Since late 1995 we have supplied casing exit mills to cut the last few feet of casing exits done in chert bearing dolomites in West Texas. We started with an industry standard speed mill type exposed diamond mill used for years to cut steel casing and hard formations when carbide mills dull in inches of use. They have only one layer of diamonds and once they fracture or dull off the mill life is ended...sometimes also in just inches of use.
We did our own simple in-house tests to see the dulling mechanism of both carbide and diamonds and found that the inherent dense nature of chert requires an intense and concentrated energy transfer at the cutter/target interface and that during that transfer there is considerable frictional heat created. The heat of course dulls the edge of any carbide cutter almost instantly. We have learned from our core bit use in the same formations that PDC cutters can cut the chert but will fracture easily in the application.
We introduced diamond-impregnated mills for the application as early as 1997, but rarely were they run as the second or third mill in the application. By the time our mill was called into play there had been multiple carbide mill runs that had work hardened the casing...making our first mill's double hard.
Our simple tests indicate that the diamond-impreg cutting structure works best at medium to high RPM (250 to 400) with relatively light WOB (150 to 300 psi of projected mill area).
The enclosed video shows a test piece cutting at 300 RPM. The piece cut well until the frictional heat cracked the piece from inside.