How might I make a tetrahedron?

[tq60]

As suggested make 3 sides on end of round stock.

depends on what machines are available but the last cut is the critical one.

Our suggestion is as follows.

Get the round stock with 3 sides finished in the lathe in a 4 jaw so it can be dead nuts centered and true.

Get some sacrificial material such as PVC that the part will fit in.

First clean part with acetone then coat it with a thin brushed on ) layer of epoxy.

Give it another coat to insure no surface not covered.

Mix some bondo and fill the PVC and place it over the part and get it to run true as possible.

Let it cure fully and place a PVC cap on end and face it then center drill it.

Now make tiny cuts to get the PVC to run true.

Now use cutoff tool to remove part.

Hold by PVC and take tiny cuts until you get the correct face.

Finish polish then toss it in the freezer.

The part should break loose.

The epoxy usually will break off of a polished surface and can be cleaned up where the bondo will not let go and will be difficult to remove.

 

[whitmore]

OK. I got making a cube now basically figured out. I have a a PM25MV mill. And some extra stuff (vise, end mills, caliper, clamps, angle plate, rotary table...). ...

How might I machine a tetrahedron? (4 identical equilateral triangles as the four sides).

Four plate triangles should be easy enough, you have a mill and rotary table. It'll take
some clamping, and shifting the clamps between edges.
Bevel the edges appropriately (angle plate UNDER the rotary table will help).
Assemble with superglue.

 

[Bill Kahn]

WOW! There is some absolute genius on this list. Thank you all. The weekend is here. I have some round stock. I am going to at least start making the pointy stick. (Yeah, how hard can it be--every project seems to start with that feeling.). And then for goodness sake, how hard can cutting off the point possibly be?

Oh, and yes, no purpose to it. I just make stuff that I find entertaining. Shapes seem to be interesting to me. I guess I am easily entertained. Like the month of weekends to final make a cube that visually looked cubic. And, yes, also, to show off to remote family at Christmas--they have no interest, but if they can wear Christmas sweaters I can have a tetrahedron on the tree.

Thanks again.

-Bill

 

[Jimsehr]

I have never made one before so out to shop I went. After a few hours I have one almost done. Took longer then I thought it would. Now I plan on taking a threaded
Piece of stock with a bore in it and a threaded cap with a hole thru end and facing
End of part to finish it. Part is faced to length on outer part of triangle just need to face center cut off to blend to outer part of faced off end.



https://photos.app.goo.gl/g4nPlrxDc0xltCzq1

 

[Wreck™Wreck]

If you like making things for the fun of it as I did at one time good on ya, I have very little interest in doing so today as 30 years working in the business dampens the enthusiasm.

However, one thing that I found to have very interesting properties and may be done in a home shop with VERY careful work is the Rolamite bearing.
I have built several in the past just for fun and they confuse most people.
This is the the basic theory.
Make one and report back (-:

https://en.wikipedia.org/wiki/Rolamite

 

[JerryK]

I have never made one before so out to shop I went. After a few hours I have one almost done. Took longer then I thought it would. Now I plan on taking a threaded
Piece of stock with a bore in it and a threaded cap with a hole thru end and facing
End of part to finish it. Part is faced to length on outer part of triangle just need to face center cut off to blend to outer part of faced off end.



https://photos.app.goo.gl/g4nPlrxDc0xltCzq1

Good thought for finishing the fourth side

 

[JerryK]

OK. I got making a cube now basically figured out. I have a a PM25MV mill. And some extra stuff (vise, end mills, caliper, clamps, angle plate, rotary table...). And I finally can make a 2" cube that is good to .005 on any measurement. I am thrilled. (Was talking to my machinist brother-in-law over Thanksgiving--he mass produces to .0002" so he wasn't impressed. I think he said he was doing .005" when he was 12 years old. Did not dampen my excitement though.)

How might I machine a tetrahedron? (4 identical equilateral triangles as the four sides). I am a good enough mathematician to work out any of the angles I may need. But geometrically, how do you grab hold of it to machine it? I don't want a hold down threaded hole in one side. I can make an "extruded" triangle--e.g. --a triangular "cylinder." So, I have one of the four triangles. Then I can grab the two parallel sides in a small vise along just the edges, angle it up, and get the second side milled.

I just can't think of how to hold it from there.

Any thoughts/suggestions/references/youtubes?

Thanks.

-Bill

Hi Bill,
Doing any good with your Tetrahedron ?

 

[Bill Kahn]

OK. Made a tetrahedron. (Well, a study piece really. Surface is ugly. And smaller than I would have liked.)

2" Round aluminum. Into my BS-0 dividing head on the mill. Angle up 19.47 degrees. (arcsin (1/3). Was fun to work that out). Mill a flat. Rotate 120 degrees. Do again. And again. Now have sharp point on the end of the round stock. To my surprise all pretty symmetric. Now need to cut off. Put on lathe. Cut off tool. Well, yeah it worked, but the surface finish of that 4th side is terrible. Note though, I am easily amused. the 1.7" a side tetrahedron is, indeed, on the mantle. Maybe next I mount with clay/glue or such and get a safe pass on that last side with the mill. I am more of a mechanical type so better machining through chemistry, while clearly smart, is not part of my intuition. I will note that the cylinder I started with was a big chunk of material. The tetrahedron waiting to come out was pretty small. I think the respective volumes are 4.44 and .61 in^3. I can't think of how to geometrically place a larger tetrahedron in a cylinder other than axially symmetric. And the real puzzle was the work holding challenge. Thanks list for the idea of starting with a cylinder. I am still puzzling too how to start with a cube and work hold a corner to be straight up. Mill that flat off. Repeat (ahh, but how hold?) for three more corners and get tetrahedron. Lose just 2/3 of the material, much less loss than cylinder.

-Bill

 

[Fabrickator]

Hi Bill,

Check out some threads from @Fabrickator

http://www.hobby-machinist.com/threads/a-diamond-is-forever.36780/
http://www.hobby-machinist.com/threads/soccer-ball-or-truncated-icosahedron.43936/#post-376170
http://www.hobby-machinist.com/threads/super-bowl-trophy.26504/
http://www.hobby-machinist.com/threads/4-hollow-sphere.61323/

There are many great ideas in those threads for work holding and positioning.

-brino

Thanks for the honorable mention Brino. I've certainly had to hold some impossible projects before! I've found that using tenons is the best way, the trick being-how to lose the last tenon so no one can tell.

Advice to Bill: Always think 3-4 steps ahead on what you're doing to be successful. I follow the 7-P's:
Proper Planning and Preparation Prevent ****-Poor Performance. It's served me well.

 

[Bill Kahn]

Thanks for the honorable mention Brino. I've certainly had to hold some impossible projects before! I've found that using tenons is the best way, the trick being-how to lose the last tenon so no one can tell.

Advice to Bill: Always think 3-4 steps ahead on what you're doing to be successful. I follow the 7-P's:
Proper Planning and Preparation Prevent ****-Poor Performance. It's served me well.

Simply fabulous. Thank you for sharing.

After learning to make a cube (well, within ,005, which is visually a cube), I then figured out how to mount the compound angle and mill off each of the eight corners. Octo-cube, but I think it has a real name too. This one in steel (have no idea what kind.)

And then I put a square cross section on the lathe and played with the taper angle for the first time to observe that indeed, a plane parallel to the axis of a cone gives a parabola. Have done in brass, steel, and aluminum. The one in the snapshot is magnesium.

Nothing at your level. But the same general idea--geometry is beautiful.

Regarding planning ahead--I can't seem to do it. I just sort of see a next possible step and do it. Once there I puzzle on what the next step might be. Sort of like chess--can puzzle on the next move options, but hard to see much past that. Fortunately, as a hobby, the mess-ups are fine too. (Though the broken drill bit/broken tap thing is getting old. I need to develop some better skills.)

-Bill