I normally use my homemade ball making jig like everyone else does, to make balls. But I also make hemispherical hollows. Today I started work on some small boxes that will hold globes, mounted so they can spin in their box. I used lignum vitae reclaimed from old bowling woods.
When the ball making jig pivots, the cutter travels through an arc. It makes either a ball or a hollow. It depends where the cutter is in relation to the timber and the jig’s pivot point. When the cutter is directly above the pivot point, it doesn’t move. To cut a hollow with the jig, I just have to advance the cutter past the pivot point. The pivot axis is set a little inside the plane of the box rim. Then I can later mount the globe a little below the rim. Swiveling the jig then cuts the hollow very accurately so it fits the ball nicely. Lignum vitae is very hard, and gritty cracks in the wood blunt the cutter. A round nosed tungsten carbide tip brazed to a steel shaft stood up to it.
Setting up the ball making jig
Once set up, the jig works well, but it is not as easy to set it up for hollows as it is for balls. The principle is straightforward. But the cutter shaft and tool holder tend to foul the blank when they are cutting at the box rim. The trick is to align them so they clear the blank but the tip finishes its arc at the centre of the hollow.
The cutter needs room to work, so a very stout tool is only possible for large hollows. Fortunately, that is where rigidity is most essential. In order to use a reasonably robust tool, I made a couple of gouge cuts at the beginning. This started opening up the hollow and made room for the cutter. I could have drilled a pilot hole instead.
When the box bases and domed lids all have their hollows, and the corresponding balls have been shaped, the work with the ball making jig is finished. But there is still some turning to do, shaping and fitting each box and its lid. I shall do this after the holiday. I have to fit the axis pins to the balls, and they will later have the map gores applied. But that happens after they leave my workshop.