Two common arbor styles are shown in Figure K-10. A style A arbor has a cylindrical pilot on the end opposite the shank.
The pilot is used to support the free end of the arbor. Style A arbors are used mostly on small milling machines, but they are also used on larger machines when a style B arbor support cannot be used because of a small-diameter cutter or interference between the arbor support and the workpiece.
Style B arbors are supported by one or more bearing collars and arbor supports. Style B arbors are used to produce rigid setups in heavy-duty milling operations.

Arbors have one or two full-length keyways machined on them. Square keys are inserted into these keyways to drive the cutter. These keys should always be long enough to extend through the cutter and both adjacent collars to provide the driving force. Without these keys,
the cutter is driven only by frictional forces. Occasionally, a delicate cutter is used without the drive keys to give the operator sensitive control while hand feeding the machine table. Arbors are best stored in a vertical position to keep them from bending.
Spacing and Bearing Collars
Precision spacing collars take up the space between the cutter and the ends of the arbor. Shims may also be used to obtain exact cutter spacings in straddle milling operations. Bearing collars are larger in diameter than spacing collars.
These collars ride in the arbor support bearing. On style A arbors, the end of the arbor rides in the arbor support bearing.
All collars are manufactured to close tolerances with their ends or faces parallel and also square to the hole. It is important that the collars and other parts fitting on the arbor be handled carefully to prevent damaging the collar faces. Any nicks, chips, or dirt between the collar faces will misalign the cutter or deflect the arbor and cause cutter runout.
Arbor Support Bearings

The arbor support bearing has an important function in supporting the outer end of the arbor (Figure K-11). The arbor bearing collar or arbor pilot fits this bearing, which is located in the arbor support. The arbor support bearing may be a sleeve bushing, or instead on some mills, a sealed ball bearing may be used. On mills where a sleeve bushing is used for the arbor support bearing, the fit of the arbor bearing
collar or pilot can be quite critical.

A provision for adjusting the fit of the arbor support bushing to the arbor collar or pilot is provided. Too loose a fit will cause inaccuracies in the mill cuts or permit chatter.
Too tight a fit will cause frictional heating and can damage the arbor collar, pilot, or the arbor support bushing. An arbor turning at high rpm will require more clearance than at slow rpm.
Arbor support bushings must be lubricated properly.
Some mills have an oil reservoir in the arbor support for supplying oil to the arbor bushing. Check the oil level and method of lubrication before operating the machine, and consult with your instructor regarding the adjustment of the bushing to fit the arbor you are using.

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