With the holidays rapidly approaching, the ownership and entire team of professional CNC machinists at Advanced Precision Machining (APM) want to wish everyone seasons greetings and best wishes for a Happy New Year. We’d also like to thank our customers for their continued support and loyalty. The milling and machining business has remained productive for us, and the hard work of our employees has grown APM into one of the Front Range’s premier machine shop operations. Part of this work is dedicated to education and keeping our customers informed with regards to the many milling and machining techniques used in our sector of the precision manufacturing industry. So, as we reflect upon another successful year of operations and look forward to 2023, we want to take this opportunity to revisit what may appear on the surface to be a seemingly simple procedure we offer as part of our CNC turning services; the various methods utilized for generating screw threads. 

When looking at the simple threads on any screw, to the layperson, it might seem as if the machining techniques used in the manufacturing processes are among the least complicated on the machine shop floor. But in reality, due to generally higher cutting forces and tight tolerances, machining screws (aka thread turning), is significantly more challenging. When you factor in the additional demands required of micro-threading, utilized in products such as watches, eyeglasses, cell phones, and medical devices, you’ve upped the workload substantially. Herein lies the difficulties many shops face. From one-off specialty items, to mass-quantity production runs, more screw threads are produced in a given year than any other CNC machining element. 

While threading is used across many varying industries and takes on many roles, from the large scale down to almost micro levels for custom precision machining projects, every type of thread is ultimately machined to provide specific functions for specific applications. The processes involved in creating a screw thread are varied, encompassing a wide range of subtractive machining, deformative, transformative, and even additive methods. Today’s revolution in modern machine shop technology means precise and accurate screw threads can be produced more quickly, accurately, and efficiently regardless of size. What follows below is a brief look at the most common technique; the subtractive method of cutting screw threads.

Bearing in mind that thread turning involves both external and internal threads, with the latter being more demanding, choosing an appropriate application for machining screw threads is based upon the constraints of time, money, equipment, and the degree of precision needed. Subtractive CNC machining methods involve the removal of raw material from a blank, and are universally accepted as the most efficient, cost effective, and least complex methods to choose from. These include: Thread Cutting, Thread Grinding, and Thread Lapping techniques.

Thread Cutting (Tap and Die, Single Point, and Thread Milling Processes)

Tap and Die Method

• Metal screw threading using taps and dies goes back to the eighteenth century. A tap is designed to thread the inside, or female surface of a hole, such as a nut. This can be done manually using various tap wrenches, or power assisted with a CNC lathe or high speed milling machine. Dies form mates for tapped components by threading the outside, or male surface, of a metal cylinder such as a bolt. Again, this can be done with hand devices, or with CNC machining tools.

Single Point Threading Method

• Single-point threading is less common, and uses a single-point tool on a CNC lathe to impart a thread on the inside or outside of a rotating part. Single-point threading is favored over tap and die, or other methods, but only when certain manufacturing factors dictate it.

Thread Milling Method

• Thread milling is a more accurate CNC milling operation in which a rotating multi-point tool threads the surface of a workpiece if a helical toolpath can be maintained. It’s a useful alternative to standard methods for producing threads with good form, finish, and dimensional accuracy. Faster work, less breakage, and left or right-handed threads can be machined using the same tool.

Thread Grinding

The precision manufacturing of screw threads can also be performed on a variety of grinding machines utilizing specialized wheels accurately profiled to the desired thread pattern that traverse along the revolving workpiece. Thread grinding is typically used to produce threads to very exacting tolerances, or in extremely hard materials. The most common machine, the universal type, specializes in threading gages, screws, plugs, rings, hobs, and even other threading tools such as taps and dies.

Thread Lapping

• Lapping a screw thread involves a process of abrasion in which threads are finely finished on a workpiece by successively traversing, or lapping the workpiece as it revolves. Lapping helps achieve the highest precision and surface finish available, and is normally performed after thread grinding. Two surfaces are typically moved together via a machine with the abrasive material between them, producing the desired result.

Although the subtractive techniques involved in machining screw threads described above garner the most attention on the machine shop floor and are most commonly employed there, deformative or transformative methods, and even additive (3D printing techniques) warrant mentioning. Thread forming, for creating internal threads, and thread rolling, for creating external threads, both involve deforming or transforming the metal used to manufacture screw threads. In short, the threads are formed by pressing a shaped tool into a metal blank, commonly called a ‘thread rolling die’. Used in large production runs due to their efficiency, thread rolling/forming dies have the capability to produce one new piece per second and produce none of the waste, or swarf, that’s associated with subtractive methods. Additive manufacturing utilizing 3D printers is gaining increased potential for creating threaded parts, but the technology is not quite where it needs to be just yet. The good news is that further commercialization is on the horizon, and the industry is making some inroads into the dental implant and medical bone screw fields.

Need help with your next threading project, or want to learn more about the range of precision machining options available to you, then contact the CNC machine shop and thread generating experts at APM. Our reputation for quality parts, on-time reliability and exceptional customer service cannot be matched! We’re happy to discuss any machining needs you have. Call us at 303.776.1910 or send us an email.

For more APM machine shop information; Like us on Facebook, follow us on Twitter @APMLongmont, or connect with us on LinkedIn.

About the Author

Gerry Dillon is a co-founder, current owner and certified CNC machinist at Advanced Precision Machining (APM), a full-service machine shop located in Longmont, Colorado. Before making his home in the United States in 2000, Gerry was born and raised in Ireland and took an interest in milling and machining from an early age, ranking #1 in the Irish National Apprenticeship Program. In 2005, he began what’s grown into a leading Colorado machine shop. Gerry brings over 30 years of machining experience to the shop floor, and is certified in all aspects of geometric dimensioning and tolerancing.