As was explained in the previous article, while the movement being made will be unique, it will also be compatible with wheel and pinion sizes from the Unitas/ETA 6498-1. It was hard to find any existing documentation on the gear-train sizes for the 6498, so I decided to take the measurements myself.
In particular, I needed to measure the major-diameter (tooth-end to tooth-end) of each wheel and pinion, as well as count the teeth/leaves. From this the all important 'module' measurement and pitch diameter can be derived.
In order to take these measurements I used an AmScope 3.5X-90X Trinocular Stereo BOOM Microscope+10MP Camera +4-Zone LED Light which was purchased for SMD soldering work (nothing to do with watchmaking). However, it turned out that through the included AmScope software, it could measure wheels and pinions to a high degree of repeatable accuracy. Bear in mind this does require accurate calibration each time the zoom level is adjusted.
6498-1 Wheel and Pinion Measurements
The Major Diameter was measured for each wheel and pinion in the 6498-1 movement and number of Teeth noted. The appropriate Module Constant was also used to calculate the final Module - in accordance with the recommended method from PP Thornton. With this information, all other dimensions can be calculated as needed. The dimensions from the 6498-1 movement are shown in the table below:
| Name | Type | Major Diameter (mm) | Teeth | Module Constant | Module |
|---|---|---|---|---|---|
| Setting Wheel | Wheel | 3.85 | 16 | 2.76 | 0.205 |
| Intermediate Setting Wheel | Wheel | 3.85 | 16 | 2.76 | 0.205 |
| Minute Wheel | Wheel | 8.3 | 36 | 2.76 | 0.214 |
| Minute Wheel | Pinion | 2.51 | 10 | 1.61 | 0.216 |
| Driver Cannon pinion | Wheel | 3.05 | 12 | 1.61 | 0.224 |
| Ratchet Wheel | Wheel | 13.76 | 56 | 2.76 | 0.234 |
| Crown Wheel | Wheel | 8.86 | 35 | 2.76 | 0.235 |
| Centre Wheel | Wheel | 12.48 | 80 | 2.76 | 0.151 |
| Centre Wheel | Pinion | 2.54 | 12 | 1.61 | 0.187 |
| Third Wheel | Wheel | 8.63 | 60 | 2.76 | 0.138 |
| Third Wheel | Pinion | 1.71 | 10 | 1.61 | 0.147 |
| Second Wheel | Wheel | 8.42 | 70 | 2.76 | 0.116 |
| Second Wheel | Pinion | 1.26 | 8 | 1.71 | 0.130 |
| Escape Wheel | Wheel | 6.34 | 15 | ||
| Escape Wheel | Pinion | 0.97 | 7 | 1.71 | 0.111 |
| Barrel | Gear | 17.3 | 88 | 2.76 | 0.191 |
| Balance Wheel | Bal | 14.05 | |||
| Hour Wheel | Gear | 8.73 | 40 | 2.76 | 0.204 |
It should be noted that the 'Module' in the table is the absolute module. Cutters are only available in certain modules, so I will take either the nearest cutter module size if the number is very close, or else I will round down to the nearest available size.
I also have the wheel and pinion dimensions plus tooth count for the 6498-2. Please leave a comment on the forum if this is of interest to anyone.
6498-1 Measurement Images
Below are images of the measurements I took of the 6498-1 movement using the microscope. It may however be easier to download them from here as the filename corresponds to the part being measured. Note that the measurement text is on the image itself, but is small and may only be visible when the photo is fully expanded.
Validating Measurement Accuracy
Measurement accuracy was verified in several ways. Firstly, some of the wheels were checked using a Mitutoyo 293-821-30 Digital Micrometer. Secondly, the centre distances were also measured using the AmScope and calculations performed to derive the diameters from the centre distance and number of teeth. Finally, dimensions were originally measured for the 6498-2 movement and were remeasured on a separate occasion for the 6498-1 movement (after recalibration of the scope). For all critical dimensions the three methods agreed within 0.07mm and in some cases the disagreement between measures was less than 0.005mm. Given the movement being examined was also used and noticeably worn, the agreement between calculation using centre distances and major diameter was quite remarkable.
Next Steps
Next I will likely use these measurements to recreate a CAD model of the 6498-1 movement. I will post the drawings for this in a future post.
