I would kine to know what the consensus is out there on spindle taper hardness is for vertical CNC knee mills. I have a vertical CNC knee mill with a CAT40 spindle taper. The spindle is supposed to be "robust" according to the distributor. It has a 4-1/8" diameter quill with class 7 bearings. I bought this machine brand new about 2 years ago. I use it in my garage as mostly a hobby tool. I do a little contract prototype work now and then for a couple of shops but absolutely no production. After about a year of use, ..i looked up into toe spindle taper opening and saw a lot of small dents that looked like impressions of chips. I have always been careful to wipe the tool holders and the spindle clean before changing tools. I cut mostly aluminum. I didn't like the looks of the taper, so I contacted a local in-place spindle grinding outfit to have it cleaned up. He put some transfer ink on his taper standard and inserted in the spindle taper and pulled it down with the power draw bar. After removing it, it was apparent that the tool holder was contacting the spindle on about the upper third of the taper. In other words, the spindle appeared to be bell-mouthed. After a thorough inspection, the guy said I would be wasting my money to grind it because it was too soft and would end up the same way in another year or so. This guy has been doing spindles for over 30 years and has a real good reputation so I tend to believe him. I did some checking with some other manufacturers and the ones who knew their specs had a taper hardness ranging from an Rc 56 to 62. I am working with the company I bought the mill from to see what they will do about it. I'm waiting on them to find out from the factory what their spec is on taper hardness. It sounds like "posturing" so far. So... I would like to tap the knowledge base in the 'zone to find out what you guys know.
Sorry for the lengthy rant, buy I wanted to give you enough information to go on.