How do concave disk--e.g. very shallow spherical bowl?

B

Bill Kahn

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I have a commercial Euler's Disk.
is one of a zillion youtube videos if you have not seen it. Yes, have made alternative disks to the 3"X.5" steel one it comes with. (Same general effect, but slightly different sounds, timing, visuals, and ending accelerations) But now I want to make my own base--the one it comes with is not as strong/stable as I'd like to try (it is not rigid steel).

(If you spin a disk on a flat metal plate the disk tends to wander a-ways, and as these are pretty heavy and energized pieces of spinning metal when they come off the steel plate they knock stuff around and dent normal household items (don't ask))

So, I want to put a gentle concave curve (think a telescope mirror shape, but I don't need anything near that sort of precision, a section of a sphere, a very shallow bowl) into an 8" diameter (by .5") steel base disk. I can make the disk (I use a 2" diameter by 1" long stub screwed in from the back to grab with the three-jaw.) This concave bowl shape keeps the heavy spinning Euler's disk safely spinning freely in a constrained space.

I have measured the sagitta from a 6" ruler, and it being .082" indicates I need a radius of like 55". I have a PM1040V lathe (and PM25MV mill).

My question--any ideas how I make a steel part like this? Geometrically I do want the shape to be a section of a sphere.

Thanks for any pointers as to where this may have been discussed before too.

-Bill
 
I have made concave spherical mirrors in a past life by mounting a long boring bar on the the compound rest and loosening the set screws on the compound pivot. However, my mirrors had a radius of curvature of 7.5" which is a long way from 55".

Perhaps you could make a special extension for your lathe bed with a pivot point at the required distance. A stout bar with appropriate cutting bit would do the actual cutting. The cross feed/compound could be set up to provide support for the bar and to control the feed rate. Your pivot point axis and cutting plane will have to be set up to intersect the spindle axis in order to cut a spherical profile.

It would be an interesting and challenging project but I think it is doable.
 
Any way of rigging up some sort of tracing unit on your carriage ? I would think you could lay out a template close enough to what you're looking for and maybe rig an air cylinder and stylus up to your carriage somehow . You may have to do some polishing as that surface speed is really going to change over that distance . Seems like a cool little project to experiment around with . I'm interested to hear some other ideas other than a buying a cnc . :)
 
Another way to rough that puppy out would be to trig out that radius and full depth with a ball mill , then bore in your z axis slightly shallower as you move your x axis out . Think of a G71 roughing cycle on a cnc . Still gonna need polishing and I'll bet that thing will be ringing !! :grin:

Just did a quick calc in my head , difference in the z axis would only be .146 , definitely do-able . But time consuming also .
 
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Easy to program but difficult to do manually.

Think about how a taper attachment works, the Z axis moves along the ways and the taper attachment slide forces the X axis to generate the taper.
A large radius attachment would work similarly, a rod the length of the radius is attached to the carriage and terminated in a position often beyond the length of the machine, when the X axis is moved toward center this forces the Z axis to generate a radius.

This may be more work then you want to do unless you are making many parts.
 
Bill Kahn said:
I have a commercial Euler's Disk...

I want to put a gentle concave curve (think a telescope mirror shape, but I don't need anything near that sort of precision, a section of a sphere, a very shallow bowl) into an 8" diameter (by .5") steel base disk.

I have measured the sagitta from a 6" ruler, and it being .082" indicates I need a radius of like 55". I have a PM1040V lathe (and PM25MV mill).
My question--any ideas how I make a steel part like this? Geometrically I do want the shape to be a section of a sphere.
Click to expand...

One way is to get two quarter-inch plates, cut 'em circular, and weld the edges. Anneal. Hydraulic oil hand pumps can get
to about 10kpsi, if you can weld a suitable boss onto the assembly. Pump oil into the interior, until the center bulges appropriately.

Grind off the weld, and stack the bowls, then re-weld at the edge. Polish the concave surface 'til it looks pretty.
 
I recall my dad doing this in a shop class but don't recall or have photos of all of the set up. I was 10 or 11 at the time. He was making some shallow saucers for some reason. He mounted a rod with one end attached at the tail stock and supported the other end on the compound. The radius cut was the distance between the tail stock and the work.

I don't recall how he attached the tool in the TS. Could have been a rod slipped through a nut, hole drilled through both and pinned so the rod could rock back/forth in the nut. The nut was held in the tail stock chuck.

The other end had a tool bit attached to it somehow. It would have been HSS hand ground. He might have used a 1/4" rod for the tool holder, made a bushing with set screws in the sides on either end, one to attach the bushing to the 1/4" rod, other to hold the HSS bit.

The compound had a block of steel with a 1/4" hole drilled through on lathe center line. He probably did that with a drill mounted in the head stock chuck to make sure the hole was dead on center. The 1/4" tool rod went through the 1/4" hole in the block on the compound. He probably left the compound nuts loose so it could rotate freely. Then moved the carriage close to the work (rod would slip through the hole in the block) and moved the cross slide back/forth to do the cut. Advanced the cut by advancing the tail stock quill, then repeated the cross slide back/forth.

Hope that makes sense, I'll work on a sketch and post it. My dad was pretty good at coming up with ways of doing these types of odd jobs.

Bruce
 
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