Page 22 - MetalForming September 2010

In-DieTapping
Die protection/sensoring: Physi- cally detecting the tap’s successful pen- etration through the workpiece is an important step in striving for zero- defect IDT. Inductive proximity sen- sors, photoelectric/optical sensors or physical contact switches may find use, according to the end-user’s preference. Turn the sensoring equipment to “look on” for the tap’s presence generally around bottom-of-stroke, between 180 to 190 deg. after top-dead center. A good tap sensing setup can detect bro- ken or worn taps (when partial misfeed- system actuation is involved), as well as improper tap-setup positions at initial tryout and each successive production startup.
Optimizing Tap Life
Tap life: A common—and difficult- to-answer—question from IDT users is, “What kind of tap life can I expect in this application?” Material type, tap size and depth, press speed and stroke, the amount of press stroke used for tapping,
prehole diameter, tap coating, and the type and quantity of tap lube all affect tap life. For IDT manufacturers, it’s generally safer to answer such ques- tions with actual case-study data.
With a new application, it’s advisable to optimize and maintain the key parameters and then benchmark the resulting tap life. In healthy IDT appli- cations, taps generally will wear out before they break. In one mode of wear, the tap continues to fully penetrate the part, but the forming lobes become so worn they’re no longer producing a deep enough thread (major diameter), and the “go” side of a pitch-diameter thread gauge no longer will go through the part.
Another way a tap can wear out: The lead-in threads become too dull to immediately engage the prehole, and a partial actuation of the misfeed spring (safety system) occurs while the tap struggles to enter the hole. As the tap skids against the hole opening, the safe- ty system’s misfeed spring compresses
and its preload increases. Eventually, a sufficient amount of preload develops and the tap finally engages the part and begins to form a thread. However, in such cases the tap invariably loses use- ful revolutions upon entry and, in so doing, usually falls short of tapping the part completely to depth.
Watch the Speed Limit
Speed restrictions with IDT tech- nology almost always result from the physical limitations of the taps, rather than the IDT units themselves. These limitations are mandated by the amount of friction-generated heat that can be tolerated by the tap’s base material and coating, and the lubricant being applied. So, the next time you’re hoping to go 60 strokes/min. in an M10 x 1.50 stainless- steel application, don’t blame your IDT supplier when it tells you it can’t be done.
Fig. 5 shows peripheral tap speeds for three different IDT alternatives for a stainless-steel application. Each of the
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18 METALFORMING / SEPTEMBER 2010
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