CadCam Corner: Let’s Get Milling!
Milling burs, endmills…another consumable we as dental technicians are buying more of as we move further along the route to full digitisation.
However, like everything else there is a range of qualities available at hugely different prices. How do we make any judgement?
First I will take a look at the range of materials used for dental tool production, then an overview of the various coatings available, and finally any other factors affecting the performance of a milling tool.
- High Speed Steel (HSS). HSS is made from wrought (rolled) steel. molybdenum, tungsten and vanadium can be added to increase what is called the ‘red hardness’. The red hardness dictates how the sharpness of the edge is maintained when subjected to the heat of milling. The hotter the bur gets, the softer the metal of the cutting tool gets, the quicker it loses its sharpness and the shorter its working life. Powdered HSS can be used and these produce a stronger material and a more homogenous alloy structure. Moving further up the quality scale, cobalt, up to around 10.5%, can be added for even more hardness. Already we can see there is a range of qualities in HSS, and of course, price implications for increased performance.
- Tungsten Carbide. Tungsten is a metal with unique properties, making it an essential component in many industrial applications. It has a very high melting point (3422 Deg C), a very high density (19.25g/cm2) and a hardness of 3430HV. Its importance is noted by the EU categorising it as a ‘critical raw material’ In 2015, 84.5% of world production of the tungsten carbide ore was mined in China! Total world production was 87,000 metric tons in 2015. Only 600 tons were mined in the UK in our only mine, Hemerdon, in Devon, which opened in the autumn of 2015. Two important factors affect the quality of tungsten carbide: the amount of cobalt added, which will be between 6 and 12%, and the grain size of the powders. Tungsten carbide is harder than HSS and will therefore last longer.
There are many different coatings available. Here I will look at five of them from titanium nitride all the way to diamond itself.
The main purpose of the coating is to act as a heat shield and therefore reduce heat. Heat will affect the substrate and then it will wear faster. We want to increase the resistance to heat (called: ‘the red hardness’) and thereby increase the longevity of the tool. We can do this by protecting the tool from excess heat with various coatings.
a) Titanium Nitride (TiN)
TiN is a hard ceramic material and is normally a gold colour.It has a Vickers Hardness (HV) value of around 2400. It is a very thin coating of between 5–24nm and was first developed in the 1980s.
b) Titanium Aluminium Nitride (TiAlN)
The addition of alumina oxides reduce the thermal conductivity (so better insulation of the tool) and increases the hardness to between 2660 and 3300 HV.
c) Titanium Aluminium Carbonitride (TiAlCN)
This time carbon is also added to produce an even greater hardness, more heat resistance and a lower co-efficient of friction. Together with a greater thermal stability, this coating has a hardness of over 3000 HV.
d) Diamond Like Carbon (DLC)
This produces even more hardness, we are now in the 5000 to 9000 HV range. There are several types of DLC with a wide range of performances.
Diamond is the most expensive but hardest coating. 20 to 40 layers of diamond and yttrium are applied. Grain size is important (smaller is better) as is the amount of cobalt in the tungsten carbide base. The layer thickness is 3 to 20 microns depending on the method of deposition. Diamond has a Vickers Hardness value of 10,000.
A diamond coating does protect the substrate, but in a slightly different way from other coatings: as the tool is used, tiny chips of diamond are lost and take some heat with them. A diamond-coated tool does not degrade gradually as with other coatings, its life ends quickly when the diamond layer has gone. This gives high performance until the end of its life.
Other Performance Factors.
It is obvious that the type of zirconia or other material you are milling will affect the life of your tools. Also clear is how the quality of your chosen milling machine will also have an effect on the longevity of the cutting tool. Poor cutting strategies can also have a bearing on these outcomes. Here, I would like to finish by mentioning that for a given mill and material, the quality of manufacture of your tools will also make a difference. The accuracy of machining, the resultant concentricity of the tools made, can change the amount of heat produced during the milling process. Poorly manufactured tools made on cheaper equipment will likely have less accurate flutes, increasing wear and so increasing heat at the cutting point. As a consequence, the red hardness value is lowered, and your tool life reduced.
I hope this introduction will help you with your purchase of the tools you need for your milling system.
Tim Brothers, Director, Bristol Cadcam Co Ltd., Bristol Crown Company Ltd