With milling becoming so popular in today’s dental laboratory industry, it’s important to not underestimate or even overlook the important role that cutting tools play in a lab’s milling success. The condition of the tool and the use of the right one for the particular job can make all the difference in the integrity of the milled piece.
What tool is ideal to use for the task depends on the type of material you are milling, the quality of the carbide, the cutting speed (RPMs it’s spinning at), and the speed velocity (the speed that the spindle is fed through the material. These are often referred to as “feeds and speeds”. The end of the tool’s useful life is often evidenced by chips, cracks in the material and marginal breakage. A close examination may reveal wear and small chips in the cutting flutes of the carbide. If these are detected, it is essential that the operator change out the tool for a new one. The cutting efficiency will be greatly improved and the risk of damaging the milled units will be significantly decreased.
1. What to use and when? If a milling wax or basic unfilled PMMA (polymethylmethacrylate resin) is to be milled, a carbide tool will work well and will be able to be used for quite a few milling jobs. In most cases, a carbide tool can mill several hundred wax units before it loses its effective ability to cut sharp and clean.
2. PMMA takes a bigger toll on the cutting surfaces of carbide tools. The number of units able to be milled may be reduced by half, depending on the quality of the tool and the set of conditions listed above.
3. Zirconia is a different animal altogether. While it can be cut very cleanly and efficiently, it requires the use of diamond-coated cutting tools for milling in order to do so. Zirconia powders are extremely abrasive and will wear cutting tool surfaces very quickly if not cleaned after each use. In addition, they should be handled gently and with care.
A good rule of thumb is to ensure that the end of the tool never touches anything besides the material being milled. Unfortunately, it’s necessary to load tools into machines from time to time, and this operation is still a manual process. However, by treating these precision tools with care and respecting their fragility, you can extend the life of these instruments and ensure optimum performance for perfect restorations time and time again.
Other families of milled dental products include filled composite resins and lithium disilicate-type glass ceramics, all of which are tough on cutting instruments. The tools made to cut those properly and last through hundreds of milling operations are often called hybrid tools. They may be titanium nitride-coated, which enhances their effectiveness and milling efficiency. They are usually more expensive than conventional carbides, but not quite as expensive as the diamond-coated ones. In many cases, the high number of units they can mill justifies the higher price.
There are many choices available for milling your restorations. Just do some research and use the proper instrument for the best, most predictable results.