What to Know When Choosing a Combination Unit
are typically used for trimming, grinding, smoothing, and polishing stones. Traditionally these units come with a trim saw, 2 or more grinding wheels, a smoothing wheel, and a polishing station. These units allow you to efficiently cut a gemstone from start to finish without ever having to switch machines. It saves money and even time because it all can be done with one machine.
Covington Combination Unit
Covington’s 5-station machines have withstood the test of time and they stand behind their product quality and durability. Each of their combination machines is constructed of heavy-duty aluminum castings and come with heavy-gauge steel accessories.
The main difference between Covington’s 2 standard combination machines is size. The 6” Combo
is compact and portable whereas the 8” Combo
is large and heavy.
The 6” machine comes with a 6” trim saw, 6” grinding wheels and a 6” polishing head. The unit requires about 4 square feet of table space.
The 8” unit features a 10” trim saw with power feed, 8” grinding wheels and an 8” polishing head. This machine is approximately 50” long and 18” deep. The advantage to the larger unit is it has the capability to cut and grind larger pieces. The larger wheels also have more surface area and therefore will last longer. The 8” machine also features more space in between wheels.
The 6” machine has a 6” trim saw that is attached directly to the main shaft. This means that the saw is always running when the machine is running.
The 8” machine has a separate trim saw with its own motor. The saw and wheels run separate from each other.
The 6” Combo units come with a small slab vise to hold pieces for trimming and small slabbing. This little vise requires you to re-clamp the piece for each cut.
The saw on the 8” machine has a slabbing mechanism which allows for multiple cuts before re-clamping is required. This saw also comes with a power feed to automatically cut the pieces without needing a complicated weight and scale system.
Both the 6” and 8” machines come with the same grinding wheel set up. Whether you choose the silicon carbide system or the diamond, the grits remain consistent in both machines. Machines come with one 100g wheel, one 220g wheel, and one soft wheel with 400g sand paper.
The 100g wheel is designed to do all of your coarse grinding and shaping. The 220g wheel is designed to remove the coarse wheel scratches and prepare the pieces for the 400g smoothing step.The 400g wheel is soft and designed for removing the 220g scratches and preparing the piece for a final polish.
Although this system is still slightly cheaper than the diamond, Silicon Carbide is more messy. Silicon Carbide is a 9 on the Mohs scale and is a tried and true media used for grinding stones. Replacement wheels are slightly less expensive.
Diamond is slightly more expensive than silicon carbide but long-term customers tend to be happier with Diamond wheels. They are cleaner to use and easier to work with. Diamond wheels have come down tremendously in cost and are now only slightly more than silicon carbide wheels.
There are several techniques for polishing stones. Two of the most common are discussed below.
Polishing with diamond compound paste is still a popular form of polishing stones. The techniques involves spreading paste onto either a polishing plate or a leather disc. You need to have a dedicated disc for each mesh of compound so as not to contaminate the finer compounds with coarse pieces of diamond.
Cerium Oxide is a great general purpose polish. It is typically used in a combination with leather in Lapidary applications. For the first application of cerium to leather, simply create a small container of cerium paste in a bowl and paint it onto the spinning leather head. The greatest amount of Cerium will be used on the first application. Additional Applications can be made using a heavy duty spray bottle and cerium. We like to use 3 parts water to one pert cerium in a spray bottle. Shake it very well, and apply to the disc. This tends to be the most effective choice for polishing stones.