Tap holder technology for super alloys

In aerospace machining, new solutions to common problems are key to remaining competitive. Aerospace is largely characterized by machining high-temperature superalloys into complex parts with extremely tight tolerancing, low volume production, scrutinous quality checks and controls, and high tooling costs. With many large firms outsourcing up to 70% of their manufacturing, every machine shop looks to new technologies and solutions that help reduce production costs.

High-temp alloys are notoriously difficult to machine due to their wear and stress resistance at high temperatures. While this characteristic makes them perfect for the aerospace industry, it also makes them very difficult to machine. Specifically, tapping this material requires highly specialized tooling, running at low surface feet per minute (SFM), flooded in high lubricity cutting oils, all while hidden from view. Many have switched to milling threads in these materials to get away from single-digit hole counts when tapping; however, this is not always possible due to time constraints in a just-in-time environment and increasingly complex designs. Short tool life, tap breakage, and thread quality problems eat away at profit margins. Having the correct tap and lubrication in a CNC environment is simply not enough to give the tool life and reliability the industry demands.
 

Superalloys challenges

Taps are unique in the cutting tool industry because they are the only tool in which the feed rate remains static and must be synchronized with the tool’s ground pitch. Any deviations from this precise feed rate causes problems including oversize threads, premature tool wear, and tap breakage. While the CNC machine will introduce a few of these errors during the tapping cycle because the machining process is slightly imprecise, the largest variances in aerospace applications come from the materials used.

In an iron or steel application, high speeds generate high heat that make the materials softer and more easily machined, reducing pressure on the cutting tool. Nickel-based superalloys remain tough and reflect heat toward the tool tip, causing increased wear and a breakdown of the cutting edge. This dulling makes the tap push the material instead of slicing it, introducing work-hardening at the contact point.

Because the tap is surrounded by tough material on all sides, it must also deal with high temperature/thermal expansion. The hole will tend to clamp down onto the tap after it cools following cutting. This is why high-lubricity tapping oil or fluid is absolutely necessary. All of this leads to exponentially higher torque load on the cutting teeth, which increases axial forces acting on the tap.

After the tap fights through the work hardening, increased wear, and thermal expansion forces, it must reverse from a fully clamped position to break a very tough chip and evacuate it in a blind hole application. In a rigid tapping environment, all of these extra cutting forces directly impact the cutting edge of the tap, leading to premature tool failures. Emuge’s solution is to absorb and dampen these forces via its modular Softsynchro tap holder.

High-temp alloys are manufactured to be heat retardant, forcing increased temperature onto the tool tip and away from the material. Increased temperature at the tap surface elongates the tap, and the difference in length from when the tool was measured at room temperature to its operating temperature is a few ten-thousands of an inch – enough to cause excessive wear on the thread flanks.

Emuge’s patented toolholder that physically separates the transmission of torque and axial force during the tapping cycle. The modular, collet-type holder, KSN/HD/Softsynchro, uses a synchronous spindle to provide optimum tool life and thread surface quality. Two precision-ground sections, a body and shank effectively separate the spindle from the tap via a patented elastomer spring that absorbs axial forces, boosting tap tool life and performance. Torque from the spindle is transferred to the tap via ball bearings riding in precision ground grooves. These ball grooves minimize rolling friction on the torque transmission balls to guarantee precision micro-correction of lead errors in a rigid tapping cycle. The tap is effectively riding on shocks to prevent excessive wear due to small pitch variances during the cutting process, while ball bearings allow for smooth rolling motion and complete transfer of torque from spindle to tap.
 

Softsynchro benefits

After three torn threads and eight broken taps, a New England aerospace manufacturer contacted Emuge for a solution to tap an STI 1/4-28 thread in 718 Inconel, 0.600" deep. An Emuge territory engineer recommended the STI REK.1DF-NI TICN tap with variable helix angles designed for Inconel. With the use of Emuge tapping fluid, a few changes in the running parameters, and support from technicians, the tap made 10 threads before slight chipping of the first tooth. While the customer was satisfied, Emuge engineers wanted to push the process to the next level. Working closely through distribution and territory managers, technicians were able to provide the customer a tap holder for testing purposes for further cost savings. The Softsynchro holder more than quadrupled the tap’s tool life, making even cleaner threads. At six threads per part, the system would pay for itself after only 10 parts, delivering a total solution from spindle to workpiece.

The next difficulty was the number of different machines the customer was able to use for tapping. Buying 10 different tap holders for different machine spindles, when they would not all be used consistently, was costly. Emuge offered the customer a single 1" cylindrical shank with Weldon flats that could be adapted to a wide variety of spindles via precision, stubby shank adapters. From CAT to HSK, the number of holders was reduced to two, with four different adapters. At a fraction of the cost, the adapters were an efficient, cost-saving solution.

Emuge Corp.
www.emuge.com

About the author: Jose Alvarenga, tap holder product manager at Emuge Corp., can be reached at jose.alvarenga@emuge.com.