Thread Milling Takes Off

March 24, 2009
Advanced machine tool and cutting tool technologies reduce the cost and boost the quality of producing internal threads in aerospace materials.

By Mark Hatch, Emuge Corp.

Edited by Jim Benes
Associate Editor

The diameter of a thread milling cutter is smaller than the drilled hole, and the cutter turns in an elliptical orbit to reduce cutting torque when compared with machine tapping.

One of the most challenging metalworking applications is the production of internal “J” threads in difficult-to-machine aerospace materials such as 718 Inconel, titanium, and 15-5 PH/17-4 PH stainless steels.

Traditionally, “J” threads have been produced by machine tapping, hand tapping or a combination of both methods. But even with the latest high-tech tap designs, problems with inconsistent performance, low tool life, poor thread quality and tap breakage continue to plague shops that produce aerospace components.

Given the high cost of aerospace metals and machining time, and the fact that internal threaded holes are normally completed in a final operation, thread-quality problems requiring rework or the scraping of a part can be extremely costly.

CNC machine tools that have the computer power, flexibility and mechanical reliability to accurately generate 3-axis tool paths, along with advanced thread-milling technology, allow shops to go beyond the limits and costly problems associated with tapping. Now, the many benefits of milling internal threads can be used to generate significant cost savings and to improve part quality.

The advantages derived from thread milling aerospace materials are primarily associated with innovative designs of new solid carbide thread mills and the basic process difference between milling versus tapping an internal thread.

The trouble with tapping
Taps remove material by a spaced sequence of cutting edges that start at the chamfer and are in the cut continuously. This type of cutting tool produces a long chip that must be mechanically transported out of the workpiece by means of the spiral fluting of the tap. Also, taps are manufactured to the dimensional size of the thread they are to cut. Thus, they are fully engaged with the workpiece during the cutting process, creating high cutting torque.

A tap is in the cut continuously, and the resulting long chips must be removed by its flutes.
The degree of profile correction required on a thread mill depends mostly on the ratio of cutter diameter to thread major diameter. The larger the cutter diameter, the higher the degree of profile correction needed. A precisely ground cutter thread-groove correction is required to avoid over-cutting of the flanks.

To deal with chip removal, the emphasis in tap design is on the form of the flute and rake; while, to reduce cutting torque, the emphasis is on the design of relief angles.

However, aerospace materials present particularly difficult combinations of toughness and squeezing, or jamming, characteristics that even the most advanced tap designs have not resolved. Also, many aerospace applications require blind holes with full thread to within a single pitch of the bottom.

A tap ground with a one-thread chamfer is a poor cutting tool and is likely to break due to excessive cutting torque. As a result, many aerospace thread applications are started on a machine with a long chamfer tap and the final two or three threads are completed by hand with a short chamfer tap. However, in an effort to avoid broken taps, a costly secondary operation is added and quality problems often result due to poor thread finish and oversize gauging.

Thread milling advantages
In contrast, circular thread milling is an interrupted cutting process in which material is removed by cutting short, broken, coma-like chips that are shaped like a pig-tail. Short chips need little space in the flute and are removed easily by the high velocity of the spindle and the high-pressure of through-the-tool coolant when it is available. Also, the diameter of a thread milling cutter is smaller than the minor — or drilled-hole — diameter.

Therefore, only a small surface area of the tool is in contact with the work. That reduces cutting torque significantly. Having the cutter diameter smaller than the drilled hole also allows for roughing passes to be used, and that also reduces cutting torque.

These primary differences in the cutting process give thread mills the advantages of having more efficient chip formation and evacuation and significantly lower cutting torque when compared with taps. In addition, thread mills do not require a multi-tooth chamfer to optimize cutting performance as taps do, so they can produce a full thread to within one pitch of the bottom without affecting tool performance.

In addition to the basic differences in thread cutting processes, the design of thread mills and the way they are made also increases their benefits and security for milling aerospace materials.

Thread mills made from a micro-grain grade of sintered carbide and formed through hot, isostatic processing, such as those from Emuge Corp., provide a balance of heat resistance, wear resistance, deflection resistance and impact resistance. Those properties allow for thread milling at high speeds and feeds, the machining of materials that have hardness to 58 Rc, and they provide long tool life.

Thread milling cutter characteristics
As it does for taps, the cutting geometry of a thread milling cutter influences the performance of the tool. The main characteristics of a high-quality thread milling cutter relate to the rake, helix and clearance angles, and precise correction of the cutter’s profile.

The conformity of the finished thread profile is determined by the way the tool is ground. Circular thread milling cutters have thread grooves that are ground without continuous pitch, but the finished thread must be produced with a continuous pitch. To accomplish that, the milling cutter turns in an elliptical orbit with respect to the thread angle. The elliptical orbit creates a difference between the angle of the thread grooves on the cutter and thread angle on the finished part. To do that, the thread grooves on the cutter must be ground precisely with a corrected profile to ensure that the flanks of the finished part are not over-cut.

The degree of profile correction required on a thread mill depends on the ratio between the cutter diameter and the major diameter of the thread, and the lead angle and flank angles of the thread.

However, the primary factor is the ratio between diameters. Higher degrees of profile correction are required for larger cutter diameters. Larger cutter diameters are desirable, because they improve tool rigidity and strength. However, as a cutter diameter increases in ratio, finish grinding becomes significantly more complicated, and highly advanced grinding technology is required to ensure true conformity.

Producing thread mills with a large web diameter is the best way to allow for longer flute length and the addition of internal coolant supply without sacrificing rigidity and strength. An internal coolant supply ensures that the cutting edges are well cooled and that chips are washed out of the hole, so that the cutter can work freely to prevent chipping or breakage from re-cutting of chips.

Spiral-fluted thread milling cutters that have multiple spiral flutes use the spiral to reduce cutting forces, rather than to transport chips out of the hole, as with spiral-fluted taps. In addition to the substrate and finished ground geometry, an effective coating, such as titanium carbon nitride (TiCN), can improve edge wear and thermal stability.

Editor’s note: Information for this article was provided by Mark Hatch, thread milling manager at Emuge Corp. Photo and illustrations courtesy of Emuge Corp.

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