core wire profile grinder
Project Highlights
This has been one of the most challenging projects I have undertaken thus far due to the state-of-the-art technological issues, magnitude and project timeline required.
Project Background
In 1996, I was approached by the medical device company MicroSpring Inc. to help them build a custom guidewire grinder that could grind complex shapes in medical grade 304V stainless steel wire down to 0.001" with a tolerance of +/- 0.0001". At the time, there were no grinders commercially available to grind shapes of that complexity. I was assigned the task of building seven production grinders and have them up and running in a new facility in less than 18 months with a capital budget of approximately $1,000,000 (give or take a few $, as I was jokingly told by the CEO).
The Team
I was project manager / design lead, heading up a cross-functional team of over fifteen people.
The Work
Highlights of the design process
The Problems of grinding a shape in a thin piece of wire thinner than a hair on your head.
No stiffness
When wire is as small as a human hair, it has no stiffness; it deflects if you breathe on it. Hold a piece of human hair and try filing it and you will quickly get the idea.
This grinder design had to focus on this primary problem; this was the issue which would determine its success or failure.
Parts with column strength (won't bend) can be supported between centers and their surface ground as shown.
Very long
Grinding the profile into a wire with no stiffness wasn't the only issue. Guidewires are typically very long (150-300cm).
Moving a long delicate wire in front of the grinding wheel was also a challenge.
Pushing a piece of wire 200cm long and only 0.010" in diameter is like pushing a piece of string. It's uncontrollable due to lack of column strength.
So a mechanism to provide column strength was needed.
Design Engineering
After many hours of brainstorming, a design concept was selected and detailed design work began. Here are the highlights of the build.
I deliberately engineered the machine with potential for upgrades. The design was modular in approach to allow subassemblies to be exchanged or added in the future. Automatic part loading and unloading was an example of this.
Design Concepts Explored
Note: Due to non-disclosure reasons some of the innovative design aspects of the machine cannot be shared.
With all the research information understood, machine concept ideas were explored.
The grinder requirements demanded a lot of electronics and wiring schematics.
I worked on the design of the motion control language for the three machine axis: machine wire rotation, wire cross-feed and wire infeed.
We also needed a GUI (Graphical User Interface). I selected industrial touch screen PCs and Visual Basic for coding.
A fellow manufacturing engineer and I began learning Visual Basic which proved to have a big learning curve. After several days of struggling to learn the essentials, we started to make progress and the interface began to take shape.
Mother nature has the best materials
The base of the grinder needed to be extremely stiff and thermally stable...what could be better than a thick slab of granite.
Solving how to push a piece of "string" towards a grinding wheel
The wire would be pushed towards the grinding wheel using a motion control axis. To stop the length of wire in front of the pusher mechanism from buckling, a moveable column strength support mechanism was devised.
Gripper fingers providing column strength would swing out of the way as the wire is pusher moves close to them.
The Grinder Prototype Build
After...
Hours spent on Engineering
Cups of coffee consumed
Building the protype machine was challenging and an extensive amount of work. A large number of parts to manufacture internally and externally.
Soon we just about were buried alive with parts piling up everywhere, waiting for us to begin putting together the giant jigsaw puzzle.
Machine base structure
To achieve a stiff structure meant BIG parts, and big parts meant heavy parts with the heaviest one being a spindle at 300-2000lbs for the granite.
I had never moved parts this big before; it required a crane which we needed to order.
I soon appreciated that lifting, moving, accurate positioning and levelling of parts this heavy is SLOW. Improper handling could cause expensive and extensive damage to parts and/or people.
Prototype Testing
Some parts of the machine testing worked well enough. Some were simply poetry in motion.
The first set of the grippers swung out of way after they finished providing column strength for the wire as pusher mechanism advances forward.
With no grippers, wire has no column strength.
Other machine tests proved more challenging.
We quickly found out that conventional grinding wheels just couldn't cut it.
During testing we ran into problems with the conventional stone grinding wheels. The profile in the circumference of the wheel kept breaking down too rapidly, causing the part size to be continually adjusted to hold tolerances. This would not be economical in production.
Enter Super abrasive wheels
To allow a delicate wheel profile to hold its form, we needed to utilize super abrasive technology. This solution meant the machine could grind thousands of parts with minimal wear and more importantly minimal machine size correction adjustments.
Headaches
The biggest headache I faced was software debugging. Getting Visual basic and Compumotor motion software to play well together was very challenging. Many hours were spent troubleshooting the handshaking subroutines.
Success
Finally the prototype was built. After all the debugging, it performed as expected. This was a pivotal growth point for the company and my career as a project manager and designer. This was a project of a life time. It was a great design challenge with a lot of pressure but very rewarding and a big career accomplishment. The original machine has made over 500k wires and still going. An additional seven grinders have now ground over 3 million wires.
Continued Improvement
Despite the success,there was always room for improvement. After the intial release of the seven grinders, I was heavily involved in process improvements. These improvements were heavily focused on wheel cutting technology. Cutting faster meant shorter cycle times, and as a result, cheaper core wires. I also, redesigned the tooling so it was 60% faster to change in and out for different jobs.
What I would have done differently
This project was on a killer timeline to say the least. To hit the deadlines meant that many decisions were based on vendor delivery dates and not necessaarily on cheapest bid. This was frustrating to me. I updated management on this problem and was guided to do the my best to control costs; hitting the deliery date was the primary goal due to potentail lost product sales. In the future, I would have place more emphasis on setting up extra resources earlier in the project to chase down competive bids to better control costs.