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Laser Scanning Helps Define Complex Thread
Geometry
 A
manufacturer recently designed an internal-return recirculating ball nut
that offers major advantages over conventional external-return
recirculating ball nuts. But in machining the area of the thread where
its direction changes, measures taken to overcome helical path
interference produced such a complicated geometry that it was very
difficult to mathematically define it.
A coordinate
measuring machine (CMM) would not have been able to capture enough
points within such a small area to define the geometry to the required
level of accuracy. Laser Design Inc. / GKS Inspection Services Division
(LDI/GKS), Plymouth, Michigan, a laser scanning service bureau, solved
the problem by cutting the nut in half, using a CMM to measure key
features, scanning each half of the nut to capture the internal
geometry, then assembling the scans by using the CMM data as reference.
“Laser scanning helped us get the new ball nut defined by accurately
measuring the geometry of our prototype so we could establish
dimensional limits,” said the company’s President.
 The
company’s internal recirculating ball nut has a unique design. For about
2/3 of a revolution, the thread is nearly identical to a conventional
ball nut with balls traveling along the grooves in the forward
direction. When they reach the end of this section, the balls enter a
reversing groove which transports them up and over the land of the
screw. When the balls reach the opposite end of the reverse groove, they
are directed down into the screw at the beginning of the forward groove,
at which point they begin the cycle again.
The design offers
considerable advantage over conventional ball nut designs. One of the
biggest advantages is its simplicity which eliminates the need for
additional components such as a crossover insert or external
recirculation tubes as required in conventional ball screws. The new
design is also substantially more reliable because the recirculation
system is so much simpler and has far fewer possible points of failure.
The elimination of the external recirculation system also makes the new
design considerably more compact and provides lower friction losses than
conventional ball nut design.
 The
simplicity of the new system also makes it substantially less expensive
to manufacture than conventional ball screw assemblies. The nut in the
new design can be machined in a single setup on an internal grinder. The
new design also avoids heat treatment movement problems inherent in ball
screws that have holes through the wall and cutouts for longitudinal
inserts. The company is offering to license the design and has already
licensed it to a manufacturer of automotive power steering systems.
Company engineers
defined the simple geometry of the internal threads on the ball screw
nut on a solid modeling system. But it was impossible to build the
geometry exactly as it had been designed.
The thread form is
machined or ground with a milling cutter or grinding wheel that has a
special form designed to produce the nut thread. As with most internal
thread grinding, the internal curvature of the nut interferes with the
wheel/cutter, producing a form that is slightly different from the
wheel. The engineers were able to calculate this effect, called helical
path interference, for the forward and reverse threads. They compensated
for it by correcting the form of the grinding wheel, despite an off axis
spindle and a “live” articulating helix axis that is swinging the
spindle as it grinds the passageway. They were never able to
mathematically define the form produced by these components at the point
where the wheel changes direction to cut the reversing passageway.
 “We
had to precisely define the inside dimensions of the nut so that we
could define manufacturing limits,” said the company’s President. “But
the form was much too complex to measure using manual gauging
techniques. We considered trying to measure it with a CMM but the
geometry was so complex that it would have taken an enormous effort to
capture a sufficient number of points. It would have taken tens or
hundreds of thousands of points to provide an adequate definition.
Obtaining all of these points one-by-one by manually moving the CMM
probe around the part without having a good solid model to work from
would have taken forever.”
 He
searched for an alternative method of digitizing the model and
discovered the technology of laser scanning. Laser
scanning systems work by projecting a line of laser light onto surfaces
while cameras continuously triangulate the changing distance and profile
of the laser line as it sweeps along, enabling the object to be
accurately replicated. The laser probe computer translates the video
image of the line into 3D coordinates, providing real-time data
renderings that give the operator immediate feedback on areas that might
have been missed.
Laser scanners
are able to quickly measure large parts while generating far greater
numbers of data points than probes without the need for complex
programming or part fixtures. Since there is no contact tip on a laser
scanner that must physically touch the object, the problems of
depressing soft objects, missing small details, and capturing complex
free form surfaces are eliminated.
 Instead
of collecting points one by one, the laser scanner picks up tens of
thousands of points every second. This means that the scanning of the
most complicated parts can often be accomplished in a few hours or less.
Laser scanning can collect data on parts that are so complex that they
would be practically impossible to digitize one point at a time.
Finally, the software provided with the scanner greatly simplifies the
process of moving from a “point cloud” to computer aided design (CAD)
model, making it possible in minimal time to generate a CAD model of the
scanned part that faithfully duplicates the original part.
The company’s President did not think it made
sense to purchase a machine and train staff members in its use for a
single project so he searched for a service bureau that would provide
laser scanning services on a project basis. “I selected GKS Inspection
because of GKS’ long experience working with laser scanning technology
and its application to real world engineering problems,” he said.
 “We
spoke with the company and first addressed the question of how to gain
access to the critical internal thread forms,” said Steve DeRemer,
General Manager of GKS Michigan. “We determined that the part should be
cut lengthwise using a wire EDM machine. The wire EDM process allowed
the part to cut without subjecting it to undue stresses that might alter
the dimensions of the part.” To allow the two pieces to be aligned after
cutting, the entire part was first measured on a CMM. The internal and
external diameters, end planes, notch and other features were picked up
for later reference.
After cutting,
temporary reference spheres were adhered to each of the two halves of
the nut. Then each half was digitized on one of GKS’ laser digitizers.
By laser digitizing the parts, hundreds of thousands of points were
picked up on each half of the nut.
 After
digitizing, the point data sets from the two halves were aligned to each
other using the CMM data as reference. Then the point cloud data was
used to construct a solid model of the nut in SolidWorks. Having so
much point data aided in modeling the complex grooves found on the
inside of the part. After modeling, the surface data was compared to the
point cloud data to ensure that it conformed to the digitized part. The
finished solid model was then sent to company engineers who compared the
solid model to the physical part using both manual methods and a CMM.
“We are still in the
process of evaluating the accuracy of the model but we are far enough
along that we can say the accuracy is very good,” said the company’s
President. “GKS estimated that they could achieve 0.002 inch accuracy
but our measurements show they have done better than that. Laser
scanning made an important contribution to this project by providing
accurate dimensional measurements that would have been difficult or
impossible to obtain any other way.”
For further
information, contact GKS Inspection Services at 952-252-3433 or by email
at:
measure@gks.com
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