Machining a Segment of a sphere

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Re: Machining a Segment of a sphere

Postby mtylerfl » Thu Sep 21, 2017 6:28 pm

Leo wrote:Here is a 10 degree slice from a sphere.

Done with a 2 rail sweep.

The outside radius is 3" and the spherical curve is 3" radius.

Because the sides are almost vertical it is going to be really difficult to machine

This is a toolpath preview

Material size is 7" x 4" x .6"

Tool used is 1/32 tapered ball nose - stepover is .003
I don't think there will be good definition on the spherical cut otherwise



Not to mention your example would have to be a two-sided carve to get the undercut portion of the curve! This kind of carve is a bit more problematic than first meets the eye.
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Re: Machining a Segment of a sphere

Postby Leo » Thu Sep 21, 2017 8:43 pm

No - not 2 sided

The 36 slices just as they are will form a perfect sphere.

I went through the layout in solidworks with the slices as in the toolpath preview and they form a perfect sphere.

NOW - how accurately they are machined will depend on setup in the machine.

The layout is correct.

Sphere slices.jpg
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Re: Machining a Segment of a sphere

Postby mtylerfl » Thu Sep 21, 2017 9:33 pm

Leo wrote:
The 36 slices just as they are will form a perfect sphere.

Hi Leo,

No, I'm afraid not.

Of course it looks perfect in your model layout. However, look very closely at that slice when brought in for 3-axis machining.

You will see the curvature becomes a straight vertical wall at the point at which the bottom curvature becomes an undercut. Actually, this takes place during the import process UNLESS you opt to center the slice model's position in the material and choose to create both sides.

As you have it now in the software, it's such a tiny flat spot on the curvature, the parts could just be glued together anyway then sanded to blend all the flat spots.

Other issues could be how to achieve the "knife edge" along the tapered end of the wedge without chipping and to secure the parts during carving. Tabs are an option - just more sanding. Perhaps vacuum hold-down if done carefully. Maybe best would be CA glue and masking tape technique.

I don't think there is any way to avoid chipout on the knife edge. Most of that would be hidden inside the sphere, but could be visible as tiny gaps on the top and bottom of the sphere.

It's an interesting concept, and will be even moreso if anyone actually tries it.
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Re: Machining a Segment of a sphere

Postby Soapyjoe » Thu Sep 21, 2017 11:29 pm

Thanks for illustration

In the end I made very similar but machined 2 components at a time and made my segment with a small cut out to make the sphere hollow.

If I knew how to...... I would post an illustration

Any help with this would be appreciated .... whilst I have been messing about with CNC since almost its inception I still have an awful lot to learn
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Re: Machining a Segment of a sphere

Postby Leo » Fri Sep 22, 2017 1:11 am

mtylerfl wrote:
Leo wrote:
The 36 slices just as they are will form a perfect sphere.

Hi Leo,

No, I'm afraid not.

Of course it looks perfect in your model layout. However, look very closely at that slice when brought in for 3-axis machining.

You will see the curvature becomes a straight vertical wall at the point at which the bottom curvature becomes an undercut. Actually, this takes place during the import process UNLESS you opt to center the slice model's position in the material and choose to create both sides.

As you have it now in the software, it's such a tiny flat spot on the curvature, the parts could just be glued together anyway then sanded to blend all the flat spots.

Other issues could be how to achieve the "knife edge" along the tapered end of the wedge without chipping and to secure the parts during carving. Tabs are an option - just more sanding. Perhaps vacuum hold-down if done carefully. Maybe best would be CA glue and masking tape technique.

I don't think there is any way to avoid chipout on the knife edge. Most of that would be hidden inside the sphere, but could be visible as tiny gaps on the top and bottom of the sphere.

It's an interesting concept, and will be even moreso if anyone actually tries it.


My model never goes to an undercut situation. The flat area around the perifery is not a result of that. Also I did not import the model, I created it with a 2 rail sweep in aspire.

The bottom surface is flat against the table and the curve of the sphere starts from that point. Basically, the spherical curve is 90 degrees to the table at that point. At ALL points on a curve, a line drawn from center WILL be normal to the surface - 90 degrees to the curve. The table is 180 degrees and the wedge is 170 degrees as measured from center.

The model is correct. The toolpath, or the graphical representation of the toolpath is what is showing as flat.

I know that as we get the walls closer to vertical, the software starts to falter. I think that is where the problem is.

I may just try cutting this out this weekend to see what happens. Double sided tape may hold it, I don't have vacuum.

I don't have much faith in the knife edge.

If this is even close to working, perhaps a 22.5 degree wedge may be more appropriate.
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Re: Machining a Segment of a sphere

Postby mtylerfl » Fri Sep 22, 2017 1:17 am

Hi Leo,

Sounds good. It'll be great to hear about the observations and result of your real-world trial!
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Re: Machining a Segment of a sphere

Postby 4DThinker » Fri Sep 22, 2017 1:24 am

The knife edge should remain there if it is endgrain.
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Re: Machining a Segment of a sphere

Postby Leo » Fri Sep 22, 2017 5:08 pm

I am going to try this over the weekend.

I don't have a warm fuzzy feeling about it.

1) trying to 3D the outside of the sphere whereas no matter how it's positioned is going to be a compromising position for 3D machining. No matter what, there is going to be a 90 degree wall.

2) trying to machine the sharp corner transition from the side of the wedge to the spherical radius is not going to leave a good definition on the sharp corner.

I am going to make the wedge - but it is seriously inefficient.

I am going back to my first suggestion of making wedges on a table saw or something like that, then to finish it on a rotary axis or a lathe.
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Re: Machining a Segment of a sphere

Postby mtylerfl » Fri Sep 22, 2017 5:15 pm

Leo wrote:I am going to try this over the weekend.

I don't have a warm fuzzy feeling about it.

1) trying to 3D the outside of the sphere whereas no matter how it's positioned is going to be a compromising position for 3D machining. No matter what, there is going to be a 90 degree wall.

2) trying to machine the sharp corner transition from the side of the wedge to the spherical radius is not going to leave a good definition on the sharp corner.

I am going to make the wedge - but it is seriously inefficient.

I am going back to my first suggestion of making wedges on a table saw or something like that, then to finish it on a rotary axis or a lathe.


Hi Leo,

My thoughts exactly on all the points you and I outlined. Glad you are going to forge ahead and try it anyway. It will be an interesting experiment.

When I first read the OP wanted to do a sphere with 36 wedge slices with 10° angles, I said "oh boy!" out loud! (It would be challenging enough doing just four wedge slices that would fit together well. Thirty-six? - doubtful.)
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Re: Machining a Segment of a sphere

Postby Leo » Fri Sep 22, 2017 5:33 pm

Couple more things I see

Tooling
- A tapered ball nose is stronger, almost necessary, but with a 90 degree wall, the side of the taper will cut the sphere
- A straight ball nose cutter will just break

Stepover
- Step over is a horizontal amount, but the 90 degree wall is vertical. When we stepover .003 - the vertical distance is huge. To get any sort of reasonable vertical stepover, the stepover value in the tool database would need to be something like .0001 in order to get a reasonable vertical move.
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Re: Machining a Segment of a sphere

Postby mtylerfl » Fri Sep 22, 2017 5:39 pm

:D As mentioned earlier - more problematic than first meets the eye, eh?
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Re: Machining a Segment of a sphere

Postby mtylerfl » Fri Sep 22, 2017 5:55 pm

If I was determined to do 36 slices, I think I would just make some long wedge strips with the Moulding toolpath, then cut out the shapes from those strips with a Profile toolpath. Glue the wedges together and sand them to blend.
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Re: Machining a Segment of a sphere

Postby mtylerfl » Fri Sep 22, 2017 7:31 pm

Must allow for glue thickness somehow. Otherwise, glue thickness x 36 won't allow the wedges to fit. (Using thin CA glue might help diminish this problem.)
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Re: Machining a Segment of a sphere

Postby Soapyjoe » Fri Sep 22, 2017 11:25 pm

My version of the 10 degree slices
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Re: Machining a Segment of a sphere

Postby mtylerfl » Sat Sep 23, 2017 12:25 am

Soapyjoe wrote:My version of the 10 degree slices
Soapy


Ok. Enough talk...cut out 36 of 'em, Soapy. :D

(Hopefully they aren't standing on edge!)
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