Just Droopy Enough
A new approach to coiling tooling and coiling machine control helps
springmakers meet medical spring flexibility requirements
By Howard A. Greis, Kine-Spin/Sleeper Division - Kinefac Corp.
Until recently, the most common use of tightly wound springs was to provide controlled tensile force for all sorts of mechanical devices. Spring users were interested in how much force was required for the spring to begin to extend. As a result, springmakers learned how to create what has been known as “back tension” while making continuous coils. This produced an initial tension in the spring, which has been traditionally measured in pounds.
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Figure 1: Diagram of droop-distance test.
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 Figure 2: Micro-Coiler with micro-align tooling.
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Figure 3: Starting end of a 0.015" diameter coil of 0.003" 304V SS wire, with maximum back tension and pitch tool beginning contact with the coil.
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Figure 4: Droop distance vs. pitch move for 0.015" diameter coil of 0.003" 304V SS wire.
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The other common use of continuous tight- wound coils was casings for flexible shaft drives, push-pull cables, sewer snakes and similar devices for the transmission of motion. In these cases, the requirements for flexibility were specified in a variety of ways, but the precise level of flexibility was generally not important.
With the advent of medical procedures that involve the introduction of catheters into the urological or vascular systems of the body, the use of micro-diameter tight-wound coils in such medical devices has become commonplace. With it came the need to define, measure and specify coil flexibility. As a result, a relatively new term has come into the coil manufacturer’s vocabulary – “droop.”
Essentially, droop is the cantilever deflection of the end of a coil under its own weight, extended out a specified length. This provides a good repeatable indication of the coil flexibility. It is most commonly specified and measured as shown in the Figure 1 diagram (above). First, the coil support height is defined. For small coils, it is generally in the ½" to 1" range. Then, while the coil is being held down manually against the support surface, the coil, under its own weight, must just touch the base surface within a specific range of the extended length. This is defined as the “droop distance.” An alternate method specifies the length of the extended coil and defines the allowable range of the drop of the coil end from the initial horizontal plane of the coil support.
To meet these new medical coil flexibility requirements, the wire and coil diameters have been continuously reduced and the droop limits have followed. This has resulted in the need for a new approach to coiling tooling and coiling machine control requirements.
With single-point mechanical coilers, the setup person adjusted the relative position of the wire guides, coiling point and pitch tool by “experience” to achieve the required back tension and produce a coil with the required droop. In general, the ability to quickly repeat such a setup on fine wire to a controlled flexibility level was marginal. The coiling points had round shanks and lacked precise groove shape and location. Therefore, the angular position and lateral position of the coiling point groove with respect to the base wire line was difficult to set up repeatedly.
The lateral position of the coiling-point mounting pocket relative to the wire line and arbor was uncalibrated. Plus, the initial positioning and the lateral adjustment of the coiling point were coarse and uncalibrated. The same held true for the pitch tool. Even for the highly skilled setup person, the balancing of these four interrelated variables to achieve a specific droop level was difficult enough with large-diameter wires; but it became close to impossible as the wire diameters went down below 0.004". With this as background, our Kine-Coil/Sleeper Division incorporated in its Micro-Coiler a number of unique features.
To achieve tight control of droop required for these medical coils and to make it possible, where needed, to modify the droop within a single continuous coil, it was first necessary to develop an ultra-precise, integrated CNC control system for feed, diameter and pitch. Then it was necessary to provide an easily set wire-guide system with very precise wire contact surfaces. The next requirement was the ability to consistently locate the angle of the axis of the groove in the coiling point with respect to the wire line. To do this, the tool system was designed so that the coiling-point groove axis is always aligned perpendicular to the coiling axis. This was achieved by locating the coiling point on a square shank with its groove axis at 45° to the flanks of the shank and centered on the flanks. Figure 2 (page 17), shows this micro-align tooling system on a Micro-Coiler.
Parallel to this, the Micro-Coiler was equipped with an ultra-precise differential micrometer system. This enables the setup person to adjust the coiling point groove laterally with respect to the wire line in increments of 0.000025". With this arrangement, the setup person, having made the coil to the initial diameter, can precisely adjust the back tension by moving the coiling point very gradually back behind the wire line.
The maximum basic back tension is reached by introducing just the amount of backward deflection necessary for the lead end of the coil to deflect over the incoming wire without deformation, as shown in the enlarged picture of the starting end of the coil (Figure 3, page 19). This generally occurs when the end of the coil contacts the incoming wire at the outer quarter of incoming wire diameter.
Having established the minimum-droop stiffest coil and having brought it to the correct diameter tolerance by the CNC radial positioning of the coiling point, the next step is to gradually remove such back tension to the degree necessary to reach the desired droop (flexibility) level. To do this, the pitch tool is brought forward to make initial contact with the back of the coil. It is then moved forward by the CNC control to achieve the required droop.
As shown in the Figure 4 graph (page 19), this droop follows a precise and critical relationship with the pitch tool position. As the pitch tool is moved forward as little as 0.0001", the droop can change by as much as 1". Since some medical coils have a droop tolerance range as small as ½" to 1", the pitch-control system is adjustable in increments of 0.00005" and must be repeatable to that level.
The graph also shows that, if the pitch tool continues to move forward, the droop increases until the individual coils are no longer in contact with their adjacent coils. At this point, the coil is visibly open. Once that occurs, the droop becomes a function of the overall pitch. Since open coils are rarely used for such medical casing requirements, we have done no droop analysis in the open pitch area.
The interrelationship of droop to wire diameter, coil diameter, coil material and tooling design is too complex to be readily reduced to mathematical formulae. However, once a tool design is established and the previously described setup procedure is followed, the droop-to-pitch relationship in the CNC Micro-Coiler with micro-align tooling is very repeatable.
As a result, it is possible to produce continuous closed coils in which there is no visible change in the coil, and yet the flexibility (droop) can be precisely controlled and modified at various positions along the length of the coil. Currently, with full CNC control of the feed, diameter and pitch motions, and the ultra precise axial adjustability of the coiling point, the Micro-Coiler provides a practical means to make long coils as small as 0.008" in diameter from 0.002" wire with controlled flexibility. These coils can be made “just droopy enough” to help physicians open clogged arteries, implant stents, take biopsy specimens and perform many other important medical procedures.
Howard A. Greis is the president of the Kine-Spin/Sleeper Division of Kinefac Corp., which is based in Worcester, MA. Readers may contact him by e-mail at sleeper@kinefac.com, phone at (508) 754-3249 or Web site at www.microcoiler.com.
