Unit 5 Creep Feed Grinding

Creep feed grinding places unusual demands on the grinding wheel because of the long arc of contact, many more cutting edges are involved than in the reciprocating grinding process. This is true even when the specific material removal rate remains constant.

The mechanical load on an individual cutting edge is lower in creep-feed grinding than in reciprocating grinding. This is because the material is not penetrated as deeply as in the case of reciprocating grinding.

The chips generated by creep-feed grinding are long but very thin, while chips generated by reciprocating grinding are short and thick. The information of a chip can be observed theoretically- by comparing the cutting process of a single cutting edge (abrasive grain) with the hardness test.

The “point of cut” is not as deep as the cutting edge is penetrating the workpiece. This means that, due to elastic and plastic deformation, the infeed is higher than the actual depth of cut.

The elastic and plastic deformation accounts for quit a large share of energy invested in the total cutting process. as a result, a lot of heat is generated that ha to be taken away by the coolant to prevent thermal damage to the workpiece.

There is no exact calculation available for practical use which would help us determine which proportion of energy goes into the cutting process and which into friction. the creep-feed grinding process shows an extremely high portion of energy used up by friction.

What can be done to minimize friction?

1. Machine parameters: reduce the wheel-peripheral speed. Reduce the depth of cut and increase the table speed proportionally to maintain the material removal rate.

2. Dressing parameters: when using a diamond roll, use the unidirectional mode.

3. Coolant system: use a coolant which combines the features of lubrication and heat removal.

4. Grinding wheel: the most critical problem- burning of the workpiece- can be avoided only when a self-dressing grinding wheel is used. Self-dressing means that the worn abrasive grains fracture and generate fresh cutting edges constantly. In creep-feed grinding process this self-dressing can be maintained only by increasing the grinding wheel’s porosity. The use of high-porosity wheels ha been widely accepted.

Ceramic Fills Gap between Aluminum Oxide and Superabrasives

Grinding shops are faced with a triple threat. The first is the appearance of harder more wear-resistant alloys, which are tough to grind and easy to burn. Next comes automation- CNC grinders need unattended operation and maximum uptime to provide a reasonable payback. Third is steadily rising customer expectations, especially in the areas of delivery and quality.

Such considerations inspired researchers to take a new approach to aluminum oxide. They could see that a niche existed for an abrasive which would exceed the performance of conventional aluminum oxide on ordinary drilling machines, at a price closer to standard abrasives than superabrasives.

Ceramics technology provided the answer. A manufacturing technique based on a sintering process enabled them to control the microstructure of the ceramic grains. The resulting non-fused aluminum oxide abrasive has substantially greater life than fused aluminum oxide products.

In use, the grains fracture along sharp crystal edges rather than flat crystal planes. As the grains fracture, new sharp edges are exposed.

The ceramic aluminum oxide can be used alone or mixed with other abrasives it can be used on ferrous and non-ferrous metals. Metal removal rates can be 50% higher than possible with standard aluminum oxide wheels. While the new type of wheels can take heavier infeeds than regular aluminum oxides, they are no harder on dressing tools than conventional abrasives. They perform best in oil or dry grinding.

Production grinding of end mils was one example of successful use of the wheels. There are 14 steps involved in producing the average end mill, and eight of them involve grinding.

Superabrasives such as diamond or CBN (cubic boron nitride) can produce astonishing productivity improvements, especially in applications involving hardened steels, tool steels and superalloys.

CBN, for example, retains its strength to more than 1000C and doesn’t chemically react with steel. Finishes are excellent and the product has twice the hardness and four times the abrasion resistance of standard aluminum oxide.

Use of a wheel fabricated with superabrasives will not compensate for a machine in poor condition. The system must be checked for tight spindle bearings. Vibration must be eliminated. This is critical to wheel life, surface finish, and dimensional accuracy. In order to eliminate chatter the operator should closely fit machine slides. Spindle speeds must be steady and high. Vibrations in spindle speeds will reduce wheel life and cutting efficiency. Reliable feed system is necessary for long wheel life, surface finish quality and dimensional accuracy. At last, high volume of coolant should be delivered, either oil-based or dry.