Adjustable Knife-Edge Cam Follower for the Pilkington & Gibbs Helio-Chronometer
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Allows for mechanical adjustments between the follower knife edge & alidade to compensate for wear & tolerance issues.
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- Physics & Astronomy
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Introduction
The original Pilkington & Gibbs Helio-Chronometer design does not have any adjustment provisions to compensate for any long-term wear or mechanical tolerance issues that might exist between the cam & the alidade knife-edge follower. This could result in small angular offsets that would need to be calibrated out of the sundial to accurately read time on the main hour plate. This is usually not a problem. However, if one prefers perfect geometric/mechanical tracking, this redesign addresses the issue by allowing the distance between the alidade follower & cam to be adjusted slightly by using a simple screw & Allen key, thereby cancelling out any unwanted time offsets.
NOTE: The new parts listed in the file section replace those listed in the original P&G design. The original design is available here: https://www.printables.com/model/1542266-working-replica-of-an-early-20th-century-pilkingto
Assembly instructions will be published at a later date. In the meantime, use the attached photos for reference & the P&G assembly instructions in the above link.
Additional Hardware Required for Assembly
(1x) M3-35mm Cap Head SS Hex screw;
(1x) M3 SS square nut;
(2x) 3mm dia. SS rod 41mm long (acts as guide rails).
How to Adjust the Offset
Rotate the month dial to EoT= 0 by choosing either one of the following dates:
April 15
June 13
September 1
December 25
Under perfect conditions, the middle index mark located at the base of the sight should exactly align with the 6 o'clock mark on the main hour plate. If not, then perform these additional steps:
Insert a long (> 85 mm) M3 Allen key into the hole located on the side of the hour plate (opposite the 12 noon mark), making sure that you are making proper contact with the follower screw head located on the alidade cam follower;
Turn the Allen key either clockwise or counter-clockwise so that the center mark located at the bottom of the Sight is perfectly aligned with the 6 o'clock mark on the main plate. The offset is now properly corrected.
References
M. Nadeau: Making a Working 3-D Printable Replica of the Pilkington & Gibbs Helio-Chronometer, BSS Bulletin, 38(i), 8-12 (March 2026).