Rolling provides design opportunities

Although many design engineers may not realize it, during the last century the tools which doctors or dentists utilize for treatments and procedures have been partly produced by the rolling process. Rolling was predominately used to produce diamond and straight knurls to improve the doctor’s grip on the tool. Most of the rolling was done on cylindrical die attachments on screw machines, but where longer knurls were required, standard two-die cylindrical rolling machines were used. For much of that period, there was very little medical device development which required more sophisticated rolling processes.

However, as medical device designs became more complex, and the need to produce very small threads, knurls, grooves, and other forms on shafts, tubes, and other cylindrical parts, conventional machining methods were frequently not practical. As product designers searched for new methods to produce such forms, they rediscovered the capability of cylindrical die rolling machines. Today, a wide range of applications have evolved where rolled forms provide the required functionality and manufacturability which is critical in any new medical product development design.

The first application area was the roll forming of fairly large acme and ballscrew thread forms for use in positioning and supporting surgical tables and various other units which requires adjustable positioning for installation in a hospital or laboratory environment. Today, these are commonly roll threaded shafts in the 1" or below diameter range. These parts are produced with a wide range of pitch diameters and thread forms, depending on the designer’s requirement. In many cases, low lead angle forms are used to eliminate the downward drift of the supported elements under load. Where frequent adjustment is required, ballscrews or acme screws with plastic nuts provide a good solution. All of these thread forms and sizes are readily accommodated by typical 50,000 lb radial die load capacity cylindrical die rolling machines, such as the MC-10 FT/I Kine-Roller with a Kine-Mat short rod feed unit.

Applied with Ease
As the designer’s saw the ease with which long, larger diameter helical threaded parts could be produced, they began to apply the process to the other end of the size spectrum. Since the thru feed rolling process is scalable, it is capable of processing forms on parts as small as 0.8mm in diameter, if the thread form and part material are rollable. There is virtually no limit on the length of the parts of the thru feed rolling process, but most of the medical parts are less than 6ft in length.

In addition, these Kine-Roller machines can be used to produce rolled forms at discrete points on the blank or continuous forms running part of the length of the shaft. This has led to a whole new family of parts being produced on smaller rolling machines such as the MC-4 FT/I Kine-Roller with precision size and skew adjustment. These shafts, produced by the machine, are used in a wide variety of medical devices, many of which have been miniaturized to take advantage of the very small diameter actuation screws that are available.

Front view of the Convergent Axis Kine-Roller.The larger diameter actuator shafts have generally been produced on conventional materials such as 4140 medium carbide steel and 304V or 416 stainless steels. This is also true of the very small diameter lead screws.
However, as the location of the rolled part has moved step by step from the doctor’s office to the hospital bed, to the operating room, and finally into the patient, the part material requirements have evolved and are more tightly controlled. As a result, the primary determination of rollability is not the size but the shape of the form and type and condition of the material into which the form is to be rolled. To some degree, it is also controlled by the ability to produce dies which have very small tooth forms, requiring the root of the die to have a radius as small as 0.001".

With the Kinefac built, 3-axis helical path CNC grinders, equipped with 2-axis diamond wheel dressing, producing small dies for the thru feed rolling of 60° and acme thread forms is possible. However, in addition to the precision dies, the tooling for the rolling machine must introduce the blank precisely into the dies, and hold it into position, while the thread is being formed. Then, it must discharge it into the output tube. This requires tooling with a special bushing and blade system that is individually tooled for the thread form being rolled. Finally, the pre-rolled blank must be held to a diametrial tolerance of approximately 0.0005" and must be straight within approximately 0.003"/ft.

With precision dies fully encompassing tooling, and a rolling machine which has very low spindle runout, it is not difficult to produce threads which have less than 150tpi, the depth in the form of not more than 15% of the diameter, and helix of less than 10°. If it is necessary to exceed these ranges, the tooth form may have to be modified in depth, number of starts, or root form.

Rolling any of the foregoing helical forms also requires that the material being rolled has an elongation ability of at least 10% and a hardness of less than Rc 40.

If any part of the device enters the patient’s body, then the material must be chemically inert, and if it is to remain in the patient’s body, it must be able to remain inert and in no way have any effect on the patient’s health and well-being. Some of the materials now being rolled for parts which are used for internal implant applications are 304V SS, titanium, and Nitinol.

Large diameter actuator screws – maximum screw shown is approximately 1"

Larger Parts
Parallel to this evolution of very small diameter actuator screws, the larger actuation screws have also undergone an evolution with more complex forms, tighter lead tolerances, and multi-start requirements. Many of these shorter screws have a continuous tooth form down most of their length. The unrolled area is then used for bearing support or connection points to the rotational drive element. They are used to axially move components or create part of the device function. This is generally the movement of one structural part with respect to another, but they can also be applied to feed components which are used in surgical applications. These parts enter the patient’s body during some type of operation but are removed at the conclusion of the process. They are made of various stainless steels or other hard, but formable, nickel-based alloys.

Another rapidly growing area for the application of lead screws is the automation of medical testing processes and research analysis systems. In many of these applications, a servo or stepping motor drive is used and special leads are required to achieve the motion or positioning characteristics the system requires. The MC-10 FT/I Kine-Roller is typical of the machines used for the production of these actuator screws, which are below 6ft in length. These lead screws generally convert rotary motion to axial motion by the use of some type of low backlash plastic nut system. The primary design goal for this type of element is low torque with a zero axial nut shake. In addition, these lead screws must have excellent surface finish to minimize stick/slip during the initiation of motion.

Recently, the requirement for precision grooved, small diameter, tubular parts has developed. The rolling process for these parts deforms the diameter of the tube inwardly with either a continuous helical form or a series of grooves. These applications are generally needed to provide flexibility, circumferential stiffness, position marking, or a connection point. Depending on the application, as well as the process requirements, these parts are frequently made from hypotube for small diameters were stiffness is required, and from welded and drawn stainless steel tube in larger diameters. At Kinefac, regularly rolling grooves as small as 200 per inch, on tubing which is approximately 0.050mm in diameter with a wall thickness of only 0.100mm, is common.

As Kinefac worked with the medical device manufacturers on means to create tube and coil fastening systems, a new rolling opportunity developed. There is a need for very fine pitch tapered parts which must be formed by rolling. These small parts, generally with a maximum diameter of 0.040", with multiple starts, cannot be practically produced by any other means. Therefore, Kinefac developed a Convergent Axis Kine-Roller machine in which the die motion, as it is actuated, closes on the workpiece and behaves like a beveled gear set. This means that the apex of the conical form on the die, and the apex on the conical form on the part, must be concentric and remain concentric during the rolling penetration operation.

Looking Ahead
There are a series of other parts with a coarser pitch and slightly larger diameter which are being considered for other connection devices much like a tapered thread used on oil well casings. The Convergent Axis Kine-Roller is also capable of rolling very small diameter tapered reamers and special tapered thread micro-fasteners, currently being evaluated for medical purposes.

To meet the medical device parts rolling requirements, Kinefac engineers get presented with a number of unique challenges, but with Kinefac’s rolling application knowledge, die design and manufacturing capabilities, and rolling machine functionality, machines can be provided to fit the needs of the medical device manufacturers.

Kinefac Corp.
Worcester, MA
kinefac.com