Fifteen most asked questions about diamond tools.
by: Nico van Tongeren, Sub Micron Tooling B.V., 1998.
This article has been awarded with the "Global Contact Award" for the year 1998.
Published in "Global Contact", no. 20.
Especially contact lens, IOL, and mould manufacturers aren't afraid asking about diamond tools, because they know the diamond tool plays a very important role in the final lens quality and production price. In general they have a lot of knowledge regarding lens design, (lens) materials and even lathes.
However during the last 15 years, visiting, and talking to customers, I became more and more aware of the need for explanation regarding diamonds in general, their use, parameters etc.:
1. Why use diamond tools for contact lens cutting?
The fact that diamond is the hardest material, and a single crystal (the cutting edge can be grind extremely sharp) makes it very suitable as a cutting tool.
2. Are all diamonds the same?
Diamonds are found all over the world and have been formed under different circumstances. All kind of elements, impurities are "built in", so one can imagine that this has it's influence on the behaviour of the diamond tool.
3. Is there a difference between "industrial" diamonds and diamonds for jewellery?
Diamonds for jewellery (gem stone quality) are selected to perform, after polishing, as a jewel: shiny, clear, near white and no inclusions to be seen.
Diamonds used for cutting tools, are selected based upon their use: so yellowish stones are acceptable, but inclusions can affect the performance of a diamond tool and have to be avoided, at least near the cutting edge.
4. Toolmakers always mention diamond orientation, what's the influence on the life time of my tool and the quality of my lenses ?
Diamonds grow as a single crystal, and diamond tool manufacturers knew there were directions, the diamond was more difficult to grind than other directions, so they choose for this "hard" direction to be the best suitable for cutting.
Now we know, there are applications, a different diamond orientation can give better results; especially when you use tools on 2-axis lathes, it is important the tool wears equal along the cutting edge.
To determine the exact required crystal orientation, X-ray diffraction techniques can be used.
5. What's the difference between natural and synthetic diamond, and what's better for me?
There are tools made of very small particles of synthetic diamond, sintered together, called PCD ("poly crystalline diamond"). Some people call this PCD: synthetic diamond.\PCD only can be used for diametering or rough cutting, because the cutting edge of such a tool cannot grinded sharper than the grain size of the PCD.
When we mention synthetic diamond, we are talking about single crystal synthetic diamond.
Natural diamond has grown in the inside of the earth, and due to it's environs, changes in temperature and pressure, they become diamonds with more or less inclusions, different colours and more or less disturbed crystal structure.
A synthetic diamond has grown in a vessel, under the same circumstances (temperature and pressure) as natural diamond. These circumstances can be controlled, even other elements can be added. Therefor the quality can be more consistent than natural diamond. The synthetic diamond, used for cutting tools, mostly is yellow. This colour, caused by the "built in" nitrogen, gives the advantage of a higher heat conductivity, so less wear during cutting, and harder to certain contact lens materials than natural diamond.
In short: for cutting contact lenses, it can be an advantage using synthetic diamond.
6. Is a bigger diamond better than a small one?
Although tools with big diamonds look as though you get more value for money, you must consider that smaller diamonds, mostly grew less distorted, and having less inclusions. The specific area, used for cutting, and the height, define the final dimensions of the diamond.
7. Why does my diamond tool wear?
Being the hardest known material, it doesn't mean diamond won't wear. While cutting contact lenses, two types of wear can be distinguished:
(1) mechanical wear: constant applied forces, during cutting, eventually will wear the cutting edge.
(2) chemical wear: the fact that diamond is no more than carbon, this carbon reacts with other elements, especially at elevated temperatures and with the presence of oxygen from the air.
8. Does the use of cutting coolants really help the diamond tool life or cutting performance?
Yes, due to the fact that a coolant, especially when sprayed as a mist, helps to reduce the cutting forces, keeps the cutting temperature lower and partly replaces reactive oxygen from the air.
When cutting metal moulds, it can prevent material "built up" on the cutting edge.
9. What is a "relap", and is a "relapped tool" as good as a new tool ?
Relapping a tool is: carefully removing all damage and wear of the cutting edge by means of removing the upper layer of the diamond. Important is to remove just enough diamond, to be sure all damage is disappeared. It isn't enough only checking the cutting edge under a microscope with a high magnification (up to 800x), but one also have to check if the original form ( waviness) of the radius is re-established. Only in that case the relapped tool is as good as new.
At Sub Micron Tooling, we ensure this by determination of the radius form of every tool, and measure its form before and after relap. It also enables us to take off only that part of the diamond, that has worn, so the total life time of a tool will increase.
10. Why does a new "sharp" tool sometimes generate a worse surface finish than a tool that has already been "run-in" ?
A new sharp, or relapped tool has an extremely sharp cutting edge. Grinding or relapping can introduce micro cracks into the cutting edge. When cutting the first lenses, the micro cracks can pass into micro chips, which generate a worse surface. After cutting a certain number of lenses, the cutting edge wears and the microchips will disappear.
By using sophisticated lapping machines with air bearings, these micro cracks can be avoided.
11. Should I use cylindrical or conical tools ?
For single point turning a cylindrical form of the radius is suitable. The advantage: the radius size doesn't change after every relap. However keep in mind that from the point of the tool towards the sides, you will have less clearance.
For 2-axis lathing a conical shaped tool is preferable. Over the entire used "window" the clearance stays the same, so also your cutting conditions. Disadvantage: the radius becomes smaller after every relap.
It is of great importance knowing the exact radius size of every new, or relapped tool. Our optical measuring system ensures the radius size within 0,5 micrometer. this measurement is provided with every new tool and after every relap.
12. What's the difference between "Non Controlled Waviness" tools and "Controlled Waviness" tools, and what do I need ?
We, at SMT, believe every diamond tool should be "controlled". We rather like to talk about "micron controlled waviness", or "sub micron controlled waviness" tools.
Waviness means: the maximum peak to valley height of the profile, compared to the best fit circle.
Tools for single point turning don't have to have an extremely low waviness, but since we know that irregularities can affect the surface quality, it is of importance to keep the waviness value <2µm.
For 2-axis machining it is important to keep the waviness value <0,5µm or better: <0,25µm, depending on the lathe, you are using. Be sure, in this case, always use a conical radius, combined with 0° rake angle, this ensures you a true circular cutting edge.
One can understand, the lower the waviness value, the more expensive the tool. Therefor it is important to know, whether the lenses are polished after cutting, or not. When polished, what's the max. allowed polishing time?
Knowing all information, a suitable tool can be recommended, giving the best results at minimal costs.
13. Does a standard tool exist ?
Of course, there are certain tool parameters, which can be used for cutting different contact lens materials. Because every individual contact lens-, IOL-, or mould manufacturer has different requirements regarding the kind of material to be cut, production speed, surface-, and optical quality, etc.
It is highly recommended to obtain a tailor made solution from your diamond tool manufacturer.
14. Should I use insert type, or solid shank type tools?
Until some years ago, it was common to use a tool with a solid shank, sometimes the diamond was clamped, or set in brass. Later on the first tools appeared with a vacuum brazed diamond.
For some applications: left-, right-, or straight tools were needed. A new lathe often required new tools.
Nowadays ISO inserts are available, with vacuum brazed diamonds. Per lathe, only one or more ISO holders are needed. A standard ISO insert fits very precisely In these holders, so very accurate repeated mounting is possible. Whenever relap is necessary, you only have to ship the insert. The use of an ISO insert system, can keep your diamond tool inventory low.
15. Can I touch the diamond, and what's the best way to clean the cutting edge ?
Always try to prevent touching the cutting edge (the extremely sharp diamond cutting edge can cause a nasty wound).
After cleaning your hands very thorough, very small abrasive or metal particles can remain on your finger tips, and can damage the cutting edge, when touching.
Use appropriate tools, to mount your diamond tool on the tool post.
For ISO inserts, use the special torx wrench, supplied with your new tools, or holder.
If, for any reason, you have to clean a diamond, use pure cotton wool, soaked with pure ethanol or iso-propanol. In case you're not sure, whether your tool is damaged or not, send it to us for a free check-up.
I'm aware there are many more questions, even more questions than I can answer right away, but co-operation between tool-, lens-, material- and lathe manufacturers, finally will result into a better knowledge of cutting processes.
The most easiest step can be made by the contact lens manufacturer and the tool supplier.
Relapping tools isn't just an exchange of repaired tools for money, but also an exchange of knowledge and experience: called Tool Management®, which finally will lead to: a more clear insight into the complex phenomena of cutting lenses, improved lens quality and overall cost reduction.
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