New Ways to Make Medical Device Coils

Unlike coil winding where the wire is pulled around a constant diameter wire mandrel while the mandrel is rotated, a new deflection coil machine pushes the wire with a set of feed rolls past the arbor, against a coiling point which deflects the wire around the arbor to form the coil. By moving the coiling point radially, the diameter of the coil can be changed. As the coil wire passes beyond the coiling point a pitch tool moves axially with respect to the formed coil to change the pitch.

Finally, after the coil reaches the required length the wire feed stops and a cut off tool is actuated to shear off the wire against the arbor to produce a completed coil. All these functions including pitch, diameter and stiffness changes are done automatically and the machine delivers a completed, complex coil in seconds without any manual operations.

To successfully apply the deflection coiling process to micro coils in which the diameter of the wire is greater than the typical diameter tolerance of the coil itself requires that the complete process be looked at from a system viewpoint. First, each of the four basic functions, wire feed, diameter control, pitch control and cutoff have to be performed at a level of precision well beyond that of a typical coiling machine. Next, the tools for each function have to be positioned precisely and repeatedly in the machine structure, and the wire must be fed to the coiling machine with minimal but controlled tension. Then, all system elements must be located for easy operator access and operators provided with some vision enhancement to enable them to set up, observe and control the process. Finally, the system must have tooling with micron level tolerances. This requires tools with forms and locations designed for the actual material size and material characteristics.

Using a system developed by the Sleeper Division of Kinefac, the wire is fed by a set of servo driven feed rolls with virtually zero run out. The servo drive controls feed length to 10 microns (.0004") and can feed at a rate of up to 2250 cm per minute (900 in./min). The wire feed path to the coiling area is controlled by short wire guides, which are precisely located 38Today’s Medical Developments November/December 2005It's possibe to produce a wide variety of medical device coil elements as small as a human hair. cad/cam technology Micro-Coilingagainst the arbor.

As the wire passes under the arbor it is deflected around it by the coiling point to form a coil. The coiling point slide is servo driven by direct cam actuation. By this means, it is possible to position the coiling point radially to an accuracy of one micron (.00004"). In addition, the adjustable lateral position can be positioned to the same level of accuracy.

As it leaves the arbor, the pitch tool is operated by a similar servo slide system and positioned to the same level of accuracy, creating any required pitch changes. When the coil is complete the feeding is stopped. Then the servo actuated cutoff tool shears the completed coil. The lateral position of the cutoff tool must be located correctly over the specific coil to be cut. This requires its position to be adjusted laterally in one micron increments.

The CNC control of the system uses a standard industrial computer with simple programming. The program is sequential and coded in accordance with the function involved. Increments are specified and can be entered in inches up to five decimal places.

In most systems the wire is unwound from the spool and passes over series of low friction pulleys mounted on a dancer arm. This adjustable balance arm maintains the precise wire input back tension required and at the same time controls the spool unwind rate.

All system components are generally bench mounted, and the tooling area of the coiler is elevated for open, comfortable operator access. To enable the operator to easily set up the small tools and effectively monitor the process, a microscope, and in some applications, a video output is required.Finally, with all production elements in place and in operation, it is necessary to measure the output. Here, there are two basic options: mechanical contact or visual measurement. For coils that are tight wound and have no open pitch, controlled pressure, mechanical indicators or micrometers can be used. But for coils with variable diameter or pitch or having length tolerances, visual measurement systems are required. These optical systems need at least 400 power magnification for most medical micro coils.

With the current Micro-Coiler system, associated micro coiling tools and the requisite inspection, it is now practical to automatically produce a wide range of very small, precise catheter casings, marker coils, connector coils, heart screws, aneurysm filler coils, support coils, and similar medical device coil elements at cycle times measured in seconds. Outside diameter tolerances of ±5 microns [.0002"] and pitch to pitch tolerances of ± 3 microns [.0001"] are practical. Within any one coil it is possible to have a number of pitch and diameter changes, and these can be simultaneous.

Length tolerances are more difficult to control since length is a function of pitch, coil diameter and wire diameter variations. Therefore, if the coil element contains many individual coils, the length can be controlled by an external sensor rather than by the total amount of wire fed. In either case the sheared ends of the wire are perpendicular to the axis of the wire and substantially burr free.

Almost any wire of a ductile material can be deflection coiled. Both round and flat wire can be coiled, but flat requires special straightness control in the plane of its width. As diameter and pitch tolerances become smaller, material homogeneity and uniformity of diameter, cast and camber become critical. However, wire manufacturers are steadily upgrading their capabilities to meet these new requirements.

With design support from system and tool suppliers, the medical device designer’s range of options for the application of micro coils is greatly enhanced. TMD

Howard Greis, President
Kinefac Corporation
kinefac.com