# Thread: Tetrahedral Drilling Jig for Molecular Models

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## Tetrahedral Drilling Jig for Molecular Models

I've been searching the Internet for a machine, jig and/or plans to make something to accomplish this to drill wooden balls. So I was wondering if anyone knows of a plans or has PERSONAL experience doing this.

http://lumberjocks.com/projects/68088

-JW:

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Sorry I don't have a specific answer for you right now, but I'll think on it. I had a fascination with Platonic solids, of which the tetrahedron is the simplest, and made jigs to make some of them on the table saw.

For a while, I made these:

Last edited by ed_h; 01-06-2019 at 03:17 PM.

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Like I said, I haven't found any drawings/plans to make something. I found a few pictures but that's about it. This one would do the trick that's for sure.

-JW:

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Pics in your link show pretty well how the jig is used.

Figuring out the needed jig angles shouldn't be rocket science either. I'm sure one of the resident geometry/math wizards can whip up the needed angles in no time.
(my gears would need some WD-40)

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Hold the work on a titling rotary table using a 3 jaw chuck. It could also be done on a dividing head but the RT has the advantage of setting the angle directly on the dial.

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One thing that makes this problem really simple is to realize that the tetrahedral angles are just a subset of the angles of the corners of a cube.

Set up a jig that holds a cube in a drill press so that the press will drill a hole from the upper corner of the cube to the center. Then rotate the cube to select the appropriate next corner. Drill a total of four holes on alternate corners and you will have the tetrahedral angles of 109.5 for each pair of holes.

You can do the same thing by modifying the original cube so that it carries the ball and acts as a drilling jig. Drill four holes using the cube as a drilling guide and you have your holes. I pushed a slide-fit nail into each hole as a dowel pin after I drilled the hole into the sphere to maintain registration between the hole in the corner cube and the sphere being drilled.
Last edited by Dan_the_Chemist; 01-06-2019 at 07:30 PM.

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Originally Posted by Dan_the_Chemist
One thing that makes this problem really simple is to realize that the tetrahedral angles are just a subset of the angles of the corners of a cube.
I'm not sure I totally understand but that's exactly what I wanted to try and make.

-JW:

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Here's another approach:

If you set a ball onto a hole that has a diameter 94.3% of the ball diameter. three of the tetrahedral vertices will lie on the contact circle between the hole and the ball. The forth vertex will be at the top of the ball. The three lower vertices will be spaced equally around the circle, or 120 degrees apart.

So, center the large hole on the axis of a drill press, and place the ball in it. Drill your first hole. Put marks around the edge of the hole at 120 degree spacing. Align the new drilled hole with one of the marks. Drill the second hole. You should then be able to align the two drilled holes with two of the marks and drill the third hole. Then, if you can align the three drilled holes with the three marks, you'll know that I didn't make a stupid mistake. If that's the case, drill the last hole.

Ed

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Something 'like' this?

-JW:

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Something 'like' this?

-JW:
Yes, but Ed_h's approach is quicker and easier.

Having played with molecular models for 50 years, I can tell you that physical models are fun, but they are totally out for any real research. The angles don't have to be that exact anymore. Anybody who wants exact answers uses molecular modeling programs which account for all sorts of deforming forces in a molecule and calculates the results far more accurately than any physical model.

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