Problem- Very rough finish when turning 6061-T6 ... why?

[cnczoner]

Hi all,

I am experimenting with turning alum tube, but getting a really odd finish. The photo below will explain the problem, but some important data -- this is 2.25" OD, 6061-T6 round tube. I'm spinning at 1500 rpm, so sfm works out to 883 sfm. That sounds high from numbers I've seen elsewhere, but I'm using cheapo C2 carbide tool bits (the 1/4" shaft one of these... http://shars.com/product_categories/...09/E_Threading) and don't have any data on what is the correct speed is for this.

I've tried cutting .005" and .010" DOC at feeds of 0.5 ipm, 1 ipm, 2 ipm and 3 ipm, and the best seems to be .005" at 1 ipm. And I've tried both dry and spritzes of ATF, but the finish is still dull, even in the rate cases that I manage not to get this odd bumpy effect. Any thoughts on if this is related to my SFM speed, cutting bit, DOC, feed, or something else?

Thanks,
-Neil





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[mrcodewiz]

I always spin the aluminum fast and use finishing hss tool with a large radius and copius amounts of wd-40. Comes out like a mirror.

I just looked at those bits - they are threading tools. The radius is waaay too small.

[The Blight]

Looks like a dull cutting edge. You might want to use a cutter with a sharper edge (angle actually) and a small radius at the tip. Also check if your tool is in center.

Edit: Took a look at the tools you are using (just read mrcodewiz's post), and you should definitely use a tool with a radius at the tip. If you can grind HSS tools yourself, you should grind a tool with a sharp cutting edge and a radius at the tip with a chipbreaker.

[ronfournier]

I turn ala. on a regular basis you need to get your surface footage up to 1200 and you shoould be using a high shearing insert and feeding at .0035 per rev.

[Eurisko]

Make sure that the tool is cutting on center. Also make sure that the spindle is running in the proper direction don't laugh, I've done it myself.

It sounds like the tool is rubbing, not cutting.

[cnczoner]

Thanks for the replies.
My max rpm is 1500, which is the 883 sfm. How should I adjust feed and doc for this? I did verify direction, but I'll check to verify that the tool is centered properly. I also have one of these tool bits in a flycutter currently (which gives me a beautiful finish!), so perhaps I should try that bit instead. It's C2 also. I intend to pick up indexable tooling later, but I wanted to experiment with el-cheapos first.

I'm avoiding grinding my own bits for now since that introduces another (major) variable, and other for other "logistics" reasons.

So what is a "high shearing" insert? As I continue to build up info on the type of inserts I'll need for indexible tools, I have figured out that I'll need a positive rake or a sharp chipbreaker (so it essentially forms a positive rake). Is that related to "high shearing"?

BTW, any recommendations on tool bits (not indexable) that work well on aluminum, and that I can pick up from Enco, McMaster, etc?

Cheers,
-Neil.

[cnczoner]

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[mc-motorsports]

Carbide inserts for aluminum are great for production work where you want to keep the machine running and not worry about tool wear changing your finish too much or changing your tolerance, reduces down time by not having to change tools out or compensate for wear to stay in tolerance. HSS is a better tool for nice finishes on aluminum, especially if your using a conventional lathe.

[skmetal7]

1500 rpm seems a bit fast, especially for how slow the tool is moving, try running round 650-1000 rpm and 5ipm and yeah you should use a larger radius tool.

[mrcodewiz]

I hand grind them but mine look very similar to the following. If you make your own, Just round the nose and stone it up real nice - You will get a great finish. Otherwise just get an HSS bit like this. Of course you will rough it with a normal LH or RH tool.

http://littlemachineshop.com/product...967&category=8

-CW

[cnczoner]

I don't know if indexable HSS inserts exist. This will be low-volume production (~50-100 pcs per batch), and definitely CNC'ed. The way I'm looking at it is that if production shops can get a great finish with indexable tooling, then I can too -- once I figure out what I need (tooling and machine). For now though, I'm experimenting with a chuck mounted in my mill, which makes it a vertical CNC lathe.

[cnczoner]

So I did some tests and had major successes. I'll tell you now that the tool bit was the issue.

First, I dug up this toolbit from a box of parts for my 7x10 lathe. IIRC this came from LMS some years ago, but I can't remember if it was for turning steel or alum. Either way, the finish was MUCH better...




After some experimentation, I figured out that too fast (1500 rpm at 10ipm feed) caused problems, as did too slow.




I later tried the C2 carbide bit from my fly-cutter and that produced some excellent results. I first got the best finish at 850 rpm (~500 sfm), but later discovered that I can go up to 1500 rpm if I increase the feed, but too much doc (over .010") was a problem. Ignore the lowest section in the photo below, as I was experimenting with brushing with scotchbrite pad to smooth out the surface and eliminate burrs.




That said, I've just ordered some C2 tool bits from Enco, so I can run more experiments, but it'll be indexable after that.

Thanks,
-Neil.