Triangular Cams

[Oldmechthings]

How do you machine triangular lobed cams for one of Elmer Verburg's Paddle Wheel Engines? Well, first you clamp a bar of metal in a wobble fixture and machine it skiwompus.

Then you put it in a regular three jaw chuck, and drill and ream the bore, turn the shoulder for the hub, and part it off.

Here are the finished cams, just 5/8" across, nestled in their respective cam followers. The shape provides a quick action to the steam valve at each end of the piston stroke. The unique thing about them is that they maintain contact with both sides of the follower through out their rotation. (With a couple thousands clearance taken into consideration)

For something that out of round, that is pretty amazing if you ask me. But nobody asked me.
Birk

 

[Bernd]

Now I know why mine didn't come out as they should have. I machined "kittywompus" instead of "skiwompus". :big: :big:

Nice firxture for turning those cams. Care to elaborate on it a bit Birk? :-*

Regards,
Bernd

 

[Oldmechthings]

Bernd
The offset fixture used was patterned somewhat after the one shown in Marlyn Hadley's book "Model Engines". Some time ago I posted some other pictures of it that show it much better than this particular shot.
As you may already know, Mr. Hadley was a prolific model maker from Oregon.
Birk

 

[hmember]

"The unique thing about them is that they maintain contact with both sides of the follower through out their rotation."
(see Oldmechthings' post above) . . .

I agree -- I love weird stuff like that this. If you do also, check out this website: www.howround.com and an associated video:
[ame]www.youtube.com/watch?v=270qEZKXAfQ[/ame]
-- some of this is too cool. I don't know how you'd machine those buckeye-shaped pyramidal items ("Solids of Constant Width") but they're remarkable.

 

[kvom]

If you draw an equlateral triangle and then used a compass with center on each vertex to draw an arc between the opposite two, you will get a shape like this cam.

Logic tells me that the lobes could be turned using a 4-jaw chuck to center the piece on each of the vertices in succession. I'm assuming Birk's fixture performs the similar function.

 

[mklotz]

The figure you are describing is properly known as a Reuleaux triangle:

http://en.wikipedia.org/wiki/Reuleaux_triangle

I've made Reuleaux rollers for demonstrations before using nothing more than a 3jaw chuck.
Chuck three short pieces of hex stock in the chuck and turn down the exposed portion. Now loosen the chuck and rotate each piece of stock 120 degrees, reclamp and turn down the exposed portion. Repeat this one more time and the exposed portions will be Reuleaux triangles. Part them off and you have three constant width rollers. Measure their diameter and you'll find that it is the same no matter how you orient the figure.

 

[Tin Falcon]

here is another interesting link
http://upper.us.edu/faculty/smith/reuleaux.htm
Tin

 

[rake60]

This is all very interesting.

The Reuleaux triangle brings Felix Wankel's Rotary Engine to mind.

I've been watching that animation for some time now, and I've almost
convinced myself that with proper flywheel inertia, a small model based on the
design could be made to run on air pressure.............

Maybe?

Rick

 

[kvom]

The link posted by Tin brought to mind our visit to Randolph's shop. Randolph does have the chuck and bit to cut square holes, and hex holes too. I noticed he had cut a screw with a female hex pocket, and I asked how he did it: 6-sided gude hole and a 5-sided bit.

 

[Philjoe5]

This is fascinating stuff. I often wondered if a square hole could be machined, now I know. But then I have a question. I'm still new at this but I haven't seen a commercially available tool from the common suppliers, Enco, Grizzly, LMS, WT, etc that would cut a square hole. Why is that?

Cheers,
Phil

 

[rake60]

Drilling square holes is a rather old idea.
It's rarely used today.
Modern machining practice is to drill the hole to it's nominal size, then
cut the flats in with a small end mill.
If sharp corners are required the square is formed with a broach in a heavy
press, the same way the internal hex is formed in socket head bolts.

It's still great stuff to read about!

Rick