Mirror finish on aluminum casting?

How to get the best machined finish on aluminum casting? With my face mill, I get a near mirror finish, but with internal boring bar, I get a terrible finish, despite variations in tool geometry. A cutting fluid? TIA, Dave

Reply to
David Anderson
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use a hi speed steel tool that is honed and polished on the cutting edge.

John

Reply to
John

Yes, use a cutting fluid by all means. If the cutter gets gummed it will produce a really awful finish.

Check for chatter. You may need a stiffer boring bar.

Reply to
Don Foreman

Maybe you need to tell us about the machine, how the boring bar is mounted, etc.

I got a 3" boring head and set of carbide boring bars a long time ago. I got horrendous chatter and finish that looked like the scales on a Carp! I had the boring head mounted on a 1/2" straight shank arbor into a 1/2" collet on a Bridgeport M head. The speed was probably too high, and the spindle/arbor wasn't stiff enough. I eventually upgraded my machine to a Bridgeport J head, and got a boring head arbor that was a straight mount into the R-8 taper. Suddenly, I could do no wrong! I've never had chatter with the boring head since. I sometimes bore aluminum dry, but usually brush on a film of dark cutting oil with a toothbrush.

Jon

Reply to
Jon Elson

I always use cooling fluid. That helps. But I have to admit that it took quite some time to get the right tool geometry. Now, I use very aggressive angles and a lot of honing the bits surfaces to a mirror finish. Use very low rpm.

Also check that the lower end of the bit doesn't touch the bore.

Nick

Reply to
Nick Müller

Jon Elson asked the right questions. The face mill worked fine on my vertical milling machine. The trouble began when I moved the same workpiece to my lathe and used a 1/2 inch dia Williams boring bar in a toolpost holder to bore 1.500 inch dia x 3-inch deep, using a 3/16 HS steel bit (not carbide). Both operations dry. I will try again with a freshly honed cutting edge plus cutting fluid and maybe reduced SFM . Dave

Reply to
David Anderson

According to David Anderson :

That is also a lot of extension for a plain steel boring bar -- six times the diameter. You could possibly get away with that much extension with a solid carbide bar (with HSS or carbide insert tooling), but for a plain steel boring bar, I would suggest that you use a 1" diameter boring bar for everything that you can use it for. (Perhaps start the hole with a tailstock-mounted drill bit, then take it up to a

1" bit (if your tailstock will handle a 3 MT shanked drill bit), use the smaller boring bar to get enough diameter to clear the chips on the 1" diameter boring bar, and then switch to it for the remaining work, including the finish cuts.

Good Luck, DoN.

Reply to
DoN. Nichols

There's your problem. Sometimes, I use an end mill as a boring bar. A 3/4" or 1" dia end mill would make a much better boring tool than your 1/2" bar. Since you need a 1.5" hole, there's no need to use a small bar. And, since you need over 3" extension, you need something stiff. Since you are using HSS, you can't go crazy on the SFPM, but usually faster gives better finish. You should be able to run it at

1000 - 1500 RPM to get 400 - 600 SFPM.

One other trick. If you have a long boring bar, with a "tail" sticking out the back of the tool holder, it can vibrate, and make a sort of tuning fork. I have rubber-banded tool blanks and other stuff to the back end of the bar to damp out these vibrations. If you can feel the back end of the bar vibrating at ALL, then you need to make up some kind of damper for it.

Jon

Reply to
Jon Elson

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