Advanced Taps Dies

Advanced Tapping

Old machining centers usually cannot match the rapid threading and traverse rates of high-speed tapping systems. But with self-reversing, constant-speed tapping attachments, older full-sized horizontal and vertical machining centers can meet or beat the holes-per-minute rates of such high-speed systems.

Rigid tapping, which is the most common tapping method used on full-size machining centers, is relatively slow because spindles must reverse twice for each tapped hole produced. A tap that begins cutting at 2,000 rpm, for instance, stops at the bottom of a hole, reverses and accelerates to 2,000 rpm as it backs out of the hole and then reverses again for the next hole. Self-reversing or constant-speed attachments, on the other hand, eliminate these spindle reversals, so machining centers thread more holes at lower speeds and feeds.

While a self-reversing tapping attachment is not a complete replacement for a new tapping center, it does make an older machining center more efficient without additional setups.

Self-reversing tapping units keep machine spindles running in one direction at constant programmed speeds and reversals occur instantaneously within the tapping head when the spindle retracts. Thus, spindle speed dictates the holes-per-minute rate. Reducing speed in the middle of a cut is bad for tool life. So eliminating speed fluctuations associated with machine-reversal tapping also increases tap life.

In addition, self-reversing tapping heads reduce required machine power.

Correct tapping speed

Several factors, singly or in combination, cause variations in permissible tapping speed.

These include thread pitch, tap-chamfer length, percentage of full thread to be cut, tapped hole length, cutting fluid used, straight or tapered threads, machine tool used and workpiece material. Feedrate and spindle speed must work in tandem to tap threads accurately, and boosting spindle speed means that operators must change the feedrate accordingly. Rigid, or synchronized, tapping synchronizes feedrates with spindle speed via a feedback mechanism.

Of the many factors that affect self-reversing attachments on high-production applications, correct tapping speed is perhaps the most critical.

Every tap manufacturer provides tap charts with a recommended speed range for specific taps in specific materials. Machine operators must select the best speed within that range, which is not always the highest speed. Each tap performs best and lasts the longest at a specific speed.

Faster speeds shorten tap life, induce tap changes and reduce the holes-per-minute rate due to increased downtime. Spindle speed also affects the life of self-reversing tapping attachments, which operate most effectively in the mid-range of their speed capability.

When using a self-reversing tapping head for cold-forming threads, acceptable torque output is limited by the size of the unit's components and is less than the output of the machine itself. Cold-form, or roll-form, tapping requires approximately 20 percent more torque than an equivalent cutting tap in the same workpiece material. Although the torque required increases as cold-form taps dull, they may produce acceptable threads even after doubling the required torque.

For maximum cold-form-tap life, a rigid tap driver and machine reversals are recommended. However, if the objective is reduced cycle time at the cost of more frequent changes, then self-reversing, constant-speed tapping heads are the best choice.

With cold form tapping, material grain fibers are not cut as in conventional tapping. Pressure forms internal threads and grain fibers follow the contour of the thread. A cold-forming tap has neither flutes nor cutting edges and, therefore, produces no chips. Cold-forming taps work well for threading ductile materials such as low-carbon steels, leaded steels, austenitic stainless steels, aluminum die-casting alloys (low silicon), wrought aluminum alloys, zinc die-casting alloys, copper and copper alloys.

It is a good idea to start at speeds at the low end of a recommended speed range when cold forming threads using coolants without EP additives or if the lubrication quality of a coolant is unknown.

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