Cutting 1/2 of hollow sphere?

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mcman56
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Cutting 1/2 of hollow sphere?

Post by mcman56 » Thu Sep 12, 2013 2:44 pm

Are there any recommendation on how to machine 1/2 of a hollow sphere? See the drawing and think of a small cylinder head. A shape that is close or similar would also work.
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machine sphere 8-29.pdf
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ken572
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Re: Cutting 1/2 of hollow sphere?

Post by ken572 » Thu Sep 12, 2013 4:49 pm

mcman56 wrote:Are there any recommendation on how to machine 1/2 of a hollow sphere? See the drawing and think of a small cylinder head. A shape that is close or similar would also work.
mcman56,

Click on this link:
http://www.homemadetools.net/radius-cutter-6

Make one of these for your lathe, and make your part :wink:

Ken. :)
One must remember.
The best learning experiences come
from working with the older Masters.
Ken.

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Lew Hartswick
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Re: Cutting 1/2 of hollow sphere?

Post by Lew Hartswick » Thu Sep 12, 2013 9:30 pm

If you have a radius turner similar to this:

http://home.earthlink.net/~lhartswick/radiusturner.jpg

All you need is a different sort of bit for it. I've done internal hemispheres, even with a slight recess.
ie. the max diameter of the hemisphere was below the surface by a small amount.
I don't seem to have a picture of the tool but I can take one tomorrow.
The one I made for a friend (an "artist" type) :-) was, as I recall about 1.5" diameter and the recess
was on the order of 1/8". This was for some thin sheet metal forming operation. It was 3 or 4 years ago and I didn't take any pictures at that time.
In general terms the bit looks like a boring bar with the actual cutter at an angle to the main shaft.
...Lew...

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Bill_Cook
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Re: Cutting 1/2 of hollow sphere?

Post by Bill_Cook » Mon Sep 16, 2013 5:56 pm

mcman56 wrote:Are there any recommendation on how to machine 1/2 of a hollow sphere? See the drawing and think of a small cylinder head. A shape that is close or similar would also work.
(Edit-added:) These calculations are for a hemispherical cavity 1.900" ID. A small solid boring bar or one with the tool bit held at 45º or an insert would be ideal. An insert would already have an accurately formed radius but may need a little clearance ground under the cutting edge. The job wouldn't require any special tooling, but it won't yield a really smooth surface. A little sanding would give a wavy appearance - rounding the ridges. Also keeping track of each cut will be tedious. (end edit)

If you were to use Marv’s BALLCUT.EXE program with a radiused tool bit you could cut it in steps using the compound and cross slide. The radius on the tool bit would make it smoother than a sharp point. The radius of the tool bit would need to be subtracted from the radius of the hemisphere.

Example: a radius on the tool bit of 1/32” (.03125) from .950 would be .91875.

http://www.myvirtualnetwork.com/mklotz/

The program input requires a diameter, so multiplying by two makes the entry for Sphere diameter 1.8375 rounded to 1.838.

I used a .020 depth of cut for the demo.
program.gif
The Coordinates are in a file - BALLCUT.OUT. Changing the file extension to .txt will open it in Notepad.

Stock diameter = 1.8380 in
X increment = 0.0200 in

N = cut number
XF = axial (along lathe bed) position of tool
DX = increment in x from last cut
YF = depth of cut
DY = increment in y from last cut
WD = work diameter resulting from depth of cut YF

N XF DX YF DY WD

0 0.000 +0.000 0.919 +0.000 0.000
1 0.020 +0.020 0.728 -0.191 0.381
2 0.040 +0.020 0.651 -0.077 0.536
3 0.060 +0.020 0.592 -0.058 0.653
4 0.080 +0.020 0.544 -0.048 0.750
5 0.100 +0.020 0.502 -0.042 0.834
6 0.120 +0.020 0.465 -0.037 0.908

There are 45 steps in all. A smoother surface would result from a smaller depth of cut, or a larger radius on the tool bit if the setup and lathe will handle it.

The through hole in the drawing will give clearance for the tool bit to start. The zero point will be the center of the tool bit radius lining up with the face and center of the work. The center could be referenced from a recess cut in the face of the work the depth of the radius.
Last edited by Bill_Cook on Tue Sep 17, 2013 3:04 pm, edited 1 time in total.
BC

If there was only one way to do each machining job, the smell of sulphurized cutting oil smoke would have fewer fond memories.

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Lew Hartswick
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Re: Cutting 1/2 of hollow sphere?

Post by Lew Hartswick » Mon Sep 16, 2013 6:34 pm

Do you want to cut the outside of a sphere or the inside.? Most of the answers seem to be to make a
"Ball" which I would call the outside. I have the pix of the tooling to do the INSIDE of a hemisphere if
that is what you want.

mcman56
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Re: Cutting 1/2 of hollow sphere?

Post by mcman56 » Thu Oct 24, 2013 10:40 am

Yes, I want to cut the inside of a sphere and could certainly use a picture. Thanks,

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jim314159
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Re: Cutting 1/2 of hollow sphere?

Post by jim314159 » Tue Oct 29, 2013 1:17 pm

I struggled with this for some time on the lathe before I found the below thread with an ingenious way of cutting perfect, no fuss hemispheres on a mill with nothing more than a rotary table. Take the arcsin of the cutter's radius divided by the desired sphere radius, and you get the degrees your mill head needs to tilt.

It's a treat to see the cutter in action; you get a beautiful result with a boring bar or a plain ol flycutter.

http://www.practicalmachinist.com/vb/ge ... re-138401/

--jim

mcman56
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Re: Cutting 1/2 of hollow sphere?

Post by mcman56 » Wed Dec 25, 2013 3:06 pm

Thanks for your help. I came up with this but have not tried it yet. It was pointed out that the tool needed to be on a 45 degree to cut a full 1/2 sphere. I thought the thick pieces were CRS but are actually some kind of stainless. The interesting part is that the inside piece sprung out and the outside piece sprung in. I did some straightening in a press but this still allowed me to take clean up cuts to square things up and get a nice fit. I welded some small angle on that clamp to work as a operation handle.

Image

Image

jpfalt
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Re: Cutting 1/2 of hollow sphere?

Post by jpfalt » Thu Dec 26, 2013 1:30 pm

As I read your question, I could see two ways to manually cut a hollow hemisphere. One would be to set up a swinging arm on a support of some sort that also will fit inside the finished cavity. Then set up a bar to pull the swing arm through it's arc. Then you need a way to pull on the bar to swing the cutting point.

The second way is similar, using a took holder built like a pantograph.

Now that I think about it some more, you could set up a boring bar with a small radius round cutter at the tip. Then set up a template clamped tot he lathe bed and set up a follower attached to the cross slide that can follow the template. Use the compound rest to adjust the final diameter of the cut.

mcman56
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Re: Cutting 1/2 of hollow sphere?

Post by mcman56 » Sat Dec 28, 2013 7:14 pm

It took some aligining to set up but worked quite well on my test piece. The finish looks good. I was expecting chatter or the tool wanting to grab but cutting was smooth.

Thanks fopr all of the input.

Image

Jaxian
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Re: Cutting 1/2 of hollow sphere?

Post by Jaxian » Sun Dec 29, 2013 2:47 am

I use one of these. The home made ones work great...to a point. You can buy these on eBay used in a number of different sizes any day of the week. Just have to be patient to get one the size you want. Mine is for 16" lathes so is a bit bigger than you might need.

I talked to Ellery Holdridge just a few weeks ago about some new indexable tooling for the thing and they are still in business and doing fine. They are more pricey than building your own depending on how you value your time. But they do a range of operations that most of the home made ones can't match and at a quality level that almost none of them do.

Anyhow the one pictured is a Model 8D, they are about $1900 and change new and half that used in good condition and complete. Ignore the 'Machinery Dealer' thieves asking $1k to $1.5k and just wait for the one in the size you want at the price you want to pop up. I paid like $800 for mine with a couple hundred bucks in extra cutters and it's mint. Good luck.
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Holdridge Radii Cutter 8D.jpg

Millbo
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Re: Cutting 1/2 of hollow sphere?

Post by Millbo » Sun Mar 02, 2014 7:20 pm

You may could drill it with a ball nose end mill, this can get you close if you don't have one with the correct radius and save a bunch of hogging with the radius tool.

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