A standard arbor which you should have been provided with should look like this...
The standard arbor size is 1 inch.
A used arbor is okay--but if it has rust on it, you should remove the rust with a wire-wheel mounted on a bench grinder (and be sure to use eye protection because fast spinning wire-wheels routinely throw off wires).
Any arbor (especially new arbors) have to be lapped into the spindle using a fine lapping compound which can be easily found at auto parts stores.
Apply a small amount of lapping compound onto the mating surface of the arbor and insert it into the spindle with the machine turning. Hold the arbor tight and let the lapping compound grind away for about 40 seconds.
Remove the arbor and reapply another small amount of lapping compound--repeat the grinding process one more time. Remove the arbor and using a rag clean the lapping compound from the arbor.
Clean the lapping compound from the outer spindle seat. The outer seat is easily accessed--the inner seat of the spindle is harder to access. This is cleaned by doubling a rag over the threaded end of the arbor. Holding the rag, insert the threaded end into the spindle and clean the inner seat.
Turn off the machine and insert the arbor into the spindle. Insert the spindle "draw bar" (which you should also have) through the hole in the drum gear box (Infimatic gearbox) on the right hand side of the machine.
Holding the drive pulley, tighten the draw bar.
Next you have to check the arbor run-out using a magnetically mounted dial indicator as shown....
Mount the dial indicator on the machine's twin cutter (tightened down) and crank in the disc-feed carriage until the dial indicator can make contact with the arbor.
Start the machine. The arbor run-out should be no more than 1 1/2 thousandths. If the run-out exceeds greater than 8 thousandths it cannot be straightened. Arbor with slight run-out problems can be straightened.
(Note: If the dial indicator vibrates too much to get a reading--you will have to manually turn the machine using a wrench as described below).
First try loosening the draw bar, and rotating the arbor 180 degrees and then re-tightening the draw-bar. Many time this will solve the problem or at the very least lessen the run-out or bring it within spec.
If the arbor is still exceeds spec, then you have to straighten it. To straighten an arbor, manually turn the machine with the "Ammco wrench" (which you should also have) using the nut on the draw bar. Turn the machine until you find the high spot (straight up), then remove the dial indicator to prevent damage.
Using a copper dead blow hammer, give the arbor a modest whack, and then recheck the run-out. Repeat the process until you manage to get the arbor within spec.
Please note that straightening an arbor using this method will sometimes break the arbor off. You are rolling the dice when straightening an arbor--but this is the nature of the animal. Even brand new arbors sometimes need straightening.
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