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SLABS TO CABS: How to Make Lapidary Cabochons

www.inlandlapidary.com ©2006-10 Inland Lapidary

Creating beautiful, polished cabochons from slabbed rock is a rewarding process that (with a bit of practice) anyone can master. This guide will help you work through all of the steps required. There is no exact calculated time for each step, but with experience you will get a solid feel for what amount of grinding is necessary at each step of the process. No matter which step you are on, the following safety precautions and guidelines always apply:

 It is extremely important to wear proper eye protection when using any machine. We highly recommend that you wear full coverage safety goggles rather than just glasses.

 DO NOT wear loose clothing or any accessories (long necklaces, bracelets, shirts with long fringes, and similar) that might get caught by the machines during operation.

 Never use any of the diamond discs, blades, or drums dry. Doing so will permanently damage them and may well damage the slabs you are working.

MATERIAL SELECTION

Today there is a wide variety of pre-slabbed rocks available from which you can create beautiful cabochons. You want to:

 Select slabs that are free of fractures, cracks, and pits that could potentially cause the piece to come apart in the process. Make sure to check both sides of the material. Inclusions may enhance or detract from the finished project.

 Pick a slab size and thickness that suits the size of the cabochons you want to create. You will find most cabs are made from 1/8" to 1/4" slabs. Larger cabs may be made from material closer to 3/8" thick.

 Look for pieces with interesting colors, patterns or design that you can bring out in the cabbing process. If the material is translucent, look at it through a strong light. Hold it up to the sun to look for colors and interesting banding. It is important to remember that your finished cab will be significantly thinner and even more translucent than the original slab. Wet the material to see what it may look like when polished.

 Gemstones come in varying “hardness’s” which will affect how you grind and polish them. In general, harder materials (like agates) take a nice polish easier than softer stones (like opal). Initially you will have more success and better results if you use harder stones such as Brazilian Agates. Knowing the hardness of the material you are working will help you obtain the best results in the sanding and polishing steps.

CREATING THE ROUGH CABOCHON

If you're going to use the cabochon in a commercial finding designed to hold a standard sized stone, it's important to cut it accurately to a specific outline so it will fit. Templates are available to help layout specific sizes and shapes. Move the template around on your slab to find the most pleasing pattern for your finished cabochon. Take a permanent marker, aluminum or brass marking stylus and run it around the inner edge of the template as close to the edge as possible. Another option is to use the marker or stylus to lay out a freeform shape.

 

TRIMMING THE SLAB

The next step is cutting the slab close to the desired outline on the trim saw. When you trim away excess material keep in mind that some of it could be used to create other cabochons or be used for inlay, intarsia beads or tumbled. So, as you cut away the excess material, do so in a manner that will maximize the rough remaining.

Always start the flow of the coolant before beginning your cuts. Coolant lubricates the diamond and keeps the working area of the blade clear of debris. You should not have water flooding the saw table. If a paste forms around the cutting area, increase the coolant flow; sawing dry will severely affect the life and performance of diamond blades. With practice you will soon develop a feel for the speed that does not slow the motor while giving you a good sawing rate. An alternative to this trim saw is to use a diamond band saw to rough shape the cabochon. Accurately cutting using a band saw can minimize the grinding step, allowing you to create more intricate shapes while conserving precious and expensive rough.

CREATING THE ROUGH CABOCHON

If you're going to use the cabochon in a commercial finding designed to hold a standard sized stone, it's important to cut it accurately to a specific outline so it will fit. Templates are available to help layout specific sizes and shapes. Move the template around on your slab to find the most pleasing pattern for your finished cabochon. Take a permanent marker, aluminum or brass marking stylus and run it around the inner edge of the template as close to the edge as possible. Another option is to use the marker or stylus to lay out a freeform shape.

The next step is cutting the slab close to the desired outline on the trim saw. When you trim away excess material keep in mind that some of it could be used to create other cabochons or be used for inlay, intarsia beads or tumbled. So, as you cut away the excess material, do so in a manner that will maximize the rough remaining.

Always start the flow of the coolant before beginning your cuts. Coolant lubricates the diamond and keeps the working area of the blade clear of debris. You should not have water flooding the saw table. If a paste forms around the cutting area, increase the coolant flow; sawing dry will severely affect the life and performance of diamond blades. With practice you will soon develop a feel for the speed that does not slow the motor while giving you a good sawing rate. An alternative to this trim saw is to use a diamond band saw to rough shape the cabochon. Accurately cutting using a band saw can minimize the grinding step, allowing you to create more intricate shapes while conserving precious and expensive rough.

Take your time and carefully cut close to the outline using a series of eight or so cuts. Some stones will lend themselves to fewer cuts, and some will require more than just eight. All cuts should be made outside but close to your template line. Allow enough space, about

1/16” –1/8”, for the material that will be removed in the grinding, sanding and polishing process. Careful cutting now will make successive steps easier and greatly enhance the appearance and quality of your final product.

After sawing, clean the cab and check your work. The next step in the process is to establish

a smooth and accurate outline to remove any margins or small corners of material left over from the sawing operation.

CREATING THE PREFORM

Now you want to remove any margins or corners so that the cabbing “blank” is the shape and size desired. Use a back and forth motion to remove material and shape the piece. Periodically check the stone’s size with the template or the mounting. It should be just slightly larger (1/32") to allow for material removed in the sanding and polishing processes.

Do not grind until it slides through the template, or it will end up being too small.

Any time a paste forms around the grinding area, stop and make sure adequate coolant is being supplied to the diamond drum. Grinding dry severely affects the life and performance of any diamond product. With some practice you will get a feel for the optimal grinding action and pressure. You may want to try this technique using a piece of scrap material first.

Finally you will want to establish a reference mark, or girdle line around the perimeter of the cab. It marks the outermost edge of the cabochon, helps you judge the progress and obtain more uniform material removal during the shaping process. Make the line using a permanent marker or aluminum pencil at about two thirds of the slab's thickness and closest to the back side of the cab, leaving at minimum 1/16". The girdle height should be narrow enough to fit down into the finding and if it has a bezel, you should be able to roll the bezel of the finding over it.

ATTACH THE PREFORM TO A DOP STICK (Please note that this is an optional step)

Dopping is the process of securing the stone to a stick (dop stick) using a special (dop) lacquer wax. Doing this gives your cabochon a handle so you can more easily manipulate the stone on the flat lap machine. Dop sticks can be fashioned from a variety of materials; the most simple is

a piece of wood dowel about 4”-5” inches long.

1. It is important that the stone be clean and dry.

2. To create a good bond, the cab needs to be warmed by placing it on top of the wax heater. A good bond between cab and stick is important: If this bond breaks while a cab is being worked, it is possible for it to be thrown and possibly broken or irreparably marred. It’s also possible that you or someone else could be struck by a flying rock. This is also why the prudent lapidary always wears eye protection while working with these tools!

3. When the cab is sufficiently warmed, place the dop stick into the wax and spin it around to pick up a gather of wax and then push it down on the back side of the cab.

With the wax still liquid, wet your fingers and blend the wax from the dop stick down to the surface of the cab making a nice fillet. This feathering creates a supporting platform and insures a secure bond between cab and dop stick.

4. NOTE: Dop wax is hot and will burn your fingers if it sticks to them! Have a small container filled with cool water handy to wet your fingers so that you can shape the wax and properly secure the dop stick to the cabbing stone. Alternately, you can feather the wax out with something that the wax will not readily stick to, such as a cold knife blade.

5. The cab and dop stick are returned to the heater for a few more minutes, giving the wax time to flow and bond. Then the assembly is removed and allowed to cool to room temperature.

6. Test to make sure the cab is securely bonded to the stick. Once satisfied that all is properly prepared, the next step is grinding the face of the cab to a dome shape and generally rounding and smoothing it.

7. Many lapidaries do not rely on the dopping system, and instead simply hold the cabs in their hands for the grinding, shaping, and polishing steps. However, it takes a lot of experience to enable you to do this safely and effectively.

 

 

CREATING THE CABOCHON SHAPE

To establish the basic cabochon shape you will begin with the lowest grit diamond wheel and work your way up to the highest after each stage is completed..

1. The goal in cabbing is to produce a smooth and properly domed surface on the face of the cab while creating uniform wear on the diamond disc (to optimize its life). Use light to moderate pressure and inspect your progress frequently. You want to use a sweeping, j-shaped motion with the cab, pulling it towards you and turning the cabochon (about ¼ turn) as you go. Always keep the contact points moving on both the cab and the diamond disc. This is probably the most difficult part of cabbing to learn. There's a certain feel when the motion and the pressure are correct. With a bit of practice you will learn that feel.

2. Always start the coolant drip and then turn on the machine. Any time white powdery residue appears on the lap it means that you are not using enough coolant and may risk damaging the diamond lap and/or your cab. Increase the coolant flow accordingly.

3. Start by holding the cab about 45° to the lap. Grind completely around the cab at this angle until you reach the girdle line. This will make a smaller flat area on the top of the cab. Watch the tendency to grind down the corners too much: The girdle reference line helps you avoid this pitfall.

4. Now increase the angle (so that your dop stick is closer to the vertical) and repeat the process, grinding from the edge of the flat toward the girdle line. Each time your repeat this process, the size of the flat area (in the center of your cab) will decrease until it is completely gone and you will then have a domed cabochon.

5. When you have formed the dome, work up and down over the center of the cab to the girdle line. This can be accomplished by rocking the dop stick back and forth like a pendulum, as you rotate the cabochon slowly from end to end. Do it in one direction, then turn the dop stick 90° and do it again. Repeat this process two more times so that you developed during the grinding process. When you reach the point where you now have a relatively uniform domed shaped it is time to move on to fine grinding and final shaping.

Thoroughly rinse the stone, dop stick, and your hands.

Now that you have the cabochon shape it is time to fine tune it and remove the scratches left from the rough grinding.

1. Move to the next highest grit.

2. Use the same motion as before to continue to refine the shape; check the cabochon as you grind to be sure you are grinding symmetrically. You want to end up within a fraction of the girdle marking line. Frequently rinse and dry the stone to reveal the remaining scratches. When you no longer have any scratches from the previous wheel and the surface appears uniform, you are ready to move on to sanding. Thoroughly rinse the stone, dop stick and your hands.

Depending on the hardness of the rock and the desired finish you may wish, this may be a single or multi-step process. Practice, the type of material, and experience will determine just when your cabochon is ready for the final polishing step. When starting out we suggest you use both the 600 grit and 1200 grit wheels. You may find that some stones will be ready for polishing after the 600 grit step while others may require even finer grit laps or sanding media.

3. Continue fine tuning the cabochon shape on the 1200 grit lap. Double check size against the template or mounting. It should fit the template or mount precisely at this point. The stone should have a semi-gloss appearance as you now have only 1200 grit scratches. Rinsed and dried you should not see any scales (aka dimples, facets, flats, etc.) or scratches. Check by holding the stone up to a bright light and look for sparkles or deep lines. Scales are most easily observed by watching the piece dry. Because the “scales” will have deeper puddles of water, they will evaporate more slowly. Wet your piece and watch for this phenomenon. If you see any scratches or scales, you may not have smoothed long enough. If after additional smoothing you can still see scratches or scales, they are too deep to remove. If they are present, you must return to the 280 grit wheel and follow through the steps, always using the complete order of grits.

©2006-11 Inland Lapidary

 

Please note that these directions were modified and adapted from the Inland Lapidary company. They were initially intended for use with their flat lapping machines.

Gordon Burkholder

                             

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