Page 44 - MetalForming April 2011

Tooling Technology
IDT misfeed system’s compression spring must supply sufficient preload force to cause the tap to immediately bite into the prehole upon contact. Rollforming/cold-forming taps pre- dominantly are used in IDT applica- tions and require much more axial force (to engage the prehole) than does an equivalent cutting-type tap. The natural tendency of the first thread of a rollforming tap is to skid against the
prehole’s opening–particularly in the presence of a chamfer or rolled edge. This tendency increases as the tap wears.
Thus, the axial force required for tap engagement also increases over time, which can sometimes present a confusing problem to diagnose. When the tap is new and sharp, parts are pro- duced with threads fully tapped to the required depth. However, as the tap
wears it can start to skid if the required axial engagement force exceeds the available preload force stored in the misfeed spring. As the tapping unit turns the tap, the female lead bushing retracts inward and the misfeed spring undergoes further deflection, because the tap is rotating but not entering the prehole.
To Better Envision this Scenario
...consider how a jack screw oper- ates. As the misfeed spring further com- presses, the force it exerts against the tap increases. Once the spring force equals the tap’s required axial engage- ment force, the tap bites into the part and tapping begins, albeit late. As a result, valuable rotational movement required for tapping has been expend- ed in compressing the misfeed spring upon entry, resulting in an incom- pletely tapped thread (Fig. 3).
In the pressroom, this condition can easily be recognized when per- forming a QC check and the thread gauge nearly makes it through the part cleanly but goes tight within the last turn or two. Note: Most IDT suppliers add some safety margin (generally an extra half-revolution, or half of the tap’s pitch) to insure against this con- dition. However, it’s worthwhile to rec- ognize that the male and female lead bushing of the pitch insert (tap car- tridge) will undergo accelerated wear whenever partial misfeed spring actu- ation occurs.
For optimum results, the misfeed spring must remain in a purely static condition throughout the entire tap- ping cycle. Stainless steel and HSLA typically are more resistant to initial tap engagement and require greater preload in the misfeed spring. Under- sized prehole diameters also increase the tap engagement challenge, and, as a reminder, attempting to tap more than 75 percent of theoretical full thread is never recommended.
To combat the problem of incom- plete-depth threads, use a sensor to look for the tap at the end of its stroke (Fig. 4). Since mechanically driven IDT
connect everywhere
Network your factories and offices with SmartPAC 2 and LETS (Line Efficiency Tracking Software)
Take advantage of
SmartPAC 2’s built-in Ethernet connectivity to communicate over your network.
Send and receive text messages, display setup sheets stored on a central server, back up your tools to a remote location, and view pressroom status over the network and internet. When you connect your equipment to LETS, you can collect production data, feed critical parameters reports automatically, view machine status in real time, and schedule a prioritized job queue for each machine.
To learn more about Wintriss Controls, please call 1-800-586-8324 or visit www.wintriss.com. ®2010 Wintriss Controls. All rights reserved.
42 MetalForming/April 2011
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