How to cut a ???

[GrampaJim]

I have been asked by my only granddaughter (another words it's a must do ) to make a plane "globe" for her.  No oceans or mountains, etc.  Why she wants it? - don't ask.  About 16" in diameter.   I don't have a large lathe or know anyone who does, so I need to find a way to do it with my scroll saw and sanders.  I envision making it with layers of 3/4" clear pine which I calculate to be about 20 layers.  What I need  help with is determining the angles to cut each layer.  I know it will not be perfect, but my drum sanders should be able to fix that.  Any thoughts??  Thanks in advance.

Jim

[WayneMahler]

If I calculated this correctly your angle of cut would be 18 degrees. 360 degrees in a circle, and 20 layers would work out as 360 / 20 = 18. You would need to calculate the circumference of each layer divide that by2 to get the radius of the circle to cut, then divide that in 2 to setup your compass to draw the next circle. Another way which is simpler would be to to take your compass. put it in the center of the piece you just cut on the shortest side. Keeping the it on the center expand the other side to meet the outer circumference of the piece you cut ( on the shorter side ). Once it is equal on both sides 180 apart you can draw your next circle to cut at 18 degrees. Repeat till all pieces are cut. 

[preprius]

Here is some pictures. For a non segmented approach.

If you cut angle at fixed angle for all slices you wont get a curved sphere. 

So here is pics that helped me get close to a sphere.

You have to change angle for each slice.  

it is good you are starting with 3/4". This allows you not to calculate past 45 deg. If it did go past 45 you would just cut from other side of board. Assuming your table only goes to 45.

Ok I have not cut this out. but i did pencil it down at scale.

1st pic shows scale drawing. notice how the bowl gets thinner walls. I had started with 1inch thick. 

2nd pic shows calculation and side view of flat board.

I added 1/8" to each offset. Then I calculated the angles. 1 equation is shown. 

notice that the top will still stack and match the thickness of the previous ring.

Build 2 halves, use magnets to hold together. That way you have 2 bowls.

just remember what the mathmetician said when he stubbed his toe.

SOH CAH TOA.

Sin angle = Opposite/Hypothenuse  SOH

Cosin angle = Adj / Hypotenuse 

CAH

Tan angle = Opposite/Adjacent 

TOA

 

Here is my bowl, the 3rd and 4th pic, that cut at 22.5 deg for each slice. It looks like a boat.

 

It took me 3 hours to figure this out.

I have not used geometry for awhile.  What was hard was to figure a way to illustrate it. Easy to make after i drew it to scale.

 

[preprius]

ooops just in case here is the pic of the rings for the bowl.

[Joe W.]

Or you could take the easy way out and cut this Steve Good pattern.globe.pdf

[crupiea]

You could just make it actual size but you might need a lot of wood and a huge saw.  

[Rockytime]

Lots of knowledgeable folks here. Thank goodness caus I couldn't even understand the question

[Dan]

I agree with @WayneMahler, then sand the hell out of it.

[Jim Finn]

Lots of sanding!  Many years ago I made a full sized wooden football.  LOTS of sanding.

[NC Scroller]

Try here:

ScrollMania!