Machining a hemispherical cavity in the end of a bar. Any tips?

[Parlo]

I have to make several Aluminium mould cavities of different sizes. The finish is more important than the size and they need to be polished. The cavities are a full half sphere.

I have 3 options:

1. Cnc machine the cavity with a boring bar or similar.
I'm not keen on the zero cutting speed at the centre and may be difficult to polish out any toolmarks. Getting the tool centre height spot on could be a pain.

2. A ball turning attachment on the centre lathe.
See above.

3. Cnc machine a "u" profile on the mill with a large radius ball end cutter and rotate 180 degrees at small angular movements. ( 1 degree movement = 180 cuts! )
This does away with the zero cutting speed at the centre but takes ages to get a decent overlap. It might be difficult to remove the cusps with abrasives as the cloth will follow the contours, leaving the geometry similar.

I think it may be possible to reverse the boring head technique for machining balls. Tilting the dividing head up at an angle to clear the tooling seems ok but I'm not sure if I will cut an ellipse?

Does anyone have any other methods I could explore?.

TIA.

 

[Dabbler]

You can do the trig and cut it manually using an insert with a larger nose radius (or grind it a little larger). Finishing is going to have to be done anyway.

You can make a form tool to do the work also. finishing still required.

 

[Jimsehr]

I have to make several Aluminium mould cavities of different sizes. The finish is more important than the size and they need to be polished. The cavities are a full half sphere.

I have 3 options:

1. Cnc machine the cavity with a boring bar or similar.
I'm not keen on the zero cutting speed at the centre and may be difficult to polish out any toolmarks. Getting the tool centre height spot on could be a pain.

2. A ball turning attachment on the centre lathe.
See above.

3. Cnc machine a "u" profile on the mill with a large radius ball end cutter and rotate 180 degrees at small angular movements. ( 1 degree movement = 180 cuts! )
This does away with the zero cutting speed at the centre but takes ages to get a decent overlap. It might be difficult to remove the cusps with abrasives as the cloth will follow the contours, leaving the geometry similar.

I

I think it may be possible to reverse the boring head technique for machining balls. Tilting the dividing head up at an angle to clear the tooling seems ok but I'm not sure if I will cut an ellipse?

Does anyone have any other methods I could explore?.

TIA.

I made a simple hemispherical radius tool to show someone how to make molds for their canon balls.
I found the tool has to enter more than half way into the cavity to get a complete half ball shape. My other ball turner tools would not work because the tool axis hits the face of the cavity before reaching the the center of the ball. So I made a radius tool that cuts from the inside of the radius. For a cutter I made a 1/4 round piece of hss and hand ground a cutting tip. By grinding 1/4 round to half it’s dia and rounding the end. It’s easy to set the ball dia to size needed. I did not have a 1/4 hss handy for a cutting tool so I used a dowel pin and it worked fine on aluminum. Sounds complicated but it was really simple. Finish shown is how it
cut. I did not polish it. It only took a few minutes to make the radius cut
after I set it up. If you need more info email me. Jimatcf@ hotmail.com
The way I made it I drilled and reamed it on the centerline of lathe and it cut a true radius within a thousand or two and by simply moving the tool and using a micrometer changing to another radius.

 

[Parlo]

I

I made a simple hemispherical radius tool to show someone how to make molds for their canon balls.
I found the tool has to enter more than half way into the cavity to get a complete half ball shape. My other ball turner tools would not work because the tool axis hits the face of the cavity before reaching the the center of the ball. So I made a radius tool that cuts from the inside of the radius. For a cutter I made a 1/4 round piece of hss and hand ground a cutting tip. By grinding 1/4 round to half it’s dia and rounding the end. It’s easy to set the ball dia to size needed. I did not have a 1/4 hss handy for a cutting tool so I used a dowel pin and it worked fine on aluminum. Sounds complicated but it was really simple. Finish shown is how it
cut. I did not polish it. It only took a few minutes to make the radius cut
after I set it up. If you need more info email me. Jimatcf@ hotmail.com
The way I made it I drilled and reamed it on the centerline of lathe and it cut a true radius within a thousand or two and by simply moving the tool and using a micrometer changing to another radius.

Thanks for the reply. I like the simplicity and robustness of your design. It looks easy to make discs for various size ranges or even dedicated sizes which I would consider. The issue of no cutting speed at the centre did not affect the performance of your fixture which is helpful to know.
I may consider a radiused die sinking cutter which will also cut at the back when the tool travels past the centre.
I will email you later for better images if that is ok.

 

[whitmore]

For fast cutting on axis, a tailstock drill or maybe a ball-end mill can be
more efficient than doing it all with the ball turner. How hard, I wonder, is it to
sharpen a Morse taper drill to cut a ball-end?

 

[Parlo]

For fast cutting on axis, a tailstock drill or maybe a ball-end mill can be
more efficient than doing it all with the ball turner. How hard, I wonder, is it to
sharpen a Morse taper drill to cut a ball-end?

I considered machining a 90 degree radius on a piece of tool steel the radius wide and relieving the cutting edge with an angled cutter. A sort of D bit that I could hold in the tool holder and plunge it in.

 

[pontiac428]

Parlo, I don't think that's a good plan, since you will get chatter on a plunge cut with a round-nosed D-bit. D-bits can leave good finish if you can mess with it, but if you are making a die I assume you have to be on dimension the first operation and onward. Chatter probably isn't acceptable if any remnant of a center tit is also unacceptable. The ball turner is probably your best bet, and the cavity can always be deburred.

 

[rwm]

I did the same thing:






A conventional ball turner really won't work.

 

[pontiac428]

I have a Holdridge. I'm not suggesting you go out and spend, these are pricey kits, but this brochure shows the design, and that's what I want to show you. This is a very well thought out tool, you could adapt some of the concept to your project. Also note how the radius is set with precision using an indicator jig.

 

[rwm]

It looks like only the larger model of Holdridge comes with the fancy inside radius cutter? You will need that or the pivot points will interfere with the perimeter.