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Over the top ball turning attachement

#41
Had the day off today so I took the opportunity to gut the swing.

PARTS.JPG

So I now I have the base, two supports and the swing roughed out.

Never in my wildest dreams did I imagine a 1" thick piece of steel would be such a challenge to work with.
 
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#44
More progress today. Mainly drilling and tapping mounting holes.

ASSY1.JPG

The tool point ended up within 1mm (high) of the chuck center. I can correct that with a little milling.

Should have given more thought to the swing pivot. Drilled holes and standard cap screws don't have a very tight tolerance. There is a wee bit of slop there.

Mounting the sucker to the cross slide is the next challenge. Cross slide only has provision for 2 M6 screws.
 

PeterT

Ultra Member
Premium Member
#45
When you figure out final centering, it might be better to err on the high side as you can always grind back the tool steel but hard to add on. having said that I think some of the cheapo carbide brazed shank tools stand proud of the square shank section. You could even use like a 60-deg threading point, but better to stick with HSS which you can shape yourself

You can also get round section HSS bits if that helps. It probably would provide more contact area & tool rigidity in your tool holder vs the edges of the square. OTOH its light cutting anyway & maybe more work grinding a flat on the side of the round for your your set screws & trying to get a consistent cutting profile.

What I found is you want a smooth swivel action but no play or it shows up in the finish. Are those cap screws acting as combination axles & tensioners?

If you already know this then disregard, but its helpful to know the distance from the edge of the U ends to the center of your axles. You then add this to the radius when you set the cutting tool tip position so it will turn a ball of a specific diameter.
 

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#46
If you already know this then disregard, but its helpful to know the distance from the edge of the U ends to the center of your axles. You then add this to the radius when you set the cutting tool tip position so it will turn a ball of a specific diameter.
Boy.... you lost me! I thought the ball diameter was controlled by the distance between the tool point and the swing pivot point?

I mounted an indexable carbide insert tool but could switch to HSS if needed.

The cap screws are intended to act as axles. I don't follow the tensioners question?
 

PeterT

Ultra Member
Premium Member
#47
I thought the ball diameter was controlled by the distance between the tool point and the swing pivot point?
It is, but its typically not easy to measure from the center of your pivot axles to the tip of the cutter. Maybe this markup helps explain. There are other ways of doing this but you have similar flats at the end of your U frame.
 

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PeterT

Ultra Member
Premium Member
#48
The cap screws are intended to act as axles. I don't follow the tensioners question?
I guess I'm wondering if the frame is too free floating (with play) or loosens up over time, you anticipate cranking in the bolts a bit to compensate?
 
#49
OK. I see what you mean now about measuring the tool tip to swing axis dilemma.

As far as the swing axle goes, I was hoping for better tolerance. Had no intention of cranking the bolts to compensate. Do you think a brass screw tensioning the axle in the perpendicular would remove the slop?
 
#51
May I suggest a shoulder bolt for the axles in your ball turner.
The shoulder portion is a precision cylinder, length and diameter wise. You can get them from local bolt suppliers.
Man.... I wish I had known such a thing exists! I have already drilled and tapped the swing and support legs. May be too late for this iteration.

I checked Fastenal's offerings. They spec cylinder dia, len and thread len but no thread dia and TPI? How are you supposed to work with that info?
 
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#53
I think the thread specs are dependent on the shoulder diameter. For example I think all ½” S bolts have 3/8x16 threads.
Upon revisiting Fastenal I discovered that if you select the details of each offering the thread dia and TPI are provided.

This may very well be the way to go. I could bore out the current drilled holes to the next size and obtain a much better fit.
 
#54
Getting there.... slowly but surely.

FIRSTFIT1.JPG

FIRSTFIT2.JPG

Tool is 1mm too high (as predicted) and I managed to screw up locating the support leg holes by 1/16"... Grrrrr

Need to revisit the axle bolts next, as Johnwa suggested.

Also, maybe a couple of holes and roll pins to keep the supports in alignment with the baseplate?
 
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#55
Today I upgraded the axles to cylinder cap screws per Johnwa's suggestion. Drilled the 3/8" support pivot holes 1/32 less than 1/2" and bored the holes out to match the 1/2" axles with a nice tight fit. Much better.

Tried her out with a 1" piece of aluminum round stock I had kicking around.

BALL1.JPG

I'm a little confused (nothing new) now. Why am I getting 1/2 a ball?

BALL2.JPG

A second go with a smaller dia yielded this.... Getting closer. I'm wondering if the tool moved on me the first go?

In retrospect I should have mounted the tool hold down screws on the bottom of the swing. Guess I could flip the swing over and run the lathe in reverse.

BALL3.JPG

Couldn't resist.... I parted off that knob I had turned and mounted it on the end of the swing handle.
 
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#56
Decided to try a different stock mounting approach today.

BALL4.JPG

Mounted a piece of aluminum round stock on a bolt and chucked that up.

BALL5.JPG

Almost got a ball, but the finish ended up poor. Too much deflection by the bolt. This arrangement could really use some tailstock support. Hmmmm another project? A narrow/thin tail stock dead center perhaps?

ASSYFINAL.JPG
 
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RobinHood

Active Member
Premium Member
#59
Almost got a ball, but the finish ended up poor. Too much deflection by the bolt.
Good job on getting this far with your ball turner.

You may want to try a “shear tool” cutting geometry on your cutter for a better finish. You can grind one from a HSS blank. Because of the much lower tool pressure a shear tool creates, you can better get away with the amount of stick-out you have.
 

PeterT

Ultra Member
Premium Member
#60
Ball turning is one of those 'seems simple but actually more than meets the eye' type deals. I suspect your true swivel radius (tool tip to bolt axle center lines) may be a bit larger than your target ball diameter because there are remnants of a cylinder. But if your tool tip is not centered on the spindle axis looking at it in top view (the in-out dimension across the bed) and/or not centered on the spindle axis looking at it from front view (the vertical tool height setting in your case) that will impart what results in egg shape. So your final shape could be a blend of these discrepancies & hard to distinguish which is the main culprit. Not that we are trying to make perfect bearing balls, I'm just talking about cause & effect. That's why I was saying if you had a means to measure the tool tip off the C end flats & deduct some fixed dimension, that would be a more precise way of setting radius & that eliminates one of them. Centering is a bit easier to spot visually.

Another complication is we set do our best to set these variables but we have to advance the cutting in progressive steps. Which means the pivot center is moving & so we have to know when to stop. Too short or too long also affects shape.

Maybe this sketch helps. We want to make a 1" dia ball inscribed in the imaginary 1" stock. But I have exaggerated the radius to something larger & (tried) to show it advancing right to left. You can see the shape is something like what you have. And this assumes exact X & Y tool centering, just messed up the radius a bit.

What I have found useful is
- nail the tool radius with certainty because everything depends on that
- measure your stock diameter with micrometer. Make contact with your tool & zero your dial position. Now you just advance the leadscrew by half the amount & this is more precise way of establishing center vs eyeballing a cut
 

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