[mactec54]

What is the OD of the main housing on most BT30 spindles? I'm asking because my main bearings are pushing me for a big housing. The bearings are 75mm OD so when I cut the housing 90mm then I'll have 7.5mm wall. And I'll drill 6x 4mm threads for the outer part of the labyrinth seal. So that the 6 holes will be weak points. In short I have 2 choices: 90mm or 95mm housing. Please see the attached render. What do you think?

Another question: is stainless steel acceptable for the housing?

303 stainless or 416 stainless are both free machining and ideal for most things like this, you want at least a 6mm cap screws in the front cover, this is not only part of your labyrinth seal, you need good strength for your bearing preload also, so 4mm cap screw is not enough for a bearing of this size, bigger the better in that area, you can step out the diameter of front section for a length of 20 to 25mm, just so you can use a good size cap screw, around a 12mm or more wall thickness in the front where you need the cap screws

When you machine the housing for you Bearing fit, make sure the housing is cold before you do the final measurement, and then cut for your Bearing fit