Bellpush For Sale

I’ve finished the bellpush and put it up for sale on eBay.

All the previous posts about the project are here.


  • 2″ (51mm) wide, 2 ¼” (57mm) tall, 1 3/8″ (35mm) deep.
  • Hand cut stainless steel front plate.
  • Light green glow-in-the-dark vinyl membrane behind the fretwork.
  • Stainless steel button and front plate screws.
  • Varnished oak backing box.
  • Brass electrical screw terminals.
  • Heavy duty sterling silver contacts, suitable for electromagnetic bells.




Bellpush Final Details

The bellpush is almost finished. It just needs another two or three coats of exterior varnish, then I’ll put it up for sale (haven’t decided how yet – probably an eBay auction). Here are pictures of a few of the final details.

Deeply countersunk holes for three mounting screws (I thought two might be a little wobbly if the wall/door isn’t completely flat, particularly as there wasn’t room to space the screws evenly):


My maker’s mark carved on the back. This is a different thing to the Holden Concertinas logo (which isn’t on the bell push because I haven’t designed it yet). The HC logo will be the company brand and will be externally visible; my maker’s mark is my personal signature and is usually hidden away somewhere (I put it on most other things I make too).


A brass serial number plate (visible when you remove the top plate), rather crudely stamped with a worn set of letter punches. The letters stand for “Holden Concertinas Bell Push 1”. Also seen in this picture is a little drain slot so any rainwater that leaks into the switch recess can run out of the bottom.

First coat of exterior clear varnish (thinned down so it soaks into the grain), after sanding the bellows valleys because the bottoms of the trenches were a bit fluffy. The grain of the plain oak looks really pretty. I debated with myself for quite a while over whether to stain it darker to look more like a vintage rosewood concertina. In the end, after staining and varnishing a test piece, I decided against it because it looked a bit fake and the end grain stained much darker than the side grain.



Fiddly Fretwork

Because I made the bellpush fretwork before I started this blog, I thought I would take a step back and write a bit about how I did it. Here’s a picture of the finished item (remember it’s only 50mm wide):

Bell Push Top

What I didn’t do was to design it 1:1 with paper and a pencil. Given my limited artistic ability and the level of fine detail involved, using computer aided design software saved me a lot of time and almost certainly led to a better end result. I could zoom in and out, automatically turn wonky hand-drawn curves into nice smooth ones, repeatedly draw and erase (without leaving smudgy marks each time), undo, redo, tweak, retweak, and basically fiddle with the design for hours on end until I felt it was good enough to hit ‘print’. That’s my little secret: I’m not very good at freehand drawing (I wish I was), but if you’re patient enough CAD lets you keep on making hundreds of tiny incremental improvements until you finish up with something that looks pretty good. In theory also I could go from a vector drawing to a control program for a CNC tool like a laser cutter, which is something I want to try in the future, though this project was about learning to cut an end by hand.

The software I used is an open source vector drawing program called Inkscape. I’m using a snapshot of the current development version for two reasons: it has a couple of whizzy new features that make it easier to do this kind of design, and it finally runs natively on Mac OS X rather than via the rather clunky X11 interface. The two new features are both path effects: spiro spline and power stroke (stupid name). Spiro spline basically makes it easy to draw really smooth curves. Power stroke lets you draw variable width lines (you can later convert a power stroke into an outline path – i.e. two curves that define the edges of the power stroke line; unfortunately that’s a one-way conversion). Rather than me spend a few hundred words attempting to explain more clearly why the combination of these two features is really useful for drawing the scrolly shapes you find in a Victorian-style concertina end, take a look at this video. (Incidentally, he would have been better off using the Path->Union command at the end to combine multiple powerstrokes into a single path.)

It took me three attempts to come up with a design I was happy with. Here are the three versions side by side:

top1 top2 top3

I spent a lot of time studying pictures of vintage concertina ends between attempts 2 and 3: I could tell my version looked wrong but it took a lot of analysis before I figured out exactly why and how to fix it. Something you might notice if you look closely is that in the first two the right and left hand sides are almost exact mirror images of each other. In the third one there are many very small differences between the two sides – this asymmetry, I found, makes the design look far more organic.

If you carefully compare the final design to the photo above, you’ll spot where I made a mistake with the saw in one place. I carefully tweaked the design to work around the mistake and it almost looks like I did it on purpose now!

Hand piercing a stainless steel concertina-style bell push

I cut the end by hand with a jeweller’s piercing saw, following a paper template glued to a piece of 1mm stainless steel. I used stainless because the bell push needs to be weather resistant – metal concertina ends are more commonly made of nickel-silver AKA German silver, which is actually a type of brass and is much easier to cut, though it would quickly tarnish outdoors. The biggest mistake I made was putting wax polish on the paper – I thought it would lubricate the saw blade and save me having to keep manually waxing the blade. Bad idea. The sawing generated a very fine black dust which stuck to the waxy paper and smudged it so badly that eventually I couldn’t see the lines clearly (hence the mistake mentioned above, caused by carrying on cutting where I thought there was supposed to be a line). After that mistake I printed out and glued a new template on top of the smudged one – very tricky to get the two perfectly registered – but then I had lots of problems with the bottom, waxy layer coming unstuck from the metal when I was cutting the points of the scrolls, hence why many of the points are slightly misshapen.


Another problem I ran into was blunting and breaking 1mm drill bits because I centre-punched where I wanted the holes to go, which work-hardened the stainless enough to cause my HSS drill bits to rub and go blunt instead of cutting. I switched to solid carbide bits (actually re-sharpened PCB drilling bits from eBay, which are good and surprisingly cheap). They cut through the work-hardened stainless OK, but I broke several of them because they tend to snatch as they break through the back of the work. Carbide is so brittle and delicate that the bits just instantly snap when that happens. In hindsight a better solution might be: a. mark the drill points using a centre drill or a tri-cornered centre punch and a very light tap to avoid work hardening the material, b. drill on top of a sacrificial piece of brass or aluminium to reduce the likelihood of snatching on break-through. I could also try using good quality cobalt bits instead of carbide.

The final problem I encountered was that of blunting and breaking jeweller’s saw blades. I must have gone through two or three dozen of them. I’m talking about very fine blades here – the teeth are so small that you can barely see them with the naked eye. They need to be small so you can cut the tiny details and tight curves inside the piercings. I started out using a highly-regarded brand that I have had success with in the past when cutting silver and brass, but they turned out to be not hard enough for stainless – they tend to go dull in one or two strokes. I found another brand that stays usably sharp for a bit longer (perhaps two piercings), but it’s still extremely easy to snag the blade in the cut and snap them because they are so tiny and delicate. It’s also very hard to follow a line closely with a dull blade because it tends to want to drift off in one direction or the other depending on which side happens to be sharper. One trick I might try in future is cutting out the bulk of each piercing out with a thicker, more robust blade, then going back later and doing the inner points and other details with a fine blade.


Swanky Switch

I’ve spent what seems like an inordinate amount of time getting the bellpush switch right. This photo shows my first attempt:


The long contact strip is made from a roughly T shaped strip of 0.7mm brass. It’s bent into a zig-zag at one end to make it more flexible (I also had to file it narrower for the same reason). One of the problems I ran into was I initially didn’t recess the contacts deep enough to ensure the terminal screws can’t press against the underside of the top. The top of the flexible strip is slightly curved so that when the lip of the button pushes down on it, it flexes a little and ‘wipes’ the contacts against each other to (in theory) break through any oxide film that may have developed since the last time it was used. The contacts are made of short sections of flattened 4mm diameter sterling silver rod (for good corrosion resistance), silver-soldered onto the brass parts. The screw terminals are salvaged from a UK 13A mains plug, filed a bit narrower due to the limited space available. I did all the soldering with the Eclipse spirit blowpipe described in an earlier article. They are held down by M2.5 stainless steel machine screws, mated to square S.S. nuts morticed into the back of the box.

When I got to this stage I thought the switch was done. The button action felt nice and the contacts closed when I pressed it. Unfortunately there was a problem, as I discovered when I proudly showed it off to my friend Juliet. She pressed it normally a couple of times and it worked fine, then she tried pressing it really gently. It didn’t work. She tried the same thing a few more times with intermittent results and proclaimed it faulty.

It was a user interface problem: press the button down firmly all the way to the bottom and it worked fine. Press it very gently and it was possible to feel the slight increase in resistance as the contacts began to close and stop pressing too soon. Result: the bell doesn’t sound, and you might not realise if you were outside and the bell was inside. I suppose some people seeing such an ornate bellpush might think that it looks delicate and press the button gently in fear of damaging it (in fact you’ve probably got more chance of breaking your finger than the button). This graph illustrates the problem (figures are estimates):


The problem is that first step when the button lip touched the flexible contact strip and the switch began to close. If you were pressing gently enough, it (wrongly) felt like that was the bottom of the button travel when you actually needed to press a tiny bit harder to close the contacts.

I won’t go through the list of ways I tried to solve this problem. I now have quite a collection of discarded springs! What I eventually settled on was a second helical torsion spring attached to the top of the flexible contact that applies gradually increasing pressure to it over the  full length of the button travel. The result is that the switch closes smoothly without any detectable step increase in force, and the contacts are fully closed at about 80% of full travel. If you press the button firmly enough, it bottoms out against the top of the contact strip and wipes the contacts as originally intended.

I also had to remake the first spring with more and bigger coils to weaken it (I could also have used thinner wire but I didn’t have any in stock), because the combination of the two springs made the button force uncomfortably high. I didn’t want to remove the first spring and just use the second one to return the button because it’s set such that the pressure on it is zero at the top of the button travel so as to ensure the contacts release properly, which means the button wouldn’t return to the top as cleanly with that spring alone.

Here’s a cute picture of the little bracket I made to attach the new spring. I filed it from one of the brass 13A plug pins that I got the screw terminals from and silver-soldered it onto the contact end of the flexible strip (that’s a 0.8mm diameter hole):


This photo shows the final setup. The brass peg to the left of the flexible contact (a screw with the head filed down) is there to ensure the torsion spring can’t swing to the left and disengage from under the button:



Now that I had a working bellpush, I wanted to make a little video to show it off. I don’t have an electric bell here and the continuity buzzer in my multimeter doesn’t sound impressive enough, so I hooked it up to something else instead:




Bellpush Spring

Not much progress on the bellpush over the past week because most of my attention has been focussed on another non-concertina-related project. Yesterday was a day off though, so in the evening I managed to stop thinking about the other project for long enough to tackle the problem of the spring that provides the button resistance.

My original plan had been to use a single strip of brass as both the button spring and one of the switch conductors. This turned out to be impractical though because the button has a vertical travel of 5mm and there isn’t enough room inside the box (once you’ve allowed room for mounting screws and electrical screw terminals and things) for a really long spring strip. A short strip probably wouldn’t be able to flex by 5mm without permanently deforming and even if it could, it would likely result in fatigue failure before long. I experimented with folding a brass strip back and forth in a zig zag to increase its effective length, but it looked like it would take up far too much space and probably still wouldn’t work very well.

So the solution I eventually came up with was to separate the two functions. Make a concertina-style spring from coiled phosphor bronze wire with 5mm travel, and a separate brass strip switch that only has to move by 1mm or less at the bottom of the button travel. I haven’t yet made the switch but I have made the spring. It took me four attempts before I developed a shape I was happy with, using a hole in a block of scrap wood as a test rig. It has more turns than a standard concertina spring because of the large amount of movement relative to its length.


Here it is installed in a recess chiselled into the back box. It’s in a slightly weird location because of where I intend to put the mounting screws and switch contacts:


I won’t bother posting a photo with the top installed because it doesn’t look any different on the outside! The action feels pretty nice though.



Bellpush Backbox

I had a day off yesterday, so I managed to make a fair bit of progress on the bellpush. I made the captive nut plates in the morning, then in the afternoon and evening I made the backbox. I decided to make it from a solid chunk of seasoned oak for good weather resistance. My first job after planing the top surface smooth was to spot through the locations of  the two top mounting screws the same size as the clearance holes in the top (i.e. 2.5mm). In order to get them exactly the right distance apart, I drilled the first hole and stuck the shank of a spare drill bit in it while I drilled the second one. For this kind of delicate wood drilling I like to use a hand cranked drill because it gives you a much better feel for the amount of pressure and torque you’re applying to the bit than an electric drill does.


After drilling the pilot holes, I used a screw inserted through the nut plate to locate it in exactly the right position for chiselling it into the surface of the block.


It’s trickier than it looks to inset them neatly. The second one is less squiffy than the first! I also must have mis-calculated the width of the nut plates because they were supposed to end up flush with the outside of the box, but there is actually about a 1mm step in. At least both the tapped holes are in exactly the right place, which was the most important thing.bellpushbackbox3


Next I drilled a small diameter pilot hole in the middle of the button hole and used it to guide a sharp flat boring bit to cut a slightly-oversize recess for the large diameter lip of the button (a forstner bit would probably be better but I haven’t got one of those and it worked well enough). The depth of this recess will set how far the button can be pressed in (minus the thickness of the spring).




Finally I opened up the rest of the pilot hole to slightly over the diameter of the bottom part of the button:


I had to chamfer the bottom of the button a little to get it to slide smoothly in the hole (should really have thought of that when I was turning the sleeve).bellpushbackbox6

Next I sawed the block to the same shape as the top using my new gent’s saw:


After a bit of planing to smooth off the saw marks and get it down to the exact size, I thought it looked a bit plain:


So I scribed three parallel lines near the back of the box: bellpushbackbox9

And carved some fake bellows with my Ashley Iles Vee gouge:bellpushbackbox10


The plastic film behind the fretwork is a tasteful pale green in daylight:


But at night it glows in the dark, which looks really cool if I do say so myself! 😉 (Sadly, it only glows this brightly for a few minutes after charging it up by shining an electric light at it.)bellpushbackbox11


More Captive Nut Plates


Following my earlier, unsatisfactory, attempt at making a pair of captive nut plates for the bell push by soldering stainless steel nuts onto thin brass sheet, I bought a set of M2.5 taps and a piece of much thicker (3mm) brass sheet for my second try.

I stained the brass with a permanent marker to make the lines show up more clearly, and marked it out using a steel ruler and scriber under magnification:



I also upgraded from a cheap, dull wood-cutting countersink bit to a small good quality (Dormer) HSS three flute metal-cutting countersink. The resulting countersunk holes for the wood screws that will hold the plates to the back box are way smoother, cleaner and more accurate as a result. I tapped the M2.5 holes by hand using Trefolex tapping compound (I bought that small pot at a Model Engineering exhibition about 20 years ago and it’s still half full!).



A brief aside about the heads of the visible machine screws/end bolts that hold the top on. This is what the brass bolt heads look like on my antique Lachenal:




It turns out that head style is called a slotted fillister (I’d not heard that word before I started searching through screw catalogues, and it isn’t in my computer’s spell checking dictionary). I couldn’t find any suitable ready-made M2.5 stainless steel slotted fillister head screws for sale (I much prefer the appearance of slotted heads for this application), so I instead got some cheese-head screws and domed and polished them in the lathe. The next photo shows them before and after modification. Not quite right but I think they look reasonably good.



The final photo shows the two finished nut plates. I’m far happier with how these turned out than I was with my first attempt. The brass wood screws are bigger than necessary, but I happened to have a packet of them sitting around and there is enough room to use them. I don’t think there’s much risk of them pulling out of the oak!



Turning Acetal

Tonight I had my first experience of turning acetal (often known by the trademarked name Delrin). It’s an engineering thermoplastic that is relatively expensive but is designed to machine well. I’ve turned other kinds of plastic in the past and usually struggled to get a good accurate smooth finish – they are usually soft and gummy and don’t cut cleanly. Acetal in comparison was a joy to use. With a sharp HSS tool and a high spindle speed, it cuts almost like it’s not there and leaves a lovely smooth finish straight off the tool. You can even take really fine cuts without it rubbing and melting. Lovely!


The part I was making was the sleeve for the bottom part of the bell push button. It will act as an electrical insulator to isolate the switch contacts from the metal part of the button. It also needs to slide smoothly into a hole bored in the wooden backing box underneath the contacts.

Because I have read that most types of glue don’t stick well to acetal, I designed the interface between the two parts of the component in a slightly unusual way. The stainless steel pin was slightly flared towards the end (actually caused by deflection from cutting forces when I turned it, but I expected this to happen and deliberately didn’t do anything to prevent or correct it). Also, the hole in the acetal was drilled 0.5mm larger than the diameter of the pin for roughly the bottom 90% of its length. The combination of the two produced a small tapered gap between the two which, when filled with epoxy resin, should act as a mechanical fixing that will prevent them separating even if the glue doesn’t bond to the plastic at all.


The finished two-part button:



Bellpush Button

Some photos of turning the button for the concertina bellpush this evening. The externally visible dimensions are copied from the metal buttons of my Lachenal English. Underneath the top it’s totally different because it won’t have a conventional lever action (so no need for a cross hole, but it does need a larger diameter lip to stop it coming too far out of the hole in the top). I turned it from 10mm stainless steel rod using a brazed TCT tool bit on my manual Taig lathe.








Captive Nut Plates

One of my intentions for this blog is to document techniques that I learn don’t work as well as those that do. Here’s one of my ideas that didn’t go as planned.

I decided to use a pair of M2.5 stainless screws for the end bolts of the bell push (I bought them with slotted cheese heads, then re-shaped them a little and polished them in the lathe). M2.5 was the closest modern metric equivalent to the size of the end bolts on my antique Lachenal.

This meant I needed to make a pair of little metal captive nut plates to screw to the wooden back box with an M2.5 threaded hole in each one. Unfortunately I don’t have any M2.5 taps (M2 and M3 but not M2.5) to thread the hole with, so I thought I would instead try silver-soldering commercially made square stainless steel nuts onto thin brass plates.

Bad idea. The solder wicked into the threads of the nuts, pulled them out of position due to capillary action, and generally made a bit of a mess. I could probably clean the solder out of the nuts – if I had an M2.5 tap. But if I have to buy a tap anyway then I might as well buy a full set of them and then I can just make one-piece nut plates from thicker brass and not have to mess about with soldering and cleaning the flux and oxide off afterwards.