Many spindle rebuilders do not have the equipment or skill to manufacture the ball cages that are essential for proper preload of the bearings. The practice of reusing ball cages is usually done because they have no other option. They have not invested in the machinery and skill sets needed to manufacture parts correctly.
One of the more common failures we see from other rebuilders is a ball cage that no longer functions freely. This causes excessive preload to the bearing set as the spindle experiences thermal growth. That eventually causes premature bearing failure.
Don’t get short changed on your next repair. Use a quality spindle rebuild company that continues to reinvest in machinery and the proper tooling to meet your repair needs.