blade It's a good bet that grinding is
the metalworking process most associated with "black art" and
"smoke and mirrors." Much of this mythology is perpetuated by a
general lack of understanding of the basic fundamentals that make grinding
applications successful. Of the grinding processes,
centerless is certainly among the least understood from a fundamental
perspective. On the surface it indeed does tend to defy much of what we think
we know about cutting metal. Most obvious is the apparent
lack of workholding. The part just lies between the regulating and grinding
wheel seemingly free to exit the cutting zone at will. Yet it
doesn't—usually. So how does centerless grinding
work, and what are essential components that need to be understood to achieve
successful part processing, consistently and predictably? We visited Landis Gardner's Hebron, Kentucky, facility where it manufactures
the Landis Cincinnati line of centerless grinding machines. We spoke with
Greg Payne, the company's bid and proposal manager, about the fundamentals of
centerless grinding specifically, why the process works and how understanding
that translates into successful part processing. What Is Centerless Grinding? Centerless grinding is an OD
grinding process. It differs from other cylindrical processes in that the
workpiece is not mechanically constrained. On traditional OD machines, the
work is held between centers or chucked and rotated against the faster
spinning grinding wheel by an external motor usually located in a workhead. Unlike center-type or
chucker-type grinding, parts made using a centerless process do not require
center holes, drivers or workhead fixtures. Instead, the workpiece is
supported on its own outer diameter by a workblade located between a high
speed grinding wheel and a slower speed regulating wheel with a smaller
diameter. It's the relationship among
these three basic components—grinding wheel, regulating wheel and
workblade—that makes the difference between a successful centerless grinding
application and one that gives headaches and bad parts. Why Centerless Works
In cylindrical grinding,
understanding the process is relatively straightforward. A workpiece,
supported by centers or a chuck, is rotated against the grinding wheel, which
is fed in a prescribed distance radially (plunge grinding) or traversed
axially across the workpiece to create the desired OD dimension. On the centerless grinder there
are no centers, so it would appear that there is no means to control the
workpiece size or roundness. However, quite the opposite is true. Centerless grinding, if set up
properly, will achieve roundness, surface finish and dimensional tolerances
that are among the best available in metalworking. Moreover, the process is
able to improve the roundness of out-of-round workpiece blanks. Creating roundness from
out-of-round conditions is the claim to fame of the centerless grinding
process. It does this by virtue of the relationship among the grinding wheel,
regulating wheel and the workblade. Side-viewed, from the operator's
perspective, the centerless grinder has the grinding wheel on the left,
workblade in the middle and the smaller diameter regulating wheel on the
right. For most applications, the centerline of the grinding wheel and
regulating wheel are in the same plane, at equal heights above the machine
bed. To achieve rounding action, the
workblade must be set so that the centerline of the workpiece is above the
centerline of the grinding and regulating wheels. This is a critical
relationship for successful centerless grinding. If the workpiece rests on a flat
workblade that is on center with the regulating and grinding wheels, the
contact points form three sides of a square. As the part is ground in this
setup, any high spot on the workpiece will shift the work slightly on the
blade, allowing the grinding wheel to cut a directly opposite low spot. Over
time this setup will create three lobes on the workpiece that may be
dimensionally accurate but far from round.
Setting an angled workblade so
it slopes toward the regulating wheel and supports the workpiece centerline
above the centerline of the regulating and grinding wheels is how the
centerless operation is able to generate roundness. In this setup if a high
spot comes in contact with either the blade or the regulating wheel, it does
not create a directly opposite low spot because of the angle created between
the centerlines of the wheels and workpiece. Instead of grinding a lobe shape
in the workpiece, the high spot is gradually reduced by the action of the
grinding wheel. So rather than creating a low spot on the periphery of the
work equal to the high spot, the grinding wheel generates a proportionally
smaller low spot at its contact with the workpiece. To better visualize this
rounding action, imagine a workpiece with several high spots. Now imagine
that part being ground with only a single rotation. When the part is removed,
it would show only the high spots had been touched. The angle of the workblade helps
keep the workpiece in contact with and under the control of the slower
rotating regulating wheel to resist any tendency to "spin up" to
the speed of the grinding wheel. In some cases, a spin-up can take a workpiece
from 850 rpm to near 60,000 rpm in the blink of an eye. This is not something
you want to have happen. Setup Keys
To get the centerless grinding
operation started, the workblade height must be set. This will control the
workpiece centerline, the height above the centerline of the two wheels that
determines the rounding action of the centerless grinding operation. "One of the most common
setup mistakes we see," says Mr. Payne, "is incorrect measurement
of the workpiece height. This is a critical dimension and should not be
guessed." A rule of thumb for setting the
correct height for a workpiece that is up to 1 inch in diameter is to have
one half of the workpiece diameter above the centerline of the grinding and
regulating wheels. So, for a 1 inch diameter workpiece, the height should be
a half inch above the wheel's centerline. "This is the primary starting
point for most centerless grinding setups," says Mr. Payne.
Most centerless grinding
machines give the distance from the top of the lower slide to the centerline
of the grinding wheel and regulating wheel as one of the specifications. For
example on Cincinnati's DE model centerless grinders, this distance is 9.875
inches. Mr. Payne points out that this measurement varies among machine
manufacturers and models. With this known dimension, the
next step to get the correct blade height calculation is to add the workpiece
height above center to the distance between the slide and wheel centerline.
For a 1 inch diameter workpiece on a Cincinnati DE model, the workpiece
center height needs to be 10.375 inches measured from the lower slide. "There are numerous ways to
get this measurement," says Mr. Payne. "Ultimately though, the
machinist needs to end up with one half of the workpiece above the centerline
of the wheels, resting one-third of the way down from the top of an angled
workblade and against the regulating wheel. This will provide good rounding
action and stable grinding conditions." Centerless grinding workblade
angles range from 0 to 45 degrees. For most centerless grinding applications,
a top blade angle of 30 degrees seems to provide the best results. "The
basic rule is the steeper the angle of the blade the faster the rounding
action," says Mr. Payne. "There are limits, however. For larger
diameter and longer work, a shallower blade angle is best. Choosing 30
degrees is a good general starting point for top blade angle." Also, setting the regulating
wheel slow, at about 30 rpm, is a point to begin optimizing the centerless
grinding process. Obviously, this regulating wheel rotation speed is
dependent on work diameter and stock removal rates required. But 30 rpm is a
good place to start. Kinds Of Centerless Grinding Like center-type cylindrical
grinding, which can use plunge or traverse motions to grind a workpiece,
centerless grinding too has different ways to get the job done. The two most
common centerless grinding techniques arie infeed and thrufeed. Setting up for infeed is the
less complicated of the two. Although workpiece height is important, proper
workblade and wheel profiles are a must to obtain the required step
relationship of the finished workpiece.
The workpiece is placed between
the wheels, and the grinding cycle begins. After spark-out, the finished part
is removed and the next one goes in. Infeed centerless grinding is analogous
to plunge grinding on a center-type machine in that the workpiece is static
in the axial direction. rFor continuous, production-type centerless grinding,
the thrufeed method is very effective. Thrufeed grinding is accomplished by
passing the workpiece between the grinding and regulating wheel. Basically,
raw material goes in one side of the grinding zone, and finished workpieces
come out. The work is driven axially
between the wheels by inclination of the regulating wheel relative to the
grinding wheel. Because all of the points to be ground on the workpiece
contact all the points on the wheels, this centerless grinding method is best
applied to straight cylindrical parts without shoulders or other interfering
features. In thrufeed grinding, the
regulating wheel is inclined, creating a feed angle relative to the grinding
wheel. This feed angle allows the regulating wheel to perform the dual
purpose of rotating the workpiece against the grinding wheel and driving the
workpiece across the face of the grinding wheel. The regulating wheel can be
swiveled relative to the grinding wheel. However, to do its job and use the
full width of the grinding wheel, line contact between the workpiece,
grinding wheel and regulating wheel must be m˜aintained. Therefore, the
regulating wheel must be trued with a diamond block located on the truing
attachment. The diamond block is a device that adjusts the diamond contact relative
to the workpiece height above the grinding and regulating wheel centerline. "A quick check to see if
the thrufeed setup is running correctly," says Mr. Payne, "is to
look at the wear line across the top of the workblade. This line should be
parallel to the top of the blade. If it runs uphill, from front to back,
there is too much diamond block offset. It's the opposite if the line runs
downhill." Is Centerless Grinding For You?
Traditionally, centerless
grinding is found in shops involved in high volume production runs. That's
still an important segment for the technology. Increasingly, though, the
advantages of centerless grinding, such as rapid rounding and accuracy, are
finding application in shops that run relatively shorter job lot sizes. The
enabler for this is the application of CNC and servomotor technology to the
centerless grinding machine. With electromechanical actuation of slides,
dressers and truing attachments, setup of the centerless machine is a much
less daunting and time consuming exercise. Moreover, the ability to program
grinding and regulating wheel contours using CNC actuated truing attachments
eliminates the need for profile cams, which take time to manufacture and are
usually less accurate. Centerless grinding is successfully applied to
manufacture of parts ranging from hypodermic needles to bowling balls. The
principles of the process remain the same regardless of the workpiece. Like
most metalworking processes, centerless grinding is not magic, if you
understand the fundamentals. |
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