- Published: October 7, 2011
by Mary Scianna
Whatever your machining process, external or internal threading is likely a part of your manufacturing process.
It's a given that selecting the right threading solution will improve quality, productivity and cut production costs. Of course, selecting the wrong threading process can have disastrous effects.
Cutting tool suppliers have done a good job - some might say too good of a job - of developing innovative threading tools to meet diverse market needs. There are many diverse multi-function inserts for thread turning and vast array of inserts and solid carbide tools for thread milling and tapping.
"With so many tools in the market for threading applications, it is imperative to consult with cutting experts to choose the best tool for your application,” advises Mark Hatch, director of taps and threads for Emuge, West Boylston, MA. "Doing so will improve productivity and quality, while reducing cost in your holemaking operation. It pays to carefully analyze the threading application at hand before any chips are made.”
The dilemma many manufacturers face is figuring out which process is going to work best for their metalworking applications. To further complicate the decision, advances in cutting tool technologies have blurred the lines between thread milling and thread turning.
"It doesn't matter if you talk about thread milling or thread turning because both can be used for making internal and external threads,” says Steve Geisel, senior product manager for Iscar Tools, Oakville, ON. "In the past people used thread milling over thread turning for ID work because with smaller diameters it wasn't possible to get an ID threading bar inside a bore. People would go to taps, but they would break inside the hole and risk damaging the part. With today's technology we have very small threading bars and we can thread down to 80,000 thou; that's a 2 mm bore.
And, continues Geisel, with new generation CNC lathes with live tooling, you can put thread mill tools on lathes.
Considerations that will influence threading method choice
Thread turning, thread milling and tapping are distinct ways of creating threads and while comparisons can be drawn between the processes, the machining application, part size and threading parameters will determine the best method for machining your threads, advise Sandvik Coromant's Kevin Burton, milling/deep hole drilling product manager and Dave Andrews, threading/turning/parting/grooving product specialist, both based in Mississauga, ON.
It's important to select the right threading method, says Burton, because it can "influence the production cost of a component dramatically. A number of considerations should be made when deciding the most appropriate method of producing threads.
"Although not as widely used as thread turning, thread milling achieves high productivity in certain applications and offers an advantageous alternative to thread tapping. You can thread non-rotating components; interrupted cuts offer good chip control in long chipping materials, and lower cutting forces make it possible to thread in long overhangs and thin-walled components. As well, threads can be made close to the shoulder or bottom of a blind hole and no relief groove is needed.”
The type and size of part you need to thread is an important consideration. As a general rule, threading on smaller, simple geometry parts are best done on a turning machine while threading on larger, more complex or awkwardly shaped parts (e.g. a fire hydrant) are more suitable for milling machines or machining centres, say suppliers.
Thread milling gains steam
Traditionally, turning was the way to go to create threads, but thread milling is growing in popularity because it offers manufacturers cost saving solutions.
For instance, unlike thread turning which requires multiple inserts and toolholders, thread milling uses one toolholder and one mill thread for internal and external threads, and for right and left hand threads. One thread mill tool covers different threading diameters andd different pitches, all of which helps to reduce tooling inventory.
Thread milling is also faster than thread turning and offers better chip control, particularly on high temperature alloys such as Inconel and titanium, adds Tom Hagan, product manager for milling products for Iscar Tools, Oakville, ON.
"Thread turning requires multiple passes to produce the thread but with thread milling, you work from the bottom up and you're done in one pass. And because of how it works, working from the bottom up, there's no long threads or long chips; it's small chips, so you get better chip control.”
Emgue's Mark Hatch concurs. Cutting tools for thread milling are designed for high cuttings speeds and feeds, and produce threads with excellent form, finish and dimensional accuracy.
"Optimum precision and accuracy are ensured by exact thread depth and position control. Other thread milling advantages include, but are not limited to, easy machining of difficult materials, elimination of the possibilities and consequences of tap breakage, and the production of small controllable chips. When you combine thread milling with the latest CNC technology and smart controllers, you can realize outstanding flexibility, process control, tool life and part quality.”
Sandvik Coromant's Kevin Burton says the use of thread milling is a growing area "because of several expanding industries such as aerospace, energy, oil and gas. There are more higher cost and larger components that require threads and thread milling offers a more secure and reliable process.”
A place for taps?
Taps have had a bad rap in the past, in part because there has been a perception among some that the quality of taps when compared to thread mills or thread turn inserts are not as good.
"We have a line of taps but we don't promote them because we don't feel it's the best option for our customers. We prefer to go with a cut thread or a mill thread because you get a better quality thread if you turn it or thread it over tapping. And you don't need special fluids for turning or threading like you do for taps.”
Other suppliers think taps are a viable option.
"Threading taps and turning tools always give the best surface finish,” says Sandvik Coromant's Kevin Burton. Thread milling will leave feed lines; it's unavoidable although the feed lines can be reduced by lighter feed rates. Typically this isn't a problem unless the surface finish is critical.”
And, adds Emgue's Mark Hatch, today's high performance taps produce threaded holes with short cycle times, uniform quality and longer tool life. For shops that perform short and medium run threading applications across a wide array of materials taps can "significantly improve thread quality and boost output while reducing production costs, because one high performance multiple tap can handle most common materials, including carbon steel, steel alloys, stainless steel, aluminum and cast iron.”
Multi-function Thrill Tools
Another option, depending on your machining needs are multi-function thrill tools that combine hole drilling, threading and chamfering in one operation.
"Thread making with thrillers, when used in a modern CNC machine with helical interpolation ability, offer unparalleled speed, flexibility, precision cutting, tool life and value compared to conventional thread cutting methods,” says Emuge's Mark Hatch.
Of course the most obvious advantage is the reduction in cutting operations because of the multi-function aspect, an important consideration that can improve machining productivity.