Generating spherical surfaces

Pete F · Jul 16, 2016

Can anyone tell me how to go about generating a spherical surface in CAD as per this article?
Generating spherical surfaces | CTE Publications

Here's a Youtube video of it in action Kearney and Trecker Autometric Teaser - YouTube

I use F360, but it's likely the basic techniques of modelling it will be similar in other packages. I understand the principle behind it, but prefer to model things so I can work out angles, diameters etc. However I just can't get my head around how this would be modelled in F360, as it seems both operations need to operate coincidently to generate the sphere.

Hazzert · Jul 16, 2016

It looks like you always set the dividing head to 45* and set the boring head to the hypotenuse of the triangle created by 2x the radius and solve it with pythagoras a^2+b^2=c^2 which for a 45* triangle can be simplified to 2a^2=c^2.

If you were doing an incomplete radius ie not a 180* span your angle would be 1/4 of the total included angle or half of the center line angle.

Pete F · Jul 16, 2016

Hazzert said:
It looks like you always set the dividing head to 45* and set the boring head to the hypotenuse of the triangle created by 2x the radius and solve it with pythagoras a^2+b^2=c^2 which for a 45* triangle can be simplified to 2a^2=c^2.

If you were doing an incomplete radius ie not a 180* span your angle would be 1/4 of the total included angle or half of the center line angle.

As I said, I understand the principle behind it (and no, the cutter isn't always set at 45 degrees, as clearly explained in the article), what I don't know is how to model that operation. ie have the CAD package generate the sphere using the same, or similar process.

Hazzert · Jul 16, 2016

Good luck modeling it as its described, you will run into constant zero thickness geometry and intersecting profile errors on most CAD systems. If F360 will allow intersecting surfaces sweep a "tube" with the ID set to the boring bar diameter 360* around the part.

You could always just try it on a piece of scrap too, by the time you're done dicking around in CAD you could have tried it twice.

If you had read my whole post you'd have noticed I gave an example of when the head would be set to something besides 45 deg. However most people using it for balls will set it to 45 to get a complete ball.

plutoniumsalmon · Jul 16, 2016

Hello.

I am not sure about your question. Are you trying to model a sphere. If yes then it's a revolve. If you are trying to see how your cam package will generate this can you not make a custom profile tool that is an image of the action that the tool makes above. IE imagine the point of the tool soon it in your cam and then contour it around your part. Hope this helps. Probably not

Hazzert · Jul 16, 2016

He's doing the job manually (I hope) and wants to model the path the boring bar will take so he can make sure he's getting the result he desires before he puts tool to stock.

plutoniumsalmon · Jul 16, 2016

Hazzert said:
He's doing the job manually (I hope) and wants to model the path the boring bar will take so he can make sure he's getting the result he desires before he puts tool to stock.

Oh. Then part 2. Model the tool revolving in HSM works and proceed from there.

Pete F · Jul 17, 2016

Hazzert said:
It looks like you always set the dividing head to 45* and set the boring head to the hypotenuse of the triangle ...

Well I did read your whole, and this is what it said!

In fact I may be interested in using the technique to generate concave surfaces, partial spheres, etc, and using cutters other than a boring head eg an annular cutter. Of course I would never have thought of using a scrap of material instead of "dicking around" in CAD. :rolleyes5:

Now it could be that it's not possible to actually generate this in CAD per se, but that's why I'm asking. The impression I got from Tom's article was that it was relatively straight forward to do that, but I sure as heck couldn't see how it would be done (in CAD)

Dennisrech · Aug 14, 2016

Pete F said:
Can anyone tell me how to go about generating a spherical surface in CAD as per this article?
Generating spherical surfaces | CTE Publications

Attached is an illustration of the spherical tool making process from an old optics book. The formula is at the bottom. In a 3D cad program you would have to produce two figures of rotation at the calculated angle between each.

We have used this on many tools and it produces a very smooth and accurate surface quickly.

The bottom photo is a cast iron mirror making tool with a 200 inch radius of curvature. One edge of the rotary turntable is elevated equal to the sag of the tool. The fly cutter is the same as the radius of the tool although any diameter will do, the turntable angle just needs to conform to the formula for that diameter and sag.
It only takes a few minutes to set up, and maybe 15 minutes to cut.

Hope this helps,

Dennis

HuFlungDung · Aug 14, 2016

In CAD, it might be possible to sweep a circle around an axis. The circle would represent the tool as sort of a 'holesaw equivalent' to a boring bar. The angle of the axis would be whatever you chose to incline it at. Now if that generates some sort of a surface, I don't really know how you would 'prove' that the resulting surface was proper (ie, not an oblate spheroid or something with a flat spot near the axis. You'd still have to logically deduce that your starting conditions were correct, and that you knew the exact contact point of the cutting edge of the tool, in other words, there are many ways to screw up the part outside of doing it perfectly in CAD.

HuFlungDung · Aug 17, 2016

I had a try at this today, and got shapes that varied from mushroom cap shapes, to nose cones, but no sphere was generated.

CPT Crunch · Aug 19, 2016

Here's how I would model it in Solidworks.

1. Create a plane on an angle to the top plane (Plane2 in my example)
2. Sketch a circle on this plane
3. On the front plane sketch a semicircle with a line connecting it's endpoints
4. To define the semicircle's size, create two points that pierce the circle created in the first sketch. Make these two points coincident to the semicircle. Make the semicircle's center point coincident to the vertical line.
5. Revolve the semicircle to create a solid sphere.

The first circle on the angled plane represents the boring bar and the line connecting the semicircle's endpoints is the axis of revolution.

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Generating spherical surfaces

Pete F

Titanium

Hazzert

Stainless

Pete F

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Hazzert

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plutoniumsalmon

Hot Rolled

Hazzert

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plutoniumsalmon

Hot Rolled

Pete F

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Dennisrech

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HuFlungDung

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HuFlungDung

Diamond

CPT Crunch

Cast Iron

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