Fine laser cutting offers precision for medical tools

Published by: David Lloyd – Tue 05 Apr 2011
Feature Type: Cutting tools

Fine laser cutting is ideal for specialised cutting requirements in the manufacturing of medical tube tools and components, says says Geoff Shannon of Miyachi Unitek Corporation.

Where “surgical precision” is needed for sharp edges, contours, and patterns found in tools and devices, laser cutting provides higher precision, quality, and speed than traditional cutting techniques.

Among emerging laser cutting technologies is a 5-axis motion package designed to cut more challenging geometries in one pass.

Proper integration of the system’s components into a process flow that works is key.

Laser cutting is ideal for small tubes because the laser has no physical presence and makes no contact with the material.

The laser is a controllable tool with minimal thermal input (while maintaining fine control over heat in the work area) and has a width under .001"

Laser cutting can be focused up to about 25 microns, removing the minimum amount of material to make the cut while ensuring high precision and accuracy.

The technology offers fine pulse width, power and focus spot size control.

As the laser requires no contact, it is not limited by physical cutting geometry, so it can be used to make unique shapes.

Cutting systems for medical tools

Fiber laser cutting with a gas assist is often used to make medical tubes and components.

The laser is "assisted" with coaxial gas, typically oxygen (O2) or shop air (after oil filtering).

This picture shows the typical cut quality with no post processing of features and edges when cutting 0.01” thick stainless steel (304SS) tubing, showing the excellent quality of laser cut facesThis technology is used for stainless steel, MP35N and Nitinol for both on-axis (90 degree to surface) and off-axis (angled to surface) cutting.

A highly focused laser melts a thin sliver of material, at which point a 0.020" diameter coaxial gas jet nozzle blows away the molten material.

The features are produced using this continual cycle of melt, then melt ejection.

The O2 blows away the molten material and serves as a heating element, because the heated material reacts with it and heats up, adding 30-50% more heating energy to the cutting area, which increases cut speed and quality.

Dimensional accuracy is key to measuring cut quality, as is surface roughness (better than 12 micro inches) and the absence of thermal damage.

Laser cutting offers cut widths of less than 0.001" and dimensional accuracy at about +- 0.0005", which is useful for producing the jagged teeth used in some cutting tools.

Dross or burr is minimised or eliminated, reducing the amount of post-processing needed.

The recast level layer is less than 0.0005-inch.

The speed and precision of laser cutting compares favourably with EDM technology.

EDM requires up to 4 passes to obtain the same quality cut as a laser cutting machine, although multiple parts can be processed at once.

A laser cutter produced widths as small as 0.001", while EDM offers around 0.004".

The EDM process is also limited by the fact that it works best with certain geometries.

A final issue is floor space - a typical EDM machine can be as large as 10 to 12 feet square, while a laser cutting system is 5-6 feet square.

Alternatively, electro chemical grinding (ECM) removes electrically conductive material by grinding with a negatively charged abrasive grinding wheel, an electrolyte fluid, and a positively charged work piece.

It is a fast cutting method offering quality similar to EDM, although the electrolyte used must be disposed of correctly as hazardous waste and the use of hard tooling makes it less flexible than laser cutting.

A third option is water jet cutting, which slices into metal using a jet of water at high velocity and pressure, although the technology offers restricted cut geometry.

5-axis motion and system integration

Non-invasive surgery tools require motion packages that offer a new level of cutting geometry.

Keeping a part in a machine while making intricate cuts enables designers to cut more challenging geometries in one pass.

Miyachi Unitek offers a 5-axis laser cutting system with control software that manipulates the laser and the motion together, providing a rigid structure, free from vibration.

The 5-axis motion consists of 3 linear axes and 2 rotary axes, which allows engineers to choose the best axis configuration for a particular cut.

Engineers can mix and match and set combinations that make it more efficent for a given process.

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