Mind the grind: Safety, Standards & Compliance

To achieve the quality and productivity requirements for most precision grinding, the grinding wheel must be trued and dressed prior to grinding and periodically during the grinding operation. The truing process is initially performed to correct any wheel runout, create the required profile on the wheel and condition the grinding wheel face.

During the grinding operation, it is usually necessary to dress the wheel periodically to re-establish the wheel profile and recondition the wheel face. Wheel conditioning typically involves sharpening and opening up the wheel face while also removing dull abrasives, excessive bond and workpiece material (loading).

A sharp and open wheel face allows maximum possible material-removal rates by minimizing the high grinding power and forces associated with a dull and closed wheel. On the other hand, there may be occasions when a closed and dull wheel face is required—for example, to improve the surface finish. Preparing the wheel face to the desired condition, or level of roughness, is achieved during the dressing process. The wheel face is fine-tuned by modifying the dressing parameters, including the dress speed ratio.

Truing and dressing a grinding wheel can be carried out using either stationary or rotary tools. As the names suggest, rotary dressing tools rotate and stationary tools do not. The main reason for selecting a rotary tool—aka a dressing roll—over a stationary tool is that it will greatly improve dressing tool life and form-holding capabilities. This is largely due to the greater number of active diamonds around the periphery of the roll.

Although tool life is usually the chief driving force for selecting a rotary dresser instead of a stationary device, an additional benefit is that the relative speed between the wheel and roll can be adjusted to influence the grinding process. The dress ratio is the dress roll velocity divided by the grinding wheel velocity; the ratio can be in either a positive or negative direction. This article discusses different rotary dressing methods. It will cover in more detail how the dress speed ratio can be used to manage the wheel face condition and ultimately manage the grinding process, including workpiece quality and process productivity.

Types of Rotary Dressing

There are several rotary dressing methods, including:

• Traverse profile dress
• Form roll plunge dress
• Intermittent dress
• Continuous dress

A traverse profile dress process involves the dressing tool moving across the grinding wheel at a predetermined depth of dress and dress feed rate. The wheel is dressed prior to starting the grind and periodically as needed during the grinding operation. The dressing tool will rotate in either the same direction as the grinding wheel (unidirectional) or the opposite direction of the grinding wheel (counterdirectional). The traverse dressing action can dress the wheel face straight or with a complex form. Figure 1 shows the toolpath for a concave wheel profile, and Figure 2 shows a typical traverse dressing tool.

In a form roll plunge dress process, the grinding wheel plunges into the roll with no traverse motion. The diamond dress roll is the inverse of the form required on the wheel. As with the traverse dressing, the form roll plunge method can dress the wheel face straight or with a complex form. Figure 3 shows the wheel plunging into the dress roll, creating a concave form. Figure 4 shows a dress roll with a complex form.

A form roll plunge dress process can be either an intermittent-type dress or a continuous-type dress. Using an intermittent dress cycle, after the initial dress, the wheel is dressed at periodic intervals during the grinding cycle using a form dress roll. The grinding operations remove a certain amount of material or grind a certain number of parts before dressing. For example, after 10 grinding passes on the workpiece, the wheel could be dressed to re-establish the wheel form, wheel face sharpness or both.

A continuous dress process constantly dresses the grinding wheel during the grinding cycle. This type of dressing operation is common in creep-feed processes. A major advantage of continuous dress, particularly in creep-feed applications, is that the grinding wheel stays sharp and open throughout the grind. A sharp and open wheel keeps the grinding power low and allows high removal rates without form loss or workpiece burn.

The dress speed ratio discussed in this article is not the only dressing parameter that needs to be considered when setting up or adjusting a dress cycle. Table 1 lists the key dressing parameters. All these can significantly affect the dressing and grinding process. However, only the dress speed ratio applies to all three previously described rotary dressing methods: traverse profile dress, form roll plunge dress (intermittent) and form roll plunge dress (continuous).

Dress Speed Ratio

The dress speed ratio is the relationship between the dress roll velocity (V_r) in sfm or m/sec. and the grinding wheel velocity (V_s) in sfm or m/sec. The ratio can be calculated using the following equation:

Speed ratio = V_r ÷ V_s