to have invented it. There are many tools of this type on the market made by other manufacturers. These diamond point dressers are traversed across the grinding wheel, forming the profile and sharpening the wheel periphery. A fast feed will result in a coarse helix to be dressed across the wheel, leaving a periphery suited only for rough grinding, and yielding poor surface finishes. A slow feed results in a finer helix, which, although yeilding high surface finish, tends to dress flats on the abrasive grain and prevent high stock removal. It is important that the dressing process is controlled carefully for consistency. It would appear the industry uses fast dressing and slow dressing to create rough or finely dressed wheels respectively. The question to ask is what is fast and what is slow? The diamond has a finite width, so a calculation can be made to determine diamond overlap. If the grinding wheel is 400 mm (16 in) in diameter and rotates at 30 ms-1(6000 sfm), the RPM will be approximately 1400 rpm. If the diamond is 0.25 mm (0.010 in) wide at dressing depth, then the fastest traverse speed for the diamond not to overlap is 355 mm/min (14 in/min). There is a rule of thumb which recommends overlaps: for roughing there should be two to three overlaps and for fine finishing four to six overlaps. The traverse feed rates would be approximately 150 mm/min (6 in/min) for roughing and 60 mm/min (2.5 in/min) for finishing. Wear will be taking place on the diamond and the effective width should be measured after approximately 325 cmc (50 inc3) of grinding wheel has been dressed. A benefit of the fliese tool is that it does not grow wider as it wears deeper, but maintains a width determined by the width of the diamonds plated to the tile. Profiling with a diamond point is a relatively inexpensive method used to create a form on a grinding wheel, however, the speed, accuracy, and concsistency of the wheel conditioning dowes not quite match that of diamond roll dressing. A diamond roll is very expensive and sometimes prohibitive for the job shop. A CNC controlled diamond disk or wafer roll dressing system may be more economical, depending on the detail and the accuracy of the required form. Due to the finite radius on the periphery of the disk, only profiles of moderate detail may be achieved by this method. A diamond disk or wafrer roll is basically a very thin diamond roller. CNC control of the thin disk's path across the grinding wheel face allows a form to be dressed quite inexpensively. The dressing parameters that need to be controlled with this dressing method are th diamond wafer to grinding wheel speed ration, and the traverse rate, as in single point dressing. The resulting peripheral condition of the grinding wheel is almost as though it had been dressed with a full form width diamond roller dresser.
