The more you know about the geometry of solid-carbide thread mills the easier it is to select the right tool to create the high-quality threads your customer needs in the efficient cycle time that you need. Depth of hole, cutting conditions, tooling material grade, and tooling type and geometry all come into play when choosing the most appropriate tool. And thread mill choices are expanding to make threading easier.
“In the past, manufacturers would have to thread the material in the soft state and send it to the furnace for hardening, where it might pick up scale or shift a little from the heat. Today, thread milling allows you to make the threads right at the machine without transferring the component to heat treat. The threads are more precise, and there are more widgets at the end of the day,” said Don Halas, SECO’s product manager for threading, thread milling, grooving, and MDT.
“Thread mills have the spiral flute with coolant-through, but cutting quality is also found in the composition of the carbide, where a toolmaker can have the best substrate to accomplish the tool design.”
New processes for producing thread mills and inserts are giving toolmakers the ability to design and produce tools with more complex, more affordable, more effective geometries. Brian MacNeil, milling products and application specialist at Sandvik Coromant Canada Inc., said, “When an insert is produced using today’s multiaxis pressing technology, toolmakers have more creative freedom when it comes to making complex edge shapes. Before, the toolmaker would try to generate the edge configuration after the tool was pressed. Now, the edges can be created during pressing. The new technology also reduces manufacturing costs that used to make complex geometries cost-prohibitive.”
Carbide grade joins geometry in making an effective threading tool. “Geometry is important, and so is the quality of the carbide,” said Halas. “It’s a little different for an insert than a thread mill, where you have to add a spiral flute. For the thread mill, the cutting effectiveness comes from the grind, edge prep, and composition.”
Selecting the right thread mill geometry starts with knowing what is important for the customer. With a thread mill, the diameter of the thread can be changed but not the form, so the first consideration is matching the tooling geometry with the desired thread. The second consideration is how many pitches—or sections of thread—should be involved.
Steve George, global product engineering manager for solid rounds at Kennametal Inc., said, “There are the single profiles, with just one crest or V, that are very flexible. They can be used for a different pitch as well as a different diameter. Then there are the more popular multiple-profile tools, but they don’t offer that flexibility. The form and pitch have to right for the specific application.”
Geometries are broken out into light, medium, and heavy. MacNeil said you either have a heavier geometry for heavy machining with small edge preps, microscopic rounding of the cutting edge where it meets the material, or you have big edge preps with more of a rounded edge. “The top rake, which is actually the angle of the cutting edge, quite often goes negative or positive. Positive provides freer cutting and negative provides a stronger cut, but it also pulls more power. The swing can be anything from +6 degrees to -13 degrees on some tools.”
Another consideration when choosing tooling geometry is the material being cut. “Heat-resistant superalloys don’t want to be cut. They want to push tools away—they have a low modulus of elasticity. So the cutting tool needs to have a compromise between its edge geometry and the carbide. The grade has to be tough enough to deal with the push back, and the edge needs to be able to shear rather than push or rub the material off. Lighter, sharper geometries will shear the material but have a weaker edge,” said MacNeil. As the carbide grades get harder, the sharper edges can become more brittle.
Choosing the right edge prep and coatings will help prevent chipping and extend tool life. PVD coatings can offer better edge life with less flaking, added MacNeil. “They can allow a manufacturer to use lighter edge preps and keep them sharp because they are so thin. That provides more security in difficult cutting situations.” A consistent edge geometry is also important.
The depth of the threads also matters when choosing the type of tooling to be used. An indexable system will limit the depth of the hole to be threaded. “Roughly, when a hole is about ¾ in. deep, you can’t use an indexable thread mill. A solid thread mill is needed to fit into the hole,” said Halas.
If a single-profile tool or a tool that is shorter than the final thread depth is used, it will require multiple helical interpolations. That factors into cycle time. George said, “A decision has to be made regarding taking advantage of the flexibility of the single tool or the productivity of choosing a tool with a long enough profile that only one revolution is needed.”
Thread milling deeper holes has the potential for more deflection when cutting with a multiple thread tool. “That is a critical consideration,” said George. “The more threads that are cutting, the more the deflection force and pressure can become difficult to overcome. Choosing the right tool becomes a balance between flexibility and stability and quick cycle time.
Speed and feed recommendations provided by toolmakers guide manufacturers in their thread mill grade and geometry choices. But there is another source that should also be used.
“Every toolmakers’ catalog has a section that shows the hex, the chip thickness, that is suitable for a particular geometry. To get the best results when choosing a tool, a manufacturer should be as specific as possible. Chip thickness relates to the depth at which you are engaging the thread tool, but it should also be appropriate to the tooling geometry. If you apply the right hex to the specific geometry, you will get a better cutting result,” said MacNeil.
“If you get below the recommended chip thickness, you’ll end up potentially rubbing and not utilizing the geometry as it was meant to break the chip. When you rub, you create heat, and that is the first thing that kills carbide during the cutting process.”
Toolmakers join with their customers to try to reduce runout by improving the concentricity of the threads to the axis of the tool. George said, “When running a profile over and over into the same groove and there is runout, the mismatch between the different teeth and the profile being cut will create chatter and reduce tool life.”
The microgeometry in terms of how accurate the cutting edges are and how true they are to the tool shank is important to producing a good, clean thread and extending tool life. A good thread mill, said George, includes a good basic substrate, good coating, and good edge prep.
CANADIAN METALWORKING APRIL 2017
BY: SUE ROBERTS