How the diamond cutting wheel was born

Where do you think the diamonds in your reel come from? From Russia, Brazil, South Africa, or from another country?

In the mentioned countries, there are the largest deposits of natural diamond, there is synthetic diamond in diamond discs, and there is a significant difference between them.

Natural diamond

Its name comes from the Greek word " adamas " - indestructible.

The unit of weight is the carat, which is exactly 200 milligrams. The largest natural diamonds reach a weight of 3000 carats (0.6 kg).

The mining of natural diamonds is historically associated with slave labor - which is why natural diamonds were sometimes called blood diamonds. Many famous diamonds come from the Republic of South Africa. For example, the "Star of Africa" diamond, found in the Cullinan mine , is now part of the British Crown Jewels.

fig. 1 Cullinan diamond mine , where they found the famous diamond, source: khplant.co.za

The basis of most modern uses of diamond are its exceptional hardness and high dispersion of light. From the point of view of diamond discs, we are most interested in its hardness. It reaches the highest, tenth degree in the Mohs scale of hardness. As early as three thousand years before Christ, natural diamond was used for drilling rocks. The oldest surviving records of the use of diamond powder as an abrasive are from the 6th century AD, and in 1495 a device for drilling glass was produced according to a drawing by Leonardo da Vinci . In 1939, the British Military Standard prescribes diamond turning. However, it is still a natural diamond, despite Lavoisier 's 1776 proof that diamond is actually carbon whose flat atomic lattice has been transformed into a cubic one (Fig. 1).

fig. 2 The cubic lattice of diamond and the flat lattice of carbon

Since then, scientists have been trying to make a synthetic diamond, which was only succeeded in 1953 by Swedish scientists.

And today ?

Especially thanks to modern technologies, the price of synthetic diamond represents only a tiny fraction of the price of natural diamond, and synthetic diamond is used mainly in industry - in construction, engineering and other sectors as irreplaceable cutting, grinding and drilling tools, powders and pastes.

Synthetic diamond

The essence of its production is the transformation of carbon at high temperature and pressure. Synthetic diamonds used for cutting and grinding diamond wheels for angle grinders are tiny compared to natural diamonds - from 0.25 to 0.75 mm - roughly like poppy seeds.

In addition to hardness, they have one more important property - they are temperature stable up to 1100-1200°C. The downside is considerable fragility and the resulting low impact resistance.

Several methods are used to produce synthetic diamonds. The most used method is based on a combination of high pressure and temperature and relatively low cost. This process involves large presses that can weigh hundreds of tons to achieve a pressure of 5 GPa at a temperature of 1500°C.

fig. 3 Principle of synthetic diamond production ( Anvil = 5 GPa , Dies = 1500°C)

Let's take a closer look at these numbers.

First the temperature. A kitchen stove reaches a temperature of around 250°C, lava from a flowing volcano has a temperature ranging from 600 to 1000°C, and iron melts at a temperature of 1538°C. 1500°C is a very, very high temperature.

And a pressure of 5 GPa ?

At the bottom of the Mariana Trench, the deepest place on the earth's surface, the pressure is about 110 MPa - even that is 45 times lower than the 5 GPa required for the production of synthetic diamond.

So, for the production of synthetic diamond, it is necessary to develop a pressure of 5 GPa . Let's calculate how high a water column of seawater is required for such a pressure.

Result: To develop a pressure of 5 GPa , a column of sea water 489 km high is necessary. That is more than the air distance from Bratislava to Munich (410 km).

And all this just so that miniature diamond grains were produced, and we could use them to facilitate work, for example, in cutting and grinding diamond tools in angle grinders and other devices.

Diamond cutting tools

Thanks to the aforementioned hardness, diamond is an ideal material for cutting and machining tools. As the hardest material known, diamond can be used to polish, cut or wear any other material, including other diamonds.

The usual form of diamond in cutting tools is grains dispersed in a metal (usually cobalt) matrix sintered on the tool.

fig. 4 Close-up of the magnified surface of the diamond segment showing worn metal around the diamond

Common industrial applications of this capability include diamond drills, diamond cutting wheels and diamond grinding wheels for angle grinders, table saws, joint cutters and other equipment. The use of diamond powder as an abrasive is significant. Although natural diamond is also used for these purposes, synthetic diamond is more suitable, mainly because of better reproducibility of its mechanical properties and of course - also because of the price.

Diamond is not suitable for machining ferrous alloys at high speeds because carbon is soluble in iron at the high temperatures that occur in high-speed machining. This leads to significant wear of diamond tools compared to alternative ones, and is also the reason why the service life of diamond discs when cutting reinforced concrete is significantly reduced, compared to cutting pure concrete...

However, we will talk more about the lifespan and working principle of diamond discs used mainly for angle grinders in the next installment...

Sources:
https://www.technikaatrh.cz/obrabeni/diamant-jako-rezny-material
https://sk.wikipedia.org/wiki/Diamant
https://www.novau.sk/diamantove-velmoci-odkial-pochadza-vacsina-diamantov/
https://sk.wikipedia.org/wiki/Karát_(jednotka_hmotnosti)
https://en.wikipedia.org/wiki/Synthetic_diamond
https://www.technikaatrh.cz/obrabeni/diamant-jako-rezny-material

Ing. Herman Nagypál

I graduated from a military school. I learned to work with tools and equipment as a child with my father on the construction of our family house. I test each of our new products personally - with my own hands in my well-equipped workshop. In my spare time, I study trends in the field and like to try competitive products. Some of them make me laugh, and some of them make me wonder (that someone got caught and bought it), but some are really good. Almost like ours 😊.