By John R. (Jay) Doubman
In concept, not much has changed in double disc grinding since the invention of the machine in the 1890s. For years, little was done to improve machine tolerance and throughput abilities. In recent years, however, automotive parts manufacturers have realized better part quality and productivity while lowering total grinding costs. This is being accomplished with new technology in abrasive grinding systems, machine controls, and a surge in the evolution of the double disc grinder.
Disc Grinding Defined
Double disc grinding is the removal of material from a part with parallel surfaces. The metal removal takes place on both sides of the part simultaneously, with the grinding occurring on the faces of the grinding wheels. Most commonly, the disc wheels are attached by inserted nuts to diametrically opposed horizontal spindles, each contained in a heavy-duty precision grinding head assembly.
Some typical applications include the grinding of connecting rods, engine valves, valve seats and lifters, brake pads, brake rotors, clutch plates, piston rings, universal joints, and bearings. Materials vary from cast iron to stainless steel, from forgings to powdered metal, from basic carbon steel to high-tech alloys. Disc grinding has become a highly flexible process capable of many operations, from the snagging of rough castings to extremely precise lapping, holding tolerances in the millionths. The majority of disc grinders utilize wheels from 12 to 42 inches in diameter, between 1 and 4 inches thick. Typical grinding wheel grit sizes fall between 16 and 240, depending on the operation.
Disc Grinding Systems
Disc grinding operations can be described by two defining characteristics. The first is the primary method of stock removal: a shear grind, a progressive grind, or a plunge grind. The second is the method of part introduction: throughfeed, rotary feed, reciprocating feed, or oscillating feed.
Shear Grinding: The most common of grinding modes is the shear grind. In this method, the angle of the grinding heads with respect to each other (referred to as the "head setting") is such that the smallest distance between the abrasive wheels occurs at the part entrance. The theoretical set-up calls for 70 to 80% of the stock to be removed at the entrance, with the balance removed along the part's path through the machine.
Progressive Grinding: For more complex applications, stock can be removed more evenly during the time the part is in the machine. In this set-up, the head setting is "tight at exit." In other words, the smallest diameter between the wheels occurs nearest the exit guides. Some stock, usually 20 to 40%, is still sheared at entrance, but the majority of work is spread out across the abrasive wheel surface.
Plunge Grinding: For some specialized applications, the heads are cycled in and out for each part or gang of parts, closing to finished size and sparking out before retraction. This method is accomplished through hydraulic head controls and limit switches in older machines, but is much more flexible and accurate with the newer precision ballscrew-powered, servo-driven grinding feeds.
Throughfeed: The most common method of part introduction is throughfeed. The parts are supported throughout the process by rails on the top and bottom. The parts are driven at the entry side of the machine, usually by belts in contact with the side of the part. After the parts leave the belts, they are supported by the entrance guides, with as little clearance as possible. The alignment of these guides relative to the wheel position is critical for part quality. (Any misalignment will cause the parts to "swipe" or "dub.") After traveling through the machine and ground by the abrasive wheels, the part exits, supported by additional guides. Throughout the entire process, each part is driven by the part behind it.
Rotary Feed: The second most popular method, commonly used with odd geometry parts, is rotary, or carousel feed. A large circular part carrier, with pockets matching the part shape, is used to carry parts in a circular path between the abrasive wheels.
Reciprocating or Oscillating Feed: These methods of part introduction are much more specialized. They both introduce a part, or pocket of parts, to the machine, where a plunge grind cycle is used to bring the parts to size. The reciprocating method utilizes linear motion, while the oscillating feed uses rotational motion.
In the past few years, advances in abrasive wheels, machine design and controls have improved disc grinding operations. The abrasive technology has been advanced with the introduction of new bond systems and abrasive types. The machinery has been improved with increased rigidity and the addition of computer-controlled, precision ballscrew, servo drives on each spindle.
With the entry of Norton Company into the disc grinding market, the level of performance of abrasive wheels has been elevated. After an extensive R&D effort, Norton Co. introduced a flexible bond system that creates abrasive wheels capable of free cutting, low heat grinding characteristics combined with long life. This bond system (Norton B18) has given many users significant monetary savings. A few examples of the advanced applications are described below.
One of the Big Three U.S. automotive manufacturers has realized significant savings with the introduction of this new high technology bond system. In a roughing operation performed on connecting rods for a V6 engine, raw forgings are disc ground to ensure flatness for subsequent operations. The current process used a 50-hp horizontal spindle rotary feed double disc grinder with servo-controlled heads and automatic gaging. The current abrasive was dressed at regular intervals, in order to prevent burn and a loss of the flatness tolerance. With the free cutting and long life characteristics of the Norton B18 bond system, the dress frequency has been decreased 150% and the wheel life has increased. The savings amounted to 48.5% of annual abrasive expenditures for this operation.
A manufacturer of compressors was using a vertical spindle rotary feed double disc grinder to finish compressor valve plates. These plates have a void on one side that reduces the contact area by approximately 40%. Using the current process, this difference resulted in regrinding 35 to 40% of the parts for non-cleanup. This is a result of the different contact area.
The following points explain the grinding theory connecting contact area to stock removal:
- Stock removal is proportional to the normal force existing between the part and the wheel face.
- The normal force at any point is inversely proportional to the surface area of the part (in other words, at a constant pressure between part and wheel, a part of smaller contact area will experience higher forces).
- Thus, a given wheel will act "harder" on the non-void side, removing less depth of material.
In order to eliminate the need for regrinding, a process was designed in a cooperative effort between the company's engineers, operators, and Norton application engineers. The process involves the orientation of the parts, such that the void is always in the same direction. By orienting the parts this way, a different specification is used on either head to match the particular grinding conditions. With the assistance of Norton's Field Instrumentation System (FIS), this pair of grinding wheels was finely tuned to achieve the same "equivalent" grade (equal normal force between each side of the part and either wheel). As a result, the non-cleanup of parts has been eliminated, as the depth of stock removal from each side is equal. The FIS, enabling engineers to view head position and power draw of each head in real time as well as to analyze this data with proprietary software, is a key to new process development.
Improvements in the machine's mechanical rigidity, coupled with modern control systems and advanced programming, has also provided many manufacturers with significant savings, along with improved performance. For example, a revolutionary process for grinding automotive connecting rods has been developed by Giustina/C&B International, L.L.C. This machine incorporates advanced controls to grind connecting rods from rough stock to finished size in a single pass. The process combines a rotary carrier part introduction with a plunge feed method of stock removal. Each rod experiences a complete cycle, eliminating much of the material handling requirements.
Significant quality improvement can be recognized, as well, over conventional rotary carrier machines. Other unique features of the Giustina machine include automatic dress cycle and dress compensation, diamond roll dressing, and optional automatic new wheel dress. In addition, many ergonomic improvements have been made, such as electric powered wheelguard hood and advanced noise abatement/safety guarding. The development of the precision ballscrew, servo driven grinding heads has eliminated the need for any hydraulics on the Giustina double disc grinder.
This technology currently is being used with 80-90% of connecting rod manufacturing in Europe. Through the recent joint venture with C&B Machinery in Livonia, Michigan, these systems are now available in the U.S.