[ThomasAlva]

My math professor once brought into the classroom one of these things you see in the attached sketch. Anyone know what it called?

It looks sort of squarish, but it rolls when you set it on a table. How would you make one of these on a lathe? The thing I want to make is on the right, the starting shapes are the two on the left.

[elfrench]

My math professor once brought into the classroom one of these things you see in the attached sketch. Anyone know what it called?

It looks sort of squarish, but it rolls when you set it on a table. How would you make one of these on a lathe? The thing I want to make is on the right, the starting shapes are the two on the left.

I made one quite a few years ago. IIRC there was an article in Scientific American about it. I don't think it's possible to do it directly on a lathe. You could do it with a 4th axis on a mill. I made mine in two pieces and glued them together.

[holbieone]

you can make that part on a lathe in one piece

the cross looking thing in your pics is just the trimming planes

[ThomasAlva]

you can make that part on a lathe in one piece

the cross looking thing in your pics is just the trimming planes

How can you make it on a lathe? I'd love to know.

[packrat]

You need two pieces of round stock.

One piece slightly longer than it's diameter. Let's say 1" dia. x 1 1/16" long.

The second should be slightly smaller in diameter but long enough to hold in the lathe. Say 7/8" dia x 3" long.

Put the 7/8 in a mill vice with ~5/8" sticking out of one side of he vice. Center on the 7/8" (7/16" from the edge) And using a 1"diameter end mill put a radius(1/2"R) into the 7/8" round. This will look like the right side projection of the center figure in you thumbnail, the "T" shaped one.

Now if you are using aluminum for this, I'd use high strength cyanoacrylate glue, the good stuff. Not the generic "super glue". With steel I'd soft solder it. But you want to attach the 1" piece to the second piece. Center as close as possible. This will give you a "T" shape like the middle figure of the thumb nail.

Then by chucking the 7/8" diameter in the lathe, you can turn the square ends of the 1" piece to a 1" diameter. Take it easy so the pieces don't get knocked apart while turning. The cyanoacrylate will degrade if it gets too hot.

When you get to the finished diameter, use a propane torch to heat the joint and a soft face hammer to break the pieces apart. And amaze people with your machining ability.

Hope the explanation was clear to you, if not I can post a quick sketch. Believe it or not this is easier to do in a mill, no 4th axis involved.

[ThomasAlva]

Thanks Packrat!

I get it. The trick is to make a temporary bond so you can do the lathe work. I try to avoid interrupted cuts, so I'd try to use a file to get it close before using a lathe bit. That might work.

My own thoughts were to put a small pin through the 1" with square ends so that could be used to chuck it up, and then do the lathe work, but then file off the excess by hand. Somehow you've got to hold it when there's not a lot to hold.

The whole point of the thing is to hand it to someone and then watch them set it on a table and it starts to roll, but they expect it to stay where they set it. It's kind of funny that way.

If anyone makes this thing, please post it in the thread. I'm going to make one.

Incidentally, how would you make it in a Bridgeport? Would you have to get a special radius endmill?

[packrat]

Quote ThomasAlva, "Incidentally, how would you make it in a Bridgeport? Would you have to get a special radius endmill?"

To do it in the Bridgeport, you would make the 7/8" holding piece out of square stock instead of round. Put the concave radius in it, then reclamp it in the vice with the radius part up and plumb. Pick up the center of the block and set the dials to "0" in both directions. Then glue the 1"dia stock into the radius like before. Then you use a boring head in the spindle with the cutter facing inward instead of the way you would to bore a hole. Then you use the boring head to "reverse bore" the 1" part until its round and to size. A boring head can also be used for making the radius in the 'holding part', if you don't have a 1" end mill.

This trick part you show was one of the things we made in high school machine shop class. It was used to teach you to work/think outside the box. We used steel and solder though. And using cyanoacrylate to hold things together was used a lot where I worked before retiring. Parts for molds that needed to be perfectly matched in shape but had to be two separate parts to split at a parting line.

You could use a belt grinder to remove some of the excess stock on the 1" piece, if your not comfortable with interrupted cuts.