WatchTech

3D Printing for Watchmaking? Part 2

 

A simple comparison of some of the basic properties of some 3D printed materials with brass gives us a good idea of what we can expect of their use in watch and/or clockmaking.
PLA, ABS and PET are the 3 most common plastic types used in 3D printing.

Property                        Unit        Brass        PLA       ABS      PET
Density                         g/cm2    8.49           1.26       1.04      1.34
Tensile strength            MPa       338-469     38          29         41
Modulus of Elasticity    MPa       97000        2852      2030     2264

Let's compare brass with PLA, for example.  In very round numbers brass is about 6 times heavier, 10 times stronger and 50 times less flexible.
 

What these numbers don't tell us, however, is things like the resistance to wear and fatigue, which are very important properties for a watch or clock that will run for decades at least.  In many clocks and almost all watches the material properties are used to the full.  In comparison to the parts size the loads on the axles and gear teeth are much higher than, for example, in an auto.  In particular for mainsprings and for pivots watch and clock parts are very close to the material limits.  When repairing clocks and watches one sees broken gear teeth often.  For such pieces in a 3D printed clock we will have to expect much bigger parts and/or shorter times between replacement.

We have friction and surface wear not only at the pivots and gear teeth but also in the escapement.  The Graham escapement, often used in precision clocks for example, has rubbing contact almost all the time as it is a frictional rest escapement.  What we don't always appreciate is that the parts of a watch or clock are almost always under stress.  It is only during the short time when the escapement lets the gear train move that the parts are under less stress.  The rest of the time the escapement is blocking any movement and the full force of the mainspring or driving weights is continuously placed on all the train parts, from the barrel, be it mainspring barrel or drive weight barrel through to the escapement.

Unfortunately I have not found any information about the important characteristic of plastics known as creep. This is the slow deformation of the material when under load.  This shows up, for example, in plastic bearings that become non-round not because of wear, but because of the material moving. This can also cause plastic gear teeth to deform with time even if they are only rarely moving.  As I have no references I will assume that the creep will be bad and try and find constructive methods of reducing it in the mechanical design, for example a lateral tooth support structure on gears.

The teeth of the gears that are close to the power supply i.e. mainspring or drive weights, have the highest stress.  For those gears I have added a ring the height of the teeth to support the teeth.  Here we can see that support structure on the gears.  The pinions will have a similar structure.

At the level of the escape wheel the forces are smaller so I am trying an escape wheel without extra structure.  The wheel is, however, more than 3 times thicker than I would make it in brass and about 50% bigger in diameter.  We will see how this holds up.


More coming...