Introduction: Machining stones

Einführung: Bearbeitung von Steinen

1. introduction

1.1 Prehistory

The processing of stones, minerals and fossils has a long tradition. Already in the Stone Age people designed tools from hard stones such as flint, jasper and the like in order to produce e.g. knife blades, weapons etc. in the simplest way, by giving these stones by hand the appropriate form for different purposes. Already in the Middle Ages, among other things, colored stones, semi-precious and precious stones were processed into jewelry and decorative items. The processing, i.e. the grinding and polishing of these stones was a time-consuming procedure, as everything had to be done by hand in the beginning. Guilds specialized in stone processing were formed. At that time, only real minerals, colored stones, semi-precious and precious stones were processed, since synthetic production was not possible. The cut stones had already a very high value in the past, because they were used in jewelry, crowns, robe clasps, etc. and were not accessible and affordable for everyone. The grinding and polishing process, later powered by water and sandstone wheels, was strictly guarded by the grinders and was a strenuous and time-consuming process. Great exhibits such as thin-walled bowls, cups, cameos, etc. can be admired today in the gemstone museum in Idar-Oberstein. Considering the simple tools used at that time, these artists deserve the highest respect.

1.2 Present

Today the situation is somewhat different. The processing of stream stones, pebbles, minerals, precious and semi-precious stones as well as the processing of fossils can be learned by almost anyone with the appropriate tool equipment. Thanks to a wide range of machines for mineral and gemstone processing, this activity is now very widespread as a hobby as well as professional production, allowing access to a large number of interested people. The information below is the result of our own practical information, i.e. from the practitioner for the practitioner, and is not the result of scientific research.

How to work stones, minerals, semi-precious and precious stones, fossils nowadays?

This answer strongly depends on the objective of the cutter as well as on the size, texture, etc. of the stones. Some make a plane cut or a slice of the stone with a plane grinder to "bring out" the inner structures of the stone, others round the stones with a spirator, tumbler according to their natural shape, many cut pendants, cabochons, Many grind pendants, cabochons, ring stones and various jewelry with wheel grinding machines, cabbing machines from it, some grinders refine the beautiful raw gemstone with many surfaces (facets) by means of faceting machine, also balls, sculptures and various objects are cut from the stones, and much more.

Depending on the grinding goal, there is a variety of processing options that can be considered for this. A distinction is also made as to whether stones are ground by hand or machines (machine grinding) are used for this purpose.

How are stones, minerals, gemstones, etc. cut by hand?

There are a number of tools available for this purpose. Here is an excerpt:

  • Grinding on a glass or metal plate with loose abrasive grit with the addition of water(sica, aluminum oxide, corundum,
  • Grinding with an abrasive block using wet abrasive paper (sica equals silicon carbide) with water.
  • grinding with diamond hand pads with water
  • Grinding on a diamond-coated grinding disc ("lap") with water
  • Diamond-covered tools such as diamond file with or without water

In some countries, foot-powered treadle grinders are also used (similar to a foot-powered sewing machine...) with sometimes very nice results.

In many cases, the stones are separated or formatted (cut) prior to the grinding and polishing process to achieve goal-oriented results. This often saves a lot of grinding work. Cutting is actually a "grinding through" of the stones - so-called cut-off grinding. For this purpose, diamond cutting saws with galvanically coated diamond saw blades or sintered cutting blades are usually used. Various machines are used for this purpose.

Here is an excerpt:

2. how are stones cut?

In most cases, they are held securely with both hands and passed through the rotating saw blade or cutting band (for band saws). (In compliance with all relevant safety regulations!).

Steintrennsäge TS 10 - zum Schließen ins Bild klicken

Stone cutting saw TS10 (table model) with stone clamp and manual feed

Or the stones are firmly clamped in a stone clamp and then fed through the saw blade manually or by means of a feed (weight or electric feed). (In compliance with all relevant safety regulations!).

Stone cutting saw with electric feed (also referred to as automatic saw).

Cooling oils, and additives are used as cooling additives. Water alone is the worst choice, since it does not provide enough cooling of the saw blade in the cutting area. Cutting additives that are mixed with water have proven to be a favorable variant. Very hard stones are usually cut with pure cooling oil. Please read the data sheet, as it may contain substances that are harmful to health or the environment. Only cut with breath/mouth protection!
The diamond saw blades should also be sharpened from time to time, as the diamonds on the saw blade become rounded and thus the saw blade no longer cuts properly. Div. sharpeners/sharpening stones are used to make them edgy again. Saw blades should not be left in the coolant/additive mix for a long period of time, as they can be attacked by rust and can then be damaged or even injured during operation! Therefore, drain the coolant until the saw blade is no longer immersed in it. Alternatively, a circulating pump system can be used. The maximum speeds specified by the saw blade manufacturer must also be observed. This must not be exceeded!
Conclusion: the nicer the cut, the less grinding work you have to expect. The quality of the cut (roughness of the surface) depends on various factors:

  • Saw blade
  • Speed of rotation
  • Feed rate (how fast is the cut)
  • Coolant

3. grinding stones

3.1 Machine grinding with abrasive sheets (coated with silicon carbide. "Wet grinding paper". Wet grinding

This uses bonded, inexpensive abrasive applied to lightly cardboarded sheets (also available at hardware stores). It is cut to disc size and applied (glued) to a flat backing board with a pressure-sensitive adhesive (spray). Worn sanding sheets can be peeled off and the backing sheet can be resurfaced. The results are impressive, as the quality of the sandpaper is consistently good. It should be noted that the coarse grits are run at low speed (150-300 rpm) to become flat. The disadvantage of wet sandpaper is that these sanding sheets wear out relatively quickly and become dull, which leads to higher costs for harder stones. Also, the grinding time is higher compared to the diamond sanding discs ("laps"). With a careful approach, clean results are also achieved with this method.

Polishing is done on various media such as felt, cork, leather, plastic discs with diamond polishing pastes and polishing oxides. When polishing with cerium oxide and other oxides, speeds of 150-200 rpm are recommended while keeping the polishing wheel moist.

Dry grinding with water abrasive paper is generally not recommended for reasons of health hazards!

3.2 Machine grinding on wet belt grinder

A method that is much more common in the USA and South America than in Europe. Sica or diamond grinding belts are used with the addition of water for cooling and free flushing of the ground material. This method is also used as dry grinding, but this is questionable in terms of health.

3.3 Grinding with sintered diamond grinding wheels, wet grinding

This method does have its merits, especially for planing after cutting, as it is fast. The service life of the grinding disc is high to very high, depending on the covering height, and can therefore also be described as economically rational. It should be noted that the disc should be dressed with a diamond dresser from time to time to keep it as flat as possible. Many grinders avoid this method especially when using multiple sintered wheels because of the considerable cost involved. These sintered wheels are more commonly found on faceting machines.

3.4 Grinding with grinding drum, spirator (tumbler)

With such devices, the original shape of the filled stones is changed slightly to considerably. This depends on the size of the abrasive grain and the grinding time. The ground and polished stones are called "baroque" ground. Not only rough stones but also pre-cut pendants, stone slabs, even pre-cut stones can be ground and polished. The average processing time is 2-4 weeks, depending on the hardness of the stones. The stones are filled into the grinding container with the addition of loose abrasive grit (powder form) Sica or Aluoxd, corundum, and water. The filling quantity should not exceed 2/3 in order to achieve proper mutual abrasion. With the Spirator (Tumbler) the grinding/polishing process is faster, because these devices work with a high vibration speed and the stones grind each other more intensively in all directions. Many grinders add ceramic pellets or other material to which the abrasive grain bonds during the grinding process. It is also important to add approx. 1/3 of small "driving stones" (approx. 1-2cm marble or quartz chippings (depending on the hardness of the tumbled stones to be ground) which are responsible for the constant movement of the mass in the spirator. The mix of stones, grinding powder, ceramic pellets and water filled in the spirator must constantly rotate slowly in a circle and at the same time show a rolling motion. The amount of water filled in is an important criterion, on which depends whether everything in the Spirator rotates well or not. If you fill in too much water, the quantity will soon stagnate and vibrate at the stand, which will not lead to success. Therefore, with a 2.5 liter Spirator, start by adding 1/16 - 1/8 liter of water at the beginning of grinding - and do this in portions. Then observe whether the mass turns after some time and also rolls. If necessary, always add a little water with a spray bottle. Watch the process every few hours and add LITTLE water if needed. After 24 hours everything should be cleaned and if more abrasion from the stone is desired, sand again with the same (but fresh) amount of abrasive powder (about 3 Tbsp). Then do the same process with the next finer grit, and so on. Finally, when the tumbled stones have been finely ground with 1200 grit (depending on the type of stone, etc.), the next step is to clean them thoroughly, as polishing can now begin. Many grinders use a separate polishing container for polishing so that no coarse grit from the previous grinding passes can scratch the stones. Cleanliness and cleaning between each step is the first order of business here!

Spirator- 9 - zum Schließen ins Bild klicken


Polishing process:

Fill the finely ground stones into the polishing container with a suitable polishing agent, a little water and possibly the addition of polishing ceramic pellets. The small driving stones, which have now also been finely ground, are also placed in the container so that the necessary rotation and vibration can get underway. Some polishers also use polishing additions such as walnut shell pieces, small wooden cubes and more (their secrets).

The finished polished, baroque stones can also be set afterwards, fitted with a pin eye, or used loose, etc.

Spirators are also used in various fields of industry for deburring, rounding, polishing.

In this context, the flat spirator should also be mentioned, on which (only already cut) stones are ground and polished with loose grain under vibration. This method takes some time, but gives nice results, especially if the supporting surface in the tub is made of hard metal (the common aluminum tubs, on the other hand, grind out after some time and a slight depression is created where the stones then no longer lie flat). It is also helpful to weigh down the stones or stone slabs with weight (putty or similar) so that more contact pressure is created and the abrasion is increased as a result.

3.5 Machine grinding with loose grit (Sica=silicon carbide) on cast iron, metal wheel

A long-established, still very good method for mineral processing. For this, mostly cast iron discs are used, which have a slight incline (or not) towards the center.

These discs are sometimes additionally grooved, which serves to better separate the removed abrasive slurry. The speed of rotation is low (approx. 100-180 rpm) to prevent the abrasive grit from being thrown off. A mixture of water and abrasive grit is applied to the stationary faceplate with a clean brush and distributed. Start with coarse grit (K80-K120) proper hand pressure and grind the stone flat with circular motion. Afterwards, the faceplate and the stone (and also the hands and fingers, etc.) must be carefully cleaned so that nothing of the coarse grit is carried over to the next finer grit. After a few grit sizes, you start at grit 600, 800 or even 1200. After that, polishing is carried out at a similarly low speed (150-300 rpm), e.g. with cerium oxide (for the entire quartz group, among others), wet on a felt polishing plate. Due to the versatile field of application, surface grinding machines with speed control such as our EMSCH 200 RPM are recommended for this purpose.

Surface grinding machine EMSCH 200 RPM

This method is still often found among agate grinders, who use it to achieve perfect results. Every agate grinder knows the procedure with grinding time, increased contact pressure and the associated cleaning work.

We have replaced this procedure by our self-developed special quartz/agate diamond grinding pads for us and meanwhile also for many customers and achieve (almost) equivalent results after 3 grinding passes + polishing. Some sanders who use our proprietary pads do not polish anymore, because the shine after the 3rd pad is already sufficient for them. Thus, we give preference to these special pads, as the sum of the disadvantages is eliminated. Grinding agates.

3.6 How does machine grinding of cabochons work?

After some practice and accurate work a popular form of gemstone grinding. For this purpose, one often uses translucent, interesting-structured, even less valuable stones, semi-precious stones such as various types of quartz, cat tiger eye, favorable turquoise, lapis, agates, banded, patterned stones or simply the self-collected stones but also valuable stones such as opals, star sapphire, etc.. Transparent, valuable stones without opacities, on the other hand, are usually faceted .Cabochons come in various shapes and thicknesses, and before cutting, it is necessary to determine where the stone has its greatest shimmer, the best pattern, the best refraction and strongest color. To do this, cut the stone into a disk and then draw the desired final shape on it. The shape can also be transferred from templates using brass or aluminum pins. Then cut so that a little oversize remains after the cut. This roughly cut stone is then ground out by hand (if not too small) on the grinding wheel (usually diamond grinding wheel hard grit 80, 100, 120) according to its shape. After that, the stone is applied with stone putty or by means of 2K glue on a wooden dopp (sticks) and sanded and finely ground with the finer grinding wheels. After grinding wheel grit 280 soft, the stone is dried and checked for any deep scratches and camber defects, because now such defects can still be corrected by regrinding with K 280. Usually, you grind up to grit 1200 or better grit 3000. (Speed approx. 1400-1700 rpm) Finally, the cabochon is polished and removed again from the wood stop. Cemented stones are detached again by flame and 2K glue is detached with hot water. (ca.70-90 degrees) whereby it is to be paid attention which stones are heat-sensitive and one must proceed accordingly carefully with the flame. (Spirit or gas burner). We also sell an electric putty melter.

CabKing 6" for grinding cabochons

3.7 Machine grinding with diamond grinding pads

This innovative method has become more and more popular in the last 10 years (regionally still different). Diapads are available in various shapes and sizes. They have been borrowed from stonemasons, who have been using them for a long time to process natural stone surfaces wet very successfully (FLEX). These pads are used in conjunction with wet manual angle grinders. These pads have also been used in natural stone restoration (floors, window sills, kitchen countertops, etc.) for quite some time. We took advantage of this for mineral processing and have been studying their shape, inner workings, bonding, diamond concentration and surface structure for over 6 years. Through years of research and testing, we are now in a position to offer special pads for minerals, gemstone, fossil processing, etc., which lead to very good results. All you need is water. We also use these pads on the surface grinding machines e.g. for grinding agates.

We offer these agate/quartz pads with Velcro for wet angle hand grinders (FLEX) as well as glued and pressed onto sheet metal discs, which can then be clicked onto surface grinding machines with magnetic discs. For this purpose, the magnetized mounting disc should have a centering mandrel for safety's sake, so that the pads cannot be thrown off sideways at high speed. Such face plates can also be obtained from us.

The conventional diamond pads offered on the market (mostly of Chinese origin) are available in a wide variety of designs and construction methods corresponding to the various fields of application. For mineral and fossil processing these are only conditionally applicable, because they often have too little diamond grain, often no sieved diamond grain as well as a too hard bond, which inevitably leads to scratches.

Wet angle grinder for diamond pads

3.8 Carving technique

Frequently seen are also "carved" stones that have been wet-worked with the Dremel or other hand grinders. Especially for opal cutting these tools are very popular, because they can be used to grind out e.g. wavy, misshapen natural shapes, where the opal layer sits on top, without having to waste much (expensive) color of the opal. Hard, galvanized and sintered grinding tools with pointed, round, concave shapes are often used here to get to the desired areas of the stone. Diamondized grinding points are also used here, such as those offered by Diamond Pacific as NOVA grinding points. Round diamond abrasive foils on a soft surface are also popular.

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