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Topics within the text
below:
Overview - need for inspection
Shrinkage - details
Profile Scanning - for precision
Samples - output examples
Roll Testing - functional inspection
Tool Scope - visual inspection
Molded Gears NEED Inspection
The
physical dimensions of any molded gear are not cut or formed
directly. A mold cavity is constructed, shrinkage of the part
from the cavity walls is estimated, and the part is formed as
shown in the figure below.
Before
anything else, the molder must estimate shrinkage, cut the cavity,
mold the gear, and determine how close he came to the targeted
dimensions, but these types of measurements are not trivial.
The entire form of the gear, including the involute base circle
and tooth thickness must be accurately measured. Currently, no
standardized system exists for directly measuring gear shrinkage.
Kleiss
Gears, Inc. has developed its own system for this this measurement.
We directly measure and control all aspects of plastic gear shrinkage.
Simple outside diameter measurements or even roll testing with
a calibrated master gear cannot characterize plastic gear shrinkage
- the involute form must be measured independently. We employ
a scanning Coordinate Measurement Machine (CMM, left) for this
purpose.
Shrinkage and Plastic Gears
Molded
gears do not shrink in any simple fashion such as a photographic
reduction. There are a minimum of 4 distinct shrinkage rates
for any gear. Please refer to our downloadable article for more
information on molded gear shrinkage.
Even
simple features such as outside and root diameters must be carefully
inspected. A simple caliper check will often miss important features.
These diameters must be inspected for total form error as well
as concentricity to the principle bore or datum. At Kleiss we
probe the tip and root of each tooth and construct a best-fit
diameter with respect to the gear datum (as shown below).
Inspecting
the gear involute profiles requires just as much attention to
detail. Each tooth should be inspected since the molding process
can result in errors anywhere on the gear. The actual form errors
of the teeth should be measured directly so that these errors
can be eliminated in the molding process or compensated for in
the mold cavity.
Gear Profile Inspection
At
Kleiss, we scan each tooth of our molded gear to determine the
size and form error of the involute gear profile. The measurements
are taken with a computer-controlled probe scanning at a constant
rate with a constant force from the root of the tooth to its
tip. Accuracy of our equipment is calibrated to 40µ inches
with NIST standards on a yearly basis. The figure below graphically
shows the method of scanning.
 
After
the data is collected for all the teeth, it is mathematically
evaluated to determine profile form error and concentricity as
well as base circle shrinkage error and actual tooth thickness.
Below are two gear scan traces. The
first report is for an actual measured gear that has significant
shrinkage error. The second report is for a gear designed and
molded as a replacement for the same application. The original,
poorly molded gear had been purchased off-the-shelf from a commercial
supply house.
 
Gear Roll Testing
Gear
profile inspection is quite laborious and time consuming. It
is absolutely required for properly sizing a gear but is not
justifiable for production control of molded gearing. For this,
gear roll testing must be used. Gear Roll testing consists of
the following:
- Fabricating a nearly perfect
'master gear'.
- Placing the production gear
against it with a slight preload.
- Rotating the gear set while
under this light load.
- Recording both the actual center
distance and the variation of center distance for the production
gear.
If
it were a perfect gear, the center distance would be exactly
where it is specified, and it should have no variation from that
position. Eccentricity will cause a sinusoidal variation once
per revolution. Molded anomolies will affect it as well as tooth
thickness errors and variations. A picture of an actual roll
tester and a schematic of its construction are shown below:
 
At
Kleiss the center distance variations are recorded, graphed,
and kept as a history of the molding experience. A sample of
such graphs are shown below:
 
As
can be seen in these traces, roll testing can give an indication
of shrinkage errors but it cannot aid in determining the amount
of error or correction needed. This type of measurement is best
used to determine and validate process control.
Olympus Tool Scope
And finally,
we have in reserve an Olympus tool-makers microscope with magnification
from 50x to 500x with Heidenhain scales on all axes. Sometimes
nothing less than a tool scope focused and sighted by eye will
do the required job.
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