Precision Thread Rolling Process for High-Accuracy Lead Screws, Actuator Screws, and Power Transmission Components

The cylindrical die rolling process can produce helical, axial, and annular forms on shaft-like parts at high rates of speed with precision tolerances.

Why use the rolling process to produce high accuracy lead screws, actuator screws, and other power transmission components rather than traditional cutting processes such as turning, grinding, milling, whirling, or hobbing? Rolling processes and cutting processes both produce a precise form on the workpiece. But if the form geometry, tolerances, and material selection allow, rolling is the process to beat. Speed, surface finish, fatigue strength, precision, dimensional stability, and material savings are some of the primary advantages realized when the rolling process is applied.

Precision CNC cylindrical die thread rolling equipment and dies make it possible to produce high accuracy standard and custom thread forms, ACME and trapezoidal lead and actuator screws, gothic radius ball screws, involute worms, splines, and serrations on shaft-like components. The rolling process has been applied to parts below .25 mm (.010-in.) in diameter up to over 150 mm (6-in.) in diameter.

Cylindrical die thread and form rolling is a metal forming process whereby helical, axial, and annular profiles such as threads, worms, splines, and grooves are formed in the surface of a cylindrical blank by the phased interaction of two or three round dies. The dies are forced into the periphery of the blank while rotating it at nearly the same surface velocity to displace the blank material radially into the form of the die until the final geometry is achieved. Dwell is used to round-up and calibrate the final form before disengaging the dies. It is a constant volume process whereby material is neither added nor removed.

Process Origins

Screw thread rolling itself is not new. The process originated from the fundamentals of the techniques used to roll metals into flat sheets, plates, and bars. The earliest known patent for screw thread rolling or screw thread swaging, as it was initially referred to, was in 1831 by Hazard Knowles of Colchester, CT. The patent was for an improved method of manufacturing wood screws using flat dies. The dies replicated a mirror image of their form into the periphery of a cylindrical screw blank by a parallel reciprocating motion. The patent also covered the use of one round die with an external segment die, now referred to as planetary rolling. Cylindrical die rolling using two or more round dies followed in 1855 with a patent by Martin Griswold of Watertown, CT, for profiling continuous lengths of metal tubes. Cylindrical die rolling was developed in various forms over the next several decades primarily in the northeast United States, England and Germany.

Thread rolling in its infancy was considered a crude low-precision process that was limited to wood screws, carriage bolts, stove bolts, and other low-precision fasteners. This was mainly due to limitations of die manufacturing accuracy and the availability of die steels and good forming steels for screw blanks. The process saw increased industrial use throughout the late 1800’s into the early 1900’s, but it wasn’t until the wartime demands of the aviation industry in WWI and WWII, along with advances in cylindrical die rolling machinery and dies, that thread rolling became the preferred method of producing high-precision forms on critical fasteners and components. Since then, the technology has been extended to many different applications requiring high accuracy and strength. The application of the cylindrical die rolling process to the manufacture of precision lead screws and actuator screws for transport and positioning was a natural evolution.

High Speed

Rolling is generally several times faster than any cutting operation. It is not uncommon to see a 5x to 10x order of magnitude difference. A single point cutting tool usually requires multiple passes to achieve full depth of form, and depending on the length of form, it can take in the order of minutes to complete the cycle. Rolling processes generally take on the order of seconds.

The primary cylindrical die rolling modes used for producing precision lead screws and actuator screws are infeed and throughfeed cylindrical die rolling.

Infeed is used to roll discrete lengths of thread within the width of the die face using a single plunge and retract operation. Two or three dies are positioned on parallel axes with the blank, and there is little to no axial feed of the part during the forming operation. Infeed rolling can produce complete forms in two to three seconds or less. The forming operation itself is generally much faster than the time it takes to transfer the part in and out of the dies. Automated infeed thread rolling systems with mechanized feeders can achieve production rates of 15 to 60 parts per minute depending on the part configuration and forming cycle.

Throughfeed is used to roll continuous lengths of thread using relatively narrow dies tilted on opposite skewed axes. The blank rotates and feeds through two or three dies at fixed center distance while the form is progressively generated as it travels through the dies. Throughfeed rolling of long forms having lengths of four meters (12 feet) or more can produce rolled product at rates of 30 to 60 inches per minute or more depending on the capability of the thread rolling machine and design of the dies. Forms that are rolled in long lengths can be colletted in a screw machine and cut into shorter pieces for subsequent machining or finishing operations.