Page 23 - MetalForming September 2010

three curves represents the peripheral speed while the tap is engaged in the material. The red and blue curves give speed profiles for direct-drive and exter- nal drivescrew IDT devices, while the yellow curve represents the speed pro- file for a cam-actuated internal drive- screw IDT device. Because direct-drive designs generally do not disengage from the upper die shoe during operation, the tap moves continuously whenever the press operates. Thus, the tap has signif- icant velocity when it’s trying to engage the hole.
By contrast, cam-actuated IDT devices are engaged and disengaged by the upper die shoe during the press cycle. The tap gradually accelerates from rest and exhibits a low velocity upon entry into the prehole—the single most critical moment in an IDT operation. In the case of the red curve, the peripher- al speed of the tap upon entry is about 16 m/min. In the case of the blue curve, the peripheral speed is about 12 m/min. In the case of the yellow curve, the tap peripheral speed is very slow upon entry into the workpiece, which generally makes for more reliable engagement and reduced tap wear rates.
How to Maximize Speed
Key points in maximizing the speed of an IDT application:
• Understand and respect the kine- matics of the process. Design and build the die around the necessary operating parameters of the IDT unit.
• Use the longest tapping stroke pos- sible. Not to be confused with depth to tap, tapping stroke is the amount of press stroke that can be safely used to tap.
• Match stripper travel (and piloting duration) to the tapping stroke.
• Choose the shortest press stroke available. The press stroke, of course, must be greater than the tapping stroke and must allow sufficient time for feed- ing and piloting. A 12-in.-stroke press operating at 30 strokes/min. will exhib- it double the ram velocity of a 6-in.- stroke press operating at 30 strokes/min. (Fig. 6).
• Maintain the prehole at a diameter
that corresponds to 65 percent of theo- retical full thread.
• Provide ample, quality tap lubrica- tion, delivered directly to the first three to five leads of the tap on each and every press cycle.
How to Handle HSLA and Stainless Steels
When stamping high-strength alloys, the challenge of producing a quality
prehole that is uniform in size and shape increases due to the strain-hard- ening nature of these materials. In most cases, at least one shaving operation is needed after the hole has been initially formed.
Compared to a cold-rolled-steel application, the allowable tap periph- eral speed with HSLA is about half, and with stainless steel it’s about one third. MF
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