IanVV
journeyman
Reged: 07/30/08
Posts: 5
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... at CB2 (Crate and Barrel 2) ro $30. I thought others might be interested, so here is the link:
http://www.cb2.com/family.aspx?c=527&f=3192
I bought one to use for a 6 in. f5 scope. It is a bit lumpy, but I don't think this will cause a problem if I use a three point support (might even be fine with a ring). I don't know the wall thickness yet -- it is thin so I will need to reinforce the opening (to be cut) with a ring.
Cutting the hole: I am thinking of locking down the sphere to a turn table and using a stationary dremel with cuttoff wheel to cut as I rotate the turntable. I'm not really crazy about this idea... does anybody have a better way?
--Ian
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Luke S
member
Reged: 07/16/08
Posts: 65
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The first thing that came to mind is this:
secure the ball to your workbench somehow so it doesn't roll away, just setting it in a ring would probably work. Then, hang your dremel from the ceiling directly above the ball. Move the ball so the dremel hangs freely with the tip directly above the center. Then you can lower the dremel until, when held taut, the cutting tool contacts the sphere at the right height. Then to cut, simply keep the string tight and move it around the sphere (the strings path would form an inverted cone).
another way would be to use a cut off wheel in a drill press, again with the ball sitting in a ring, then spin the ball in the ring to cut around it.
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astrosam
member
Reged: 05/14/08
Posts: 52
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Hmm. I'd mark a circle for the opening, then drill a hole and cut just shy of the mark with a jigsaw. Then stick some sandpaper down on a sheet of glass and grind down to the exact diameter.
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5211
Loc: Mobile, Alabama, USA
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Yes, a plasma cutter is what you should use. Stainless steel is very hard and tears up cutting tools very quickly. I cut stainless steel Unistrut and all-thread all day, and I go through blades for a portable band saw left and right. You might butcher the sphere trying to cut it with a cutoff wheel, which can shatter and strike you with dangerous shrapnel. And you will have to use a respirator and eye protection, the dust created is definitely not good for you because of the chromium and other toxic metals that are found in stainless steel, to say nothing of the silica or carbide dust that comes off the cutoff wheel. At a temperature of up to 50,000 degrees Fahrenheit, the beam from a plasma cutter will make a neat, hairline and precise cut which will make finishing your sphere a lot more rewarding. A metal or welding shop would be able to do this for you once you lay out where you want the sphere cut for the welder.
Taras
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15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
One curious cat
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GJJim
scholastic sledgehammer
Reged: 09/09/06
Posts: 907
Loc: Western CO
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Quote:
The first thing that came to mind is this:
secure the ball to your workbench somehow so it doesn't roll away, just setting it in a ring would probably work. Then, hang your dremel from the ceiling directly above the ball. Move the ball so the dremel hangs freely with the tip directly above the center. Then you can lower the dremel until, when held taut, the cutting tool contacts the sphere at the right height. Then to cut, simply keep the string tight and move it around the sphere (the strings path would form an inverted cone).
I sometimes (and reluctantly) machine stainless steel and I can tell you for a fact that what you describe above will yield nothing but grief. The tool bit will catch in the thin metal and either break or skip around and destroy the sphere. Machining stainless requires a rigid fixture, cobalt steel or carbide tools, low speeds, and lots of lubrication. A hand held Dremel tool will not do much besides hurt somebody.
The earlier suggestion about a plasma cutter could work, and we've cut SST sheet successfully with a laser, but both are probably overkill for a one-of part. The abrasive cutoff wheel suggestion will probably work too but you might have to take very light cuts to keep from distorting the sphere from heat. Use the thinnest, smallest diameter wheel you can find. Another safe bet might be to use a hacksaw with a fine-tooth, bi-metal blade. Scribe or mark the line you want to cut and them slowly work your way around the perimeter without cutting all the way through on the first pass. Use plenty of oil.
Good luck and watch out for the burs - they will cut you like a knife.
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astrosam
member
Reged: 05/14/08
Posts: 52
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sigh...
You can't just lump all stainless steel in a pile and say "that stuff is hard to work!". Each grade has different attributes. Some are easy to machine, but hard to weld. Take some annealed 416, it cuts like butter (well ok, like any other free machining steel). But don't try to weld it, or draw it out, and if it's hardened then it will be difficult to cut or machine. Other grades like 304 are great for deep drawing or forming, and weld great, but are tough to cut or machine. It's why a sink is made of 304 and a valve is made of 416.
Your stainless steel spheres are likely made of 304 or some variant (304DD comes to mind) which is a difficult grade to cut or machine. Luckily it should be mostly annealed, although the forming process will cause some work hardening. Abrasive tooling should do the trick, but it's likely thin enough that you can cut it with a fine tooth metal cutting blade.
If it's manufactured like I suspect - 2 hemispheres welded to form a sphere, then you'll probably want to avoid cutting across the weld line if you can find it. Close visual inspection may locate the weld line. If not, the weld line may be slightly magnetic in grade 304, or would be of a different thickness and could be located acoustically.
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GJJim
scholastic sledgehammer
Reged: 09/09/06
Posts: 907
Loc: Western CO
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Quote:
sigh...
You can't just lump all stainless steel in a pile and say "that stuff is hard to work!". Each grade has different attributes.
Yes there are exceptions, but I seriously doubt a sheet metal sphere from China is made from 416 SST. I've never seen that alloy available in anything but bar stock. The common SST alloys are difficult to work with hand tools.
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IanVV
journeyman
Reged: 07/30/08
Posts: 5
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Just to clarify: I had no intention of using the dremel by hand -- I have ways of securing it and securing the sphere with some sort of jig with a pivot. I don't like cutoff wheels for the aforementioned shrapnel effect.
The weld line is obvious, so I can avoid that.
Plasma arc would defeat the purpose of using a $30 sphere. I have a few people I can talk to (one of whom might be able to cut it).
BTW: the weld line is ever-so-slightly more magnetic, and the rest is slightly magnetic (as tested with a small rare earth magnet)which I think is in keeping with 300 series depending on how it is formed. I did a test cut into it with a hacksaw -- wasn't too awful. If I manage a rough circle, I can then go back and grind it to true it up.
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theoUK
sage
Reged: 04/22/04
Posts: 283
Loc: N. Yorks, UK
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Make ring.
Draw line.
Drill hole.
Hack-saw + elbow grease.
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Vixen SP
TAL-150P
Surplus shed f5 travel scope
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stars_in_my_eyes
scholastic sledgehammer
Reged: 03/24/08
Posts: 803
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How did your efforts with cutting this sphere turn out?
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highertheflyer
sage
Reged: 07/08/05
Posts: 334
Loc: Ft. Worth, Texas
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Hi Ian.
And for those so inclined.
I would offer a great use for this stainless steel ball as a "Gazing Globe" in a garden setting too.
I'm looking at placing a home made Armillary Sphere in the garden area of our back yard alongside the observatory.
This would make an interesting addition!
Thanks for your inputs...
Have you received yours, and is it truly polished to a mirror finish?
Please keep us posted.
Jim
Here is and example of a Gazing ball with no affiliations.
http://www.1worldglobes.com/Sundials/gazingglobe.htm
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3881
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forget the scope and build something useful. like a vandergraph generator
http://flickr.com/photos/91183587@N00/3040619490/
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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NickG
professor emeritus
Reged: 11/06/06
Posts: 549
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Looks like it attracts aliens! 
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spaceghost
super member
Reged: 01/04/07
Posts: 153
Loc: Olathe, KS
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Hi Ian,
I'm curious how you get your sphere cut done. Pictures and a story would be neat! My vote would be for the hacksaw approach.
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Justin
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2407
Loc: Courtice, Ontario
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Quote:
Cutting the hole: I am thinking of locking down the sphere to a turn table and using a stationary dremel with cuttoff wheel to cut as I rotate the turntable. I'm not really crazy about this idea...
I actually think this isn't a bad idea. I cut thin stainless from time to time (~0.030") and find abbrasive disks like those for dremels to work quite well. Toothed blades will wear down very quickly and can catch easily destroying both the blade and the work. I'm sure it's fine for more rigid SS but this sphere is'nt going to be that rigid.
The abbrasive disks will shatter and catch and send everything flying if they get jammed in the slot you're cutting like what can easily happen when working by hand and get the disk slightly askew to the cutting direction. If you make a jig to hold the sphere properly and align the cutoff disk (and no slop anywhere) I can't see there being any problems.
FWIW you may want something a little meatier than a dremel like say a grinder with a thin disk. I have a king size dremel that takes 3" disks which is just perfect for these types of situations. Also work quickly, as in make many light passes. If you try to cut too deeply and slowly you will heat up the SS too much making it harder and probably discolour it too.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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