A machined taper fit between mating parts is a fast and secure way to put two
parts together. Fast mating and good alignment make taper fits critical in two
very different fields. One that most machinists are familiar with is the taper
fit that aligns the toolholder and spindle on a machining center. Whether it’s
an ISO, BT, CAT, R-8, Morse, Jacobs or HSK, the machined-in taper is what locks
the tool in the spindle.
A taper fit that most people don’t see but often rely on is the one used in
hip joint replacement. The acetabular component (hip ball socket) and femoral
stem (the part that goes into the leg bone) are held together by a machined
taper. Just like the toolholder, the taper aligns the two parts with a strong
fit that gets tighter as it is forced in.
What is most important to a successful connection is the accuracy of the
taper rate. The more accurate the taper rate, the more accurately the two mating
parts will fit. The more completely the two elements are mated, the stronger the
hold. The stronger the hold, the more stable the assembly, which is critical
both to accurately machining parts and walking down a flight of stairs.
Air gaging is an ideal way of checking the rate of taper on such parts.
Because air tooling is made to closely resemble the mating part, it compares the
deviations from the part to an ideal condition. In some applications, it is
important to measure just the rate of taper, while in others, it’s also
important to check part diameters at the same time.
When only checking rate of taper, a “jam” fit air tool is employed. This air
tool acts exactly as its name implies. It is a tapered piece of tooling that
contains two air circuits that measure taper diameters at a known distance
apart. Because there is no reference surface, the tooling goes into the part as
far as possible. If the clearance between the two diameters is the same, then
the part taper matches the taper machined onto the tooling. If the taper angle
is different, then the readout will “see” a variation in the two diameters
signifying an incorrect taper angle.
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| A medial taper plug. |
Occasionally, it’s also important to measure the two diameters for size in
order to control how deeply the two parts fit together. For example, a
toolholder that is undersized would have too much clearance between the mating
parts, resulting in a cutter that would become loose under the stress of
machining. An oversized condition would prevent the tool from being pulled to
the reference surface. This could potentially cause parts to be machined at an
incorrect size. Air tooling for this application is called “clearance fit” air
gaging, because a reference stop is included in the design. By using the
reference stop, the same two-circuit air plug can measure the taper and two
diameters at a specified location.
Air gaging is an excellent means of measuring the taper angle and the
diameters, because both of these are very tightly controlled. Diameter
tolerances typically are ±0.001 inch or better, while a taper
tolerance—typically specified as a taper per distance—is usually 0.001 inch/12
inches. While 0.001 inch may not be that difficult with today’s high performance
gaging equipment, the problem is that often there is not a full 12 inches to
measure the taper change-over. Thus, the taper gage has to measure over a small
portion of the taper—sometimes 1 inch or less. Now, that 0.001 inch/12 inches
specification becomes 0.000083 inch/1 inch which is more suited to the precision
of an air gaging tool.
Air gaging is an ideal method for measuring taper because of its
size/performance ratio. Machine tools with tapers are fairly large and have lots
of room for air channels and jets. However, hip replacements are relatively
small: mating tapers apt to be only 0.5 inch in diameter and 0.75 inch long. It
would be impossible to get any other type of measuring system squeezed into a
measuring tool as efficiently as an air gage. With air, jets can be as close as
0.150 inch and still get good performance and enough discrimination to measure
the taper angle.
Finally, the nature of air gaging makes it the ideal system to be used on the
shop floor. Using air as a medium cleans the part as it is measured. Because it
is made to simulate a mating part, it requires little operator training
virtually eliminating operator influence. Air tooling can be designed as
hand-held or bench-mounted. Hand-held gaging can also be designed for use on
parts that are still in the machining center.
Article courtesy of MMS Online.
