It’s funny how everyone has heard of Morse and Jacobs Tapers, but how often do we think of 7/24th tapers? Sure, it sounds like a rare taper, but all BT, CAT, NMTB and ISO (IT) toolholders use this taper size, so it’s a lot more common than we think. Although over 89% of all machining centers in the US use this taper interface on their machining centers, when was the last time we addressed much attention on the quality of the connection between the toolholder taper and the spindle taper?
It’s easy to assume a taper is a taper, allowing us to often neglect the interface. Instead we focus and attribute cutting capability more on what holds the cutting tool within a toolholder assembly. However, as with all quality tapers, the surface area contact between the male and female interfaces provide not only the repeatable precision necessary to assure accurate cutting tool run out, but also to establish a solid connection that can sustain the radial and axial forces associated with machining. Higher-quality toolholders ground to a closer taper tolerance achieve the ideal 85 to 90%, while lesser quality tapers can range between 60 to 70% contact. While the lesser contact toolholders may work for the time being, after time, the minute gap between both the toolholder and spindle taper creates micro vibrations. These vibrations start to wear on the surface of both components, often resulting in a shot peened finish that’s commonly referred to as fretting.
Prior to purchasing any new toolholders, verify that your toolholder supplier’s tapers comply with the ISO1947 (1980 revision) angular and diameter specifications, and clearly express that their taper quality is AT3 or better.
AT2 Angular Specification for 7/24 taper (Female Spindle Taper Tolerance)
CAT40 Angular Tolerance of Taper: ± 0.00083 degrees
CAT50 Angular Tolerance of Taper: ± 0.00069 degrees
AT3 Angular Specification for 7/24 taper (Male Toolholder Taper Tolerance)
CAT40 Angular Tolerance of Taper: ± 0.00139 degrees
CAT50 Angular Tolerance of Taper: ± 0.00111 degrees
AT2 Diameter Tolerance for 7/24 taper at Gage Line (Female Spindle Taper Tolerance)
CAT40 Diameter Tolerance at Gage Line: +52 to +80 millionths (0.000052″ to 0.00008″)
CAT50 Diameter Tolerance at Gage Line: +64 to +100 millionths (0.000064″ to 0.0001″)
AT3 Diameter Tolerance for 7/24 taper at Gage Line (Male Toolholder Taper Tolerance)
CAT40 Diameter Tolerance at Gage Line: +80 to +126 millionths (0.00008″ to 0.000126″)
CAT50 Diameter Tolerance at Gage Line: +100 to +160 millionths (0.0001″ to 0.00016″)
Unfortunately, since most machine shops do not carry equipment to measure angular and diameter tolerances this tight, consider investing in a reputable manufacturer that can show documentation of strict adherence to these standards.