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Step 4: Select a material and grinding wheel to suit your bone drill
Approximately 72% of industrial tools are made from sintered Tungsten Carbide and 28% from High Speed Steel (HSS) including HSS-Co [Dedalus, 2010].
Surgical instruments, however, are usually made from stainless steel which has better sterility properties but lower hardness and edge holding abilities. In this installment, we describe the properties of the various grades of stainless steel and give you a starting point for selecting suitable grinding wheels, feeds and speeds for working with them.
Caution: Always consult an engineer qualified and experienced in medical material selection when choosing a material for your specific project.
Properties of Stainless Steel
Stainless steel comes in many different grades. Whilst the grade to use will usually be specified on the print, it is worth having a basic understanding of how rotary instrument designers choose a grade for their designs.
Stainless steel is an alloy of Iron and 10.5% or more of Chromium. Chromium gives stainless steel its corrosion resistance, and hence it’s sterility properties, by rapidly forming a 2-3 nanometer oxide barrier when it comes into contact with air. Nickel and Molybdenum are the other common metals used in stainless steel. By varying the concentrations of these four metals, along with small amounts of carbon and exotics such as Titanium, the crystal structure and material properties of the alloy can be varied to suit different applications.
Stainless steel grades commonly used in medical applications
Non-Heat Treatable Stainless Steels
Austenitic alloys (the 300 series) are the most popular stainless steel grades. 304 is the most widely used austenitic grade in industry, common in architectural, commercial and food applications. In medical applications, the lowest 300 series alloy is typically 316 (often called marine or surgical grade). 316 has a high Chromium content (16-18.5%), quite a high Nickel content (10-14%), Molybdenum (2-3%) and a very small amount of carbon.
Nickel allows the alloy to be workable and Molybdenum helps protect from local corrosion such as pitting. Austenitic stainless steel, however, cannot be heat treated to increase its hardness and lacks the strength to hold very sharp cutting edges in some re-usable instruments.
Variants of 316 stainless steel are specifically formulated for medical applications (eg: 316L and 316LVM) and there are standards covering their use (eg: ASTM F138 – 08). Standards differ depending on whether the steel is for permanent (prosthetic) use inside the body (eg: 316VLM) or for durable instruments.
316 finds uses in prosthetics such as implant screws, but for rotary instruments that include cutting edges, the higher grades are usually more appropriate.
Heat-Treatable Stainless Steels
Rotary instruments will usually be made from Martensitic alloys. They are harder than Austenitic alloys and can be heat treated to improve their hardness even further. Martensitic alloys contain less nickel (<2%), which makes them harder than the 300 series, but more brittle. Popular martensitic stainless steels used in rotary instruments are 420 and 440. These can be hardened up to Rockwell 50 and above. We typically see heat treated stock specified at Rockwell 47 or higher.
The disadvantage of 420 over 316 is that because the Chromium content is lower (12%) it is less corrosion resistant, however this does increase slightly after heat treatment and surface polishing, particularly grinding.
Precipitation Hardened Stainless Steels
At the top of the scale are the Precipitation Hardened (PH) martensitic alloys; 455, 465 and 630 (also called 17-4PH). In these alloys, the Chromium content is higher than 420, giving them similar corrosion properties to 304. Their Nickel content is also higher than 420, allowing them to be precipitation and age hardened (at lower temperatures), resulting in hardness about three times that of 304 and a much better ability to hold a sharp cutting edge.
The hardening process is very stable so PH martensitic stainless steels are well suited to long, thin instruments like bone drills where roundness and straightness is very important.
Grinding Stainless Steel
Unlike grinding carbide, grinding stainless steel produces a lot of sparks and heat so, like with HSS, you need to have very good, high pressure coolant delivery into the cut zone to avoid burning and the risk of igniting the cutting oil.
Resin bonded CBN is the wheel of choice for grinding stainless. A fairly course grit in the range B96-B128 at a concentration of 100-125 is appropriate for most applications because stainless tends to be stickier than HSS and finer grit wheels tend to clog. Surface speeds are usually in the 35-40m/s (115-130 ft/s) range. A typical feedrate is 125mm/min (5”/min).
Dressable grinding wheels
To optimize flute shapes in rotary instruments, dressable wheels can save you a lot of time. There are two main options here. The first; dressable, vitrified bond CBN can be dressed directly on a tool grinder and used with the same coolant delivery system as resin bonded CBN wheels (ie: with a pressure of about 10-12 bar).
The second option is to use long grit aspect ratio, vitrified bond, Aluminium Oxide wheels as we mentioned in part 3. These wheels, which can also be dressed on a tool grinder, require a special coolant kit with highly focused laminar flow nozzles with delivery at 15-16 bar. ANCA provides a high speed fluting kit for this type of wheel. This kit has automatic coolant height adjustment which keeps the coolant focused on the cut zone as the wheel size changes due to dressing. It also includes a 60 bar cleaning jet aimed at the top of the wheel that keeps it open and free cutting in between dresses.
High speed flute grinding of a medical reamer
With this kit and the correct work support, fluting is possible up to 4,000mm/min (160”/min). Surface speeds for these wheels are in the 60-70 m/s (200-230 ft/s) range. These wheels are much cheaper than vitrified CBN so, considering their exceptional productivity, are a good choice for many instrument fluting jobs. Examples are the Aulos and XGP-90 from Saint Gobain/Norton; and SlipNaxos from Winterthur.
An XGP grinding wheel mounted in an ANCA TX7 with high speed fluting kit
In the next - and last - installment of this course, we’ll look at how you can apply what you’ve learnt to expand your presence in the medical components industry.
||Step 1: Apply some fresh techniques from industrial cutting tool design
||Step 2: Optimize instrument geometry to match the specifics of bone structure
||Step 3: Leverage the advantages of grinding over Swiss turning
||Step 4: Select a material and grinding wheel to suit your bone drill
||Step 5: Drill your way deep into the medical components market
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