Ball turning troubles

[DPittman]

It's my first attempt at making a ball turner and making a ball ( actually my first ball is more of an egg).
I'm experiencing severe chatter at times in the middle third of the ball. Cuts are smoother one way more than the other on both ends.

I've messed around with speeds and have things as rigid as I can but am obviously missing something. Almost anything with me could be suspect! As you might be able to see from the picture, my cutting insert does not sit flush with the " backstop", although it does not move around. I'm thinking this shouldn't matter?

I'm looking for suggestions and experience. Am I missing some critical geometry fundamental principals?

Don

 

[Tom O]

picture?

 

[DPittman]

Don't know why the pic didn't show up. I'll try again....

 

[DPittman]

Another picture attempt

 

[PeterT]

I think the important factors for spherical ball turning are
1) (viewed from the top) the ball turner rotation axis must be exactly under the lathe/part rotation axis
2) (viewed from the end) the cutter point must be centered vertically with the lathe/part rotation axis.

Setup wise, check that you don't have deflection going on. If the ball holding arbor (your bolt) is sticking out cantilevered quite a ways, it might be bending off-axis because its unsupported & rather skinny. But I think that would have to be a lot to account for your shape. I realize you cant use the tailstock because the spherical cutting has to go around the end, but if you can sneak the ball segment closer to the chuck without endangering yourself or spherical cutter, that might help. But check 1 & 2 first.

 

[DPittman]

I think the important factors for spherical ball turning are
1) (viewed from the top) the ball turner rotation axis must be exactly under the lathe/part rotation axis
2) (viewed from the end) the cutter point must be centered vertically with the lathe/part rotation axis.

Setup wise, check that you don't have deflection going on. If the ball holding arbor (your bolt) is sticking out cantilevered quite a ways, it might be bending off-axis because its unsupported & rather skinny. But I think that would have to be a lot to account for your shape. I realize you cant use the tailstock because the spherical cutting has to go around the end, but if you can sneak the ball segment closer to the chuck without endangering yourself or spherical cutter, that might help. But check 1 & 2 first.

Thanks for your insights Peter. I should have mentioned that the egg shape I ended up was because of me, I accidently moved the setup.
Yes my arbor I'm using is just 3/8 rod / bolt and I know that isn't great but I don't think that is the major cause of my chatter.
On the very ends of the ball things seem to cut alright but it goes to hell until the very middle again.

If I can figure out how to make and post a video, would you be willing to view and comment again?

Thank you.
Don

 

[PeterT]

Sure, that would be great. I have to figure out vids one day myself. And BTW I'm no expert, especially when it comes to non-linear turning but we might figure some things out with group input.

Chatter can be a number of things, but usually its related to micro-deflection somewhere in the system. The tooling, part holding, flex in the cutting assembly, resonance.... As an example, when you cutting ball end out near the end, your tool cutting pressure is pointing more inward towards the chuck. That means through the arbor so less arbor deflection. Think of it as 2" thick of resistance in this orientation mode. But when you are cutting in the mid section you are pushing 90-deg against the arbor = maximum bending deflection mode ( only .375" steel). That's why they have those rule of thumb 'stick-out' guidelines like 2-3 X diameter for mild steel or whatever it is. Even though you take a straight cutting pass, the part end may be a couple thou larger than in near the chuck.

Another issue is relative surface speed = what the cutter sees relative to material passing by. At a constant rpm, the surface speed near the end cutting is very low, theoretically zero at the axis. Then when you come around to mid SP increases. If that's the level where chatter arises you may have to adjust something like depth or feed.

 

[DPittman]

Okay Peter, no video yet but some progress...
Speed does make a huge difference, I have to go much slower than I imagined. Approx 300-400 rpm on 1-1.5 aluminum. On the ends I get better finish with much faster speed, my lathe is variable speed so it's easy enough to do.

The other thing I realized is that I'm not actually using the exact same portion of the insert in all positions of the tool. I believe that geometry is constantly changing and therefore results vary. I think my slightly crooked insert exasperates this phenomenon.

I have yet to figure out the right set up of the tool to end up with a ball and not an egg. I figured I just cranked in cross slide until I got a nice ball, but just when it starts to look like a ball it starts to elongate.

I had visions of just sweeping the tool across the material at high speed and ending up with nice round balls. That is certainly not the case for me.

Don

 

[PeterT]

The way I would set it up for a true ball is
- contact the cutter on diameter blank side closest to you, near outboard corner edge
- swing cutter 180-deg so its facing opposite side of part. An open gap means the turners vertical rotation axis is too far way from you. So for example if you had a gap of 0.100" on the other side then true center would be 0.100/2 or 0.050" towards you from your first position. Wind the cross slide back 0.050" & repeat.
- once turner rotation axis is truly centered under part rotation axis (lathe center) lock carriage down
- now in cutting mode, you set & progressively feed cutter within the turning assembly itself (not the lathe cross bed axis)

Maybe to keep the assembly aligned you could use steady? Might conflict with your left side ball cutting though.

 

[DPittman]

Peter,

So after some trial and error, I can now say I am happily turning balls. I was able to eliminate 99% of my chatter problems. I believe the source of the chatter was a few variables, rigidity as you pointed out, being a large one.
However, I found that my technique and set up, had a big role as well. To make matters worse, initially I was also using out of round stock.

I now turn at 1200 rpm and the finish is excellent (aluminum). I've posted a couple of small pictures. One of the ball without ANY sanding or polishing and the the same one after a bit of 220 paper and steel wool.

Now everything in site needs a ball!

Don