Which ball burr?

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I know you'll ask what it's for because you are certain I'll put an eye out, kill the neighbor's poodle, build a nuke or am up to no good anway, so here goes. I'm building a press that presses a dimple into thin leather. For that purpose a 3/8 ball bearing is by means of a C-clamp (not shown in drawing) pressed down through layered aluminum sheets . A wet leather strip is sandwiched between the aluminum plates. The plates are held together with M5 machine bolts and wingnuts. The ball will press dimple into the leather. I need to cut a spherical depression into a 3/8" aluminum cylinder. How do I best go about it? I was thinking of using maybe a 1/4" drill bit in my drill press to get a little pilot dimple going. Then switch over to a 3/8" ball burr and remove material down to a depth of 3/16." Looks like ball burrs come in two flavors" carbide or diamond coated. I'm not a machinist, so I am asking which burr is more suitable for my task? I guess the carbide cutter? As always, enjoy the drawing.

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If it were me, I'd try a Ball End Mill
Normally there used in a milling machine for cutting a radius along an (inside) corner.
For using in a drill press, everything would have to be securely clamped down.
Just position the cutter where you want the dimple and feed straight down 3/16"
For cutting aluminum, go with: 2-flute / high speed steel


Then switch over to a 3/8" ball burr and remove material down to a depth of 3/16." Looks like ball burrs come in two flavors" carbide or diamond coated. I'm not a machinist, so I am asking which burr is more suitable for my task? I guess the carbide cutter?
If your going to cut aluminum with a Burr & Dremel Hand Grinder,
carbide or diamond is not necessary. High speed steel would be good (and cheaper).
It would be very difficult to get a 'perfect' dimple/radius using a burr.
 
You can press the ball into the aluminum to get the exact same shape. The sides will mushroom but that either doesn't matter, or can be trimmed away on a lathe. If the leather is say 1/32" thick, you can press a 7/16 ball into the bottom plate and then will have the right inner and outer radii on the leather. Or you may try experimentation on something simpler like just pressing the ball into the leather on some wood or something.
 
I would try to approach a welder with a 3/8" steel rod and a few ball bearings and ask him if he could weld the bearing to the end of the rod.

I'm worried that, over time, the aluminum will spread out over the hard steel ball.

Also, my old BMW motorcycle has aluminum push rods with steel ball ends.
Could you use a push rod? ("All pushrods are 5/16" in diameter, except for #9605, #9635, #9647, #9648 and #9654 which are 3/8" O.D. )"
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Maybe have a welder seal up the lubricant hole, or experiment to see if you like the look of a raised center in the middle of the depression.
 
Why are you using aluminum for the cylinder?
Typically tooling is made of tool steel, of which there are many different grades. But since you are only dimpling wet leather, it shouldn't require an extremely hard material. Any steel that can get up to Rc 26-30 in hardness should be good enough for that. i.e.: SAE 1020
 
Why the cylinder? Most people who build such a contraption will simply stack two or three balls. I thought using a rod that rides snugly on top of the ball which is used to make the impression would be a slicker solution. Why aluminum? because I have 3/8" aluminum rod and because I doubt that the required pressure will deform the aluminum noticeably over time. Wet leather yields pretty easily. However, I do know a professional welder who can weld the ball to a steel cylinder, if teh aluminum cylinder fails.
 
So I got around to making this device tonight. I ended up not using a ball cutter at all. Instead, I drilled first a short 1/8" hole, followed by an even shorter 1/4 inch hole. Then I used a few pretty hard and abrasive conical silicone polishing points followed by increasingly softer and less abrasive ones.

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The contraption

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The aluminum plunger with the polished cup. Shown is an 0.31" ball. I want to be able to press balls from 0.31" to 0.375."

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The plunger has a bevel to prevent getting snagged when being pressed through the layers of aluminum.
 
Alright, so I finally got some 3/8 steel balls and did a first test with my press. As expected, I did not shape the intended cup, but I instead punched a pretty clean hole through the leather, because I have not yet sized the aperture under the leather properly. Right now, all holes are 3/8 inch, so the leather gets cut/punched when I force the leather with the ball through the hole. Here goes, for those who are still interested:


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This is the press with the leather inserted and the center of the cup marked and lined up with the center of the hole through which the ball will be forced. The ball and plunger have been inserted.

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The ball and plunger have been inserted. I used a vise to force the [lunger down until flush with the top layer of teh press. (Not shown)
I may go and use a C-clamp instead of a vise next time. Not that it matters.

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The press partially taken down and the punched leather strip.

My plan is to increase the diameter of the hole under the leather by 2x the thickness of the leather, so that instead of the ball punching hole, the ball will stretch the wet leather into a cup. I am unsure if a should put a radius on the hole edge for a smooth transistion to the cup in an effort to discourage tearing and to achieve a visually more pleasing result.
 
We got apparently only 15 minutes or so to edit our posts. I had to attend to Tinker and Blueberry, whose suddenly odd droppings have me worried, and it shows.
 
Someone was worried the plunger would undergo plastic deformation, mushrooming, when forced into the leather. I checked with my calipers and the diameter of the plunger was 0.3715 before pressing and 0.3715 after pressing. I measured 5 times before and after in different spots across the plunger and averaged the results. I cannot measure any permanent deformation.
 
The stunned silence tells me everyone is already waiting with bated breath for the next installment. Today is the day. I finally made a cupped pouch with a 3/8" cup for small ammo. Next I'll make a larger cup for 5/8 balls. All I need to do is change the aluminum plate under the leather to one with a larger hole than in the current plate. Since I am sure you get the idea, this project has now come to an end as far as you are concerned. Happy slingshooting!


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