GII Solutions Dressing and Truing Solutions GII Solutions Dressing and Truing Solutions GII Solutions Dressing and Truing Solutions
GII Solutions Dressing and Truing Solutions
G II Solutions, Inc.
Dressing and Truing Systems
GII Solutions Dressing and Truing Solutions
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Overview

    Rotary dressing and truing is becoming more popular and profitable in abrasive machining. Manufacturers have found rotary dressing and truing to be superior to single point, cluster and stick dressing for processes that require high part production and/or close part tolerances. With the introduction and increasing popularity of vitrified superabrasive grinding wheels, rotary dressing and truing has become a necessity. These harder grinding wheels offer greater flexibility and improve grinding processes, but can only be effectively dressed or trued with rotary devices.

    The rotary dressing and truing process maximizes the grinding wheel life and greatly increases consistency in the wheel profile. Rotary dressing and truing can be incorporated into the grinding machine's controls to maximize production time, reduce operator labor, and insure tighter tolerances in the finished product. Not only are higher production rates possible, but more complex designs can be produced, further reducing setup and improving the grinding machine's utilization.

    Following is a short description of the terminology used in the rotary dressing and truing process as well as some of the advantages possible from incorporating this technology.

Introduction to Dressing and Truing

    A grinding wheel is composed of bonding media and abrasive (conventional wheels) or superabrasive grits. Bonding media in either conventional or superabrasive grinding wheels may be generally classified as resinoid, metal, or vitrified, where resinoid constitutes the softest bond and vitrified, the hardest. Typical grit material in conventional grinding wheels includes aluminum oxide and silicon carbide. In superabrasive grinding wheels, typical grit material includes diamond and cubic boron nitride (CBN).

    Dressing is the operation that involves removing bonding material in order to expose the sharp grits in the grinding wheels. It is a sharpening process that is periodically required to maintain a consistent and quality grinding wheel surface finish. Truing involves manipulation of the grinding wheel geometry. The process is responsible for creating the grinding wheel profile and assuring that the profile is concentric. Oftentimes, "dressing" and "truing" are used interchangeably, even though they are separate operations. In most processes, however, both operations are accomplished simultaneously. As a specific profile in a grinding wheel is being produced (truing), the correct grit exposure (dressing) is also being generated.

Single Point vs. Rotary Dressing and Truing

    The traditional method of wheel dressing and truing users single point dressers usually made with diamond. Sometimes, a cluster of single points or a solid block is used. By moving a rotating grinding wheel into and across these non-rotary devices, forms are generated and surface finishes are obtained. (In some cases, the single point may have both infeed and crossfeed motions). The surface finish of the grinding wheel will be affected by varying the amount of material removed from the grinding wheel per pass as well as the rate at which the single point traverses across the grinding wheel.

    Rotary dressing and truing features a spindle that drives a dressing and truing roll. The spindle is generally powered either hydraulically or electrically. During the dressing/truing process, a rotating grinding wheel moves into and across the rotating dressing/truing roll on the spindle. (Again, the dressing/truing mechanism may have both infeed and crossfeed motions). As with single point devices, the surface finish of the grinding wheel will be affected by changing the amount of material removed from the grinding wheel per pass as well as the rate at which the rotary dressing and truing spindle traverses across the grinding wheel. Moreover, with a rotary dressing and truing spindle, a grinding wheel's surface finish may be manipulated by changing the speed and direction of the dressing/truing roll.

    Single point dressers generate more friction between the dresser and the grinding wheel than do rotary dressers. Because single point dressers are stationary, they inherently tend to "rub" the bonding agent away from the grinding wheel during the dressing/truing process. The increased friction generated produces heat, which is transferred to the grinding wheel. The heat causes the grinding wheel to expand, affecting the precision of the intended grinding wheel form. In extreme cases, the additional heat generated may also change the characteristics of the bonding agent in the grinding wheel. This can result in an inconsistent or "gummy" grinding wheel surface finish.

    Rotary dressers actually "cut" the bonding agent away from the grinding wheel. This cutting action produces chips as material is being removed. Less friction results with this process because most heat generated is transferred to the chips, as opposed to being transferred to the grinding wheel. Hence, less thermal expansion is observed and consistent bonding material properties are maintained.

    Superabrasive grinding wheels are harder than conventional grinding wheels. More heat is generated as the wheel hardness increases. Therefore, because rotary dressing/truing generates less heat, it is essential for dealing with superabrasive grinding wheels.

Hydraulic vs. Electric Rotary Dressing and Truing

    Rotary dressing and truing spindles may be powered hydraulically or electrically. Although electric spindles are slightly more expensive than hydraulic spindles, they have several advantages. Hydraulic systems consist of a spindle, hydraulic tubing, and a reservoir with a pump and an electric motor. Initially, hydraulic spindles were more compact than electric spindles. However, with recent technological advances, electric spindles are now as compact as hydraulic spindles. Moreover, the controlling components associated with electric spindles occupy much less space than the hydraulic system's. Electric spindle controls can also be easily integrated with the existing machine's controls. Electric spindle systems posses both the manual and automatic capabilities to precisely select spindle speed. Through closed loop velocity control, a selected speed is easily maintained regardless of applied forces. Speeds are also easily varied and verified through digital readout. With hydraulic spindles, speeds are typically selected exclusively by manually adjusting valves. The selected speed is not easily verified or maintained as varying forces are experienced. Other undesirables of hydraulic systems include leaks and the unexpected results of forces generated by pressurized hydraulic tubing.

Why Use Electric Rotary Dressing and Truing?

    The primary reason to use rotary dressing and truing is its capability to dress/true superabrasive grinding wheels. Electric rotary dressing and truing systems furthermore contribute to increases in part quantity, improvements in part quality, and enhancements in the overall process control. The characteristics of electric rotary dressing and truing that produce these enhanced results are listed below and discussed in the upcoming sections.

Characteristics of electric rotary dressing and truing:

  • Remove less material per pass.
  • Dressing/truing rolls wear at a very low rate.
  • Dressing/truing rolls wear at a consistent rate.
  • Dressing/truing rolls wear at a predictable rate.
  • Dressing/truing rolls do not exhibit diamond fade and catastrophic failure.
  • Create a higher grit exposure.

Remove less material per pass with electric rotary dressing and truing.

    To produce a desired grinding wheel surface finish, single point dressers generally take up to .010" from the grinding wheel radius per pass during the dressing/truing cycle. Smaller amounts, down to .00005", may be taken off with rotary dressers in a single pass to create equivalent or even superior surface finishes. Less material removal from the grinding wheel results in longer grinding wheel life and produces savings in the following areas.

  • Fewer grinding wheels are required.
  • Maintenance time associated with grinding wheel replacement is decreased.
  • Production time is increased as time required for maintenance is decreased.

Dressing/truing rolls wear at a very low rate.

    Dressing truing rolls wear at such a low rate, that one roll may last a year. Single point dressers, especially in high production cases, may require weekly replacement. Each time a dressing/truing roll or single point dresser is replaced, the entire process must be re-indicated. Compare one setup per year versus fifty or one hundred, and increases in part production are easily identified. Even though dressing/truing rolls are more expensive than a single point dresser, they are extremely cost effective.

Dressing/truing rolls wear at a consistent rate.

    Dressing/truing rolls wear at a consistent rate, whereas the wear rate of single point dressers is often erratic. With consistent wear, the amount of material removed from the grinding wheel is consistent. When controlled by CNC machinery, the programmed depth of pass will be closer to the actual depth of pass and actual material removed will be closer to the programmed removal amount. Thus, the actual size of the grinding wheel is exact and part tolerances are more closely and easily maintained.

Dressing/truing rolls wear at a predictable rate.

    Single point dressers wear until the onset of diamond fade, a period when the single point dresser begins to wear at rapid rate. The onset of diamond fade is unpredictable. After it begins, there is a very short time before the single point dresser catastrophically fails, suddenly shattering or fracturing. In the event of catastrophic failure, the grinding wheel may be severely damaged as well as the work piece. Most manufacturers replace single point dressers long before the chance of diamond fade and catastrophic failure. A single point dresser may be discarded with much unused material remaining. Because dressing/truing rolls wear consistently and at a low rate, they do not exhibit diamond fade and will only fail catastrophically if misused. Therefore, they may be used in their entirety until all abrasive material has been used.

Create a higher grinding wheel grit exposure with electric rotary dressing and truing.

    The grinding wheel grit exposure created with single point dressers can be manipulated only by changing the depth of the pass and the speed at which the dresser traverses the grinding wheel (and sometimes the grinding wheel speed or direction). Electric rotary dressing and truing spindles add the capability of altering the ratio of the surface speeds between dressing/truing roll and grinding wheel as well as their relative directions. Speed and direction control are the two variables that have the greatest impact on optimizing grit exposure. Grinding wheels with a higher grit exposure are able to:

  • produce a cooler cut
  • cut more material from the part per pass
  • create a better surface finish on the part.

Summary

    Process control is any procedure where selected variables are electronically controlled to produce a consistent part quality and/or quantity final product with no manual control. Electric rotary dressing and truing spindles are suited to accomplish this exactly. They offer more variables that are also easily controlled. They possess the flexibility and controllability to optimize the dressing and truing process.

    The capability to precisely control and maintain speeds over a broad range allows for the easy manipulation of surface finishes. Various grinding wheel surface finishes are necessary to accommodate various grinding processes. Since the dressing/truing process becomes less timely and more efficient, the frequency of the dressing and truing cycle can be increased. The grinding process, in turn, may be made more aggressive, increasing part output. Part output is also increased by the significant reduction in maintenance associated with changing dressers. Parts are produced at more consistent sizes and with higher quality surface finishes.

    Some processes demand electric rotary dressing and truing. Changing the grinding wheel to a style that will accept rotary dressing and truing techniques may enhance processes that currently do not necessitate electric rotary dressing and truing. As newer and harder grinding wheels are developed and their benefits demonstrated, rotary dressing and truing will become increasingly more important.

 

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