Using a Rock Tumbler to Polish Cabochons

Using a Tumbler to Help Production Cabbers - The Hybrid Method

Overview:

This article is going to go in depth about how a lapidary artist can utilize a rock tumbler in their cabochon cutting process. Using a rock tumbler during your cutting process can alleviate the time you spend sanding and polishing your gemstones. In which case, cuts your costs and the time spent on each of your projects. Certain materials lend themselves to this process better than others. Cabochons can't always be polished using a rock tumbler and this article will explain when to use a tumbler and how to do it efficiently. We will go in depth on exactly what materials can't be polished in a tumbler, why, and other quick ways to finish those stones as well.

Tumbler polishing is often used for production cutters and this article will most benefit someone who is cutting large batches of stones at one time. If an artist is only cutting a couple stones at a time, then it might be better to hand finish as you can control the outcome and finish easier.

How to Start Utilizing a Tumbler:

Rock tumblers will not shape, trim, or grind a rock/gemstone.  When using a tumbler to finish cabochons, the initial shaping and sanding should already be done on a flat lap or cabbing machine. This way your stone will polish correctly and look like a cabochon when you are finished.

Therefore, to begin this process you will need to grind 10-25 cabochons with an #80-220g diamond metal-bonded wheel. (10-25 Pendant sized stones will fit into a 4LB tumbler fairly easily. You can tumble more cabs at once if you have a larger tumbler.) This initial grinding stage is crucial to get the right shaped stones. Cut your stones with a domed top and flat back like normal.

Once you have done your initial shaping on a grinding wheel you will need to hop over to a #280g resin bonded wheel. Sand your cabs so that the heavy grinding marks are sanded away. Also ensure that the shape of the stone is correct, even, and symmetrical.

If you are cutting calibrated sizes, then you will need to shape your stones about .5-1mm over the calibrated size you are trying to achieve. The tumbling process will sand it the rest of the way. (Edges, corners, and points get rounded off the most. Create sharp edges when shaping your stone so that once your stone is done in the tumbler, they have the correct radius on your edges and corners.)

Once you have sanded with a #280g resin wheel, you will be ready for a tumbler. There are many people who go one stage further and also sand their cabs with a #600g resin bonded wheel. This allows you to be more precise when cutting calibrated stones and can also speed up the tumbling process. It depends on whether you are cutting for production, or if you are cutting higher-end stones that need to fit into exact settings. It takes more labor and costs more to go the extra step however it can be beneficial.

Once Cabochons are Cut:

Once your initial shaping and sanding is complete, you can begin using a rock tumbler. Your stones should have a matte finish that is fairly uniform, and they should be scratch free from any heavy grinding. A vibratory tumbler works a little better for finishing cabochons because it works faster and is more gentle on the stones.

Start by placing your stones into your tumbler with #600g silicon carbide powder and Ceramic media that has no abrasives in it.  The ceramic media helps give some cushion between your stones so that they do not bang around on top of each other. If you stopped at #280g on your cabbing machine and you are using a rolling tumbler, then this stage should last about 3-5 days. (1-2 Days if you are using a vibratory tumbler.) If you stopped at #600g on your cabbing machine, then it should only take about 1-2 days with a rolling tumbler. (1 day if you are using a vibratory tumbler.)

You want to keep the stones in the #600g stage until you see that there is an extremely uniform #600g finish all around the entire stones. Check the front and back of at least 5-10 stones to verify that the stage is complete. The edges and corners should be rounded to the correct radius by the time you pull them out. The next stages will not remove much from the stones so by the end of this stage you should check to make sure your stones are the correct size if you are cutting calibrated sizes.

You must let the first stage complete, then you can move on to using #1200g silicon carbide and ceramic media (Or Plastic Pellets if you are working with material that is softer than a 6 on the Mohs Scale). Ceramic Media has a hardness of 6 so any materials that are softer than that can be scratched at this stage by using that ceramic media. This stage should take 2-4 days in a rolling tumbler or 1-2 days in a vibratory tumbler.

To finish the tumbling process, you must use a polishing agent in your tumbler and a filler media. Some common polishing medias are Tin Oxide, Aluminum Oxide, Chrome Oxide, and Cerium Oxide. Find which one is most effective on the material you are working. If you are using a polishing media similar to the ones listed, then a rolling tumbler should take 3-7 days for a "glass-like" finish. A vibratory tumbler will take 2-3 days. Use Plastic Pellets as your filler when polishing to prevent your stones from bumping on each other. Plastic Pellets are softer than Ceramic so they can be more gentle on your stones. However ceramic media will polish more aggressively on hard stones like Agate and Jasper. The material must be a 7 or harder to use ceramic angle during your polishing stage.

Benefits and Drawbacks:

The benefits to polishing cabochons with a rock tumbler are vast. It allows you to polish 50+ stones at once without much supervision. This can save you hours at a time! If you have multiple rock tumblers running at once, then there can be hundreds of cabochons sanding/polishing while you are cutting more preforms. Getting this routine going can create a large amount of production at a low cost.

Another benefit is that the polish can be better than hand finishing if done correctly. Tumblers create exceptionally uniform, bright polishes. Tumblers never miss a spot, and your entire stone will by highly polished from the front to the back.

The drawback to using a rock tumbler is that it can be rougher on your material than hand finishing. Fragile stones or rocks with small fractures/cracks can break in a tumbler while they are polishing. So, you must study all the stones you put into the tumbler to ensure that the do not have any structural compromises that could lead to breakage. Vibratory tumblers seem to be slightly gentler on your gemstones than rotary if you find this to be an issue.

The last drawback is that it is a little harder to cut precise gemstones that can drop into settings. It takes a few tries to calculate exactly how much material gets worn away in the tumbler. Much of that depends on how long you keep them in the tumbler for each stage, especially the #600g. It will decide how rounded your edges and corners are as well. If this process is done correctly, then the tumbler should take off no more than .5mm to 1mm around the stone. 

Materials that Work:

Most materials work in this process as long as you tumble similar stones together. Feldspar, Agate, Opal, Quartz, Jasper, Tigers Eye, Aventurine, and other hard, dense stones all polish amazing in the tumbler as cabochons.

The materials you cannot use are materials like Variscite, Turquoise, and other soft, non-homogenous gemstones. Materials like Turquoise have areas that are hard and areas that are soft. If you tumble polished turquoise cabochons, then you will experience undercutting which does not look good. On top of that, soft gemstones tend to wear away faster, and it can be extremely difficult to gauge the time it is going to take in each stage. If you run the tumbler a day too long, you could ruin an entire batch of cabochons. In addition, edges, points, and corners will turn into rounds when tumbling soft stones. If you are looking for an easy, quick way to polish these gemstones, then I would suggest trying out a buffing wheel with Tripoli and Rouge.

Other Ways to Polish:

For gemstones like the ones listed above, Variscite, Turquoise, Fluorite, Lapis, Malachite, Amber, Rhodochrosite, Howlite, etc. you can quickly polish with a cotton buffing wheel. The stones will need to be finished to a #280g-600g resin bonded wheel.

Once that's done you will need a tapered spindle, two buffing wheels (One coarse, one fine), Tripoli, and Red Rouge or Zam. Use the coarse buffing wheel with Tripoli if you stop at a #280g resin bonded cabbing wheel. It will sand the gemstones and get rid of any final scratches. Do not apply very much pressure or else you will cause under-cutting on the surface of your stone. A light touch will give you the best-looking outcome. To apply a final polish, you will use the fine buffing wheel with Red Rouge or Zam. Again, do not use very much pressure. Rotate your stone and cross hatch your lines to ensure a bright finish and minimal undercutting. 

Overall Recap:

Utilizing a rock tumbler during your cabochon cutting process can be extremely beneficial. The process lends itself to production cabbers who plan on cutting hundreds, if not thousands of cabochons, throughout the year for resale.

Using a rock tumbler takes all the finishing away from you and does it effectively/efficiently. Playing with what grit you stop on at the cabbing machine before throwing it in the tumbler, how long you keep it in the tumbler for each stage, and the kind of tumblers you use will all make a noticeable effect on your finished stones. Don't be afraid to test different processes to better fit your way of cutting or the material you are working.

Get all the tools you need in order to make this process happen at Cutting Edge Supply. We have all the supplies you will need on our website, ready for shipping within 24hrs.