Lapidary Cabochons

SLABS TO CABS: How to Make Lapidary Cabochons

Creating beautiful, polished cabochons from slabbed rock is a rewarding process. We strongly suggest that you work through the process with practice pieces first. PLEASE, save that special slab for when you have a bit of experience with the machine. Remember to spend the necessary time at each step eliminating scratches before going on to the next step. 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.

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.

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 PREFORM
Now you want to remove any margins or corners so that the cabbing “blank” is the shape and size desired. Place the cabochon flat on the grid surface. Turn the machine on and slowly move the material into the grinding head. 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

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

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. You can control how fast you grind by using the speed controller and the position of the cab on the lap. You will grind and sand more slowly near the center and more quickly as you move closer to the outer rim. Practice will help you determine the optimum speed and position for each step of the process.
4. 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.
5. 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.
6. 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 are blending out the grinding lines 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.

POLISHING THE CABOCHON

Thoroughly wash the cabochon. We recommend you also rinse and dry the flat lap reservoir and master lap before starting polishing; you don’t want to bring any grit from the previous steps into the polishing process.

Mount the felt polishing pad to the master lap and install on the machine. The polishing pad has a pressure sensitive adhesive and backing. You can leave the protective liner on and mechanically clamp it to the master lap as in previous steps or you can remove the liner and apply the felt pad directly to the master lap. (if you have a second master lap)

Wet down the felt pad with water and then turn the machine on briefly to throw off excess water. With the machine turned off, use the sponge brush to apply a small amount of cerium oxide onto the pad. Using the brush to paint radial lines that divide the pad into thirds or quarters (like a small pizza!) should be sufficient. When a felt pad is new, it may need additional cerium oxide applied until the pad becomes "charged" with polishing grit.  Once the pad is charged you should only need to apply addition cerium oxide occasionally when you notice that use of the pad is no longer polishing.

You do not use the water drip during polishing as you would quickly rinse away all of your polishing powder. Polishing requires you to run the lap at a faster speed than the grinding and sanding stages. How much faster will depend on the material, the size of the cabochon, and your ability to control it on the dop stick. Because of this increased speed, polishing is also where heat fractures are most possible. Polish and rinse often to minimize the temperature build up at the contact point.

Turn on the machine and begin rocking and rotating the stone, working up and down over the center of the cab to the girdle line using medium pressure. If the cerium oxide cakes up on your cabochon, add water to the piece simply by rinsing it with water.

Periodically dry the piece and check your progress for scratches or scales. Remember, these scratches and scales can never be polished out and must be taken out by grinding / sanding. Continue with this polishing process until you achieve the desired degree of shine. Harder gem materials take a higher polish while softer ones will have a satiny finish.

When you are satisfied with the finish of your cabochon, you will need to remove it from the dop stick. Put the cabochon, dop stick and all, into a freezer for 10 minutes or so. Then, just peel the cab off of the wax. It will pop right off leaving you with a beautifully polished cab and a dop stick that can be re-used. Any remaining wax can be carefully scraped off with a knife.

Use a tissue or other type of soft paper with alcohol to remove any film from the wax.