Locking It In With A Good Taper Fit

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.

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.