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Tolerances
You may think of tool steel as being inflexible, but it flexes just like other materials; it's just less noticeable. A spindle is much like the chuck of a drill. It is the piece that holds the bit in place. A drill is only designed to plunge along its axis, whereas a mill is also designed to cut perpendicular to the axis, which exerts a great deal of leverage on the spindle -- and in turn must stand up to the leverage. So, spindles must be machined out of thick, high-quality tool steel, using far more precise tolerances than other cutting chucks, such as one found in a basic drill press. The tight tolerances make moving parts fit together very snugly, which will translate into an accurate spindle.
Drive Mount System
The drive mount system connects the drive shaft of the mill to the spindle. It can be designed and manufactured to splice into the drive shaft of an existing mill -- usually by threading onto the axle end. If you plan on using it with a mill you already have, adapt it to whatever the axle requires. Otherwise, there are several variations for couplings, and each requires a connection with some kind of locking mechanism such as two bolts that lock against one another or a main fastener and locator set pins to ensure there is no play in the connection.
Bearings
Because the mill must spin very fast, bearings must be used. Bearings are premanufactured components designed to cope with two moving parts that come into contact with one another, like an axle and a collar. Most bearings by themselves have a little bit of play in them, so the spindle design must isolate the bearings, often by sandwiching the out bearing cage tight along the spindle axle with locking nuts. This isolates the play inside the bearings while keeping the spindle within precise tolerances.
Collet Compatibility
Spindles are designed to work in conjunction with collets. Unlike a drill bit chuck that clamps directly to the shaft of a drill bit, the spindle clamps onto the collet -- a ferrule with longitudinal slots cut in it. The collet fits between the spindle jaws and the shaft of a cutting bit. The collet helps accurately center the cutting bit in the spindle. Spindles and collets must have compatible designs. Because many commercial collet and collet systems are available, it may be easier to design your spindle to the specifications, radius and depth of an existing collet system that will meet your needs. Otherwise, you will have to machine your own collets in addition to the spindle.