# Engineering principle behind this type connection



## Maplehead (Jan 21, 2022)

Hi All
I want to research how to make a certain type of mechanical connection. I don't know the nomenclature but basically it's the concept of a rod or cylinder in a hole/bore that turns on its axis. In the provided pic you can see a previous, disassembled radius turning jig I had made using the above concept. (I use the Jim Sehr design now.) I have two jigs I want to make. One being my Jim Sehr ball turner because my connection at the turning part slightly wobbles so I want to remake it better. In my method a screw underneath the jig pulls together the male round part into the female round part. However, if I want the two pieces snug and tight then I need to tighten the screw, but if I do then the male piece turns with greater difficulty/resistance in the female piece. On another jig I will be making a horizontal connection of this type.
So how do I make these connections fast without increasing the resistance to turning? I'm sure bearings will come into play but I can never wrap my head around how they connect in. As usual, any and all help/advice is greatly appreciated.


----------



## Dan Krager (Jan 21, 2022)

I'm not an expert here, but it would seem that a large tapered roller bearing, like a big farm machine wheel bearing could be useful. An old used one would work even if it had some wear, as long as it was still smooth. The center bolt might want a small thrust bearing against the top of the rotating plate. You could adjust out all unwanted wobble and still rotate smoothly with little effort.   
DanK


----------



## Maplehead (Jan 21, 2022)

Dan Krager said:


> I'm not an expert here, but it would seem that a large tapered roller bearing, like a big farm machine wheel bearing could be useful. An old used one would work even if it had some wear, as long as it was still smooth. The center bolt might want a small thrust bearing against the top of the rotating plate. You could adjust out all unwanted wobble and still rotate smoothly with little effort.
> DanK


Thanks for the reply Dan. What I would love is the nomenclature on this type connection so that I can research it and see diagrams on how the connections are made and affixed. I've tried looking up different things but it's that game whereby if you don't specify the exact words then you can never find it.


----------



## mmcmdl (Jan 21, 2022)

Im confused on what you're trying to accomplish here .


----------



## jwmelvin (Jan 21, 2022)

I think it’s just a rotary joint (rotational joint, pivot, etc.), and in particular one that can handle a primarily thrust load. Many examples. You’re looking for an axially compact version. I like the idea of a single tapered bearing with a thrust bearing on the preload screw. 

You could do it with bushings (plain bearings) but I’d think then you want no taper, just a bore for radial forces and a flange for axial forces. Maybe use a spring on the preload screw so you have a more repeatable and stable preload. e.g. a Belleville washer under the head. Don’t need much preload really since the forces do no try to separate the bearing. You would need very good control of your radial dimensions to eliminate slop. 

The tapered roller bearing doesn’t have that problem of tolerances, but does require reasonably good fits for the bearing races. Not as critical, and line fits or slight clearance may be dealt with by retaining compound.


----------



## Maplehead (Jan 21, 2022)

mmcmdl said:


> Im confused on what you're trying to accomplish here .


Two things: What is the technical name for this type arrangement/connection and two, how best to make it?
So referring to my simple drawing here, the disk will be inserted into the hole. I want the disk to turn easily and smoothly around the axle but I do not want it to move along the axle. Bearings or no bearings? Use a screw as the axle with a lock nut or something else like a shaft through a bushing? If I could just get the technical name for this and look it up and then find some simple, straighforward pics on how they all connect together then I'd be good to go.


----------



## homebrewed (Jan 21, 2022)

In general terms you might want to look at the problem in terms of how many degrees of freedom you want to permit.  Up to a point, grooved thrust washers can constrain radial and axial motion and permit rotary motion.


----------



## mmcmdl (Jan 21, 2022)

Why not just press a bearing into the block and machine a shaft with a very light press or slip fit for the top ? Either a lock collar or adjustable threaded ring on the bottom would pull it tight without binding . Even better , a top and lower bearing to support the business end .


----------



## jwmelvin (Jan 21, 2022)

I agree that, in general, identifying degrees of freedom is important. But you need to do more. Here, you want a single degree of freedom (rotation about the vertical axis). Now you need to identify loads. You have, as I understand it in the example, primarily axial loads. So different 1-DOF rotational joints will handle loads differently. A deep-groove ball bearing will handle _some _axial load but that's not it's strong point. A thrust bearing will handle axial loads, but not radial loads, and most will not handle radial alignment (i.e., they are not suitable for restraining all but 1-DOF). 

A tapered roller bearing is great at a mixed axial and radial load, but they require preload (a force smashing the two halves of the bearing together). Generally, they are used in opposing pairs, so that they preload each other essentially. If, as here, you have unidirectional axial loading, you could use a single one as long as the preload is sufficient. What does that mean? It depends on the applied load. If you have a pure axial load, then really no preload necessary. But as soon as the bearing needs to resist a moment from radial loading away from the plane of the bearing, then you need preload. Better than that would be a second bearing.

What is difficult about what you're asking is that is is too simple. You need much more than what the joint is called. And there are many ways that such a joint could be constructed, depending on constraints. Bushings (plain bearings) are good for low speed, high load, but will have more friction than rolling-element bearings. 

It seems like physical size is important to you. So you may think about a solution that can nest the two structures (one for radial/moment loads, one for thrust). How about a cylindrical bushing to keep everything centered? That would restrains 4 degrees of freedom, leaving 2 unconstrained (axial movement and rotation about the primary axis are unconstrained, while movement along and rotation about the transverse axes are constrained). Then add a needle thrust bearing on bottom to handle the axial load (restrains axial movement). That seems like a potentially good approach. 

You could do as Dave suggests - a single bearing. But not many bearings are well suited to operate in that configuration with your proposed loading. I've been considering the ball turner but you mention another jig that you are interested in. It's entirely possible that the jig would work really well with a single deep-groove ball bearing. Even the ball turner might if we can estimate the loads a little and check available bearings.


----------



## Maplehead (Jan 21, 2022)

Maplehead said:


> Two things: What is the technical name for this type arrangement/connection and two, how best to make it?
> So referring to my simple drawing here, the disk will be inserted into the hole. I want the disk to turn easily and smoothly around the axle but I do not want it to move along the axle. Bearings or no bearings? Use a screw as the axle with a lock nut or something else like a shaft through a bushing? If I could just get the technical name for this and look it up and then find some simple, straighforward pics on how they all connect together then I'd be good to go.


End result would be a smoothly funtioning "Jim Sehr" ball turner as seen in the simple pic.


----------



## Maplehead (Jan 21, 2022)

jwmelvin said:


> I agree that, in general, identifying degrees of freedom is important. But you need to do more. Here, you want a single degree of freedom (rotation about the vertical axis). Now you need to identify loads. You have, as I understand it in the example, primarily axial loads. So different 1-DOF rotational joints will handle loads differently. A deep-groove ball bearing will handle _some _axial load but that's not it's strong point. A thrust bearing will handle axial loads, but not radial loads, and most will not handle radial alignment (i.e., they are not suitable for restraining all but 1-DOF).
> 
> A tapered roller bearing is great at a mixed axial and radial load, but they require preload (a force smashing the two halves of the bearing together). Generally, they are used in opposing pairs, so that they preload each other essentially. If, as here, you have unidirectional axial loading, you could use a single one as long as the preload is sufficient. What does that mean? It depends on the applied load. If you have a pure axial load, then really no preload necessary. But as soon as the bearing needs to resist a moment from radial loading away from the plane of the bearing, then you need preload. Better than that would be a second bearing.
> 
> ...


There's a lot for me to digest here but thanks. In terms of loads my primary use for this ball turning jig is for turning a 3/4" ball from 3/4" mild steel rod. I also would have another hole for the cutter to do the same for a 3/8" ball made of mild steel.
The Sehr version I made does work but it's kinda sloppy. (My fault.) I have to be pushing down on the disk opposite the cutter otherwise the disk has wobble and thus does not cut cleaner and perfectly at the center, (3/8" up the 3/4" rod).
Lastly, searching on DOF came up with a lot of cool related stuff.


----------



## Maplehead (Jan 21, 2022)

Maplehead said:


> There's a lot for me to digest here but thanks. In terms of loads my primary use for this ball turning jig is for turning a 3/4" ball from 3/4" mild steel rod. I also would have another hole for the cutter to do the same for a 3/8" ball made of mild steel.
> The Sehr version I made does work but it's kinda sloppy. (My fault.) I have to be pushing down on the disk opposite the cutter otherwise the disk has wobble and thus does not cut cleaner and perfectly at the center, (3/8" up the 3/4" rod).





jwmelvin said:


> I agree that, in general, identifying degrees of freedom is important. But you need to do more. Here, you want a single degree of freedom (rotation about the vertical axis). Now you need to identify loads. You have, as I understand it in the example, primarily axial loads. So different 1-DOF rotational joints will handle loads differently. A deep-groove ball bearing will handle _some _axial load but that's not it's strong point. A thrust bearing will handle axial loads, but not radial loads, and most will not handle radial alignment (i.e., they are not suitable for restraining all but 1-DOF).
> 
> A tapered roller bearing is great at a mixed axial and radial load, but they require preload (a force smashing the two halves of the bearing together). Generally, they are used in opposing pairs, so that they preload each other essentially. If, as here, you have unidirectional axial loading, you could use a single one as long as the preload is sufficient. What does that mean? It depends on the applied load. If you have a pure axial load, then really no preload necessary. But as soon as the bearing needs to resist a moment from radial loading away from the plane of the bearing, then you need preload. Better than that would be a second bearing.
> 
> ...


Isn't my primary load radial and not axial?


----------



## jwmelvin (Jan 21, 2022)

Maplehead said:


> There's a lot for me to digest here but thanks. In terms of loads my primary use for this ball turning jig is for turning a 3/4" ball from 3/4" mild steel rod. I also would have another hole for the cutter to do the same for a 3/8" ball made of mild steel.
> The Sehr version I made does work but it's kinda sloppy. I have to be pushing down on the disk opposite the cutter otherwise the disk has wobble and thus foes not cut cleaner and perfectly at the center, (3/8" up the 3/4" rod).



I'm not really sure but it seems like axial loads there wouldn't be more than several hundred pounds, and likely quite a bit less.

Take an example bearing: SKF 6002. It has a 641 lb basic static load rating. The information about loads says that axial load can be 50% of that, but for bearings with bore less than 12mm, only 25% of the basic static load. So that puts us about in the ballpark for axial load (but not much margin).

Your load isn't pure axial, depending on where it's applied relative to the bearing diameter. You likely want a larger bearing. They do get more expensive, so you start having to balance factors. But it seems like Dave is right - a single ball bearing may work for you. And these are not expensive bearings. Even a name brand 6005 is not much. According to SKF, that size has a basic static load rating of 1472 lb. So you're starting to get to something that is more likely to survive the application. 

If you have room for almost 3.5" OD, then use something like a 6209.


----------



## jwmelvin (Jan 21, 2022)

Maplehead said:


> Isn't my primary load radial and not axial?


I thought that in this style tool, the cutting load is pushing down on your rotating assembly, along its axis. There will be some load radially away from the spindle, which combines to a radial and moment load on your assembly. It's the moment load that will be difficult to handle with a single bearing. A large one, like the 6209, would handle that better (wider base can resist the moment much better). But using an axially spaced radial bearing is better. Just depends on what you need and how much space you have. I'm thinking you are most constrained by axial space, so using a wide thrust bearing or a large deep-groove will be best.


----------



## rabler (Jan 21, 2022)

The other option that has not been brought up is to use some sort of anti-friction bushing.   You are not looking at a lot of motion, but at some high pressures, so a bushing approach would work.  Bronze or delrin ...


----------



## Maplehead (Jan 21, 2022)

Maplehead said:


> Isn't my primary load radial and not axial?


Here's a pic of my jig now along with a video showing the wobble. I can take out the wobble by tightening up the axle screw but then I cannot rotate the disk. And again, isn't the jig taking on radial load and not axial?


----------



## Maplehead (Jan 21, 2022)

jwmelvin said:


> I thought that in this style tool, the cutting load is pushing down on your rotating assembly, along its axis. There will be some load radially away from the spindle, which combines to a radial and moment load on your assembly. It's the moment load that will be difficult to handle with a single bearing. A large one, like the 6209, would handle that better (wider base can resist the moment much better). But using an axially spaced radial bearing is better. Just depends on what you need and how much space you have. I'm thinking you are most constrained by axial space, so using a wide thrust bearing or a large deep-groove will be best.


Yeah, if axial space is up and down then yes, I don't have much space, as you can see in my latest pic.


----------



## Maplehead (Jan 21, 2022)

jwmelvin said:


> I thought that in this style tool, the cutting load is pushing down on your rotating assembly, along its axis. There will be some load radially away from the spindle, which combines to a radial and moment load on your assembly. It's the moment load that will be difficult to handle with a single bearing. A large one, like the 6209, would handle that better (wider base can resist the moment much better). But using an axially spaced radial bearing is better. Just depends on what you need and how much space you have. I'm thinking you are most constrained by axial space, so using a wide thrust bearing or a large deep-groove will be best.


To me, and I'm probably wrong, the load is radial, as in the red arrow, and not axial, as in the green arrow.
To me the load is a "pushing across" load, not a pushing down load.


----------



## rabler (Jan 21, 2022)

You need to back up a bit.  Your working two problems, tolerances, and friction.
The up and down motion at the handle is an issue of tolerances.  You are working with a very short axle length.  
Imagine a shaft held at two ends, with .001 inch tolerance in the ends.  If the shaft is a foot long it will not move as much as a shaft that is only an inch long.    More angle of play in a short shaft.

You might start by using a sharpie to blue up the contact faces and see where the disc is touching the cutout.  If the contact point is near the center bolt, you have the same effect as a short shaft.  If you hollow out the center a bit so the contact point is near the edge, then you have a much more stable base.


----------



## jwmelvin (Jan 21, 2022)

Maplehead said:


> To me, and I'm probably wrong, the load is radial, as in the red arrow, and not axial, as in the green arrow.
> To me the load is a "pushing across" load, not a pushing down load.



I’m sure there is a mix of loads. The ratio will depend on the tool geometry, depth of cut, and feed rate. Let’s say for now that both loads apply. 

That means your tool needs to resist not just axial load but also moment load. Plenty of ways to do it. 

A large deep-groove like Dave proposed and I discuss above is pretty straightforward and if sized for your largest permissible diameter, should handle the loads. 

A plain bearing such as a bronze flange bushing would likely work but I’d make the flange pretty large diameter. And use a preload bolt and spring. 

Think about a needle thrust bearing. Would be great for pure axial load but as moments apply, far side of the bearing is compressed (that’s what it’s made to handle, so no problem) and the near side is pulled apart. If that pulling apart overcomes the preload, you now have a wobbly thing. The nice thing about the ball bearing is that is handles the arbitrary loads quite well.


----------



## jwmelvin (Jan 21, 2022)

rabler said:


> The up and down motion at the handle is an issue of tolerances. You are working with a very short axle length.



Very much this.


----------



## Maplehead (Jan 21, 2022)

That's a great point about the short shaft tolerances. So much to consider when designing this thing.
I'm still baffled as to why the industry just doesn't come up with a great design and then bang out these ball turners for different sized lathes?
There appears to be a tool for every other use in the world for our lathes and mills but hardly any commercial ball turners. I don't want to be spending all my time making jigs and tools. I need to be spending my time making my guitar parts. Plus, isn't spending money on more machine tools why we're all here in the first place?


----------



## mmcmdl (Jan 21, 2022)

Maplehead said:


> I'm still baffled as to why the industry just doesn't come up with a great design and then bang out these ball turners for different sized lathes?
> There appears to be a tool for every other use in the world for our lathes and mills but hardly any commercial ball turners.


They do , they're called CNC lathes .


----------



## jwmelvin (Jan 21, 2022)

mmcmdl said:


> They do , they're called CNC lathes .



Lol


----------



## mmcmdl (Jan 21, 2022)

If you ever do decide on what you're  going to do and need bearings , give me a shout out .   I have a box full up here new in their boxes . I know I have 6207s as well as smaller and larger . They come in handy for G jobs .


----------



## Maplehead (Jan 21, 2022)

mmcmdl said:


> They do , they're called CNC lathes .


Great, another expensive tool to go out and purchase.


----------



## RJSakowski (Jan 21, 2022)

With CNC lathes becoming more popular, there is little incentive to come up with a specialized radius turner for industry.  The CNC lathe will do the job and much more.  
Hobbyists are probably not a large enough market to warrant designing and manufacturing a specialty tool like this.  We have a habit of DIY, especially when costs are relatively high.  The Holdridge radius cutter was around for decades but very few of us would want to pay upwards of $1K.  Shaplane still makes less expensive radius cutter for around $400.

If I were making your design, I would use an oil impregnated bronze thrust washer on the bottom and perhaps a ball or needle thrust bearing on top.  A bronze bushing or pair of ball bearings would take care of radial loads.


----------



## aliva (Jan 21, 2022)

use a thrust bearing with a bolt, screw thru the center with a nylon lock nut. That's what I used on mine.


----------



## Maplehead (Jan 21, 2022)

aliva said:


> use a thrust bearing with a bolt, screw thru the center with a nylon lock nut. That's what I used on mine.
> View attachment 393179


Where is the placement of the bearing?
Do I bore out a space for it at the bottom of the cavity where the ball turner's rotary "disk" sits?


----------



## rabler (Jan 21, 2022)

My suggestion would be to take the bottom of the inner cylinder (hockey puck), relieve the center part and leave a 1/4" wide contact area around the perimeter of the bottom.  Polish that perimeter using progressively finer sandpaper against a flat surface..  Put some grease in there.  Then use a heavy belleville spring washer under the bolt head.  This will allow you to adjust the holding force between the two parts with just a little bit of give. If that doesn't work then you can start redesigning with bearings.

The point is this design relies on the bottom of the inner male cylinder being perfectly flat, as well as the bottom of the recess you've cut.  If the two surface aren't perfectly flat, then it is likely they are making MORE contact near the bolt.  You want the contact surface to be at the very perimeter of the circle, and absolutely outside the radius of the cutter, as that reduces the rocking action.  By cutting the bottom of the inner disc to be concave, you are preventing contact near the bolt, and forcing it to only make contact near the perimeter.   That has the same effect as the longer axle verses shorter axle in the tolerances issue.

The belleville spring washer gives you a little more adjustment on tightening the bolt down so you get some significant holding pressure but not so great that you can't turn the disc.  You may still have a slight bit of radial play, but just like backlash, it won't interfere with turning an accurate circle once it's loaded.


----------



## jwmelvin (Jan 21, 2022)

Yes try what rabler suggests. The rotating piece should not rock. A cone spring (Belleville washer) will help, as would a plastic washer between that and the rotating piece. It may be an easy way to what you really care about. Some friction may be good for these, to dump vibrational energy. 

What materials are you rotating piece and the stationary mating piece?


----------



## Maplehead (Jan 22, 2022)

jwmelvin said:


> Yes try what rabler suggests. The rotating piece should not rock. A cone spring (Belleville washer) will help, as would a plastic washer between that and the rotating piece. It may be an easy way to what you really care about. Some friction may be good for these, to dump vibrational energy.
> 
> What materials are you rotating piece and the stationary mating piece?


Mild steel


----------



## aliva (Jan 23, 2022)

Maplehead said:


> Where is the placement of the bearing?
> Do I bore out a space for it at the bottom of the cavity where the ball turner's rotary "disk" sits?


Yes. Bore between the top and bottom disk and plate


----------



## Jimsehr (Jan 23, 2022)

I like simple. I don’t think you need a bearing to make the tool better. I think I could make the tool so it could have a longer life if I wanted . I made it thinking I would only make about 100 balls in a home shop. So I was not planing on production runs. Maplehead I think you saw a video of one of my radius cutters or ball turners working. I don’t remember if it was vid of cutting a ball or vid of cutting a radius into the od of a bar. I have made radius cutting tools where swing axis of the cutter was soft leaded material and the mating base was also plain steel plate. No heat treating at all. I just used a little oil and it worked.

If you want my advice email me and I will try to help. Jimatcf@hotmail.com


----------



## Jimsehr (Jan 23, 2022)

Maplehead 
If I was making the balls I would use easy to cut leadloy material. I think I have ball turner that will cut 3/4 ball so maybe I will cut a few just for fun. Leadloy will give a good finish and cost is probably same as mild steel. Also it might be easier to form a 3/8 radius.


----------



## Jimsehr (Jan 23, 2022)

I hand ground and formed all these balls in my avatar.


----------



## Maplehead (Jan 24, 2022)

Jimsehr said:


> I like simple. I don’t think you need a bearing to make the tool better. I think I could make the tool so it could have a longer life if I wanted . I made it thinking I would only make about 100 balls in a home shop. So I was not planing on production runs. Maplehead I think you saw a video of one of my radius cutters or ball turners working. I don’t remember if it was vid of cutting a ball or vid of cutting a radius into the od of a bar. I have made radius cutting tools where swing axis of the cutter was soft leaded material and the mating base was also plain steel plate. No heat treating at all. I just used a little oil and it worked.
> 
> If you want my advice email me and I will try to help. Jimatcf@hotmail.com


Hi Jim
I took the advice of others here and used what I had on hand to update the existing turner. I didn't use any bearings as I don't have any but I did make a brass bushing and fit that in. I also drilled out most of the "hockey puck" from the middle so that the lat 1/4" of the diameter is what contacts the base. All seems to make it better but I still have one issue. I cannot keep it tight. I don't have belleville washers so I used a concave washer. The washer is on a bolt that faces up and screws into the puck. No nut involved. Maybe I need to redesign that to include a nut?
If I can just set the jig to a certain tightness, while still having smooth rotary movement, then I know it will work well now.


----------



## Jimsehr (Jan 24, 2022)

Maplehead said:


> Hi Jim
> I took the advice of others here and used what I had on hand to update the existing turner. I didn't use any bearings as I don't have any but I did make a brass bushing and fit that in. I also drilled out most of the "hockey puck" from the middle so that the lat 1/4" of the diameter is what contacts the base. All seems to make it better but I still have one issue. I cannot keep it tight. I don't have belleville washers so I used a concave washer. The washer is on a bolt that faces up and screws into the puck. No nut involved. Maybe I need to redesign that to include a nut?
> If I can just set the jig to a certain tightness, while still having smooth rotary movement, then I know it will work well now.


I am going to go out to my shop this morning and see about making a guick and dirty 3/8 ball turner. Maybe even a form tool to see if that works better.


----------



## jwmelvin (Jan 24, 2022)

Maplehead said:


> I cannot keep it tight. I don't have belleville washers so I used a concave washer. The washer is on a bolt that faces up and screws into the puck.



Do you mean if you tighten it, the motion locks (too much friction) or the screw works itself loose?

If the former, a compliance like the Belleville washer or a plastic washer may help. But your concave washer should do the same thing, right? If you mean the bolt loosens, what about some threadlocker?

Sounds like you got the mechanism fitted better, so that’s good.


----------



## Maplehead (Jan 24, 2022)

jwmelvin said:


> Do you mean if you tighten it, the motion locks (too much friction) or the screw works itself loose?
> 
> If the former, a compliance like the Belleville washer or a plastic washer may help. But your concave washer should do the same thing, right? If you mean the bolt loosens, what about some threadlocker?
> 
> Sounds like you got the mechanism fitted better, so that’s good.


I can only tight the screw to a certain point before adds too much friction to the rotary motion. However, when I start to add a load to the jig, meaning I start cutting, I notice the rotary motion is looser which allows for a slight wobble. I want zero wobble.


----------



## Jimsehr (Jan 24, 2022)

I changed the tool a smaller dia because I thought 1/4 dia was too large for a 3/8 dia ball. Too much wasted material.  Think 1/8 dia tool would be about right.
I don’t know how far the ball shape needs to be. I would need to see a finished part . I also think a 5c collet that extends out of the spindle farther would help on making these. I forget what Hardinge calls that type collet. Or a Hardinge extended length emergency collet will do the same. You can get these collets
 from  Hardinge.
Just making a one off takes about 3 or 4 passes with the tool and takes about half a minute each.


----------



## jwmelvin (Jan 24, 2022)

I still don’t quite get your wobble. With no screw and no load, am I right there is no wobble? The axial interface is the outer rim of the rotating piece, pressing down against the stationary piece? And the radial interface is the outer circumference of the rotating piece, or a central bushing?

Maybe rethink the design so the load is carried directly through the interface. That is, before loaded, the two parts are touching. So then load does not cause a change in clearance but instead increases the pressure at the already-contacting interface. 

Sketch a cross section of your design. It doesn’t sound optimal.


----------



## Jimsehr (Jan 24, 2022)

I did not have any chatter problems at all. I did not get a chance to try a form too. But that might be quicker.
How close do size of parts need to be ?
I still don’t see any need for a bearing.
Send me a pic of what finished parts look like and tolerance needed.

Jimsehr

I checked Hardinge long nose 5c collets and see their long nose ones cost $151 ea but you can bore a 1 inch extended length emergency collet for $ 64 dollars.
But you have to drill and bore them to size.

One other thing once chatter starts it is harder to control. May be nit picking but I would move the center of the ball turner closer to the center of the lathe cross slide.
Plus I would not bore the base to hold the rotating disk cutting tool. I would just put the rotating disk flat on the flat base. It would be easy to set 2 radius dia locating holes  on one disk. And it would have less chance of chatter by not having a sliding tool to adjust for different size radius. Also for production I would change the thickness design of the rotating disk. I think it would be able to cut a thousand balls before needing to change rotating disk.

Jimsehr


----------

