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Grinding’s
Blackest Art
Click here for a PDF of this article from Cutting Tool Engineering.
Centerless grinding is often viewed as overly complex, but a look at the process’ fundamentals reveals otherwise.
Shaping
materials by grinding them with hard, abrasive particles is
arguably the oldest manufacturing process. Despite its long
history, grinding is often seen as shrouded in mystery because
of the numerous cutting points and their irregular geometry, the
small DOCs that vary from grain to grain and the sorcerous
sparks shooting off the wheel. Grinding a workpiece that’s not
supported
between centers by a headstock and tailstock only adds to the
darkness. “Centerless is considered the black magic of grinding
because it’s not as straightforward as putting a part on a
fixture and then just moving a wheel,” said Mark Bannayan,
marketing manager for centerless grinder builder Glebar Co.
Inc., Franklin Lakes, N.J.
But centerless grinding doesn’t have to be
misunderstood. “You have to develop the skill level for
centerless and then it becomes a day in, day out operation,”
said Mark Schram, president of Ace Grinding, Melrose Park, Ill.,
a custom centerless grinding shop. “It’s not
really a difficult thing to do.”
Centerless Basics
A centerless grinder has three main components: a grinding
wheel, a regulating wheel and a work support blade. Because the
entire workpiece is supported by the regulating wheel and the
blade, centerless grinding provides a more rigid system,
enabling more efficient and accurate grinding compared to
grinding between centers. “If you’re trying to grind a ¼"-dia.
bar between centers, it’s very difficult because the bar wants
to move and flex,” Schram said. “In centerless grinding, there’s
no flexing because the workpiece is supported on three points.
The workpiece is rotating but not moving its position.”
Ace is the place for custom centerless
Established in the
early 1950s and owned by the Schram family since the late
’70s, Ace Grinding provides custom centerless grinding,
specializing in machining titanium, and straightening of bar
stock. “About 80 percent of what comes from the mills we have to
straighten,” said Mark Schram, the president of the Melrose
Park, Ill., company, which also operates a smaller facility in
Franklin Park, Ill., to handle Ace’s infeed centerless jobs.
The
bar stock Ace grinds is from 0.030" to 6" in diameter and up to
24' in length, but it limits bar lengths to 6' or less
when grinding the smallest diameters. The titanium bar stock is
generally smaller rounds, requiring a tolerance of ±0.0005" and
a surface finish of 16 or 32 rms. When processing titanium,
multiple passes are almost always required and it runs at a
slower speed than other materials, such as 12L14 steel.
“Everything is different,” Schram said about grinding titanium.
Some of Ace’s work for the medical industry requires bars to be
heat treated before restraightening and regrinding, and the shop
sends those out of state to a heat-treat facility that can
handle up to 12' lengths.
Ace buys remanufactured Cincinnati centerless grinders when its
needs a “new” machine. The shop’s latest acquisition was
remanufactured by Always Precision Inc., Plainfield, Ill., and
came equipped with a laser gage that measures a ground bar and
adjusts the grinding and regulating wheels via a CNC-actuated
servodrive. “That machine was probably originally made 25 years
ago,” Schram said.
When Ace purchases a remanufactured grinder, the shop trades a
machine in. “I could end up buying my same machine back 10 years
from now,” Schram said.
He noted that Ace has about 20 machines and runs two shifts,
operating up to 20 hours a day plus Saturdays when busy, but not
all the machines run at night. The company employs nearly 20
workers. “We’ve got a really stable workforce,” Schram said.
“We’ve replaced one person in the last 6 months.”
Retaining workers is important as finding qualified help is
difficult for manufacturers in general and centerless grinding
shops in particular. Schram said: “In centerless grinding, your
hands get
dirty. A lot of people don’t like doing that anymore. And it
takes a long time to be a top setup man in centerless grinding.”
To enhance productivity without adding staff, Ace is automating
more processes so one person can operate multiple machines. That
includes automating bar load and feed operations, which Ace
designs and builds the equipment for in house, as well as newer
technologies. “That’s where the laser gauging comes in and makes
it a lot easier for the guys,” Schram said. “They can look
across the room and see if the machine is running correctly.”
—A. Richter
The grinding wheel can
contain conventional abrasives or superabrasives and has a
larger diameter than the regulating wheel.
Also known as a drive or feed wheel, the regulating wheel
controls the workpiece’s speed, preventing the workpiece from
speeding up and rotating at the same speed as the grinding
wheel. That enables the grinding wheel to remove material from
the workpiece. Although a regulating wheel might have some
abrasive grains in it, it is typically comprised primarily of a
rubber-type bond material and doesn’t perform grinding. Other
regulating wheel materials include stone and knurled steel.
The regulating wheel rotates clockwise in the same direction
as the grinding wheel, but at a much slower speed. For example,
a regulating wheel’s speed might be 30 rpm while the grinding
wheel is rotating at 1,300 rpm. “If the regulating wheel fails
to do its job, you lose frictional drive from the regulating
wheel, the grinding wheel takes over and the part speed will be
controlled by the grinding wheel speed,” said Greg Payne,
centerless product manager for Cinetic Landis Corp., Milford,
Ohio.
While 85 to 90 percent of workrest blades are carbide-tipped
materials, other materials, such as ceramic and bronze, are
sometimes used, according to Payne. The blade angle varies by
application—going from a flat blade for some bar grinding
operations to as high as 45° for some small, intricate finishing
work—but the most common angle is 30°.
“The function of that blade angle is to promote roundness
generation,” Payne said. “The angle helps to not let the part
drop an amount that’s equal to how much you removed from the
high spots. It kind of cancels that out. The higher the angle,
the faster it helps to provide roundness correction, but there
are limitations.” As a blade wears, a groove forms, which can
cause profile problems. To prevent that, the blade is reground
to the desired angle and flatness.
Shims can be used to adjust blade height, but other methods
are available. For example, Bannayan noted that Glebar offers a
ramp mechanism on its machines where an operator turns a
setscrew to raise or lower blade height.
Centered grinding on a centerless
Some
grinding professionals consider performing center-type grinding
and centerless grinding in the same machine suitable for only
very special applications, but it’s frequently done according to
Frank Warthun, senior product manager for United Grinding,
Miamisburg, Ohio. The machine tool builder offers the 6-axis
Kronos L dual from Studer Mikrosa GmbH for these applications.
With the Kronos L dual, the workpiece center remains intact
because of the combination of two grinding methods, according to
the company. In addition, the workpiece is fully supported and
therefore cannot bend.
Warthun noted that grinding between centers is appropriate
for parts that require a high level of roundness accuracy on the
diameter related to the centerline, such as for bearings and
transmission shafts. Centerless grinding is suitable for parts
with a roundness requirement less than 1μm and when part
quantity is suitable. “You can’t just do a couple hundred; you
need a couple thousand,” Warthun said.
“I normally explain to customers that we are going to split a
micron by a third,” Warthun added. “You just come in with the
grinding wheel and grind these parts related to the centerline
on the diameter, retract the tailstock and the part will fall
down to the workrest plate and then you centerless grind it.”
Standard live or dead workhead and tailstock centers are
used, and the workhead does not necessarily need a driver and
programmable rotational speed. “Clamping between centers
provides enough pressure on the part that the part will be
broken down in relation to the OD grinding wheel,” Warthun said.
The Kronos S centerless grinder is another centerless grinder
from United Grinding, which provides economical processing of
small parts, according to the company. An end user can convert
the machine from any setup into a 15° inclined plunging machine
within 2 hours, including dressing. This enables applications
that were only possible between centers to be ground
centerlessly, such as for diesel injection needles and bearing
housings. In this way, the Kronos S is “two machines in one,”
stated United Grinding. —A. Richter
Another alternative is available from Micron
USA Inc., Kentwood, Mich., which offers a flexible slant-bed
machine for quick changeover on throughfeed applications,
according to Brad Darr, manager of North American sales and
marketing for the grinding machine builder. The regulating
wheel’s lower slide is set at 15°, and the upper slide is
parallel to the ground. By manipulating the two slides
simultaneously, the end user is able to raise or lower the
regulating wheel centerline, maintaining the workpiece contact
point with the regulating wheel. “For example, when going from a
40mm-dia. part to a 65mm-dia. part, we have the capability of
changing the regulating wheel center height through the CNC
rather than manually changing the rest blade height, greatly
reducing one’s changeover time,” he said.
Types
of Centerless
Two basic types of centerless grinding exist: through-feed
and infeed. According to Payne, through-feed centerless grinding
is the most common. As an example, it is often performed to
prepare bar stock for Swiss-type machines.
In through-feed grinding, a workpiece is inserted between the
regulating and grinding wheels, causing the workpiece to rotate
and move forward. To provide the workpiece with suitable
rotation and feed, the regulating wheel is tilted from the
horizontal plane, with the end of the wheel where the workpiece
enters being higher. The larger the feed angle, the faster the
workpiece travels between the wheels.
Because the regulating wheel is tilted, it has to have a
hyperbolic shape—a slightly hollowed center—so the workpiece
contacts a straight line across the regulating wheel’s face. “If
the regulating wheel was a constant, true cylinder, the
workpiece would only hit the center,” Payne said.
The second type of centerless grinding is infeed grinding,
which is sometimes referred to as plunge, profile or end-feed
centerless grinding. The infeed method is for workpieces that
require multiple diameters or steps. Here, the grinding and
regulating wheels are dressed to match the finished part’s
dimensions. In addition, infeed grinding is for workpieces that
have a projection that prevents through-feed grinding. When
infeed grinding, a workpiece is placed on the rest blade between
the two wheels, a work locator, or “end stop,” blocks the
workpiece from traveling axially during the grinding cycle. Like
through-feed grinding, infeed grinding is geared towards
high-volume production, but the through-feed method generally
grinds parts quicker. “The only parts you grind infeed are the
parts you have to,” Payne said.
Valve spools, pump shafts and engine valves are examples of
parts that are suitable for infeed grinding. Schram described
one Ace Grinding job producing a bone screw drill bit for the
orthopedics industry that requires the part’s three diameters to
be ground simultaneously. “It’s turning into a really nice but
difficult job,” he said. “When surgeons are turning that drill
bit, it can’t wobble. The TIR on the part has to be very
uniform.”
contributors
|
Other Applications
Infeed centerless grinding can also produce the carbide and
steel blanks for making industrial cutting tools and punches and
core pins for the moldmaking industry. That’s the primary
application for the centerless grinders from Tru Tech Systems
Inc., Mt. Clemens, Mich., according to Ryan Michels, sales
manager. Compared to a larger, conventional centerless grinder,
he said setup is quicker and easier on Tru Tech grinders. “You
can get a pretty good setup in 5 to 10 minutes,” Michels said.
He noted that software simplifies the process. “It talks to
you and walks you through the setup process,” Michels said. “It
tells you what to do and when to do it.” The software also
incorporates a library of help videos to explain how to fix
specific problems, such as correcting wheel runout and wear.
Modules are available for making trigonometry and radius
calculations, and a database of speeds and feeds is available to
help impart optimal surface finishes.
Setup on Tru Tech machines is also fairly quick, according to
Michels, because they can use 1A1 grinding wheels, which are
flat and usually measure ½"×7"—smaller than most centerless
grinding wheels. The flat wheel, for example, can plunge a
workpiece to produce a part’s rough form and then profile it
with angles, radii, back taper and a drill point in one setup.
Unlike a typical centerless grinder, Tru Tech’s grinders use
a bearing arm with a roller bearing on the end of it to hold the
workpiece similar to a V clamp. A motor turns the roller, which
in turn spins the part.
In addition to being able to quickly set up different infeed
grinding jobs, Michels said switching from infeed to
through-feed on the machines takes about 10 to 15 minutes.
Centerless grinding has also been used to produce parts
previously thought of as only suitable for other manufacturing
methods. Glebar’s Bannayan noted the example of centerless
grinding three different tapers down to 0.002" in a 0.012"- dia.
guide wire that carries a stent to a blocked artery. Previously,
the wires were produced with chemical etching, removing the wire
from the chemical at specific time intervals to create the
tapers.
“We were able to introduce the centerless grinding process to
it and do it a hundred times quicker with much more flexibility
as far as the shapes of the wire,” said Bannayan. He added that
centerless grinding is also suitable for some small-diameter
parts typically produced on Swiss-style turning centers.
Centerline Matters
Whether through-feed or infeed centerless grinding, the
centerlines of the regulating and grinding wheels are at
approximately the same height, with the grinding wheel slightly
higher for some applications. However, the workpiece centerline
must be higher than the other two to avoid producing tri-lobed
diameters. “The most dangerous point in centerless grinding is
when the workpiece centerline is exactly in line with the
centerline of the grinding wheel and feed wheel,” said Schram.
That’s when centerless grinding generates high points on a
workpiece diameter, and those high points start magnifying as
the part rotates, generating progressively smaller low points on
the opposite side as a high point hits the feed wheel and pushes
the workpiece into the grinding wheel, he explained. If the
setup is incorrect, “centerless grinding is notorious for
threepoint out-of-round with a bad setup,” Schram said. “It’s
also very good at fixing three-point out-of-round as long as the
setup is correct.”
With knowledgeable centerless setup personnel in short
supply, software can sometimes help. “It’s not real critical as
far as centerlines go because with our software we usually can
compensate for any human error in the setup,” said Tru Tech’s
Michels. “It’s quite forgiving.”
As a builder of new centerless grinders, Payne said Cinetic
Landis designs and manufactures its machines with the spindles’
centerline heights to be precisely in line with one another,
which becomes more critical as part diameter decreases, but
rebuilders frequently overlook that during the rebuilding
process.
Payne added that a remanufactured centerless grinder has the
same static stiffness as when it was originally built, but a new
machine usually has significantly higher static stiffness
because of its advanced design, which is beneficial when
processing hardened metals and achieving tight tolerances and
high throughput. For example, he said an old Cincinnati
centerless grinder has a static stiffness from about 300,000 to
800,000 lbs. per inch, depending on the model, but a new
machine, such as the Landis Cincinnati Viking SuperSeries II,
has a static stiffness of 3 million lbs. per inch. “Over 50
percent of the projects we sell new machines for started out
looking at rebuilds,” Payne said. “The machine design has to be
from the ground up.” That design work includes conducting a
finite element analysis of a machine to understand and measure
the static stiffness.
Since the introduction of centerless grinding in the early
1920s, the fundamental principles have remained the same, but
design advances have boosted throughput and the ability to
achieve tighter tolerances and the introduction of CNC has made
the process more flexible for job shop work and even
prototyping. In addition, grinding machine builders continue to
make centerless grinding faster and easier while helping to
demystify it. “A lot of people have been afraid of centerless
grinding,” said Michels. “We’ve tried to take out all the
unknowns and make it very easy for everyone.” CTE
About the Author: Alan Richter is editor of Cutting Tool Engineering, having joined the publication in 2000. Contact him at (847) 714-0175 or alanr@jwr.com.
Seamlessly centerless
To
provide a more consistent surface finish when through-feed and
infeed centerless grinding, Abrasive Technology Inc., Lewis
Center, Ohio, developed a technology for manufacturing seamless
superabrasive wheels and has recently received patents in the
U.S. (7393370) and South Korea (849587 and 849590). In addition,
the company received an approval notice from the European Union
and is awaiting patent issuance. Butch Peterman, company
president, developed the original concept and spent 9 years
perfecting it.
Conventionally pressed wheels are produced in segments, which
are joined with epoxy and/or screws, and have seams that can
leave markings on the length of a ground workpiece, explained
Glen Rosier, business development for resin- and metal-bond
products at Abrasive Technology, adding that seamless
wheels
eliminate any concern for that problem. In addition, he said
manufacturing variations can exist in the sections of seamed
wheels and make them prone to inconsistent wheel performance.
This variation causes a wheel to breakdown at a rate consistent
with its softest section.
He said the seamless wheels have virtually no porosity,
providing consistency in the wheel from end to end and from
outside to inside. In addition, compared to conventional methods
that typically produce wheels with a deviation of 7 to 10
Rockwell B-scale hardness points, the seamless wheels have
virtually no deviation in hardness points, according to Rosier.
“Thus you get very consistent wear and performance,” he said.
Rosier noted that the seamless grinding technology extends
wheel life 10 to 30 percent and enables higher stock-removal
rates. “Because it is stronger and tougher by nature, a seamless
wheel allows you to take a heavier DOC than you would typically
try to take with a conventionally pressed centerless wheel,” he
said.
Seamless wheels are available in diameters of 8", 10" and
20", and the company plans to offer a 24" wheel by the start of
2009. Currently, Abrasive Technology has not charged any
premiums for the new patented seamless technology. —A. Richter
www.ctemag.com
For more information, visit “Articles Archive Index” and select
the “Grinding” category.
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