Cirkut Gear Selector
There seem to be continuing online questions about how to select the correct gears for Cirkut cameras, depending on focal length and focal distance.
By inspecting the scales on the bed of my Cirkut #8, I find that there is a simple straight-line logarithmic chart for each distance and all focal lengths.
Note that the x-axis scale is logarithmic and the y-axis is linear. However there is no need to worry about any of that. Choose your lens focal length (in inches) along the x-axis, then the y-axis shows the tooth count of the required gear at 25, 50 or 100 ft. I guess infinity uses the 100 ft setting. A larger PDF version is downloadable from the link below.
It would be a useful exercise to check the data for other Cirkut models, but I don't have those so leave it to others - who may hopefully to publish the results!
If you don't have a gear to use as a model, here is a basic drawing. Any small lathe will be suitable.
D1 - turn to 4.1mm diameter then form a screw thread #8 UNC over the full length.
L1 - 5.5mm
D2 - 6.3mm nominal, but should be a sliding fit inside the gear socket of your motor.
L2 - 7.3mm
D3 - 9.5mm in the original, but can also be any convenient larger diameter. The right face of this section positions the gear the correct distance below the motor.
L3 - 11.5mm in the original but see below
D4 - this is the gear itself, so the diameter will vary by tooth count. Use Module 0.8, PA20° (which is the default)
L4 - 3.75mm in the original.
L4 is the thickness of the actual gear blank, and the gear you buy or make may be different, typically thicker. 3 - 5mm thickness should be no problem, if more it is probably best to face it thinner. The shaft and gear will be made separately, so the shaft will have a projection to the left to fit the bore of the gear, typically 6mm. If the gear comes with a boss, the diameter of the boss will displace a good part of D3/L3. If there is no boss, be extra careful to get the gear truly square to the shaft when you attach it. In the original I think the shaft is a press fit to the gear, with the left end peened over to be sure. I find that with a good fit, soft solder or a modern adhesive are both fine - the gear is not highly stressed in service.
The key to getting the gear at the right position against the big toothed wheel of the tripod head is getting the total of L3+L4 correct. These dimensions are as original for my Cirkut #8 but I have another original Cirkut gear whose shaft was about 9.5mm longer before I cut it down to fit this camera, so it's clear that there is more than one standard size.
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