As marketing leading dental zirconia manufactuers,we’ve spoken with hundreds of dental lab owner during last year,and that vast of conversation give us a lot of insight to improve our production,also provide us precious experiences for how to machining our zirconia product,we’d like to share with all you guys.
There are four basic principles that should be followed when machining Vsimle dental zirconia disc: slow speeds, high feed rates, climb cutting and a flood coolant.
Zirconia tends to compact and adhere to cutting tools and metal surfaces. It also tends to work harden, requiring higher than normal clearance angles on cutting tools to penetrate the work-hardened surface and cut a clean coarse chip. 12 to 15 degree positive radial rake give optimal surface finish and tool life.
Cutting rates ae from .005” to .010” per tooth at 150 to 250 SFM. Since small cutters are used with small step over values are used, small chips are created therefore Radial Chip Thinning adjustments must be made to cutting rates to avoid excessive tool wear, poor finishes and chipped margins - yet another reason for flood coolant as the heat generated by the machining process is typically not transferred to the chip (either due to inappropriate feeds/speeds or the fact that zirconia is a pretty good insulator) but instead remains in the cutting tool which tends to gall on the tool cutting and relief surfaces.
Because of its brittleness, Heavy rigid machines and fixturing give best results and it is important to maintain a non-interrupted, constant load, climb cut when machining.
The machining “slurry” associated with dry machining zirconia is very fine, caustic and highly abrasive. Regardless of fine filters and massive vacuum systems it is very difficult to keep the dust from penetrating “sealed” areas of the machine (ballscrews, ways, transport mechanisms, electronic enclosures, etc.). This penetration will lead to premature wear on critical machine components and in many cases escapes the enclosed machining area and accumulates in the work environment.
There is also the noise and added expense and maintenance associated with filters and vacuum systems.
Since the zirconia tends to gall, it should not be left to dry and accumulate on metal machine surfaces for an extended period of time. Periodic rinsing of the machining compartment of the machine using a tube attached to the coolant nozzle while activating the coolant pump is a very quick and easy way to accomplish this. Also, a quick shot with an inexpensive steam cleaner into the machining “chamber” periodically will keep the area looking shiny and new. This whole process takes about 5 minutes or so.
Distilled water is best to use. We use it for both zirconia and glass-ceramics. Extended tool life can be obtained by utilizing a lubricant. We are currently testing a water soluble, residue-free coolant that looks very promising. As mentioned previously residual lubricant on zirconia or glass-ceramics is all bad.
Machining wet does add another dying step to the workflow as units with residual moisture can crack in sintering and cause color shades to appear lighter.
An inexpensive toaster oven works quite well. Drying times vary. See below.
Single (70°C)): 15 minutes+
Single (140°C)): 5-10 minutes
2-4 units (70 °C): 40 minutes+
2-4 units (140°C): 25 minutes+
5+ units (70°C): 50 minutes+
5+ units (140°): 25 minutes+
Our customers have found the machine cleaning (which should be done anyway),coolant swapping (if necessary) and drying are worthwhile trade-offs for increased tool life, lower operating expense, lower noise levels, cleaner working environment, extended machine life, higher quality and more aesthetic units.