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.
