First, determine the type of grit that is best for your job. The most popular grits used at Tanis are CeramiXTM and silicon carbide, which work well on a range of applicaions. Diamond is ideal for tough materials, but it's a pricer option; for finer polishing applications try aluminum silicate.
After you have chosen the grit type, you will need to select the right grit size. Grits available at Tanis range from 46 for the heaviest deburring application to 1000 for the very finest polish. Selecting the perfect match for your application is an art, so when in doubt, start with a finer grit and then move to a more aggressive size until you have the desired finish.
You have a one inch bore with .250 cross bore holes in an aluminum part and you need to deburr and polish bore holes. For this application, I would suggest a two-step process. Start with a 320 grit silicon carbide to deburr the one inch bore. On the cross bores, move to an aluminum silicate at 1000 grit for a smooth finish.