Lattice Clocks

Added 24 June 2021

This 10-inch diameter clock is made from Sycamore and Cherry.  The minute markers on the chapter ring are matt black painted copper rivets, and the centre movement mounting was turned from brass.  The rings are 16 segments except for the centre and the chapter ring, which is 12.  I did consider making them all 12s for symmetry, but the segments in the outer ring become too wide.  Another consideration was the number of cuts in the lattice.  Logically it would be 12 or 24, but 12 would be too few and 24 would have left the attachment too flimsy at the centre given the width of cut I wanted to use, so I chose 16. 

The biggest design consideration is that the segment rings must be glued to an undercut in the adjacent ring, so that there is a significant glue area where the joint is in shear.  There is also an undercut in the brass centre, so that the epoxy resin joint to the 12-segment centre is also partly in shear.  If you put a ring in a ring, then all the joints will be in tension if there is a change in water content, and it will very likely fail, I have had several clocks bite the dust from this error.  Incidentally, that’s why the small ring joints in the base of segmented woodturnings can fail, if you don’t allow for it in the design.   I’ve never had a large ring in the body of a piece fail, they get support from their neighbour by the very strong shear joint between them.  

April 2022.  

When I started making segmented woodturnings I saw it as an opportunity to make unique pieces and I was interested in the challenge of the techniques required.  I also thought it solved a lot of the problems of plain turned work, and by that I am referring to the effects of changes in humidity, especially rapid ones, on dimensional stability and the fact that the effects are different in different timber and in different grain directions.  I naively thought that the effective circular grain direction of segmented turning would solve all that.  However, segmented turning has its own problems related to changes in humidity and they are no less serious, probably worse.  What to do about the base insert in a bowl, or how to make sure that glue joints in tension, and hence weak, are supported by joints that are in shear, and hence strong.  A classic example for me was when making segmented clocks, using a simple design of segment rings inside segment rings I had several joint failures.  If there is a humidity change then every joint is put in tension, and something will give unless you are lucky or have a way of resisting the forces, and relying on luck does not make for successful woodturning.

To cut a long story short I made this clock nearly a year ago, and I've had no joint failures....  ...yet.  It involves careful design of joints, by backing up the inevitable weak joints with the strong ones, and many coats of finishing oil.  The oil does not completely prevent the effects of changes in humidity, but it will slow down the rate of change, and that seems to be vitally important. 

A sample size of one clock is not statistically significant but after a string of joint failures on bowls in the base insert, the last 3 or 4 I have made have had no problems.  I used to have a workshop attached to the house, I now have a detached shop (since January 2018) and I'm convinced that it exacerbated my potential for humidity problems.  I'm pretty much convinced that the design and finishing changes I've made are working.  

September 2024

So that the story is kept up to date, this clock is now sitting in the workshop because  it has had several major joint failures.  I have done nothing to them because I have no idea what to do with it to make it resistant to changes of humidity.  I think it will go in the bin some time but I can't bring myself to do it just yet.