Jig for Cutting a Wooden Ball

Hello and welcome to my instructable. I only decided after the event to make a tutorial about this so apologies for there not being a complete catalogue of images relating to all processes - I hope you find it interesting nonetheless.

I am studying architecture and the reason I needed to cut a wooden sphere was related to the concept phase of a project I have just begun. I found this wooden sphere in a charity shop (bought for just £1) and was immediately attracted to it, and the idea of pulling it apart!

Wanting to cut it neatly and accurately had obvious complications. The ball is highly polished as well as being spherical and as such would be difficult to grip. I was also keen not to design something that would be single use, I wanted to be able to present the jig after cutting up the sphere. Here's what I came up with.

I always like to start by sketching whatever it is I am wanting to make and then drawing it in 3D - I use Rhinoceros, Sketchup is also a great tool and often found to be more accessible, it is also free.

The design I came up with is basically two metal cups which squeeze the ball, suspending it, which allows a cut to be made using a bandsaw. The two cups are fixed to lengths of threaded rod to allow them to grip the ball very tightly.

The ball is approximately 86-89mm in diameter, it varies depending on where it is you measure, I guess it was handmade so it's not perfect.

I have recently returned to university and one of the best things about coming back has been to get back in the workshops. I am super lucky here to have access to wood, metal and CAD-CAM workshops. The tools I used for this mini project are quite extensive, tools I certainly don't have at home.

Tools Used:

• Donkey saw
• Metal Files
• Pillar Drill
• Tap and Die set
• Cutting Paste
• MIG Welder
• Angle Grinder
• Blow Torch
• Ball-Pein Hammer
• Clamps
• Drill Bits (various)
• Bandsaw
• Counter sink tool
• Gouge
• 50mm Holesaw

Materials:

• 10mm Solid Steel Square Section (250mm)
• 10mm Ø Solid Steel Round Section (70mm)
• M6 Threaded Rod (100mm)
• M6 Nuts x 4
• 2mm Sheet Steel (approx. 60mm x 100mm)
• 18mm Ply or something similar (100mm x 150mm)
• M3 Bolt
• M3 Nut x 1
• Piece of Leather (approx. 60mm x 100mm)
• M10 Bolt x 2

U Shape:

• I used the donkey saw to cut up my 10mm square section in to 3 pieces - 2 60mm long and one 130mm long.
• Using the pillar drill i made a 5mm ø hole in one end of each of the 60mm pieces.
• Using a tap and die and cutting paste I cut an M6 internal thread in those holes.
• These 3 pieces are then welded together to form a U shape.

Upstand:

• Use the donkey saw to cut a 70mm length of 10mm ø steel round section.
• Cut an M10 thread at one end at least 25mm long.
• Screw a nut up the thread and spot weld it in place.

Weld the round section to the U shape at the centre point of the 130mm long bar. This is the main part of the jig, the wooden sphere will be suspended in this U shaped frame.

The frame needs to be attached to a base for use with the bandsaw. I used a piece of oak (I think) and attached 2 plywood feet to the bottom. The feet ensure the bolt which holds the frame to the base would not make contact with whatever the jig is resting on. These feet need to be perfectly even so that the jig rests flat, make sure the screws which secure the feet to the base are counter sunk.

The last job is to make the cups that hold the sphere. I used 2mm steel plate to make these. To create the dish shape I carved out a circular recess in a block of wood with a gouge and beat the metal discs into this recess.

Cups:

• Carve a 50mm Ø dish shaped recess into a solid block of wood.
• Use the pillar drill with a 50mm Ø holesaw to cut 2 metal discs from your 2mm sheet steel.
• Using a ball-pein hammer beat the two discs into the wooden block.
• Heat the metal discs with the blow torch, quench and beat as required.
• When the discs match the curvature of the sphere you can stop shaping the cups.
• Weld a 50mm length of M6 threaded rod to the cups.

Great, we now have the two cups required to suspend the sphere from the frame. To make it easier to really tighten the cups I drilled a small hole in the end of the threaded rod on one of the cup assemblies. Through this hole I placed a small bolt for leverage. For extra grip I cut two leather discs which will be sandwiched between cup and sphere.

All that is left to do is put it all together and clamp the sphere in place. I found that even with the cups tightened well the sphere could still rotate in the same orientation as the threaded rod. To prevent this I placed a nut either side of the square section, that way the rod could not rotate.

It was a nerve racking few minutes but it went surprisingly smoothly. The only issue I faced was that as the bandsaw cut, the cups where trying to push the cut shut, resulting in the bandsaw blade being pinched. This generated a lot of friction and heat and as such smoke. You'll see in the video I try to wedge the cut open, but the smoke continues nevertheless.

I hope you enjoyed this instructable, and again sorry for not taking more pictures. If anything was unclear please feel free to leave a comment. Happy making!