Guidewire Lapjoint

Project Highlights

The Team

I served as the product designer on a 4 person team consisting of research and development engineers and a grinding department technician.

The Work

Analysing the problem

Joining two pieces of wire together may seem trivial, but taking into account a laundry list of specific performance criteria, trivial instantly becomes a complex set of exacting engineering challenges. Further, we were limited by existing patent designs, and therefore needed to be creative and efficient in our approach. Hats off to the designers in working through this tortuous path, while avoiding patent infringement issues and yet needing to develop a solution for proprietary use. The next step was to familiarize ourselves with all existing patents.

Concepts

After researching what we could do, we started to dream up joint solutions.

Crimping sections

Final concept

After many hours of designing and patent workarounds, we devised the following.
Half the wire would be ground away, achieving a paddle shape for torque transfer. Sounds simple, but the actual grinding of a wire in such a way is extremely challenging.
Sketches by Andy Whitehead

Fortunately, we had state of the art grinders I had helped design, which were up for the challenge. (See Profile grinder project.) The ground paddle faces would be accurately aligned and be soldered together. See figure below.

Then a PET (Polyethylene Terephthalate) heat shrink sleeve would be added over the paddle sections to provide additional strength and contain the paddle sections.

Engineering Design

Final concept

I converted the conceptual sketches into AutoCAD Mechanical Desktop drawings.

Grinding the Parts

I helped write the machine programs for our profile grinders to grind the tight tolerance paddle profiles. With some R&D time, we figured out the grinding wheel and the machining programs for both hybrid wire sections, even the more demanding nickel titanium wire.

Assembly

After the parts were ground, next came the task of joining them together. Again nickel titanium was the challenging material. The freshly ground surface oxidizes very quickly. Oxide is detrimental to solder adhesion and causes poor "wetting" and the solder balls up. We had to achieve good wetting in a cost effective way.

Removing the titanium oxide in a cost effective way was challenging. We tried roughening the surface with diamond abrasive bits, but the solder adhesion results were poor. Instead, we chose a chemical etching process which worked well on multiple wires at a time. However, it had to be done just prior to soldering to avoid reoxidation. Timing is everything as the saying goes.

Even with an oxide free surface, an aggressive flux was required to get the solder to wet out on the titanium surface. The parts would be tinned immediatedly after oxide removal to avoid oxidation.

Poor wetting
Good Wetting

Alignment issues

After the soldering challenge came the alignment issues. Soldering two pieces of wire together sounds easy but when you add in long wire lengths, tight tolerances, concentricity requirements the complexity rises by several magnitudes. The wires needed to be held in precise alignement while they were soldered together. This was challenging while one hand was holding the iron and the other the solder rod.

The operator needed a second set of hands. Something along the lines of the traditional clamps shown here, but infinitely more accurate.

With some creative ingenuity, we devised a very simple and cheap set of consumable tooling to align the paddles axially and concentrically. This technique was so ingenious, we determined it should be a trade secret and therefore further details cannot be disclosed.

The heat shrink PET sleeve was slid on to the wire prior to joint soldering and then shrunk over the joint to provide additional torque strength and a smooth surface.

After many hours of manufacturing headaches and creative solutions, we had a viable joint design and a process to manufacture it.

Finished Part

This is how the finished assembly looked prior to PET heat shrinking:

And with PET heat shrinking:

Testing

The hybrid wire looked the "part" with nice curves.

It achieved the goal of a smooth tight bend profile even with the joint at the apex of the curve, not easy to acheive.

In addition, it had plenty to TORQUE with great a performance statistics which exceeded the design specifications.

Road blocks

As soon as higher volumes of production parts were being ground, we started to see a distinct trend. The grinding wheel needed resharpening much more frequently compared to grinding stainless steel.

We were aleady using superabrasive wheels to cut stainless steel and figured these high performance wheel qualities would have been up to the task. They were working but not well enough.

I worked with our super abrasive grinding wheel manufacturer to develop a custom grinding wheel to stay sharper longer to help improve our nickel titanium cutting problem. With a few wheel receipe iterations, we achieved our performance goal.

Success

As mentioned, after all the engineering and testing work, this project yielded a commercially viable hybrid corewire joint which was awarded a patent (Patent US 20070282270 A1). To date, after over 200,000 manufactured parts, there have only been six field failures. Pretty good by any design standards. (The root cause of these errors was determined to be operator manufacturing process errors.)

What I learned

Working around existing patents is challenging. When many patents exist, it can often be daunting to determine if another solution is even possible. The upside is that it drives "out of the box thinking" to come up with alternative creative solutions. This is the type of challenge I thrive on.