The best designs and coatings in the world are of little value if they are not applied to the appropriate substrates. Using an end mill with a subpar substrate is like using a front door made from cardboard on a new house.

From a distance it looks the same, but a closer look will reveal the obvious flaws that make it unsuitable for its intended purpose.

In today’s fiercely competitive die/mold industry, it is critical to note the differences between carbide end mills for machining and carbide end mills for HSM. The most notable indicator of a quality substrate is grain size. Typically, the smaller the grain size, the better suited the tool is for HSM applications. For HSM, a grain size of 0.5 microns or less is recommended.
There are further elements that still dictate substantial differences even when grain size is equal. These characteristics are hardness and transverse rupture strength (TRS). TRS is a measurement of how much force (N/mm) can be applied radially to the substrate before it fractures.

Hardness and TRS are controlled by the cobalt percentage within the given substrate. Higher cobalt percentages dictate a lower hardness and higher TRS, while a lower cobalt percentage dictates the opposite. For roughing applications, when radial forces are the greatest, more TRS is desired, hence a greater cobalt percentage. Tools designed and suited for roughing applications should sacrifice hardness by increasing cobalt percentages to achieve more TRS. The inherent cost to manufacture such carbides is not cheap when compared to regular fine micrograin carbide.

Aside from the time and energy to produce a smaller grain size, other measures are taken to actually control the amount of molecular bonds between the carbide and cobalt particles. The ability to control the size of the cobalt and the number of bonds is another fundamental difference between a good substrate and a premium substrate. Since this difference is not notable to the naked eye, it is often deemed an unnecessary expense to the end user. The result of such an oversight is more cost and less profit.