Yeah, there's very little chance an aluminium crankarm can damage a hardened steel spindle. What is happening in cases where replacing the spindle fixes the loose-crankarm problem is that the threads in the spindle is worn-out due to repeated tightening of the bolt. Most people will not completely remove the crankarm and bolt and clean up everything. Dirt and grit will be on the threads of the bolt when they re-install and over time (10-years?), the threads on both the bolt and spindle will be worn out.
The most likely cause of these issues is insufficient torque.
erbfarm, to what torque are you tightening down the crankarm bolts and adjustable-cup lockring? Check out this
Park Tool - torque specifications page (scroll down). Note that crankarm bolts require 25-33 lb*ft torque! That's amongst the highest torque values used on a bike and can only be achieved with an automotive-style ratchet-wrench with 12" handle.
Well, this is a breaker-bar, but you get the idea. If you've ridden with the loose-crankarm for more than 30-seconds, it's most likely buggered and you'll need a new one. Also replace the bolt with a new one as well. In some rare cases with everything done properly, the crankarm
still comes loose. I've had good luck with using a star-lockwasher, blue Loctite on the bolt-threads, and tightening it to the maximum 33 lb*ft torque.
For the bottom-bracket adjustable-cup lockring, you need to use the proper tools:
http://2.bp.blogspot.com/_8Tly19mAQz...ool+SET+BB.JPG
It's impossible to get the lockring tight enough without spinning the adjustable cup, so the pin-tool holds that in place. Pick a spot on the lockring to hook such that the two-wrenchs are about 90-degrees apart. Then squeeze them together as hard as you can (hold the pin-tool stationary). When removing the lockring on some bikes, I've had to hit the end of the lockring tool with a hammer! They probably used a hammer to tighten it!!!