Perfect sockets for spheres?

[Evanism]

SWMBO has me making stands that she puts little terrariums in. They are hand blown spheres.

Up to now I've been making sockets for them to sit in. The sockets are cut with big (huge!) Forstners bits. There are three different spheres and three different socket sizes.

She says they are fine, but I find the flat bottoms ugly. It lacks pizazz.

I've been trying to come up with a way of making a circular and concave indentation in wood that is smooth, accurate and replicable. The bottom of the sphere would optimally match the cavity and sit flush.

There may be 3 sockets per board and each board may be up to 40cm long....so the lathe is out.

My feeble intellect can't create any ideas on how to do this..... Keep in mind this isn't a one-off hand carving job....I may have to make 400 of these for the markets....

Any ideas brains trust?

[_fly_]

you could use smaller and smaller forstner bits to get down.

Turn up a part of a sphere, big bolt from outside to the middle to put in a drill chuck.
Cover outside with sandpaper and sand away till you get the arc you want for the item to sit in.

3 different sizes for the ones you need to do.

thats about all I can offer as a method.

Peter

[clear out]

I recently had to machine bowl shapes in end of 14 inch square, metre long bits of Oz hardwood for a few TAFE autobody workshops.
I used a festo plunge circ saw and a jig.
Lots of overlapping cuts and minimal sanding.
Yours would be smaller so a lamella, grinder or one of the new trim saws could be jigged up.
H

[FenceFurniture]

What radii or diameters the spheres? The radius of the socket would need to be smaller than the sphere so that the sphere only contacts the perimeter of the socket - a perfect match would lead to problems if the socket is a part sphere.

As an alternative, what about just drilling the hole straight through in the three places, and then laminating another board underneath (opportunity for contrasting timber)?

If you need it, you can work out how deep a hole needs (or thick the top board needs to be) to be with this formula (rearrange it to get "h")

radius = ½h + w*w/h/8 where h is the height of the protruding sphere arc, and w is the width of same.

[FenceFurniture]

Or....
break out the lathe, do three part spheres in three boards and glue them to the under-board. Use 2 or 3 different timbers for the lathe work so that you make a highlight of this workaround rather than try to hide it by using the same timber.

[BobL]

What radii or diameters the spheres? The radius of the socket would need to be smaller than the sphere so that the sphere only contacts the perimeter of the socket - a perfect match would lead to problems if the socket is a part sphere.

As an alternative, what about just drilling the hole straight through in the three places, and then laminating another board underneath (opportunity for contrasting timber)?.

Along a similar vein. Drill deeper holes with the Forstmer and turn just sockets in a lathe and then glue the sockets into the holes. If the same wood is used it will be hard to see the joins.
Or use contrasting woods as per FFs idea.

Turning 400 sockets sounds like a bit of a PITA so although the following is fair bit of work it will really speed up production.

One possible approach would be to make dedicated bits.
My approach would be to cut half moon bits from an old plane blade or similar and grind a bevel on the edge similar to a spade bit or similar and attach that to a 1/2" shaft.
These bits will be large so they will be turned slowly so they don't need to be well balanced.
As there is no pilot drill tip, everything will have to be firmly held.
A pilot hole drilled almost to the bottom with a small bit will help

The chance of tear out at the top edge is high so faintly pre scoring the outer circle with something is desirable, better still would be tiny nib on each bit - this could be attached to one side of the bit at the required depth. The faint score could be sanded away if deemed a problem

If you could find i,t a small cutter head from a spindle moulder and a set of HSS cutter blanks that could be ground to shape would be the ducks nuts for this job.
These heads usually have several places to which cutters are bolted.
The head would need to be adapted to something like a 1/2" steel shaft to fit in a drill press or even a router if you could find one that will go slow enough (I doubt it!)
One head and multiple radii cutters that could be swapped on/off the head would do it.

I happen to have one of these heads in my shed that came along with a pile of stuff from a combo machine and will take a photo of it if you are interested.

[FenceFurniture]

All this is a lot of work especially if you have to do 400!

One possible approach would be to make dedicated bits.

I was nearly going to suggest that with 1200 to do it could be worthwhile getting a quote from Carb-i-tool for a TC bit to be made. If they are reasonably similar radii then you could just get one made slightly smaller than the smallest radius.

Ev, there's a good reason why I left that equation for you to rearrange. I had a go but it's been a little too long- I'm buggered if i can rearrange it to get h = blah. Maybe Bob can.

[mark david]

Hi you could make an adjustable gimbal type cradle and use a core box bit.

[Evanism]

The idea of mounting a spindle moulder blade r a panel router bit in the drill press sounds good.

I also like the idea of a hemisphere mounted with sandpaper for finishing is also good.

Let me get off the iPad and post some photos....

[FenceFurniture]

Google to the rescue:

h = r - SQRT(r² - w²/4)

where h is the depth to drill to (height of the arc), w is the width of the depression

[BobL]

The idea of mounting a spindle moulder blade r a panel router bit in the drill press sounds good.

I also like the idea of a hemisphere mounted with sandpaper for finishing is also good.

Let me get off the iPad and post some photos....

Put a ruler in the photo or post some dimensions would be good.

[Evanism]

OK, here is one prepared on a table saw using a jig that felt frighteningly dangerous. (One of the infamous Issy Swans finger-loosing opportunities: https://www.youtube.com/watch?v=uv3IxAgCS2c )

DSC09165 (1024x685).jpg

This is the terraria (excuse the dust!!!!!)... the timber is some very old Kauri I picked up. Its all cracked and nasty, so a light plane and 2 coats of NC really highlighted the cracks and dirt.... for a cool effect!

DSC09166 (1024x685).jpg

Showing the 70mm Forstner. The others are 60 and 40mm

DSC09168 (1024x685).jpg

as can be noted, the edges of the terrarium are only on a single lip. Id prefer to get the effect of photo 1. I was thinking of a table saw coving method and then rotating the workpiece, but its restricted to the radius of the blade....

and here is a cute kitten prowling in the yard....

DSC09104 (1024x685).jpg

[FenceFurniture]

Hmmmm. Well personally, I can't see what's wrong with your current forstner approach. After all, it can only be seen when they are removed - so put them back in a hurry! They don't look too bad anyway. Seems like a godawful amount of work (or cost) for a minor improvement to something that is rarely seen.

[FenceFurniture]

as can be noted, the edges of the terrarium are only on a single lip. Id prefer to get the effect of photo 1.

They won't rest satisfactorily (stability) if you want contact with all or more of the depression. You're asking for trouble I reckon. As soon as a sphere comes along that doesn't match (diameter change or aspherical - I assume they are Chinese) then you'll have a rather difficult dilemma to solve.

I'd stay within your current sphere if I were you.

[Evanism]

FF, you know me. Id rather buy a $900 tool to do a $6 job.... let me have my fun.

Yes, the spheres are Chinese, but so is everything at the moment. Makes me wonder who will make our crap when their economy implodes into a galactic black hole.

Anyway, they are bought at 400 at a time in each size. The price to my door is simply inconceivable.

I was thinking a bastardised router bit mounted in a drill press would do the trick... Ive been looking at the Carb-i-tool catalogue (what a monster!) and they get pretty close. As you mentioned, I might drop them a line to see what they can do... page 16 looks promising: http://envirocatalogue.com.au/Carbit...it/#/16/zoomed