I'm puzzled by your reference to "roughing process". Usually, engraving is done in one pass with a very sharp-pointed tool. Are you talking about relief carving of aluminum using a router designed for woodwork? In that case, your machine may not be rigid enough - it takes a much more sturdy machine to deal with metals effectively that wood or MDF. It could also be that you're attempting to use an unsuitable grade of aluminum, the wrong tools, or incorrect feedrates.