Tetrahedral Drilling Jig for Molecular Models

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Project by ironwoods posted 07-02-2012 07:07 AM 3292 views 6 times favorited 12 comments Add to Favorites Watch

Because I advertise wooden models for chemistry, geometry, and geology on my website: DrB Science Basics, I was asked by a biochemistry professor to make several custom jigs for large chemical models to be built as a summer project involving high school and university students. Those who have tried are aware that tetrahedral angles (109.47…. degrees) are very fussy and the design of a jig for accurately drilling them is not trivial. The sequence of operations for drilling a 2 1/4 ” diameter carbon atom is shown.

-- Eric, Maine,

12 comments so far

View ownafixerupper's profile


11 posts in 1453 days

#1 posted 07-02-2012 10:53 AM

The nerd part of me that remembers chemistry is definitely amused and a wee bit nostalgic. That’s a very nice-looking jig—-good job getting the angles set up just right.

View jeepturner's profile


936 posts in 1884 days

#2 posted 07-02-2012 11:27 AM

Very nice tool. Thanks for all the pictures of the sequence.

-- Mel,

View KnotCurser's profile


1945 posts in 2160 days

#3 posted 07-02-2012 11:32 AM

Love this! It’s a great jig and a great reason to build – teaching Science is SO important.

I applaud your efforts, sir!!!!


-- Robert Rhoades WoodWorks / Email: /

View Ken90712's profile


16042 posts in 2280 days

#4 posted 07-02-2012 02:17 PM

Great jig, for a great project!

-- Ken, "Everyday above ground is a good day!"

View Helkat's profile


74 posts in 1387 days

#5 posted 07-02-2012 05:32 PM

Really neat jig. I’m fairly certain I wouldn’t have come up with such a simple-looking solution that that problem.

Well done!

-- Nat, UPstate NY,

View oldnovice's profile


4665 posts in 2459 days

#6 posted 07-02-2012 08:05 PM

A number of years ago I made some models similar to this using Plexiglas spheres as part of a orientation for new employees (I also made a SOMA cube out of Plexiglas cubes that looked like ice cubes).

I can understand the requirements for accurate angles and can really appreciate the fixturing designed for these chemical models!

GOOD WORK and the students will be able to “visualize” better!

-- "I never met a board I didn't like!"

View ChrisK's profile


1416 posts in 2173 days

#7 posted 07-03-2012 02:25 PM

Tetrahedral Drilling Jig for Molecular Models, not something you hear everyday. Cannot say I miss chemistry much. Great Jig.

-- Chris K

View rance's profile


4197 posts in 2252 days

#8 posted 07-06-2012 04:57 AM

Clever solution there.

-- Backer boards, stop blocks, build oversized, and never buy a hand plane--

View ironwoods's profile


3 posts in 1295 days

#9 posted 07-06-2012 01:19 PM

Thank you for your comments. I’m a brand-new member and from the beginning have felt that this is a fine organization of creative people.

Making this tetrahedral jig requires no complex (expensive) machinery for setting a 109.47… degree angle. It requires a little trigonometry for setting the angle of the ramp. The idea for the rest of the design comes from playing with a wooden model. (Type “Tetrahedron with Enclosed Sphere” with the quotes on Google and you’ll see what I mean.) I observed that three of the spokes of the sphere are 120 degrees apart, although inclined at an angle. I only needed to set a precise 30 degree angle for the jig.

-- Eric, Maine,

View BTKS's profile


1984 posts in 2556 days

#10 posted 07-06-2012 05:04 PM

Freakin ingenious!

-- "Man's ingenuity has outrun his intelligence" (Joseph Wood Krutch)

View Zmode's profile


1 post in 388 days

#11 posted 11-07-2014 06:27 PM

This is awesome!
I have been trying to build this since I’m the DIY type and it seems that choosing the correct height for the triangular bottom rails, as well as their distance apart is very important. Do you have a formula for those distances given the ball diameter? I ask because I am trying to drill out balls of different diameters. Its also been challenging to center the sphere under the drill. How did you solve that? Thanks for any help you can give and its really cool you did all this.

View ironwoods's profile


3 posts in 1295 days

#12 posted 11-12-2014 12:28 PM

Hi, Zmode . I’m pleased to see your interest and happy to help you construct a jig for the tetrahedral angles.

1) I don’t use a formula for the height of the groove in the 120 degree bottom rails. I start the 90 degree groove undersize. After I’ve set the angle for the platform (19.47 … degrees) and attached the rails, I drill two balls to center, attach with a long dowel, place on the undersized grooved platform, and drill a second hole in the top ball, as shown in the main figure. For this second hole the depth of the groove is not critical.

2) I place a short dowel in this new hole and rotate the assembly until this short dowel hits one side of the 120 degree rail. The groove is not deep enough, so the dowel hits this angle at only one point. For this size ball I then gradually deepen the groove until the short dowel precisely lies against the 120 degree surface (see second figure). The jig is now ready for positioning the drilling stop.

3) Drilling to center of a ball is straightforward. Clamp a piece of scrap wood to your drill press table, drill a hole with a Forstner bit about 2/3 the diameter of the ball, remove the Forstner and insert the bit for the dowel hole. A sphere placed in the Forstner hole will now be drilled to center with any bit you choose.

Let me know if I can be of further help. (For those mathematically inclined, the derivation of the 109.47 … tetrahedral angle is quite interesting and I recommend trying to solve it yourself.)

-- Eric, Maine,

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