Making implants from tough-to-machine titanium

Published
April 23,2021 - 06:30am

Machining medical components cost-effectively and reliably requires high-precision tools that have been tested in practice to meet specifications that often reaches the micron level.

Inovatools, the tool manufacturer from Haunstetten near Kinding in Germany, offers the Inomed range of tools for this application. According to the company, the tool’s optimized geometries, quality documented with test reports and service life make the Inomed tools ideal for processing the kinds of materials required in medical engineering, which include stainless steels, zirconium oxides, titanium, and cobalt/chrome.

One example is the production of bone compression plates for osteosynthesis in jaw surgery. For these types of implants, the material most often used by surgeons, orthopedic technicians and dentists are specially alloyed surgical steels or highly bio-compatible titanium alloys. But, these materials are difficult to machine cost-effectively and in-line with the relevant directives such as high elasticity and low-thermal conductivity.

This is because the cutting edge can reach high temperatures due to low heat dissipation through the chips and workpiece. This can result in thermally induced stress in the tool. In addition, the high threshold load due to the lamellar chips combined with the high, localized pressure loads on the cutting edges due to the material hardness mean that conventional tools quickly reach their limits.

Nilüfer Cebic, head of product management and marketing at Inovatools, said, “Miniaturization, precision and compliance with the most stringent of standards are decisive in medical engineering, which is something that tool manufacturers have to take into account and reflect in their product ranges.”

To cope with these hard-to-machine materials, Cebric said the development of the cutting-edge geometries requires a sharp awareness of these factors coupled with the right combination of coatings and surface and edge preparation.

“After all,” she said, “only implant manufacturers that can rely on the quality of the very best tools available can perform machining processes cost-effectively and with maximum precision.”

Express shipment of special tools

Due to the difficult-to-access geometries of bone compression plates for fractures in the jaw area, machining tools are often narrow and usually designed with a long reach. To avoid vibrations caused by the long projection lengths, Inovatools matches the helix angle and cutter pitch of the tools to these special machining situations.

Norbert Geyer, head of the special tools department at Inovatools, said, “The range of Inovatools INOMED mills … opens up new options for high-precision milling in the micro range for diameters of between 0.1 mm and 20.0 mm.”

According to Geyer, the company also develops and produces  special tools for complex drilling and milling tasks with extreme precision and accuracy in the micron range. The company offers a Special Tool Express Service so that a manufacturer receives a quotation specially tailored to its requirements within 24 hours. The solid carbide (SC) special mills and drills in dimensions from 0.1 mm to 32 mm, which are custom-made for individual requirements including edge preparation and PVD coating, can then be shipped within just one week.”

Bone compression plates used in jaw surgery are titanium implants and they are available in different thicknesses and designs. Due to the stringent requirements imposed on the material, application-optimized tools are absolutely essential for machining. The tendency toward sudden strain hardening, for example, can instantly increase the friction on the cutting edge, causing the tool to become blunt. The combination of carefully selected, ultra-fine-grain carbide, the perfect geometry and chip control, prepared cutting edges and high-performance coatings is the key to commercial success and consistently high quality.

During the machining of bone compression plates to create frictional connections, the Inovatools tools can be used in a range of representative applications compared with conventional tools.

SC standard fiber end mill 132

The SC tool’s geometry offers a high machining rate during the milling of titanium alloys. Taking the jaw plate as an example, the challenge was to increase throughput without compromising process reliability. The tool was optimized with special surface treatment, edge preparation and internal cooling. The resulting special mill 998049223_HB100100326 makes the milling process 40 percent faster with Vc = 70 m/min; F = 700 mm/min.

With a second special fiber end mill (998051524_HB100100455), the upper and lower outer edges of the jaw plate are chamfered simultaneously (Vc = 60 m/min; F = 225 mm/min), reducing the machining time by 50 percent. With a Inovatools combination tool for drilling and chamfering (998044881_HB100100166), implant manufacturers can achieve time savings of more than 60 percent compared with conventional production using two tools.

“Where every micron of precision counts, our micro-tools offer outstanding surface quality, the narrowest of tolerances and long service lives,” said Cebic. “Thanks to Inomed, we can supply our customers with ‘high-end powerhouses’ almost instantly. And if customers need to perform even trickier tasks, our developers can deliver the perfect solution with smart special tools.”

Related Glossary Terms

  • alloys

    alloys

    Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

  • chamfering

    chamfering

    Machining a bevel on a workpiece or tool; improves a tool’s entrance into the cut.

  • edge preparation

    edge preparation

    Conditioning of the cutting edge, such as a honing or chamfering, to make it stronger and less susceptible to chipping. A chamfer is a bevel on the tool’s cutting edge; the angle is measured from the cutting face downward and generally varies from 25° to 45°. Honing is the process of rounding or blunting the cutting edge with abrasives, either manually or mechanically.

  • gang cutting ( milling)

    gang cutting ( milling)

    Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.

  • hardening

    hardening

    Process of increasing the surface hardness of a part. It is accomplished by heating a piece of steel to a temperature within or above its critical range and then cooling (or quenching) it rapidly. In any heat-treatment operation, the rate of heating is important. Heat flows from the exterior to the interior of steel at a definite rate. If the steel is heated too quickly, the outside becomes hotter than the inside and the desired uniform structure cannot be obtained. If a piece is irregular in shape, a slow heating rate is essential to prevent warping and cracking. The heavier the section, the longer the heating time must be to achieve uniform results. Even after the correct temperature has been reached, the piece should be held at the temperature for a sufficient period of time to permit its thickest section to attain a uniform temperature. See workhardening.

  • hardness

    hardness

    Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.

  • helix angle

    helix angle

    Angle that the tool’s leading edge makes with the plane of its centerline.

  • micron

    micron

    Measure of length that is equal to one-millionth of a meter.

  • milling

    milling

    Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

  • milling machine ( mill)

    milling machine ( mill)

    Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.

  • physical vapor deposition ( PVD)

    physical vapor deposition ( PVD)

    Tool-coating process performed at low temperature (500° C), compared to chemical vapor deposition (1,000° C). Employs electric field to generate necessary heat for depositing coating on a tool’s surface. See CVD, chemical vapor deposition.

  • pitch

    pitch

    1. On a saw blade, the number of teeth per inch. 2. In threading, the number of threads per inch.

  • stainless steels

    stainless steels

    Stainless steels possess high strength, heat resistance, excellent workability and erosion resistance. Four general classes have been developed to cover a range of mechanical and physical properties for particular applications. The four classes are: the austenitic types of the chromium-nickel-manganese 200 series and the chromium-nickel 300 series; the martensitic types of the chromium, hardenable 400 series; the chromium, nonhardenable 400-series ferritic types; and the precipitation-hardening type of chromium-nickel alloys with additional elements that are hardenable by solution treating and aging.

  • strain hardening

    strain hardening

    Increase in hardness and strength caused by plastic deformation at temperatures below the recrystallization range.