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« on: December 26, 2010, 06:29:59 PM »
The key factors in a good finish are...
*Rigidity (This is in the machine as well as the tool and the work)
*Speeds and feeds (aluminum loves high speeds and feeds as long as the rigidity is there)
*Chip evacuation (A high helix helps get the chips out of the way so you don't recut them. Recutting chips will kill your finish. Flood coolant and lots of it is a must. We use through spindle coolant when doing finish passes. They aids in getting the cut chips out of the way.
*Harmonics (Every machine will vibrate. This causes chatter. Chatter = bad finish and poor tool life. You need to tune this out of your machine by adjusting the speeds and feeds to get rid of the resonation)
*Cutter geometry (You need to have a high positive rake to give you a good finish from what I have found. We use some exotic variable flute cutters to do this. For a hobby machine I would recommend 3 flute high helix end mills. They will help with harmonics because 2 flutes will more than likely be engaged in the cut. They have good chip evacuation and have the good tool rigidity to evacuation ratio. They also are relatively cost effective.)
Surface finish is a black art. There isn't a one way works for all method. It requires experience and proper equipment. Getting a mirror like finish with a hobby grade machine I feel will be next to imposable. Even with the best equipment it can be a challenge. Most parts we produce go though vibratory tumblers to get the mirror like finish and we use industrial grade Haas CNC machines. I would say stick to multiple progressively finer grits of sandpaper. Wet sand it the best you can and then use a buffing wheel. if you don't have access to a wheel then some good ole mag wheel polish and elbo grease will go a long way.
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