THE TWISTIn my first post I said I would get back to the agate twist. Let's talk about that.
Several months ago, right after I got my first flat lap and had done a few basic shapes, I got kind of discouraged because I was unable to do concave surfaces. You'd be surprised at just how much of a limitation that is.
Still, there are things that can be done on a flat lap that almost defy one's sense of physics while actually in the act. Sure - a twist - no big deal. But when you're sliding a curly thing around on your flat lap and it's 'fitting' - well, kinda cool.
Here's one I did using the waterline portion of a waterline agate:
SIDE - SKINNY
END A
END B
END ON
SIDE - FAT
BACKLIT
Probably the most important thing I learned is that 45 degrees yields the most visually 'twisted' cab in appearance:
Any lesser degree and the twist is difficult to see. Any more than that and it loses the look of a twist and looks like you just cabbed a rod with two different directions of heads/tail on it.
And so I call it a 1/8th twist (1/8th == 360d / 45d). It's obviously not a new concept so there's probably already a name for it but I had to call it something.
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For those with flat laps who might be interested - here's the pattern (are you sitting?):
That's a SiC honing stick - it's larger than the agate slice I started with and the lines are much easier to see on it. Otherwise it's the exact shape of the agate I started with for the twist shown. The pattern on the other side looks identical. The lines running down the sides mate to the lines crossing diagonally over the ends.
You have to visualize an imaginary plane running through the rectangle from the lines on one side to the lines on the other - like a sheet of paper slicing through it. Then you remove all stock save for a few millimeters on either side of that plane, round it off and presto - twist. You'll end up with a 1/4 twist - cabbing a 1/8th twist is not so terrible once you do a few of these but it's not that intuitive straight away so I recommend the 1/4 twist pattern to begin with. Slice it in half and you get two 1/8th twists.
As you cut be sure and keep the entire thing parallel to your lapping surface. When you're finished the entire thing sits flat (though the offset weight will pull it to one side). Another way of think of it is to pretend that you're making it inside of a tube and that it fits the tube at both ends and middle at all times (which it would).
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This came to mind when I was going over curvature and uniformity of lines and, as you can see, it's quite curvacious. See how uniformly the light follows that curvature at all points? This is important to me - big time. I think nothing distracts and disappoints me more than planar disparity in reflections (ripples and bumps and skews...) coming from a cab I've made. Especially when I bang my head against the wall trying to get those reflections to look right - because they are literally a reflection coming off a surface that distorts them according to the uniformity of that surface.
What's that you say...? Ridges? ...