The economics of re-coating
The most common tool coatings are TiN, TiC and TiAlN. Other superhard nitrogen / carbide coatings are also useful, but less common. PVD diamond-coated tools can also be ground and repainted. During the re-coating process, the tool should be “protected” to avoid critical surface damage.
Often, when the user has purchased an uncoated tool, the coating is reworked when the tool needs to be ground, or a different coating is made on the new tool or the reworked tool.
Restrictions on re-coating
Like a tool can be repeated several times the same, the cutting edge of the tool can also be a number of coating. And in the already re-grinding the surface of the tool to obtain good adhesion coating is to improve the performance of the key to the tool.
In addition to the cutting edge, the remainder of the tool surface may not need to be removed from the coating or re-coating at each time the tool is drilled, depending on the type of tool and the cutting parameters used in the machining. Hobs and broaches are reworked to remove all of the original coating of the tool, otherwise the tool performance will be reduced. Before the stress-induced adhesion problem becomes prominent, the tool can be re-coated with a small number of times without removing the old coating. Although the PVD coating has residual compressive stress for metal cutting, this pressure increases with increasing coating thickness and will begin to delaminate after a certain fixed limit. When the old coating is not removed and the re-coating is carried out, the outer diameter of the tool is increased by a thickness. For the drill bit, it means that the diameter of the drill is getting bigger. Therefore, the influence of the additional thickness of the coating on the outside diameter of the tool must be taken into account, taking into account the effect of both on the dimensional tolerances of the machined apertures.
A drill bit can be coated with 5 to 10 times without removing the old coating, but after that it will face serious error problems. In the range of ± 1μm, the coating thickness will not be a problem; but when the error is in the range of 0.5 ~ 0.1μm, the influence of the coating thickness must be taken into account. As long as the coating thickness is not a problem, then re-coating, grinding the tool is entirely better than the original performance.