Improved Turning of Medical Components

Medical component machining will remain a growth industry for the foreseeable future

By Yair Selek

September 24, 2012

Working with difficult-to-cut materials and short runs leaves little room for error.

Worldwide demand for precision-turned medical parts is definitely on the rise, especially for orthopedic and dental implants. This is partly due to our aging population; the longer we mortals live, the more spare parts we'll need. Not surprisingly, so is the competition for this high-end work rising.

The business will go to providers that deliver not only extreme accuracy and efficiency in difficult-to-machine metals, but also provide prototyping capability, quick turnaround, and the willingness to deliver in small quantities.

Working with such expensive and difficult material and short runs leaves little room for error. To be competitive and profitable in this specialty, you must get it right the first time.

The good news is that advanced tooling is now available specifically designed to help those demanding medical fabrication jobs run faster and better. The proper tooling—even drop-in replacements—often can double machining rates or edge life, improve chip control, and eliminate part distortion on thin-walled or difficult-to-grip workpieces.

For example, while producing a small part, a Swiss dental product manufacturer was experiencing premature and unpredictable insert failure, often by chipping and edge breakage.

Edge life was averaging 90 pieces, but the range varied so widely as to render meaningless the word average. Consequently, the operation required constant operator attention.

With a drop-in switch to a PVD-coated insert in a rigid screw lock toolholder, the manufacturer was able to double edge life to 250 pieces per edge, and the surface finish improved to excellent. The failure mode was gradual, predictable edge wear as indicated by deterioration of surface finish. The operation could run unattended as a result. Key to the improvement was the greater clamping rigidity that held the insert in place.

Three Common Denominators of Medical Part Turning

1. Stock Removal. In medical part turning, more than half of the original metal weight is machined away, illustrating an often overlooked aspect in medical component turning. Though the parts may be small, the volume of metal to be removed is relatively large. Whether on miniature Swiss automatics or small turning centers, turning medical components often involves extreme stepped diameters machined from solid barstock.

The finished part may weigh much less than it did before machining.

2. Tough Materials. Another common denominator of medical part turning is that the material likely will be difficult to machine. The main diet is gummy stainless steel and other nickel-based alloys, long-chipping titanium, and high-temperature alloys and hardened steels.

In material selection, biocompatibility, corrosion resistance, and extreme high strength will necessarily take precedence over machinability.

3. Complex Geometries. Workpiece geometry can add to the challenge. Thin-walled tubing, especially aluminum or other nontypical medical parts, is prone to distortion unless the cutting edges are extremely sharp.

Many workpieces are asymmetrical, with complex curves interfering with fixturing and support. This places a premium on minimizing cutting forces, so the workpiece is actually cut and not distorted. In particular, RPMs must be adjusted to maintain proper surface speeds as the part diameter shrinks in unforgiving materials.

Proven Advances, Smaller Sizes

Fortunately, significant advances in tooling for difficult materials in larger-scale applications have now been applied to the smaller sizes needed for medical component work. Advanced coatings reduce friction, machining heat, and microscopic stress raisers that can lead to sudden edge failure.

New carbide grades stand up reliably to the punishing materials, even with interrupted cuts. More aggressive chipbreakers and through-the-tool coolant delivery improve chip control and evacuation while reducing recutting even in the deepest bores. More secure clamping systems keep the insert in position and eliminate the microvibrations associated with insert movement in the seat pocket.

Coatings Improve Performance

A post-coating treatment is designed to make the insert coatings smoother and more lubricious, and thereby mitigate the three key enemies of insert life: friction, heat, and stress raisers.

Field experience has demonstrated that a post-coating treatment improves efficiency an average of 35 percent, taking into account both throughput and edge life.

Matching the correct grade with the correct post-coating application also is important.

Certain grades with a post-coating treatment may be well-suited for medium-speed machining in austenitic stainless steel, heat-resistant alloys, and hardened steel because of their excellent resistance to built-up edge (BUE), which is common in stainless steel work. Other grades with a hard, fine-grain substrate and the same coating and treatment deliver high resistance to wear and chipping on a variety of materials and are suitable for interrupted-cut applications.

Extra-tough grades are useful in milling, parting, grooving, and unstable turning applications in plain, alloy, and stainless steels.

Choice of chipbreaking geometries can also make a big difference on small medical parts in difficult materials. (See Advantageous Chip Splitting sidebar.)

Thread Whirling Protects Slender Parts

For threading on slender parts, a multitool process called whirling can do the job without deforming the workpiece. Multiple tools, symmetrically spaced, balance the cutting forces as no single-point tool possibly can.

Inserts specifically designed for this process typically are successful in thread whirling of titanium bone screws.

Secure Chip Evacuation From Tight Spaces

In internal turning, chip control and evacuation have been a chronic problem, especially for gummy, long-chipping metals. Through-tool coolant delivery and a rigid clamping system that prolongs edge life and improves process security should be used in these cases.

Like other high-end manufacturing sectors, medical part manufacturing will remain very demanding and increasingly competitive. Advanced tooling, now more widely available in the required smaller sizes, can make a big difference in efficiency and profitability.

Advantageous Chip Splitting

By Andrei Petrilin,
Senior Technical Manager — Indexable Milling
Iscar Tools

The combination of the chip splitting cutting edge with a modern design and the newest technology in the latest milling inserts has substantially increased productivity of rough milling operations.

Chip splitting reduces cutting forces and power consumption and leads to less heat generation during milling.

In chip splitting, a wide chip is divided into small segments that improve chip evacuation and handling. Chip splitting reduces cutting forces and power consumption and leads to less heat generation during milling.

In addition, the chip splitting action strengthens vibration dampening. Better chip evacuation, lower cutting forces and heat, increased resistance to vibrations – these key features give the best advantage in rough milling with the use of extended flute or porcupine-style cutters.

Indeed, the cutting blade of such cutters comprises a set of consecutively located inserts. Usually the cutters work with high depth of cut and can remove a large volume of material. Consequently, the segmentation of the produced chips is highly important for their effective transportation through the chip gullets of a cutter.

The high metal removal rate causes substantial cutting forces and therefore demands considerable machine power. A tool works under hard conditions and experiences significant heat load. The chip splitting ability of the extended flute cutters contributes greatly to reduction of the acting cutting forces and the required power and diminished thermal loading of the cutter. The latter is especially important for machining hard-to-cut materials such as titanium alloys, in which minimizing heat generation during cutting is an important factor in improved performance and longer tool life. In addition, reducing cutting forces allows the cutter to be used in machine tools with low-power drive.

In rough milling with extended flute cutters, a high cutter overhang is quite common. Vibrations and bending of the cutter are well-known problems technologists and operators face. To solve these problems, the cutting data should be reduced.  However, this may affect productivity.

The better vibration dampening cutters with inserts with a chip splitting cutting edge produce significantly improves the dynamic behavior of the cutters and helps to overcome these problems.

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