New makers… a suggestion quick release

[alex_holden]

11 hours ago, JimLucas said:

Meanwhile, a complaint I have with some "modern" concertinas is that they use modern-standard machine screws, where the threads have a much shallower pitch than those on the "old-fashioned" instruments.  That means many more turns of each screw when opening and closing the instrument.

 

Okay, nobody seems to make screws of any sort these days with such steep pitch.  But anyone with a screw-making machine could do so.  Decades ago, Steve Dickinson made some for me, but he can no longer take the time to be a parts-maker for others.  I actually had thoughts of asking a Russian friend if he could do it -- I could maybe pay for a thousand... at a time, if I could sell the entire first batch, -- but since the invasion of Ukraine, I'm unwilling to contact him, for fear that contact from an American would put him in danger.

 

The problem with using a custom extra-coarse thread like the vintage makers did is you would also need a matching tap for threading the nut plates in the bellows. Making such a thing is not easy, and although you can have custom taps made they are horribly expensive and they don't last forever. Ideally you would also want to have a matching die made for threading the bolts. While it is possible to single-point thread the bolts (if you have a screw cutting lathe, which I don't), it is much more finicky, time consuming, and error prone than using a die. If you went through all that, you would then have the same situation as we now have with vintage instruments: future owners and repairers would curse you for having made an instrument with non-standard threads that they can't easily buy taps and dies for. The other reason for using a modern pitch thread is it leads to a larger diameter core for a given major diameter, so the bolt is stronger and less likely to shear in use.

 

As the maker, I probably have to put the ends on and off more times while building the instrument than the owner ever will, and I don't find the extra turns to be a major issue.

[Richard Mellish]

Some old screw standards had coarse and fine versions, such as UNC/UNF and Whitworth/BSF. Is there no such coarse/fine choice with metric threads, and if not why not?

[alex_holden]

Just now, Richard Mellish said:

Some old screw standards had coarse and fine versions, such as UNC/UNF and Whitworth/BSF. Is there no such coarse/fine choice with metric threads, and if not why not?

 

Yes there is. The most commonly available pitch is metric coarse. Metric fine is unusual particularly at small diameters. The non-standard threads that Wheatstone and Lachenal etc. used are extra-coarse. As far as I'm aware there is no standard thread that is similar to them, regardless of metric or imperial. I use 8BA because it's about the same diameter as the vintage ones (2.2mm) but has a finer pitch of 59TPI vs about 44TPI. I believe most other modern makers use either M2.5x0.45 or (less common) M2.2x0.45, both of which have a pitch of about 56.5TPI.

[Chris Ghent]

A good reason to use a fine thread for the machine screws is to increase the purchase the screw has in the captive nut. A coarse thread might only get a couple of turns in the nut.