Why--What is going on with this Aluminum?

[Diy89]

Up early..dont ask why...thought i might shoot the clock with some shop time.
I chucked up this piece of aluminum, to turn a flywheel. The first side, was roughly cut with a band saw, so i figgured it would be a bugger to cut. Which it was. Made a helluva chatter then viola started cutting smooth. I tried several cutters, HSS, carbide, etc. All produced the same chattery cut. I was using a hss when it started cutting smooth. My first thougt was that it was work hardened by the band saw. Then, i flipped it over, which was the out side of the block. It cuts the same way, up till the middle of the cut. What the heck??? I think, this is 2023....but no markings on the block i cut it from. Any ideas? I'm sneakin up on the final size here, but i'm not happy with the cut....

 

[Deanofid]

That looks like you may be turning it too fast, or the tool is not sharp.
Aluminum doesn't work harden the same way some steels will, and it can't get that hard.
Some types of aluminum are a bit gummy, and will "weld" a chip to the tip of the cutting tool. Then you will have chatter problems, too.

It looks like the piece is short enough that it can't be sticking out of the chuck too far. Was it actually mounted in the chuck, or was it on a small arbor?

How fast are you turning it, what is the dia of the piece, how sharp are your tools? We need answers to those questions to do anything but guess.

Dean

 

[Diy89]

The piece is mounted to the arbor as in the pic. Pressed tight to the chuck jaws to. Dia is 3.485. To big for the chuck that i have.
I am not sure on the speed exactly. I tried a moderate speed at first, and as i started hating this piece, i varied a bit both up and down. I dont have an rpm indicator, and didnt put my hand tach on it.
The first tool i used was a new carbide bit. The second is a reground cutter that is sharp enough to get a nice finish on a 6061 bar that i cut first. I'm not the best at grinding tools! I did notice a little balling of al on the tool tip of the carbide, that's why i changed. Strangely, the speed settings were left when i flipped the piece over, and it only chatters on the outer edge. Maybe a rubber dampner wrapped around the outside might help.
I suppose i'll grind me another tool and have a go at it tonight. Worst case the part will be undersize or i'll make another!
I think i'll have a brass nameplate made up with "Not to Drawings" for when it's done.....

 

[arnoldb]

That arbor is relatively thin, and flexing might be the cause of your headache. Try a slow speed with a fine feed and shallow cut, and importantly some form of cutting fluid suitable for Aluminium. If you do not have any special fluid, paraffin should do the job - as long as you don't let things heat up to the point where the paraffin starts to smoke - then it becomes very dangerous.

 

[bearcar1]

It sounds to me like one of three things. The first being a relatively large diameter not being supported well enough, the second would be the spindle speed or feed rates were too high and the third would be that the cutter edge was too far above center. Any or all of the above could/would cause chatter. Al does not work harden, it is 'gummy' at times but even the harder 7075 alloys will cut smooth as silk if all of the above conditions are met.

BC1
Jim

 

[d.bick]

I use a little paraffin (kerosene) in a metal can with a spout as a cutting fluid this helps with the sticky material on the tool. I also have a bigger rake angle on the cutting tool which I find helps with Aluminium
Good luck Dave Bick

 

[Deanofid]

Since we now know you are holding it by that tiny arbor, I'll say that is almost surely the cause of your problem. At the least, make it as short as you can, and put a tail stock center in it. Better would be to screw the piece to a slab of wood, and bolt that to a face plate.

That little arbor is likely putting the work piece through all kinds of crazy gyrations while you're trying to cut it.

HSS and plain old WD-40 for a cutting fluid should do this job just fine.

Dean

 

[BobWarfield]

You can never have a setup be too rigid, but, there may be another explanation...

CNC and the most recent manual lathes have an option called CSS: Constant Cutting Speed.

If you think about it, the material is moving at different speeds relative to the cutter based on the diameter where you are cutting. To maintain the same cutting speed from a larger to a smaller diameter (e.g. if facing towards the center), you have to steadily speed up the spindle as you cut closer to the center.

In this case, you may have been running too fast for the outer area and just about right near the middle. I have a continuously variable DC motor on my lathe, and its nice to be able to tweak the knob in mid-flight for situations like this. I've often noted it helps on difficult operations even when the diameter change seems relatively small.

By all means, try to make a more rigid setup first though.

Best,

BW

 

[Lew_Merrick_PE]

DIY89,

Assuming that you have good support for your part (which is what everyone else questions), how's the support for your lathe bit? It is amazing how often the set-screws (grub-screws, for you on the other side of the pond) back out a small amount and let the bit become a leaf spring. Once you have a natural frequency set-up in a cut, it can be a bloody devil trying to figure out where the "spring" is coming from.

 

[Diy89]

Thanks for all the suggestions! I had another hour to kill last night, so i chucked it back up.
First off, the tool height was a little high, and a little to far out of the holder. While digging thru my already ground cutters, i found my favorite insert tool. I love this thing! I dunno why, but this thing is sweet and always gives me a nice cut. So i flipped the insert and fired up the spindle. Turning 250 rpm (ref the hand tach) there still was a bunch of chatter. And again, i varied the speed up and down, just made no difference. I was using LPS #2, then switched to WD just to try it out. But as i made cut after cut, it smoothed out. Kinda like cutting thru a week old jelly donut! Whats strange is even though the cut is smooth an shinny, there are little spots in the material. They really remind me of the hard spots in bus brake drums i used to cut in a previous life. I'll try to take a pic and post it.
I have done what i wanted on the lathe, now to drill some holes in it to lighten it up!
;D