9. Grinding

• Grinding has two main uses,

as a surface finishing process for flat or cylindrical features that have already been cut.

to cut complex surfaces with high tolerances or hard materials.

• The basic process involves an abrasive wheel spinning at high speed that is brought into contact with the work surface, thus giving a smooth finish.

9.1 Operations

 

9.2 Machine Types

• Typical categories of grinders include,

Surface

Cylindrical

Internal

Center

Centerless

Jig

• Surface grinders have a few basic types,

Horizontal Spindle with Reciprocating Table

Horizontal Spindle with Rotary Table

Vertical Spindle with Rotary Table

Vertical Spindle with Reciprocating Table

• With centers parts are mounted so that they may rotate about fixed centers and then ground externally.

• Centerless grinding is popular as a high speed, low cost operation.

• In this operation there is a grinding wheel and a governing wheel. The part sits between the wheels and is ground by the grinding wheel. The governing wheel acts to slow the rotation of the part so that it does not spin at the same speed as the grinding wheel and reduce the surface speed of the grinding operation.

 

• If the part has a uniform cross section through feed grinding can be used. Otherwise infeed grinding will have to be used. For infeed grinding the parts are placed between the wheels, ground, and then pulled out. Through feed grinding has the parts move in a steady flow between the wheel.

 

• Internal grinding is similar to other forms of rotational grinding, except that as the part rotates the internal features are ground by a smaller wheel.

 

• Using a smaller wheel requires higher grinding speed which increases the challenge of this process.

9.3 Grinding Wheels

• The wheels are typically made with a bonded abrasive.

• common abrasives used are,

Aluminum oxide

Silicon carbide

• Grain size is typically from 6 to 600.

• The bonding mechanisms used commonly are,

vitrified

resinoid

rubber

shellac

silicate

• The Grade of the wheel is a measure of the ability to retain grit. If a wheel is Grade A it is soft, if it is Grade Z, it is very hard.

• The wheels are also given a structure number. 1 indicates a dense structure, whereas, 15 indicates an open structure.

• Loading of a wheel refers to the embedding of swarf in the voids.

• Glazing occurs when the grit has dulled, but is still bonded to the surface.

• Grinding wheels should undergo a dressing process.

• Many grinding wheels are shaped. This shaping is done while the wheel in the machine using diamond, or other hard shaping tools.

• The wheel can be made to act soft by increasing work speed, and decreasing wheel speed. The wheel can be made to act hard by reversing the parameters.

• Typical operation parameters are a depth per pass.

 

• The wheel normally turns to give a CS of 3500 to 6000 fpm and wheels range from diameters of a few inches to a few feet.

• The table feed is 80-350 fpm for finishing passes. Up to 1000 is reasonable.

• For surface grinding

 

9.4 Problems

Problem 9.1 Why would we use grinding on a part instead of turning?