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Category Archives: Toolholders

1 AprilHow important is taper contact between your machine spindle and your toolholder taper?

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?

Taper-Contact-BT-CAT-NMTB-Tapers

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.

31 MarchCutting Tool Runout – How tenths can affect your productivity

Machine Spindle - Tool RunoutWithin your toolholder assembly, one of the most important factors that can affect or limit your machining potential is the runout of the cutting tool. Albeit .0005 to .001” may not seem to be a lot, nothing affects tool life, predictability of tool life, cutting ability, surface finish, and part tolerances more than runout. To add more fuel to the fire, in many cases, runout doesn’t only cause just one of these issues, but many, if not all at the same time. This often results in longer part production time, and can lead to unnecessary down time to change out cutting tools more frequently.

Let’s take into account a simple formula to calculate your feed rates: Feed Rate= RPM X number of teeth X chip load per tooth

Now, let’s give an example of how runout can slow production under the guise of optimal machining capability. Assume we’re in the process of setting up a job to run into production. With all things being equal with RPM and the number of teeth on the cutting tool, if we were to use a chip load per tooth of .005”, a .001” runout adds 20% more cutting force unnecessarily to one tooth unevenly. Unknowingly to the circumstances of the runout, the operator will proceed to make adjustments with the feed dial during machining operations concluding his optimal feed rate is where the squeals, sound and load meter are at a comfortable level. In addition, if the operator continues to use a toolholding system that provides inconsistent runout (some good and some bad), cutting tool life predictability becomes variable, often resulting in the operator picking the lowest quantity produced as a safe benchmark of when to change cutting tools. To the detriment of the business utilizing the equipment, this example shows how costly such a small issue can not only limit the true machining productivity of your machining center, but also limit the ability to get the full cutting capability of the cutting tool.

The solution goes along the saying, “Prevention is better than a cure.” For starters, although this may take a bit more time on your set up, make an active effort to verify that cutting tools are running true prior to machining operations and see if improvements can be seen. As one may also realize how runout may vary each and every time a cutting tool is replaced, the value in investing in a quality toolholding system that provides accurate and consistent runout will become apparent.