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

[Jimsehr]

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.

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.
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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.
View attachment 436101

When I first thought up this tool up and posted the hand drawn pic seen in my screenshot Answer to parlo’s post
people told me I would first have to rough the cavity with a big drill or roughing tool. So just to see if it would work I went out to my shop and made the part. I
did it on my light duty Logan lathe. I took cuts of about .100 per swing with no roughed out cavity. I had no trouble with chatter or machine stalling. And just using one disk I could change the radius knowing exactly how much stick out
I had by simply checking stick out of tool. If starting out with a 2 inch alum disk
I knew I had a 1 inch radius then you add stick out of tool to come up with new
radius. If tool sticks out of disk 1/4 inch you have a 1 1/4 radius or 2 1/2 ball size.
one important thing is I made the tool on the lathe that I used it on and drilled and reamed holding drill in collet in spindle. That put cutter exactly on center.
the alum disk can be removed and replaced leaving it’s base on centerline of lathe. Took me about 3 hours to make the tool using what I had in stock bin.
email me for more info if needed. Jimatcf@hotmail.com

 

[Jimsehr]

I did the same thing:

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A conventional ball turner really won't work.

Nice tool . Can you cut more then one radius with it.

 

[Jimsehr]

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.

Parlo
I think you would get too much chatter doing that. I don’t know how stout your machine is .
But if you need to do this type of Cut often email me and I will tell you how to improve my tool to fit your lathe by using your tailstock using one simple disk and using a hand ground 1/4 hss or carbide tool. That will easily change from one radius to another. And my new design puts the tool on center every time with out fiddling with tool. Tell me what size balls you need to make,email me for more info.
Just to be clear a hemispherical is different than a concave radius. It is a FULL half of a radius of ball. Not just part of the radius.

 

[rwm]

Nice tool . Can you cut more then one radius with it.

Negative. In my application I only needed one size. Adjustable tool stickout is a good idea.

 

[Jimsehr]

Negative. In my application I only needed one size. Adjustable tool stickout is a good idea.

I think you can easily adapt tool to make it adjustable to cut larger radius sizes. If you center tool on lathe swing tool around to new position and drill and ream from spindle into disk then use a 1/4 inch round cutter ground down half dia and hand grind clearance under cutter and tap a 1/4 inch set screw to hold the cutter from the top. It’s easy to check stickout by depth mic from tool tip to disk dia.
Jimsehr

 

[pontiac428]

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.

If it is a true hemisphere, you are right, but the concept can also work when shrunk down to fit inside like the rim like pivoting disc tools do. It's another tooling arrangement to glean ideas from, anyway.

 

[Parlo]

Thanks for all the info.

I have decided to use the Aluminium disc type by Robert RDW. The disc can be well supported on thrust or taper roller bearings. I think that I will use a button insert on the perimeter of the disc as it is easier to locate this shape of insert. The insert will probably be mounted on the base of the disc so the swarf will drop away and I will cut the far side of the hemisphere.

 

[Jimsehr]

Thanks for all the info.

I have decided to use the Aluminium disc type by Robert RDW. The disc can be well supported on thrust or taper roller bearings. I think that I will use a button insert on the perimeter of the disc as it is easier to locate this shape of insert. The insert will probably be mounted on the base of the disc so the swarf will drop away and I will cut the far side of the hemisphere.

I think I might make a new tool based on my tailstock just for fun. Just because.
If I make it to mount on the barrel of my tailstock it will always be right on center line. So tell me what size ball shapes hemispherical radius you are looking for. Jimsehr

 

[Parlo]

I think I might make a new tool based on my tailstock just for fun. Just because.
If I make it to mount on the barrel of my tailstock it will always be right on center line. So tell me what size ball shapes hemispherical radius you are looking for. Jimsehr

The sizes will be 50mm dia - 75mm dia & 100mm dia.
I like the idea of mounting on the tailstock.

 

[Jimsehr]

The sizes will be 50mm dia - 75mm dia & 100mm dia.
I like the idea of mounting on the tailstock.

What size lathe are you using? The 100mm large size might be a challenge on my 11 inch Logan. And I sure don’t want to waste a piece of aluminum that large.
But I might make the tool just to see it work. Maybe I need some wooden bowls?
jimsehr