Thread milling, a fundamental metalworking process to create threads, is often the operation of choice when working with difficult-to-machine materials, such as titanium, tool steels, stainless steels, hardened steels and other superalloys. Thread milling has been used in the energy and automotive industries for some time, but now its popularity is growing in the aerospace and medical industries. It is typically used on large workpieces for the aerospace industry and small workpieces in the medical industry.
One of the final operations in the machining process, thread milling usually creates an internal thread, although it can also be used to make external threads. “The parts that require exotic materials lean towards thread milling since the application is on a component that typically has a higher associated cost,” said Marlon Blandon, product manager, thread milling for Emuge Corp. (West Boylston, MA). “The part may have a substantial cost and would be expensive to break a tap in it. Tapping has a continuous cut process versus thread milling, which does interrupted cutting, making thread mills efficient in exotic materials ranging from soft to hard materials. Thread milling is an attractive option for these types of applications and parts.”
Even though thread milling is usually a safe option when threading difficult-to-machine materials, sometimes a tool does break, but when it does the entire workpiece is not ruined. “If you break a thread mill during threading operation, the diameter of the actual tool is smaller than the hole, so [if the tool breaks ] it is easier to fish the tool out of the hole,” said Tim Holmer, regional applications engineer for OSG USA Inc. (Glendale Heights, IL). “You are not risking scrapping that large part that took a lot of time and material to process.”
The Proper Thread Mill
There are many variables when choosing the proper thread mill—from hole size, cutting parameters, and cycle times to the type of coating used, using coolant-through or flood coolant, and the tool design and size.
“You want the most rigid tool you can have regardless of the application, and you want to use the largest thread mill you can for the given application,” said Brett Kischnick, application and sales engineer for Horn USA Inc. (Franklin, TN). “The tool needs to be 30% smaller than the diameter of the hole.”
The type of coolant is a factor if there is the option of running coolant-through thread mills instead of flood coolant, especially for blind holes in some difficult-to-machine materials. “When thread milling HRSA or high-nickel alloys, [coolant-through] helps keep the tool cutting edges cool since the material doesn’t absorb any of the heat created during machining,” said Holmer. “The coolant-through feature helps flush chips and prolongs tool life.”
Designs for solid-carbide and indexable thread mills used to create large threads can place coolant directly on the cutting edge via coolant-through channels instead of flooding the area with coolant, according to Tom Raun, national product manager for Iscar Metals Inc. (Arlington, TX). “Difficult-to-machine materials are often used for specialized applications in the aerospace industry, which is selective about coolant,” said Raun. “There may be some elements in coolant, such as chlorine, that would contaminate an aerospace product.”
Another consideration when thread milling difficult-to-machine materials is if threading will be done in one pass or multiple passes, and whether the thread mill will be single tooth or multitooth. It is common to use the multitooth design for one-pass threading, where one full revolution completes the thread. With a single-tooth design, there is one thread for every rotation.
“These single-tooth designs are less productive in terms of speed to create a thread, but the design gives you more versatility in terms of doing different diameters and multiple pitches instead of having multiple taps or a thread mill specific to a certain pitch,” said Raun. “With single-tooth designs, you can do a wider range of thread forms.”
Also, choosing the proper thread mill depends on several variables such as the part to be produced, how hard the material is, and the required thread dimensions. For example, extremely hard materials typically have smaller part runs, according to Emuge’s Blandon. “When you have less to produce, having the ability to choose the right thread mill from a variety of options, including different thread diameters, profiles, and pitches, is most cost-effective and productive,” he said. “The depths of the required thread would guide us towards [certain tool options] as well. The deeper the thread, the more deflection and so on.”
Apps and software packages are available to help operators choose the appropriate thread mill, and thread mill manufacturers’ reps can help as well. “It is all about time savings, error proofing, and productivity,” said Raun. “Think about how a company specializes in a certain thread form, but one day they get an opportunity to do a thread they never have done, so they can rely on software where all the typical thread forms are supported.”
Machine, Toolholding Requirements
Thread milling difficult-to-machine materials requires a CNC machine tool capable of helical interpolation (simultaneous motion in three axes). “There are old machines which can only drive one axis or maybe two axes at a time,” said Raun. “Some of the newer machines have more than three axes, such as five-axis machines, and there are machines with many other configurations.”
For toolholding requirements, the general rule is to maximize rigidity and accuracy. The better the toolholder and the lower the runout, the better the tool performance. Depending on the size of the thread to cut, a rigid setup and a rigid spindle are necessities. It is important to make sure the part is held as firmly as possible and the collet is not damaged since rigidity is not easily measured.
“When you get runout in your thread mill, it’s harder to control the size you are cutting,” said Harvey Patterson, product development engineer for Scientific Cutting Tools Inc. (Simi Valley, CA). “Plus, it’s harder to keep an even chip load on the teeth.”
Todd White, sales manager for Scientific Cutting Tools, added, “When you get to the smaller thread mills and threads, the room for error is reduced significantly. If you don’t have that minimum runout on the holder, it gives you a big problem—you can just snap the thread mill.”
Also, a thread mill in a holder with the lowest possible runout will always run better, produce better threads, and last longer. “Managing total indicated run-out [TIR] is essential on thread milling, especially in smaller diameters; larger diameter indexable thread mills are more forgiving,” said Horn USA’s Kischnick. “A good TIR rule of thumb would be to shoot for 25% of the feed per tooth in TIR. Also, the surface footage that the tool runs at can be elevated because of the light radial depth of cut [DOC]. The lighter the radial DOC, the higher you can run the surface feed per minute, so you can spin the tool faster, feeding at a higher inch per minute.”
Also, shrink-fit toolholders typically offer high rigidity and low runout. “With little runout, you can create one-size threads and have little tool deflection with that,” said OSG USA’s Holmer. “The less runout you have, the less additional processing if the threads aren’t to the exact specs.”
The combination of the machine and toolholder is another important consideration, according to Blandon. For example, a small machine equipped with a good toolholder can provide stability and work well. “You can use a thread mill in this type of setup since it doesn’t require a lot of torque, and the machine doesn’t have to be huge,” he said. “It allows shops with small-spindle machines to bid on jobs that have these difficult-to-machine materials. With a thread mill, you can program it to produce the thread in several passes, to take a smaller load per edge—which allows ease of penetration with some of the hard materials.”
Milling Very Small or Large Threads
Challenges come with thread milling both very small and large threads. For small threads, ensuring rotational accuracy with small-diameter milling tools can be a challenge. “The average chip load for a tooth will be small, so if you have too much runout you can overload the tooth and you will break the cutting edge,” said Raun of Iscar Metals.
Also, low-speed machines running small-diameter thread mills won’t be able to reach the recommended RPMs to mill difficult-to-machine materials since these materials often resist heat. “When this happens, you must do all your calculations based on running slower, which may not be good,” said Raun. “When cutting something, heat is created, and that heat makes material more pliable. If not enough speed is created to generate enough heat, the material is more difficult to machine.”
When RPMs are lacking, high-speed heads can be used. One option is Iscar’s SpinJet spindles, developed for use when high RPM is required for small-diameter tools on limited RPM machines. The system uses the machine tool’s existing coolant supply, driven by a high-pressure pump (minimum 20 bars) as an energy source to rotate a turbine up to 40,000 RPM.
Another challenge is how precise intricate pieces—small medical or electronic parts—need to be. When using a solid tap it can be difficult to hold tight tolerances. “With smaller size thread mills, you can decrease the amount of cutting pressure from the cutting tool to the part and at the same time not cause as much tool deflection, which will eliminate rework—especially for tiny threads,” said Holmer.
For large threads, the rigidity and type of thread—an ACME thread or a buttress thread—present challenges. These larger threads usually are used in the oil and gas industry and have a large pitch and thread depth. “The main challenge is to figure out if the thread can be done in one pass or multiple passes,” said Raun. “This is an instance when moving to a single-tooth design rather than using a multitooth design can be a solution.”
When a chip is too big, two passes are needed. “If the chip load is too high, you may have a thread mill designed specifically to do half of the thread with one tooth and the other half with a second tooth,” explained Kischnick. “This would cut chip load in half for each insert.”
Flushing chips from deep and blind holes can be difficult, especially in vertical milling machines where the thread mill goes straight down. “The solution would be to move it to a horizontal machine,” he said.
Stability becomes important with large threads—there is a lot of force because of the amount of material being removed. “If someone wants to do a deep, coarse thread, a single-profile thread mill is needed because the pressure can be too much for [a multitooth tool],” said Patterson of Scientific Cutting Tools. “As the material gets tougher the pressure increases. A single-profile thread mill, where there is one set of teeth, takes longer to machine, but it creates a better quality of thread. You also get rid of taper issues.”
New Tool Designs
Thread milling is an interrupted cut process, so the tool itself needs to be very stable. Emuge manufactures cutting tools with the stability required to work on the softest metals as well as tough, hard metals. “To create greater stability and strength, we increased the core diameter of the tools and added more flutes,” said Blandon. “We have expanded our line of solid-carbide thread mills for advanced application capabilities.”
Emuge recently introduced a high-performance solid-carbide thread mill program called GF-VARIO-Z, offered in sizes from #10 to 1″ and form M3 – M24. The VARIO-Z thread mills feature an increased flute count and core diameter and multilayered TiALN-T46 coating. Emuge offers an array of carbide thread milling solutions such as SHUR-THREAD full thread profile carbide thread mills, THREADS-ALL miniature size thread mills and GIGANT-IC indexable thread mills for large thread diameters.
From Iscar’s MultiMaster Indexable Solid Carbide Line, with indexable solid-carbide heads, a new mill thread design suitable for long-reach threading applications has been developed. This is beneficial for applications where a threaded hole is not located on the face of a part and the machinist needs to reach 3-4″ deep, or further.
“It’s an indexable solid carbide head that screws on and it can be used in a variety of ways—[because the head is replaceable] it optimizes carbide usage and allows you to use a solid-carbide or heavy metal shank to come up with a rigid scenario for long reaches,” said Raun.
The AT-1 is OSG’s new thread mill design, released in Japan in 2017. The current industry standards are right-hand spiral, right-hand cut. The AT-1 thread mill has a left-hand spiral and right-hand cut, which is a patented design.
“The concept is when you use a right-hand spiral the cutting forces and tool deflection are at the tip of the tool,” explained Holmer. “This is like standard milling where you get high deflection at the tip of the tool. Because of the change to the left-hand spiral, you get the cutting forces along the length of the cut instead. The advantage is by having cutting start as close to the tool shank/toolholder connection as possible, it reduces amount of tool deflection.” AT-1 tool sizes range from ¼ to 1″ (6– 24 mm); 1/16–2″ British Standard threads (taper and straight); and 1/16– 2″ ANSI Standard Taper Pipe threads.
Scientific Cutting Tools offers a line of ACME thread mills in the SPTM (Single Profile Thread Mills) family of tools. The low flank angles of an ACME thread tend to distort, which must be compensated for in a thread mill design. “As the industry progresses, there is not a one-size fits all,” said White. “You must think outside the box a lot of times—that’s how we made the ACME thread mills.” The company compensates for the low flank angle by using a special software program that allows correction of the thread profile.
At Horn, a new Grade EG35, both a coating and substrate change on the Supermini Type 105 boring bar line for extremely small parts, is being applied to several company product lines. The coating comes in different substrates and adheres to carbide well. “I’ve been doing this for 33 years, and I have never seen a coating/substrate combination make such a big difference in the tool life,” said Kischnick.
New high-performance variants have been added to the Supermini Type 105. With a new coating, new substrate and new microgeometry, the coating is setting standards when boring holes between 0.2 and 6.8 mm and in lengths up to 5 × D. With this Grade, production throughput is accelerated, even when machining an increasing proportion of stainless, high-temperature alloys and inhomogeneous steels. To achieve further performance improvements with Type 105, several adjusting screws need be screwed into place in a logical order: at the substrate, the microgeometry and the coating. Coordinated development steps led to extensive internal and external test series showing a clear increase in service life.