During the extended COVID break here in Melbourne I have been twiddling my thumbs on various overdue tasks on the old Macson (1946 17") and in order to remove the carriage/crossfeed drive shaft I needed to remove the QC gearbox. What started as 'just a clean-up while I had it out' has turned into a bit more detailed teardown / clean than planned.
The Macson came with a damaged QC gearbox ratio selector knob - the bakelite end was completely missing leaving a steel centre stub that is fitted to the sprung loaded pin with an off-centre tapered pin. The knob belongs to the gear selector seen in the foreground here.
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I didn't have the presence of mind to take a photo of the steel piece before fitting my substitute knob to it but that piece is shown in this picture I copied from an eBay ad.
It is a small steel bar that had bakelite formed around it and this was anchored in a shallow machined groove.
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I knocked up a copy of the knob on the 3D printer - it is made up of two main sections with a split collar arrangement inserted in the lower section to clamp into the groove. This required a bit of faffing around as there is only about 6/7mm of the steel section in the knob and given the tapered pin was fitted off-centre it seemed less fiddling than trying to make a different steel piece.
The replacement was printed in black PLA that has been sanded and given a bit of polish and it does match an original fairly well (original bakelite on the right) so I am very happy with the result.

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In the unlikely event someone else needs to make one of these I am happy to share the .stl files.
Attached Images




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