Aluminium brush finishing problems

I have aluminium blocks 60 mm x 60 mm x 43 mm that I need to finish with a brushed finish. I use a bench mounted linisher and finish first with 80 grit to take out machining marks and marks associated with original storage and cutting of the material. Then I use 180 grit or scotchbrite to carry out the final finish. The problem is that I can't seem to get an even finish on the parts and occasionally cause them to become slightly rounded and misshapen from too much finishing. The linisher is similar to www.surtech.co.uk/products.asp?s=31&p=732&a=p without the wet operation and has a plate section that comes off to reveal a flat bed platten with a right angle plate to place the part against. Any ideas or tips? Should I not be placing the part against the right angle section?

It sounds like your parts are small enough to fit into a vibratory finisher. You can use a dry organic abrasive mix that will get you a satin finish. It will be a uniform finish but it may take you 1 to 4 hours or more.