donut holes and lever arms

[wunks]

Would someone please explain to me the function of the  "Center", "Thumb" or "Donut" Hole in the sound board.  I can't seem to wrap my brain around it (although both seem to consist of empty space). 

Also, how are the various lengths of the lever arms and fulcrum points set up to achieve uniform action response? 

Edited February 17, 2019 by wunks

[alex_holden]

You mean the big hole in the middle of the reed pan? You put your thumb through it and pull to remove the pan from the bellows frame. A concertina doesn't have a 'sound board' as I understand it.

 

The way I design the action levers is (in CAD) I draw a straight line from the button centre to the pad hole centre, then I place the pivot post at some ratio along that line, typically 1:2. That is to say, the distance from the button to the pivot is half that from the pivot to the pad; this means the pad lifts to double the height the button depresses. Sometimes I have to move a post to avoid an obstacle, if so I try to maintain the ratio as closely as possible. I then add whatever bends etc. are necessary to thread the levers around other buttons etc. Then I add a fixed amount extra to each end of the lever (you wouldn't want it to end right in the centre of the pad). Finally I ask the CAD program to calculate for me the two lengths either side of the pivot point (it does the hard work of adding up the curved and straight segments for me). After I've made the levers I print out the drawing of the action board and manually bend the levers to match the drawing.

 

 

I have seen instruments with other action ratios, e.g. 1:1 or 2:3. 1:2 is at the high end and results in a nice fast action, but it can be difficult to make work with very short levers because the distance between the pivot post and the button, where you have to attach the spring, gets really small.

[wunks]

Aha, There's a ratio!  Thanks Alex. 

 

With regards to the "thumb Hole" I ask because I noticed it is absent in many of the photos of the workings of modern instruments and I had assumed it must have some sort of acoustic function.  It seems like an awful lot of wasted space when a simple pan head screw or some such would do the trick.  This is of interest to me because on both my duets there is space for a few more buttons without disturbing the pattern or straying into the fretwork but no extra internal space for reed assemblies.

[alex_holden]

2 minutes ago, wunks said:

With regards to the "thumb Hole" I ask because I noticed it is absent in many of the photos of the workings of modern instruments and I had assumed it must have some sort of acoustic function.  It seems like an awful lot of wasted space when a simple pan head screw or some such would do the trick.  This is of interest to me because on both my duets there is space for a few more buttons without disturbing the pattern or straying into the fretwork but no extra internal space for reed assemblies.

 

Usually the thumb hole is beneath the keyboard array, where it's tricky to put chambers because you can't fit a pad hole over them. You'll probably need to make a drawing showing the action board overlaying the reed pan to figure out whether it's possible to add any more buttons.

[wunks]

1 hour ago, alex_holden said:

 

Usually the thumb hole is beneath the keyboard array, where it's tricky to put chambers because you can't fit a pad hole over them. You'll probably need to make a drawing showing the action board overlaying the reed pan to figure out whether it's possible to add any more buttons.

 

Yah.  These are both Jeff duets which have lots of variation outside the core rows to begin with and both have significant differences between them.  The objective is to:

-bring them into fingering compliance with each other (as far as possible, one is 6.25" and the other 8.5" and so will have a greater low end range ).

-bring them both into compliance with a "standard version" of the pattern ( in case by some slim chance I happen across another box).

-Add a small number of notes to the low range of each,  mainly with a cluster of thumb keys (freeing up the fingers for other use.  I use my strap over the wrist on the left hand and will eliminate the palm rest).

I'm exploring the possibilities and will have a complete plan before discussing this with a maker.  

Edited February 17, 2019 by wunks
spacing

[wunks]

On 2/17/2019 at 7:51 AM, alex_holden said:

 

Usually the thumb hole is beneath the keyboard array, where it's tricky to put chambers because you can't fit a pad hole over them. You'll probably need to make a drawing showing the action board overlaying the reed pan to figure out whether it's possible to add any more buttons.

 

The Jeffries button pattern is more like an anglo with a fan shape towards the top.  The "thumb hole" is not beneath it for the most part.  here's some pic's. to show Wheatstone's attempt to squeeze a Jeff duet into their radial reed pan.  The levers are flat and tapered 'though you can't see it in the photo.  The blank chambers should be easily filled and there is button space close by, but I still need to get a couple of extra notes in there to comply with a 58 key standard.  There's a button dropped out at the top in the photo.  I replaced it after the photo op.