Exotic metals are more popular than ever as industrial designers and manufacturers take advantage of the properties exhibited by these materials, such as high strength-to-weight ratios, excellent corrosion and oxidation resistance, and strength and hardness retention at high temperatures. But properties that help make great finished components also may make machining parts a challenge throughout production, including threading.
Manufacturers usually have two main threading methods from which to choose: tapping and thread milling. Tapping is traditionally the go-to method for cutting threads into standard metal parts but can be challenging with these hard, heat-resistant, expensive materials. Thread milling costs more and has requirements for equipment and software but is much more flexible and multifunctional and produces pristine finishes. Tapping is fast but might gall a surface, tear a thread or — worst case — break a tap inside a part. In that scenario, a beautiful part is likely expensive scrap.
The Bottom Line
When considering which process to use, threading experts point to the potential for monetary loss — both in terms of raw material and value-added processing — as the No. 1 factor.
Threads-all ZGF-S-Cut solid-carbide thread mills feature multiple teeth, a helical flute form and a multilayer TiAlN T46 coating. Image courtesy of Emuge
“Threading is typically one of the last processes performed on a part,” said Marlon Blandon, product manager of thread milling for Emuge Corp. in West Boylston, Massachusetts. “By the time you do the threading, many machines have engaged that expensive part, and the overall cost is much higher.”
While any cutting tool can break from wear and tear after enduring heat resistance, tensile strength, ductility, hardness and other factors, taps often cannot be fixed. The major diameter of a tap is almost exactly that of the inner diameter of the milled hole that the tap is grooving.
With thread milling, the major diameter of a tool is much smaller than the inner diameter. If that tool breaks, it can be removed and replaced by another tool to finish grooving threads.
“If the tap breaks,” Blandon said, “you’re left with a piece of high-speed steel inserted very tightly into a $5,000 engine casing or turbine, bone screws in medical parts or another expensive part that is now basically scrap.”
A tap might break for a number of reasons, but it often comes back to the properties of exotic materials. Exotic metals are alloys composed of a blend of metals to create a more functional material with specific physical properties. For instance, some exotics stiffen from heat, making them literally and figuratively harder to machine, he said. Emuge recently released ZGF-S-Cut solid-carbide thread mills that feature multiple teeth, a helical flute form and a multilayer TiAlN T46 coating that can thread-mill a variety of exotic materials.
Examples of exotic metals are titanium, stainless steels and hardened nickel-alloy steels. Well-known brand names include Inconel, an austenitic nickel-chrome mix, and Hastelloy, a nickel-molybdenum recipe. Both are noted for hardness and durability, resistance to rust and corrosion, and the ability to withstand heat and remain stable over a wide range of temperatures.
Nickel alloys are “incredibly good at absorbing heat,” said Executive Vice President Drew Strauchen of GWS Tool Group in Tavares, Florida. (South Beloit, Illinois-based North American Tool Corp. is a division of GWS Tool Group.) “In an aircraft engine housing, heat builds up, and you don’t want the heat getting out to the wing or electrical wiring. So you house it with an exotic metal, such as Inconel 718, and it absorbs the heat. This metal has wonderful attributes for the component but terrible attributes if you want the cutting tool to last.”
These thread mills feature a helical flute design and are manufactured from premium micrograin carbide for increased life and improved thread quality. Image courtesy of North American Tool
Mark Ford, director of global product management for threading tools at YG-1 Tool (USA) Co. in Vernon Hills, Illinois, said heat and points of contact on a tap quickly could wear out the tool. Taps traditionally are made of various grades of high-speed steel with a physical vapor deposition coating or surface treatment, and they work well with a range of materials up to about 50 HRC. Thread mills typically are made of carbide and can mill material up to 60 HRC.
“In tapping, the tool is fully engaged at once, and that generates a lot of heat that gets put back into the tool,” he said. “Temperatures above the oxidation point will begin to oxidize the coating, and then it gets down to (cutting with) the bare metal.”
Carbide is the material of choice for thread mill makers. While harder, carbide is not impervious to the crystalline structure of martensitic steel or other properties of superalloys. But carbide can withstand things better.
“In addition, through CNC programming and the smaller major diameter of the cutting tools, users can select how much material that (they) want to remove per pass and adjust spindle speed to achieve the best thread quality,” Ford said.
Tapping is a much simpler procedure that can be done quickly on a variety of machines. Thread milling exotic materials, however, requires at least a three-axis CNC machine capable of helical interpolation, which means that the machining center can move with simultaneous motion in three axes, Ford said.
He said in the past, programming this action might have been a challenge, but most modern CNC equipment is easily capable of helical interpolation, and most tooling suppliers offer software and applications to help customers. YG-1 Tool (USA) recently released a thread milling program generator, YG-1 TM Xpert, which generates G code required for thread milling and displays relevant images and data.
In addition, Strauchen said exotic solid-carbide thread mills today are made with a left-handed helix and left-handed cut, which permit an operator to run from the top of a hole to the bottom and climb-mill threads.
“This will create a right-hand thread in the part,” he said.
Compared with tapping, thread milling also can be more precise.
“We are talking microns of precision,” Blandon said.
Although taps can be used in exotics, Ford said taps might not be the better choice for several reasons, such as precision and potential scrap cost. Like the other companies interviewed, YG-1 Tool (USA) produces taps and thread mills, including those for exotic metals. He said the choice ultimately comes down to what a customer wants and needs.
“The preferred method for producing threads is tapping,” he said. “While a large majority of all threads are done using a tap, you can be far more precise with a thread mill than you can with a tap. And for many applications, that precision is critical.”
The line of thread mills from YG-1 Tool (USA) for exotic materials has short thread lengths to minimize engagement between a part and a thread mill. Image courtesy of YG-1 Tool (USA)
Ford said thread milling also achieves a better thread quality than tapping thanks to chip control.
For instance, blind-hole tapping exotic materials is a “major issue in particular because in most cases taps make long, stringy chips that love to wrap around taps and potentially lead to breakage and scrap,” he said. “It is one of the most difficult application issues to overcome with a tap because it is so limited in what can be changed. With a thread mill, you don’t have that problem, because it is designed to move in and out of the cut very fast. And you don’t have any issues with chips, because they are very small and manageable and they flush out of the hole very easy.”
Still, deep-hole threading can be problematic, creating excess tool pressure, heat buildup and tool deflection. Unlike tapping, in which forces are distributed 360° around the circumference of a tap, thread milling doesn’t have the same engagement.
“The radial load is a fraction of a tapping operation,” Strauchen said.
North American Tool offers a line of taps and thread mills. The thread mills for exotic alloys, much like Emuge’s ZGF line, are designed with only three teeth.
“So there is less stress on the tool than (with) a conventional thread mill with six or eight or 10 teeth engaged in the material at the same time,” Strauchen said.
As for tool deflection, Blandon said thread mills experience radial side pressure during cutting and that cutting forces are not balanced. Tools and workpieces should be clamped securely into chucks, shrink-fit toolholders and other devices, he said.
For toolholding requirements, he said the general rule is “the better the toolholder and the lower the runout, the better the tool performance.”
Emuge offers the FPC line of toolholder solutions, including chucks that feature “mechanical clamping via worm gear that provides superior clamping force and concentricity,” Blandon said.
While both methods have advantages, when it comes to exotic materials, thread milling may be the better choice. A thread mill can perform many functions. Depending on the product, a mill can drill a hole, chamfer it and machine the thread. A single tool could be used in thread milling to make a wide range of hole sizes, interior and exterior threads and right-handed and left-handed threads. Thread milling also allows users to closely and consistently control pitch diameter and achieve tight tolerances. And thread milling provides a good finish to a thread, which is always an issue with tapping, particularly with difficult-to-machine exotic materials.
Related Glossary Terms
Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.
Hole or cavity cut in a solid shape that does not connect with other holes or exit through the workpiece.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
Ability of a material to be bent, formed or stretched without rupturing. Measured by elongation or reduction of area in a tensile test or by other means.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
Machining grooves and shallow channels. Example: grooving ball-bearing raceways. Typically performed by tools that are capable of light cuts at high feed rates. Imparts high-quality finish.
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.
- inner diameter ( ID)
inner diameter ( ID)
Dimension that defines the inside diameter of a cavity or hole. See OD, outer diameter.
Process of generating a sufficient number of positioning commands for the servomotors driving the machine tool so the path of the tool closely approximates the ideal path. See CNC, computer numerical control; NC, numerical control.
- machining center
CNC machine tool capable of drilling, reaming, tapping, milling and boring. Normally comes with an automatic toolchanger. See automatic toolchanger.
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- physical vapor deposition ( PVD)
physical vapor deposition ( PVD)
Tool-coating process performed at low temperature (500° C), compared to chemical vapor deposition (1,000° C). Employs electric field to generate necessary heat for depositing coating on a tool’s surface. See CVD, chemical vapor deposition.
1. On a saw blade, the number of teeth per inch. 2. In threading, the number of threads per inch.
- stainless steels
Stainless steels possess high strength, heat resistance, excellent workability and erosion resistance. Four general classes have been developed to cover a range of mechanical and physical properties for particular applications. The four classes are: the austenitic types of the chromium-nickel-manganese 200 series and the chromium-nickel 300 series; the martensitic types of the chromium, hardenable 400 series; the chromium, nonhardenable 400-series ferritic types; and the precipitation-hardening type of chromium-nickel alloys with additional elements that are hardenable by solution treating and aging.
Tough, difficult-to-machine alloys; includes Hastelloy, Inconel and Monel. Many are nickel-base metals.
Cylindrical tool that cuts internal threads and has flutes to remove chips and carry tapping fluid to the point of cut. Normally used on a drill press or tapping machine but also may be operated manually. See tapping.
Machining operation in which a tap, with teeth on its periphery, cuts internal threads in a predrilled hole having a smaller diameter than the tap diameter. Threads are formed by a combined rotary and axial-relative motion between tap and workpiece. See tap.
- tensile strength
In tensile testing, the ratio of maximum load to original cross-sectional area. Also called ultimate strength. Compare with yield strength.
Process of both external (e.g., thread milling) and internal (e.g., tapping, thread milling) cutting, turning and rolling of threads into particular material. Standardized specifications are available to determine the desired results of the threading process. Numerous thread-series designations are written for specific applications. Threading often is performed on a lathe. Specifications such as thread height are critical in determining the strength of the threads. The material used is taken into consideration in determining the expected results of any particular application for that threaded piece. In external threading, a calculated depth is required as well as a particular angle to the cut. To perform internal threading, the exact diameter to bore the hole is critical before threading. The threads are distinguished from one another by the amount of tolerance and/or allowance that is specified. See turning.
- titanium aluminum nitride ( TiAlN)
titanium aluminum nitride ( TiAlN)
Often used as a tool coating. AlTiN indicates the aluminum content is greater than the titanium. See coated tools.
Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.