Fastener Technology International — June/July 2011
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Advances In Thread Rolling Load & Torque Calculations Offer Industry Insights
David C. Willens

Company advances the art of thread rolling by developing new equations for more accurately calculating thread rolling radial die load and torque, lifting the burden from an industry often limited by available data and essential practical experience.

Kinefac Corporation is a producer of world-class cylindrical die rolling machinery focused on meeting the thread rolling demands of the world market. With nearly 50 years of machine design and application experience, Kinefac has contributed significantly to the thread rolling industry by applying its advanced technology to improve basic manual rolling machines to programmable servo controlled hydraulic machines. These machines automatically supply the required radial forces and torque to roll. Kinefac CNC thread rolling machines offer sophisticated load monitoring systems and an ability to vary die penetration rates during a roll cycle to take full advantage of machine efficiency and decrease cyclical fatigue of the rolling dies.

The development of new equipment is tailored to continuous market changes. Industry has not only focused on smaller and larger diameter fasteners, but also on higher hardness exotic alloys. Hardnesses being rolled in the Rockwell C 46/48 range are no longer uncommon. Over 90% of the world’s externally threaded fasteners are produced by rolling, making it an essential manufacturing process for a variety of worldwide markets3.

Latest Technology to Meet Market Demands

Kinefac meets the market demands by developing its specialized equipment with the latest technology to perform the application requirements. The lack of existing data available in an industry highly based on practical experience and observation has left open windows for missed opportunities for the most efficient machine designs. Designing and building thread rolling equipment starts with an understanding of the application process parameters involved. Two of the most important parameters for thread rolling equipment are the radial die load and torque requirements to roll a given thread.

Cylindrical die thread rolling can be described as a continuous yet intermittent forging operation achieved by two or three cylindrical dies. The dies produce an external form on a cylindrical blank through a controlled application of radial force and torque. The rolling machine must deliver adequate radial force to achieve a desired rate of die penetration into the blank while also delivering sufficient torque to maintain die/part rotation.

Using Kinefac’s understanding of cylindrical die thread rolling fundamentals, new research has led to the development of more precise methods for determining the radial die load and torque required to roll a thread of any geometric form, pitch, and material at any rate of die penetration. The accuracy of Kinefac’s newly developed radial die load and torque calculations is typically within 2% of actual recorded values.

In order for plastic deformation to occur within any material, a force must be exerted over a specific area that exceeds the yielding strength of that material. The Kinefac radial die load and torque calculation methods were derived from industry knowledge of conventional flat-rolling process parameters. The fundamentals of the flat-rolling process were studied and then applied to the thread rolling process. This resulted in the development of two new equations, one for calculating radial die load and the other for calculating rolling torque. These new equations provide more precise estimates, which have exceeded the capabilities of existing charts and references and can be used for infeed and thrufeed cylindrical die thread rolling applications. Thread form specifications, material specifications and forming rate requirements define the application input parameters.

The visible likeness between the flat plate rolling process diagram and the thread rolling process diagram can be seen in the following illustrations. The directional vectors for rolling force and torque are similarly applied to the rolls in both cases with the work flow path being the major difference.

calculated and compared to Kinefac’s rolling test results.

A course-pitch thread of just over 6" in diameter made in 4340 material Rc 40 hardness was calculated to require 572,000 lbs radial die load. The actual test results showed it required 580,000 lbs to roll. For large-diameter applications such as these, there is virtually no existing comparative data available, therefore these equations are essential to determine the rolling requirements.

The formulas have yielded several benefits allowing Kinefac engineering to eliminate the guess work for machine sizing of customers’ applications in the most cost-effective manner. Customers can now realize the benefit of better knowing whether or not their equipment is capable of handling production of new potential applications.

The equations also serve as an inspection tool, which provides closer monitoring of a rolling application. Once the expected load and torque signature is established, it can be used to ensure a process is performing optimally. Having the capability of monitoring the radial load and torque during the rolling cycle provides the ability to determine if the machine is rolling good parts. When loads are abnormally lower or higher than the normal signature band, it should be an indication to check the process parameters. Unnecessarily overloading the dies will quickly result in premature low-cycle fatigue tool failure.

Continual self-improvement of Kinefac’s products relies on technological developments to make the best engineering choices toward machine structure, stiffness and versatility. The demand for smaller and larger fasteners in the aerospace, nuclear and power generation industries is encouraging Kinefac to look into new machine designs capable of rolling threads in the Rockwell C 36 to 48 range with thread diameter capacities in excess of 8".

Kinefac has developed and expanded its PowerBox family of Kine-Roller machines to meet the customers’ requirements. The PowerBox family currently includes its smallest MC-8 FI (V) PowerBox Kine-Roller with 18,000 lbs radial die load capacity up to the MC-300 FI CNC PowerBox Kine- Roller with 660,000 lbs radial die load capacity.

These machines were designed to handle the thread rolling loads typical to the appropriate range of fasteners. Both Kine-Roller machines are designed for versatility while providing a stiff, fully enveloping, rolling structure.

To obtain additional information and technical specifications on the thread rolling equipment available from Kinefac Corporation, visit the website listed below. Www.kinefac.com
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