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):

bellpush_details1

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).

bellpush_details2

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.

bellpush_details4

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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.

halfway

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.

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Pint-size Printing Press

Here is my new toy essential concertina-making workshop tool (mug of tea is for scale):

adana1

adana2

It’s a little Adana High-Speed No.1 (also known as a HS1 or 3×2)  self-inking platen printing press, possibly dating back to the 1930s. I plan to use it to print decorative bellows papers and maker’s labels, and possibly also stationery and marketing materials – compliments slips, business cards, bookmarks, beer mats, etc. I could perhaps even take a few small commissions as a sideline (wedding invitations and the like!). The maximum size of forme you can fit in the bed is 3 3/4″ x 2 5/16″ (95 x 59mm), though obviously the paper can be a little bigger than that if the design has margins around the edge. I got it cheap because it looks a bit tatty and it’s missing several parts, many of which are no longer made.

Some of you are probably wondering at this point, “why bother?” I much prefer the appearance of Letterpress printing over modern laser or offset printing because the type makes a 3D impression in the paper (you can also use proper metallic gold or silver ink, which you can’t do with a laser). It’s almost certainly also the method that was used on original Victorian concertinas. Although there are still a handful of commercial letterpress printers around who would probably be willing to print my designs for a fee, I want to do the printing myself on a machine in my own workshop with plates I’ve designed and made myself, for the same reasons that I’m planning to make all the other parts of the concertina myself.

Luckily you can download the original user manual for free. It is ‘arrestingly written in a non-technical style.’

printing_made_easy

From these instructions I gather the way you use it is to place an arrangement of (missing) movable metal type and furniture called a forme in a (missing) chase or clamp it directly into the bed, use a (missing) hand roller to transfer some ink onto the (missing) inking disk, then move the handle most of the way down and back up to pick up some ink on the (missing) pair of inking rollers and transfer it onto the face of the type. Next you place your piece of paper or card on the (missing) tympan (which is held to the platen by the (missing) tympan clips), lining the bottom edge up against the (missing) lay gauge. Finally you push the handle all the way down to press the paper onto the type. When you release the handle the (missing) spring-loaded gripper finger pulls the paper off the type.

adana3

I’m not bothered about the fact that it didn’t come with any metal type, furniture, chases, quoins, etc. because I’m instead planning to make my own printing plates after designing them on a computer. I have a few different ideas for ways to make the plates. The most obvious one is to use sheets of photo-sensitive polymer that are designed for the purpose (I already have a UV exposure box for PCB photo-etching); the main drawback with them seems to be that they aren’t super robust and they have a limited shelf life; perhaps a few years at best. For one-off print runs that would be fine. I suppose it wouldn’t be too much of a hardship to make new polymer plates every couple of years.

Alternatively I could use the same techniques that I use for PCB etching to make etched brass printing plates (using spray-on photo-resist). If I can get that to work and produce clean sharp edges, brass is hard enough that the plates ought to last pretty much forever as long as I don’t abuse them. Another idea I’m considering is to cut brass or aluminium plates on the CNC milling machine with a small vee engraving bit. That ought to produce excellent clean results, although it would probably be quite a slow process and any sharp internal corners might require cutting by hand with a fine graver. It might not be trivial to convert the design into G-code either. I’ve even considered using the mill to engrave a positive “matrix” into a piece of aluminium and use that to cast plates in type metal. One potential advantage of this method is I could mill a single matrix for a bellows paper or logo design and then cast as many copies of it as will fit into the bed of the press.

I’ve had to buy a new pair of inking rollers because, although I probably could have made the mechanical parts, I don’t have any practical way of casting the special soft rubber compound onto them. Luckily they are still made, though a pair cost me more than I paid for the entire machine!

I’ve also ordered a cheap soft rubber hand roller made by “Speedball” (isn’t that slang for an illegal substance?) to ‘mill’ the ink (work it until it’s totally smooth) on a sheet of glass and transfer it to the inking disk.

The inking disk shouldn’t be too hard to make. I’m planning to cut it from 3mm engraver’s brass sheet with a scroll saw, round off the edges, and braze a steel hub to the bottom. I haven’t yet decided how to make the ratchet on the bottom that rotates it to evenly distribute the ink, but I have a couple of ideas. The simplest way seems to be a circle of holes drilled in another disk soldered/glued to the bottom. The point of the pawl is a bit worn, but interestingly it looks like it was made to be reversible when the first point wears out:

adana4

The tympan clips will be trivial to make from stainless steel sheet. The tympan/packing is a bit of mystery to me – it would probably be easy to make if I knew exactly what to make it from (it seems to be some combination of card and paper). The purpose of it is to provide a soft surface between the paper and the platen so that the type makes a visible impression in the paper and isn’t damaged by being forced hard against the metal platen. More research and probably experimentation needed.

The lay gauge will probably not be too difficult to make from a strip of brass or aluminium and some small screws and wing nuts, though I don’t fully understand the illustrations of it in the manual – it has various cutouts and holes in it that I haven’t yet worked out the purpose of.

The most difficult replacement part to make looks like being the gripper arm and gripper finger. This is the part that holds the paper against the tympan so that when you release the handle the paper doesn’t remain stuck to the type (if it did, the inking rollers would probably run over the back of it before you had a chance to manually remove it). It needs to be adjustable for different sizes and thicknesses of paper and there needs to be a long spring so it can easily be lifted up to replace the paper. I’ll also need to track down a nut that fits the shaft so that I can attach it to the machine (almost certainly won’t be metric; possibly 2BA or similar).

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Steppers

The stepper motors have arrived that I’m going to use to convert my Taig Micro-Mill to CNC.

steppers

It’s hard to explain how exciting this is. I’ve dreamed of owning a CNC machine tool for probably twenty years and yet somehow never got around to building one. I bought the Taig about a decade ago for another project with the intention of fitting it with DC motors and encoders and building my own servo controllers (in hindsight that was a bit of a silly idea considering steppers are relatively cheap and work OK on a machine as small as the Taig). I got as far as buying the motors, encoders, pulleys and toothed belts, and a large block of aluminium that I was going to machine the mountings from, before the project ground to a halt. There are some photos on my old website of the mill when I first received it. I have used it occasionally as a manual mill but what I really want is to be able to use it to machine complicated parts under automatic control.

The motors are bigger than I expected, and I went for the small end of the range people tend to put on this machine. There are a couple of different theories on stepper motor sizing for the Taig mill: 1. use small motors and keep everything well adjusted and lubricated so you don’t need lots of torque. 2. use big motors so you have lots of torque in reserve and it doesn’t matter so much if the leadscrews and slides get a bit stiff. The advantage of the smaller motor option, as well as lower cost (both in terms of the motors and the electronics to drive them), weight, power consumption, and heat generated, is that smaller motors have lower winding inductance and armature mass. Lower inductance means the torque doesn’t drop off as rapidly as the speed increases; lower mass means they can accelerate and decelerate quicker. I’ve read people advocating both paths and claiming to get better results with small/large stepper motors, but the smaller, nimbler option appealed to my sensibilities more (as somebody who is happy driving a small, light car with a 48Bhp engine).

 

 

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Swanky Switch

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

switch1

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):

buttonfeel

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):

switch2

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:

switch3

 

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:

 

 

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Clico Swiss Bench Shear

Here’s another lovely vintage tool; one that almost certainly will get used in my concertina production. It’s a small Swiss-made bench shear of a design that I have never seen before. One of the blades is a helical shape (that must have been pretty tricky to make!), and as you rotate it the shearing point slides along the fixed straight blade. The maker’s stamp appears to say ‘Clico’ but other than that I know very little about it. It’s quite small and very precisely made; I suspect it was probably intended for jewellery or clockmaking. It was a little bit pricey for a second hand tool (about the same as a new Chinese-made bench shear of more conventional design), but it’s such a lovely thing and works so well that I regard it as a bargain. As long as I don’t abuse it and hone the edges occasionally, it should outlast me. When I got it the blades were quite blunt but it still cut surprisingly well. I sharpened them last night and it now cuts really cleanly with more control and much less effort than a pair of hand shears.

shear1

The state the edges were in before sharpening:

shear2

The unclear maker’s stamp. I think it says “Clico”. The number doesn’t seem to refer to a patent (too short). Perhaps it is a serial number.

shear3

 

A brief video showing how it works (I would normally use two hands but one was holding the camera):

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Moore & Wright Spirit Blowpipe

I love old well-made tools. Recently I picked up a vintage Moore & Wright spirit blowpipe (I thought they only made measuring tools!). It’s a very nicely made brass wick burner with a pipe that allows you to blow a stream of air across the flame. This creates a very hot, concentrated flame that can be used for hard soldering. I may not actually use it in concertina production (I have several modern gas torches than are more convenient and powerful), but I thought readers might be interested to see it.

blowpipe1

blowpipe2

I haven’t quite got the hang of it yet; it’s much trickier to use than a gas torch. It also doesn’t seem to produce much heat. I suspect it would work much better on a jeweller’s charcoal soldering block because then any excess oxygen in the stream would make the charcoal burn and generate extra heat. I tried it with a piece of ordinary barbecue charcoal but it crackled and spat sparks at my face so I quickly gave up! I also suspect the meths I’m using may be too dilute to produce a really hot flame.

Here’s a video of me experimenting with it. As you can see the brass got hot enough to anneal it but I doubt it was hot enough for silver soldering.

Update: I spent a while this afternoon practising with the blowpipe and successfully made several silver-soldered brass joints on the refractory brick in the video above. The key seems to be to blow very lightly and consistently (using circular breathing technique) in subdued lighting so you can see the parts of the flame clearly, and at all costs don’t let the blue inner cone dwell on the work. It is far colder than the outer flame. It also helped to push out a bit more wick to get a slightly larger flame. The job took longer than it would have with a cheap butane gas pencil torch, but I got there in the end.

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Crafty Concertinas

I’m in the early stages of designing a logo for Holden Concertinas that will also be used as the maker’s label on my instruments, and I mentioned to a friend that I was considering including the phrase “Hand crafted in England.” I intended that phrase to put across concepts like small production quantities, hand assembly, care and attention to detail, tradition, built by an individual who really cares about the end product. In other words the opposite of “Mass produced on an assembly line in China.” My friend pointed out that hand crafted may be a controversial claim to make because I am intending to use CNC machine tools to produce some of the parts for my concertinas. I wondered whether I would be safe if I dropped the word hand, e.g. “Crafted with Care in England” or similar, though I’m uncertain if that is any better (and it doesn’t sound as good to my ear).

Since then I’ve been thinking about what the word craft and the term hand crafted mean to me and reading other opinions on the subject. I came across an interesting essay this morning about the ongoing debate over the meaning of the marketing term craft whisky (apparently there is such a thing – I had never heard of it) that made me question whether I can use the term craft at all when I am only just starting out in the business of musical instrument making. After all I certainly wouldn’t dare to call myself a master craftsman at such an early stage in the development of my new career, though neither would I say that I am completely inexperienced or lacking in craft skills (including the use of traditional hand tools).

In the case of my concertina-making operation I will be assembling all the parts and tuning and finishing the instruments by hand. A fair amount of skill will be required and little or no use of power tools, so I think this part of the process certainly ought to count as hand craft.

What about the parts that I will be assembling? Most of the screws and the action rivets I will probably buy ready-made because they are so generic that there would be little point to making them myself. The rest of the parts I intend to make in my own workshop, with the possible exception of laser-cut metal end plates (a machine powerful enough to do that kind of work is far outside my budget). The raw materials I’ll be making the parts from will be things like seasoned hardwood, brass sheet, spring steel strip, various kinds of tanned leather, card stock, wool felt, hide glue, shellac. That is to say, pretty much the same types of raw material that traditional concertina makers like Wheatstone and Lachenal used back in Victorian times (though I don’t intend to use bone, ivory or endangered woods, and may also use make some use of more modern materials and adhesives when appropriate). The tools I will be using to make the parts can be divided into four categories:

  1. Hand-powered tools that require a significant amount of manual skill to operate. Things like files, hand saws, hand planes, chisels, skiving knives. Many would include treadle-powered machines like my scroll saw in this category because they are also propelled solely by the muscles of the operator (incidentally, Geoffrey Crabb told me that his reed pan routing machine that was made by his great grandfather in the 1860s was originally treadle powered). Purists would argue that this is the only category of tool you can use if you want to describe your product as hand crafted. In some cases (e.g. chainsaw carving) I might even agree with them! 😉
  2. Externally-powered (typically by an electric motor) machine tools that require a significant amount of manual skill to operate. This includes things like manually controlled lathes and milling machines, drill presses, bench grinders, as well as woodworking machines like bandsaws and table saws, planer/thicknessers and drum sanders, even sewing machines. Non-purists will usually allow items made with this type of tool to still be called hand crafted – after all a skilled pair of hands was still required to twiddle the knobs or push the material through the machine in just the right way. It is an area of considerable debate, particularly in cases where the end result is indistinguishable from an item made entirely with hand tools.
  3. Hand-powered tools that require little skill to operate. This is mainly things like punch and die sets used in hand presses. You stick a piece of raw material into the tool, pull the handle, clunk, briefly inspect the part that pops out, and place it in a box if it looks satisfactory. Repeat the operation over and over until you have replenished your stockpile of that part. Anybody could be shown how to do it in five minutes. Personally I would describe parts made this way as hand made but possibly not hand crafted due to the lack of skill required. Obviously the design and manufacture of the die sets is another matter – that does require skill, and I intend to perform that part of the process myself too. Does that elevate the parts I’ll be making this way to the status of hand crafted? I don’t know.
  4. CNC machine tools. I intend to make extensive use of small CNC machine tools including a lathe, a wood cutting router, a milling machine, probably a laser cutter (though in that case I’ll have to send my designs out to a commercial machine shop), and possibly a surface grinder for profiling reed tongues (which I may need to build myself because commercial ones are rare, expensive and larger than I need). Wire EDM is also worth considering, specifically for cutting the reed frame slots. The main reason I’m planning to use CNC where it’s practical to do so is to reduce the amount of time I spend doing highly-repetitive machining operations that a robot can do faster and more consistently without getting tired or bored. I’m pretty sure this doesn’t count as hand crafted because there isn’t a human hand guiding or powering the tool as it cuts. Can it be called craft though? This brief article on the subject argues that it can. Considerable skill is certainly required to teach a machine to make a new part, and in many cases also to make jigs to hold the parts securely and accurately while they are being machined, but once that has been done you can pretty much load a piece of raw material into the machine, press ‘start’, and go off to do something else while it works its way through the list of instructions. Admittedly the kind of machine I’ll be able to afford will require a lot more manual tending (e.g. changing the tool bit part way through a job, or turning the workpiece over so that it can machine the other side) than the big machining centres used by commercial machine shops that can go all the way from bar stock to bucket of finished parts without human intervention. Most of my parts will also require some manual finishing-off, e.g. deburring edges, polishing (where externally visible), and tapping threads. The reeds in particular, arguably the most important parts in a concertina, will require a considerable amount of skilled hand work to finish them off after the components have been roughed to shape using the CNC milling machine (traditionally, this roughing out stage was usually done using punch and die sets in a fly press).

Enough rambling. I’d like to know what you think. Will my concertinas be crafted or merely made? Is it valid to describe them as hand crafted if some of the parts in them were machined by power tools and even robots?

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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.

bellpush_spring_1

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:

bellpush_spring_2

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.

 

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Email Subscriptions

By popular request (i.e. one person has asked for it!) I’ve added the ability to subscribe to the blog by email. Add your address using the subscribe box at the bottom of the right-hand menu and respond to the confirmation message, and you will be sent an automated message every time I post a new entry. Your email address won’t be used for any other purposes or shared with anybody else.

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