Posted by Phil Rockwell on June 08, 2002 at 12:43:13:
In Reply to: True Position versus Timing of Spline teeth posted by Zachary Young on June 06, 2002 at 14:05:35:
You have three issues:
1. How to measure the nominal location of a spline tooth.
2. The range over which the tolerance applies.
3. Suitability of datums.
1. To the best of my knowledge, drawing standards, such as ASME Y14.5M-1994 and
ISO 1101, do not specify how to measure the nominal location of a complex feature
like a spline tooth. On a simpler feature, like a keyway, you would determine
the location of the largest key with perfectly flat and parallel sides (but the
key would not necessarily be perfectly oriented) that would fit in the keyway,
then figure how far out of position of the worst point is on a plane exactly
halfway between the faces of the perfect key.
Applying this approach to your situation, you would somehow determine the
location of a pair of involutes of perfect form (but not necessarily perfect
orientation) that would fit against the flanks of the spline tooth, then figure
how far out of position of the worst point is on a plane exactly halfway between
the faces of the perfect involutes.
A simpler approach would be to measure points on the flanks of the spline tooth
at the pitch diameter, plot two straight lines representing the high spots of the
two sides of the tooth, and figure out the position error of the worst point on a
line exactly halfway between the two plotted lines. It is between you and your
customer to decide whether this would be an acceptable criterion.
2. A tolerance applies only within the limits of the size of the feature to
which it applies, so your interpretation of larger implied tolerance at a larger
radius is correct.
3. It is incorrect to use a feature as a datum if it is unfinished to the extent
that variations of location of points on the surface of the datum exceed the
tolerance of the other features relative to the datum.
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