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The Anchor

The anchor of a clock has always been a most mysterious object to us. We've never been able to understand the shape it takes on, relative to the shape of the teeth on the escape wheel. We're talking about the shape of the pallets (circled in red) here:

The shapes, and their interactions are all very obvious while watching a running clock, as here

Anchor escapement animation 217x328px

but not before, in the design phase.

This is the reason why the anchor in the plans here is "tunable"--it does not have a fixed shape, as the two screws can be adjusted to make the clock work. However, after watching this video, we became more convinced that "trial and error" may be the technique for making a clock anchor. We also note that when looking at clocks in places like this and this, anchors come in all different shapes and sizes. So what gives?

Well, an anchor needs to stop the escape wheel, while also giving the pendulum an impulse, and do it all with low enough friction, so that the clock will run. How to proceed then, and perhaps help us to get rid of the (ugly) tunable anchor? Why not use the tunable anchor as a template! Here's what we did.

We built our escapement up until it was ticking away. Then we pulled the anchor out and measured the heck out of it. So this

became this

We noted that, relative to the center of the mounting hole, the tip of the rounded acorn nut was 0.7425" below the horzontal diameter of the hole on the right. The straight screw on the left was 0.8615" below the same diameter. The tip of the straight screw to the tip of the acorn nut was 1.518". Also, only the right side of the acorn nut is ever used as the clock runs.

So now what? We hit Autocad, sketching up the facts in the above paragraph.

(Note we only included the right part of the acorn nut.)

Now, we realized something: As long as the right edge of the left screw is unobstructed, and as long as the right edge of the acorn nut is unobstructed, the anchor should work in the escapement. Thus, we are free to connect the rest of the shape as we so desire. Hence, anchors can have a bit of artistry to them! Here's what we did (boring):

So we learned a bit why anchors and pallets have the shapes they do. The overall anchor shape is somewhat arbitrary, constrained only by the needed contact edges with the escape wheel's teeth. The pallet shapes are important for reasons of friction, and to mesh properly with your escape wheel given the teeth geometry.

In our case, we needed a straight vertical edge on the left to outright stop the escape wheel, and the 1/4-circle of curvature on the right to also stop the wheel, but a little less abruptly (and with a smooth curve to let the given tooth slide off--as per the acorn nut). Likely the vertical edge on the left could be shaped a bit (curved), but we'll leave that for the next iteration of the anchor.

Off to the Shapeoko 3 CNC machine, and we made this:

Off to the 3D printer for this:

Did it all work? You bet! Here's the wooden one:

And, here's the 3D printed, PLA version:


Files: 2D anchor DWG, 3D extruded anchor, Anchor .STL file for 3D printing, Carbide Create .C2D file,

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