Is it possible to turn a 'perfect' sphere? If so how?
Is it possible to turn a 'perfect' sphere? If so how?
How perfect? We have some diamond turning machines(Precitech) that will turn a perfect sphere within 1/4 fringe which is about 8 millionths of an inch. It's not easy though....
:cheers:
I don't know much about anything but I know a little about everything....
i'm just thinking hypothetical situations and i just want to know if it's possible and how they do it. i'm 20 and in cnc machining classes at school and it's just something that i was like 'is this even possible? cause i don't have any idea how to hold it'.
Hadn't seen this one before I answered your other.
If within better 0.001" is considered perfect then it is not too difficult on a CNC machine. I have one sitting on my desk; sphere that is not CNC machine .
The trick is holding it to do a complete sphere. A partial sphere with a neck on it is dead simple but then you have to make a fixture to hold that partial sphere running as true as possible so you can machine off the neck and finish the whole sphere.
If you want pictures I can do them tomorrow.
pics would be appreciated.
I dont know why you couldnt cut it with a neck on it,then part it off. Then bore jaws to the diameter of the sphere and machine off the neck from the first operation. The neck wouldnt even have to run exactly true for the second operation only the sphere would.
Well yes, that is what you do, more or less.
Parting off is not advisable because when your spinning sphere drops down and bounces around inside the machine the surface gets damaged.
Just having the jaws bored cylindrical is also not advisable because they only contact the sphere on a very small line and can easily damage the finish when the sphere is gripped firmly enough to machine off the neck.
bookwurm99-
You can always machine a rough sphere and then lap it to near perfection with some relatively crude aparatus. To see what I mean, check out the Yahoo group "Spheres" http://groups.yahoo.com/group/spheres/
I have wanted to make a floating granite ball (about 18" in diamater, floats on water pressure as a garden ornament) for a long time, and I had come across this group. I have been interested ever since I saw the 5' diamater granite ball floating on a socket pressurised with water in Epcot - and kids were spinning it one way or another.
It's funny that cdlenterprises mentions Precitech, as when I worked there we used a similar technique to lap 1/2" test balls to ±1 millionth of an inch, and we had them sent to NIST for calibration. Also, I have a long history working with air and hydrostatic bearings, and I have always found near frictionless motion addictive.
NEATman
Here are the pics.
The partial sphere is first turned with a thick neck then the neck is thinned down before cutting it off. If the neck is made too small the ball bobbles around during the turning and chatters.
The holder for the finishing is held in the chuck and the inside liner skimmed true before use. This makes sure the partial sphere is held true.
Just enough protrudes to turn off the neck and finish the sphere.
This might seem a lot of effort just for a big aluminum ball. It is but it has a purpose. I am establishing the procedures for making aluminum spheres from around 2.4" diameter up to 3.4" both solid and hollow with varying wall thickness. I am working with a guy to start manufacturing really big ball Trackballs for computer access by people who have a physical disability so they cannot operate a regular mouse or trackball. We want to experiment with different size balls with different weights. Some potential users will have Cerebral Palsy so they cannot control fine motor function; for them a heavy ball that does not spin readily may be preferable. Other users may have Muscular Dystrophy or another muscle weakening disorder and they may find it better to have a very lightweight and easy to move hollow ball.
And maybe the whole venture will fall flat on its face and I will be left with a whole bunch of aluminum marbles to play with .
Geof-
That is one clever fixture!
Also, that sounds like a great application. I hope your business venture works out well.
NEATman
i lived in singapore for four years and they had one about 36" in diameter at a public park. it was always surrounded by kids and even adults playing with it. i absolutely loved it and i have been looking for 10 years if there is one in houston and i haven't found one yet. i would also love to make one myself someday but i don't even have a machining job yet, much less my own machine to even think of doing it yet. probably do it when i get my own machine or when i retire in 30 years or so. i might buy one before then but it would be more fun to make one myself though. if you ever make it post pics so other people might get inspired.
btw thanks for all the posts everyone.
Wow Geof, that is really fantastic. Is there a name for the holder you use for finishing the sphere and is it in widespread machining use or did you design it yourself?
Also you mention trueing it up, does that mean its disposible and you dump it after making one sphere?
Thats a great project your working on by the way.
I designed and made the holder on the machine it is used on.
The trueing up is done on a replaceable aluminum insert in the holder and trueing up is only needed at the start of a batch of spheres so you do whatever is the batch size each trueing up.
Each insert should be good for many 'trueing ups' because it is only necessary to take off a few thousandths of an inch.
Geof,
The sphere making device is inspired.
Your skills and experience are a godsend to those who will benefit from these trackball devices. Best of luck !
I also wanted to mention Ripley's Museum in Myrtle Beach, SC.
Outside the museum there is a 2000 pound granite sphere, floating on a water-filled pedestal.
Takes very little force to set it spinning. It is truly amazing.
What is about spheres that causes such wonderment?
Imagine. A Yahoo group devoted to spheres. I may join...
Boring bar and a G2/G3 line in a short program, would be my method.
www.integratedmechanical.ca
Just like Darebee says, use a boring bar.
Turning or boring partial spheres is fairly easy. The difficult part with boring a full hemisphere is going past zero, or even getting to zero, on the X axis. The boring bar needs clearance and of course the sphere gets in the way.
Regarding my holder the slight truing up cut needed before use served two purposes; one was to get a true running hemisphere, the other was to establish the Z offset with respect to the center of the sphere. Both the truing program and the sphere finishing program used the center of the sphere as Z zero; once a cut had been taken it was certain that Z zero was at the center.
Some people have mentioned those floating granite ball fountains. Only a few companys in the world can make those really big ones (2-3 meters diameter) with the accuracy that is needed to make them work. Does anyone know what CNC-machines can handle something that big and with an accuracy of a few hundreths of a millimeter.
An open mind is a virtue...so long as all the common sense has not leaked out.