What causes this finish on 6061 aluminum?

M

Mike Gibson

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I have tried several different tool shapes and nose radius but don' seem to be able to get rid of what I term "ripple" in the finish on 6061 aluminum. I've also varied speeds. Any ideas on what I need to do?

I believe most of it is due to my lack of ability to grind the tool properly.

part 001.JPG

part 001.JPG
 
I'm no expert, but for added information:
  • could we get a pic of the toolbit?
  • what's the feed rate?
Those ripples seem to spiral around the workpiece at thread-like rates.
 
Looks like work chatter. I use a live center as the small chucks offer limited stability
 
What is the dia. of the stock? How far is it projecting from the chuck? Are you supporting it at the other end? If your live center is not firm enough (pushing on the stock) that would be the cause. The larger the dia. the less you can have hanging out in relation to the size of your chuck. If your lathe is not solid anchored that might even be the problem. The front of your tool should have (approx.) 7 degree relief or from the center of your stock the tool needs 7degrees so its not rubbing below center and your tool should be on center or .005 approx below. If you are looking down on the tool it should be something less than 90 degrees. So nothing but the point is touching. Also a .010/.015 radius on the point. Also if your chuck jaws are not tight or sprung your stock (workpiece) may be moving.
Whew I'll bet that's confusing. If I was there it'd be a lot easier to explain. Good luck and keep trying
dickr
 
Yup, a classic case of chatter.

It's happens when the tool bit digs into the work and then releases. If that happens at the natural frequency of the structure supporting it ,in this case your tool post, compound rest and saddle, it will resonate; sort of like playing the violin. You can get rid of it by changing the natural frequency of the set-up. Change the rpm of the spindle, move the workpiece farther into the chuck, grind a smaller radius on the point of the tool, reduce the amount of tool overhang, or maybe even tighten the gibs in your cross slide and compound rest. Some or all of those things may be required.

Let us know what worked for you.

Tom
 
How large is your lathe? It is chatter,but possibly a bent leadscrew could be lifting your carriage a bit with each revolution if it is not secured to the bed properly so it can't rise up and down. Could be a loose spindle. Do you have ball bearings or plain bearings with pick off shims that need to be tightened up a bit.

A lot depends upon knowing what size,make,and condition of your lathe. I think some guys here are trying to do work that is too big for the strength of their lathes,like in the cutting off hardened hydraulic cylinder rod on what appeared to be a 7" import lathe.
 
Mike Gibson said:
I have tried several different tool shapes and nose radius but don' seem to be able to get rid of what I term "ripple" in the finish on 6061 aluminum. I've also varied speeds. Any ideas on what I need to do?

I believe most of it is due to my lack of ability to grind the tool properly.

View attachment 35951
Click to expand...

Try adjusting your gibs. My lathe, purchased used, gave a finish EXACTLY like this until I tigthened everyting up.

JOhn
 
I once got this kind of chatter and it turned out to be the cheapo drive belts on the lathe, causing some kind of vibration...I've heard that lathes (like mine) with the motor attached to the chassis are more prone to chatter from motor vibration...
 
Sorry, I don't have a picture to post of the tool/tools I've ground and used. The last tool I ground was per the right hand tool ground by Tubal Cain on his Youtube video.

My lathe is a Jet 12x36 and it is not lagged down to the floor. I get decent finishes on 4130 and 4140 and I thought it would produce a better finish with 6061 aluminum. The gibs are adjusted pretty tight but I may go back and readjust. The lead screw does not appear to be moving other than spinning as it should. May have to put an indicator on it to check.

Thanks for all the replies.
 
The lathe gods have smiled. After reading all the replies it made me do a lot of checking and it turned out to be the bolt that the motor pivots on was working it's way loose. Tightened the bolt and the finish improved tremendously.

Thanks again for all the ideas and the help you were to me.
 
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