OT -- Help! Imagine a shallow plastic bowl . . .


I figure the crafty minds frequenting this site might be able to help, so I'll give it a shot. This is hobby-related, but not likely to be calculatorish.

Imagine a shallow, black, concave plastic bowl or dish, not spherical, but (I think) a surface of revolution -- overall about 24 inches in diameter. (A TV satellite dish might be a good analogue.) This bowl is a piece of a larger, heterogeneous structure composed of several materials, some of which are sensitive to heat, stress and/or moisture.

I want to mark this thing so I can make a planar slice through it. The plan is to cut out a section, flip the cut-out piece, and reattach it, thus reducing the depth of the "dish" while retaining much (if not all) of its inherent strength. (I'm assuming that a properly-oriented planar slice through this thing will give the cut piece an outline that is symmetrical about the "axis of flip".)

I'm thinking I'll position it as necessary, slowly pour in some water until the water's edge describes my desired line of cut, then add something (oil-based paint?) that will float atop the water and cling to the plastic sides. Then, I'll remove the water, let the second substance dry or harden, and I'll have my line drawn as a "high-water mark".

I'm wondering if anyone has any suggestions for what kind of "marker" substance to use? (Again, the object is black.) Any suggestions for an alternative method of accomplishing this task? Suggestions for how to accomplish it on a convex surface as well as a concave one.

I'll share images of my result when I've got it looking presentable.


Why not make a fixture that is compatible with the saw, andholds the part securely. Then you won't need to go to the trouble of marking a perfect anything.


Yeah, I thought of a jig that would let me slide it through a band saw . . .

But I don't have a band saw and I don't think I'll like the bevel of the cut, come reattachment-time.

Really, perfection isn't the goal. I'm asking only for maybe an order of magnitude greater precision than I was able to achieve by sliding a carpenter's square (with a silver marker attached) around on a table top.

Edited: 10 Apr 2006, 4:25 p.m.


Instead of putting the dye in the water, why not put it in the *air*. White particulate, a sticky white particulate. Then suck the water out.

If you want the convex side done, then put the object on a flat level table or floor, on which a child's inflatable pool is set up. Fill the pool to the appropriate level, then when all is quiescent, make the particulate fall. Then drain the pool, take the object d'art out, and cut it. (you could also level the object with shims--you don't need a level or even a flat floor)

However, the "order of magnitude" only matters at the saw. If you can't make use of the extra precision, then fugget it and use the sliding marker block.

You could put stickiness on the object, immerse in water, and then sprinke baby powder, or cocaine, whatever you have lying around...

Edited: 10 Apr 2006, 4:53 p.m.


Why not:

1) Take thin copper wire (the one used to repair Hp-41 screw posts),

2) Soak it with turquoise blue food-compatible dye,

3) Pass it around the object at the level you want to cut,

4) Tense, tie & knot the wire

5) Let dry

6) Take off the wire and cut along the mark left by the wire.

From Etienne (the copper wire) & wife (the turquoise blue food-compatible dye used on cake icings...)

Thanks for the family challenge & hope you succeed.



This is a really shallow, rounded bowl shape, so I won't be able to tension a wire at the desired level, but it may be the only suggestion that involves cake icing. (A real "sweet" idea . . . )


Can you put a little rotatable support in the middle of your bowl?

You could then put a laser pointer on the spindle, rotate it and mark the laser dot as you go. (Connect the dots as necessary!) Use a white crayon marker, perhaps, for contrast against your black bowl.

There are also laser levels that project a wide, narrow straight line. If you could put one of these in the middle and rotate it, you could also mark the line. You'd get about a quadrant at a time.


Now THAT's an idea! (I could probably justify the laser level as "necessary" for home improvements! ;-> )

I don't know if there's room inside the concavity, but for the convex side of things, that should work great.

I'll look into it. Thanks!



You could also suspend whatever laser gizmo you use from a string above the concave bowl.

"Home improvement" Always a good excuse to buy something!!! (I spend so much time trying to figure out where to put all the improvement equipment that I hardly have time to improve anything!)


mark the inside with a water-based ink, or even chalk, then pour water? My guess is the high water mark would be very clear.

Greeetings from Brazil.



Also, for a convex structure, just mark or paint the outside and dip it into a larger water recipient.


Fill the bowl to the desired line with strong coffee. Let it sit in a very warm place for a day or two and then remove the remaining coffee without removing the "coffee ring" that will have attached. Reuse the coffee to mark the outside in the same fashion.


Maybe my mental imaging skills are deserting me, but if the intention is just to reduce the volume of the bowl, then wouldn't *any* shaped cut be acceptable? Okay, a neat one is, well, neater :-), but whatever you cut out can be flipped and fitted back into the hole. The only issue is the bevel if the material is thick.

I'd test with a cheap plastic bowl of some description first, just in case. :-)

For artistic effect, you could draw a pattern onto the inner of the bowl and cut around that.

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