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[June 3, 2014] At Advanced Precision Machining in Longmont, spring has sprung at our Colorado CNC machine shop, and business is at an all-time high. The hard work of our CNC machinists has grown APM into one of the Front Range's premier milling and machining operations. Part of this work is dedicated to educating our customers about the techniques used in our sector of the precision manufacturing industry through our machine shop blog. With this theme in mind, we'd like to shed light on a seemingly simple subject; the various methods utilized for generating screw threads. 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. The processes involved in creating a screw thread are varied, encompassing a wide range of subtractive, deformative, transformative, and even additive machining methods. Here we take a brief look at the most common technique; the subtractive method of generating screw threads.
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 the most common method of creating screw threads. They include: thread cutting, 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 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.
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.
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.
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 Advanced Precision Machining in Longmont today.
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 on the emerald isle of Ireland and took an interest in milling and machining from an early age, ranking #1 in the Irish National Apprenticeship Program. In 2005, he partnered with his friend and colleague, Kirk Tuesburg, currently APM’s machine shop manager, together launching 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.