Tools – Page 3 – Holden Concertinas

Reed Prototypes Part 2: Tongue

Reeds, Techniques, Tools / August 3, 2016

The way a free reed works is that a tongue made from a springy material (usually spring steel or brass) is made to oscillate through a close-fitting window in a metal frame by the flow of air through the reed. Each time the tongue passes through the tight part of the frame it briefly interrupts the air flow. This regular chopping-up of the air flow produces a tone with a significant harmonic content (it’s a long way from being a pure sine wave).

I’m using hardened spring steel for my reeds (IIRC it was 0.8mm thick on this size of reed), which I found practically impossible to cut with hand shears, so I bought an old bench shear on eBay. I got it cheap because it was seized up with rust and the blades were blunt and dented, but it is a nice heavy-duty machine:

After restoration, it works really well (though it would be nice to have an extension tube on the handle):

I need to come up with a better way of cutting the strips consistently to the right width. To complicate matters, they are slightly tapered. Initially I scribed them and lined them up under the blade by eye, which worked better than I expected but was rather fiddly and time-consuming.

The shearing action bent the tongue slightly so I straightened it before doing any more work on it:

Next I cleaned up the edges by draw-filing while it was held in a toolmakers vice:

It’s important to make the tongue a very close fit in the frame, but not so tight that it’s prone to catching if the reed pan expands and presses on the sides of the frame:

A low-power back-lit stereo microscope is a big help with fitting the tongue to the frame. Although the gap looks off-centre in this picture, that’s because you’re only seeing the view from the right eyepiece.

In a concertina, the tongues are usually not a consistent thickness along their length: they are profiled so as to bring the pitch up or down and to balance the volume of the reeds across the range of the instrument. Because I don’t fully understand all the parameters yet, I decided to start out by copying the profiles of the reeds in an antique instrument. I took the tongue I was copying out of its frame and measured it in several places with a point micrometer (I found it wasn’t difficult to put it back in the same place, and it still produced the same pitch afterwards).

I did the profiling by hand using a triangular Bahco saw file and an Eclipse hand vice, on top of an oak block with a shallow step cut into it. You can tell roughly what pitch you are at by ‘pinging’ it. I found, at least with this size of reed, that it is very easy to take a hair too much thickness off the belly area and the pitch suddenly drops by a couple of semitones. You can bring it back up by taking a lot of thickness off the tip, but then you have a weak reed that sounds slightly odd.

After clamping the profiled reed into the frame, you have an extra bit of tongue sticking out of the back of the reed that needs to be removed (you deliberately shear the tongue too long so you have something to grip while profiling and fitting it). Because it is hardened steel, the extra bit is very easy to break off, and the fact that the clamp doesn’t quite reach the end of the frame means that the sharp stub doesn’t stick out significantly past the end of the reed:

Here I am doing the initial rough-tuning of the reed before trying it in the instrument. Note that removing some metal from the belly (using a 600 grit diamond needle file) caused the set of the tongue to alter, i.e. the tip bent down slightly. This caused the reed to choke the second time I tried to sound it. There needs to be a slight gap between the tongue and frame when it’s at rest or no air will flow through it and it won’t start oscillating by itself.

From left to right, we have the original antique Lachenal reed I was copying, my first working reed (using the best of the aluminium frames), and my first brass-framed reed:

My first brass reed in the instrument:

The profiling of this first reed isn’t a perfect copy of the original: rather than being a smooth curve from the belly up to the tip, the profile curves up too sharply and then plateaus before the tip. The effect of this is that although the pitch is right and the dynamic range seems OK, the tone has less upper harmonics. When I compare it in the instrument to the original reed next to it, it sounds ‘softer’ with less of the piercing ringing overtones of the original. I suspect this is because most of the bending action is happening near the clamp rather than spread out along the full length of the tongue. Something to work on improving in my next prototype!

Reed Prototypes Part 1: Frame and Clamp

Reeds, Techniques, Toolmaking, Tools / August 2, 2016

Recently I made my first prototype concertina reeds. There’s a lot to write about so I’m going to divide it into two articles, this one will cover the frames and clamps, and the next one will cover the tongues.

My plan was to make a drop-in replacement for one of the low ‘B’ reeds in my vintage Lachenal English. I think this instrument was probably one of Lachenal’s higher-end ‘broad scale’ models, because I also have another steel-reeded Lachenal that in many cases has narrower reed tongues in the same pitch. I’m not totally sure why the narrow scale models existed or were originally cheaper than the broad scale equivalent (the extra metal and labour is fairly trivial). They were probably quieter at the top end of the dynamic range, which might make them better for a student instrument.

I understand Wheatstone’s earliest reeds were made totally by hand, piercing them with a jewellers’ saw and cleaning them up with files. This must have been very labour-intensive, highly-skilled work, and prone to inconsistency. At some point fairly early on, perhaps when Louis Lachenal was hired to mechanise production(?), they changed to using fly-presses and dies to punch out the reed frames. This was much faster, worked well, and the presses could be operated by relatively unskilled workers, but the disadvantage is that precision dies are very expensive to make. To save on tooling costs, instead of making a different set of dies for every pitch of reed, they made do with a handful of sizes and made up for the gaps between them through careful tongue profiling. Until relatively recently, the need to invest in a set of press tooling was a significant barrier to entry for new reedmakers.

Enter CNC machining. I understand other makers have successfully used laser cutting or possibly wire EDM, but I have my own small CNC milling machine so that is the process I am going to use. It is fairly slow (certainly compared to a a press tool), but it works pretty well and I hope to get to the stage where I can load in enough brass for half an instrument worth of reed frames, and set it going with minimal supervision while I work on another task. As well as cutting out the shapes of the frames and clamps, it can also cut the vent slots (albeit with filleted corners), drill the clamp holes, engrave the note labels and a logo, counterbore the clamp screw holes, and even chamfer the top edges of the frame so they fit nicely into the dovetailed slots of the reed pan. Initially I’m planning to copy all the dimensions of my prototype reeds from the Lachenal instrument, but in future when I understand the design parameters better I will be able to make frames that are the optimal size for each pitch.

My first attempts at milling frames were using scrap aluminium. It took me quite a few failed attempts before I got one that seemed pretty good (the antique Lachenal frame I was copying is on the right):

Next I moved onto brass prototypes, immediately running into problems with it cutting very badly and breaking 1/16″ end mills:

A microscope view of an end mill with clogged flutes, from a run that I aborted before it snapped:

I think the reason for my problem was that the chips weren’t clearing from the slots properly so on subsequent passes they were getting re-cut and generating a lot of heat. I experimented with a lot of parameters, but basically what worked was making the depth of cut shallower, increasing the spindle speed to 10K RPM (the maximum my machine’s spindle can handle), significantly increasing the feed rate (to make bigger chips), and adding a compressed air blast to blow the chips away. I also changed from two flute HSS bits to three flute cobalt bits, though I’m not certain that helped with the chip clearance (it did allow me to increase the feed rate a bit more). It also proved necessary to make some proper mechanical clamps to hold the plate to the spoil board, because double-sided tape wasn’t holding it securely enough:

Here is a (21 minute long) video of the process of milling a single reed shoe prototype. Don’t bother watching the whole thing unless you’re really fascinated! This isn’t quite the final program: I subsequently altered the bevelling operation slightly so that the frame wedges more securely into the reed pan.

The CNC program includes small tabs that prevent the parts coming loose during machining. Afterwards these need to be manually cut. I found that it was possible to break them out with a small chisel but it left rough stubs that I then had to clean up with a file, so I changed to cutting them with a jeweller’s saw:

Because I cut the vents using a 1/16″ end mill, this leaves 1/32″ radius fillets in the corners, which should ideally be dead sharp. I’ve been manually cleaning these up using a fine square needle file with one edge ground smooth. I put the reed frame over the small square hole in my bench peg (see previous photo), hold the safe face of the file flat against the end of the vent, and carefully file sideways into the corner until it’s as sharp as possible without leaving a nick:

The clamp screws I’m using are a bit smaller than the originals; they are M1.6, stainless steel, with 2.5mm diameter allen heads:

In my testing they are strong enough for the purpose and take up less space than the originals, and the finer pitch and allen heads make them easier to tighten and loosen without damaging them.

When tapping the clamp screw holes in the frame, it’s very important to keep the tap perpendicular to the hole. After researching tapping machines and complicated guides, I came up with this simple method that works surprisingly well (though I still managed to break a tap the first time I tried to tap one of the brass shoes!):

I added counterbores to the holes in the clamps because it was easy to do and significantly reduces the height of the reed without weakening the clamping ability. It also improves the accuracy of the location slightly. Because I was already using an engraving operation for the note labels, I added a simple brand to the clamp (HC=Holden Concertinas):

Because the screws start out a couple of mm too long, I put them in the frame and grind them almost flush with the bottom of the frame, then finish them off with emery paper on a sheet of glass:

One of the defining characteristics of traditional concertina reed shoes is that the underside of the vent is relieved (i.e. the bottom of the slot is slightly wider than the top). My current understanding is that this allows the reed to work properly even at very low bellows pressures, i.e. it enables you to play quietly if you want to. It also has an effect on the tone. I’m not doing this on the milling machine because there are good reasons to cut them out from the top and it would be a bit tricky to turn them over and accurately register them for an extra operation. Instead, I made a special jig that allows me to file the vent to a consistent angle using a flat needle file with two safe narrow edges.

The clamp part of the filing jig started out as an old war-surplus hand vice with damaged jaws:

I trued up the jaws and modified the profile of the front jaw so that there is room for the file to tilt down below the level of the back jaw:

Next I added an adjustable brass frame and a PTFE roller to guide the file, as shown in the following video. I align the top edge of the vent with the top of the back jaw, paint the inside of the vent with a black marker pen, and file until I’ve almost but not quite removed all the ink.

Here we have my first brass reed frame in my Lachenal reed pan. You can see how much lower-profile the new screw heads are: I think this might help with air flow inside the reed chambers. It took several prototypes before I was totally happy with the tightness of the fit in the dovetailed slot. There is a small area around the clamp that isn’t fully bevelled, giving a nice friction fit without compressing the sides of the frame adjacent to the vent.

Tuning Bellows

Techniques, Toolmaking, Tools / June 25, 2016

Recently I’ve been working on making myself a new set of tuning bellows. The first tool I made towards this goal was a new craft knife for cutting leather and card. I ground it from a piece of HSS machine hacksaw blade, because I had read that the steel is more wear-resistant than a simple carbon steel, and I wanted to test the theory.

I’m not convinced it holds an edge better than a well-tempered O1 blade, however it does work pretty well. Unfortunately the spalted beech scales (glued on with epoxy resin) popped off after a couple of days. It would have been better to have riveted them on, however drilling holes in hardened HSS is easier said than done. For the time being I have just bound them on with a length of thick elastic.

On a roll with the knife-making, I next made myself a leather skiving/paring knife from the same materials as the craft knife:

This video shows how I use it:

I do of course also have a Schärf-Fix 2000 skiving machine, which I wrote about previously:

The Schärf-Fix is good for long strips, whereas the knife is better for skiving small pieces (particularly gussets) and odd bits here and there. Both tools are tricky to use, and I wasted a frustrating amount of leather while getting to grips with them. The Schärf-Fix has a tendency while edge-skiving a long strip to suddenly dig in and cut a chunk out of the strip. The key is that the blades have to be absurdly sharp, because the thin leather tends to stretch badly if you have to use any significant force to pull it through the machine. I need to spend some more time figuring out a way to resharpen the disposable blades (stropping didn’t help much), or I’ll end up going through at least a couple of them per concertina.

I used my Käfer Dial Thickness Gauge to make sure I was paring it consistently:

I needed to make end frames for my bellows, so I first needed a way to accurately make the corner reinforcing blocks. Here’s the jig I came up with. There’s a bit of a knack to using it, but the results aren’t bad:

And here’s one of my new corner blocks next to an antique Lachenal one:

I cut the sides of my bellows frames from 9mm plywood using my Nobex Proman 110 mitre saw, and glued them together with hot hide glue:

After a bit of shaping, I checked that they fit in the scrap Jeffries bellows I was copying my dimensions from:

Next came the bellows mould. This proved quite a large sub-project in its own right. It has six forms (one of which is split in two to make it possible to remove the forms), screwed to a hexagonal core, suspended from a stand. Making the forms was the hardest part. I started by gluing blocks of pine to strips of plywood, with the grain running across the form, being careful to avoid including any large knots:

Then I used the bandsaw with the table tilted over at 45 degrees to relieve the under-sides of the forms:

Then I used the CNC milling machine to cut the valleys into the top sides of the forms (video sped way up: it actually took about an hour to machine each form):

After I had spent days making all six forms, I laid them out next to each other and realised I had made a silly mistake: five of them were spaced wrong, and in fact all of them were pitched slightly too tight to fit comfortably inside the Jeffries bellows:

I could have tried to unglue the blocks and glue them onto new plywood strips with the correct spacing, but I decided it was easier just to start again and remake them all. By the end, I was getting really tired of the noise the milling machine made as it cut the valleys, not to mention the dust everywhere!

I made the core of the bellows mould by mitring the edges of six pine boards on the bandsaw and gluing them together. I deliberately made it slightly oversize, then hand planed it to final shape/size (a good idea as it turned out slightly wonky, plus I wasn’t certain exactly how big it needed to be until I tried assembling it inside the Jeffries bellows):

The stand is a simple affair with the vertical ends roughly dovetailed to the base. A nice feature of this style of core is that if you turn the central bar one way up, it presents the sides uppermost, and if you turn it the other way up, it presents the corners instead. This picture also demonstrates that the scrap Jeffries bellows fit on the mould:

Finally time to start making the bellows! I cut the 108 individual cards out by hand, using a template I cut from a piece of scrap aluminium to match the shape of a card taken from the Jeffries bellows:

I hinged pairs of them together using strips of fine-woven linen cut on the bias and bookbinders’ maize paste (very similar to wheat starch paste but supplied pre-cooked and with some anti-fungal stuff mixed in):

Then I hinged the pairs together into six strips. Note that the hinges are both on what will become the inside of the bellows, and the valley hinges have to be pasted on with the hinge partly closed or they will tend to tear themselves apart when they close. There may be a less fiddly way to do this but it seemed to work well enough.

After the paste had initially dried, I noticed that the cards had all warped a bit, so I pressed them all tightly in a big wooden clamp for a couple of days (forgot to take a photo), which helped to flatten them out again.

I tied the strips of cards onto the mould with string, then hinged the corners together with more bias-cut linen, though at this point I switched to using hot rabbit-skin glue. It’s messier and more difficult to work with than paste, but in my tests it was the strongest of all the glues I tried (slightly stronger even than PVA wood glue, and significantly more flexible when dry).

I made a simple press to clamp the bellows shut, and whenever I had to let the glue dry before the next stage, I took the bellows off the mould and transferred them to the press. If I hadn’t, they would have dried in the fully-open position and possibly torn apart when I tried to force them closed. It’s also necessary to periodically take the bellows out of the press and exercise them to avoid them drying fully shut.

Next I glued on the valley leather strips. I used goatskin for all the leather on the bellows. I pared the valleys down to about 0.65mm and skived the edges for cosmetic reasons. They are simple rectangles rather than butterflies because that’s how they were on the Jeffries bellows I was copying:

Next the gussets:

Here’s a video of me gluing a couple of gussets on:

The top and end runs. In hindsight the end runs would have worked better if they were both narrower and thinner, and perhaps I need to work on my technique for gluing them on, because I was unable to get them to go round the corners without creasing, which caused the end sets of gussets to be stiffer than the rest. It probably didn’t help that I made the cards all the same size (the end ones probably should have been slightly taller because of the inset):

Pressing it all together:

To make them look a bit prettier, I made bellows papers from decoupage paper:

I screwed a plain piece of plywood onto the bottom end, with a sheet of black “funky foam” (closed cell EVA foam sold in thin sheets for craft purposes) as a gasket, and a couple of pieces of scrap lead to pull the bellows open with a consistent amount of force:

The top board has a reed holder next to one edge. It’s a simple design that doesn’t require any adjustment for different sizes of reed, though you do have to hold the reed in place with your thumb while sounding it. The top plate is slightly thinner than a reed frame so that it’s possible to file the reed in situ, and it has a slight undercut so as to hold the dovetailed reed frame more securely. It took some careful measurements and fiddling about to get the wind slot just right so that it works for the full range of sizes of reed I had available. It might require further adjustment if I ever want to use it with even bigger reeds from a bass instrument. The separate brass screw is used in conjunction with a specially shaped thin spring-steel shim (not pictured) to hold the reed tongue up above the frame while filing.

Here’s the finished tuning bellows clamped to my bench. The flap of leather is a relief valve to let the air out when you raise the bellows. Not clearly visible in the picture, there are a pair of straps tacked to the sides of the frames that prevent the bellows opening too far.

Finally, here is a quick video of me showing them in action. The reeds are the highest and lowest reeds from my antique Lachenal 48-button English, plus the A₄, which is at 444Hz because it is tuned in old pitch, meantone temperament.

I want to thank Geoffrey Crabb for all his advice on the construction of the bellows moulds and the tuning rig, and also Bob Tedrow for his bellows-making essay (although my technique is quite different, I picked up several good ideas from it).

Hand-Cut Maker’s Mark Punch

Toolmaking, Tools / April 14, 2016

There is something magical about the ability to anneal¹ high-carbon steel, work it into a useful tool, then harden and temper it so that it can hold a sharp cutting edge for a long time. It was one of the most important discoveries of the Iron Age, enabling the manufacture of tools that were far superior to those made of softer metals like copper, bronze and wrought iron.

I have made a few hardened steel tools of my own; wood carving knives and gouges, and simple punches. It is a wonderful and exciting feeling to use a tool that you made yourself.

Part of the process of hardening tool-steel involves heating it to something in the region of 760-800C² and holding it at that temperature for a while. If the temperature is too low the steel won’t harden properly, and if it’s much too high you get a coarse grain forming inside the metal that will affect your ability to sharpen the tool. In the past I’ve heated my tools using either an open solid-fuel forge or a propane blowtorch, but in both cases it is difficult to accurately gauge and regulate the temperature. Because I need to make more tools for concertina production (mainly press dies), I’ve been looking out for a better way to heat them.

Recently I came across an old electric laboratory muffle furnace on eBay. Luckily I managed to get it very cheaply because it was described as faulty and it was near enough for me to collect it in person. It looked to be in good condition in the photos, and I figured that even if the heating element had burned out, it would be cheaper and easier to re-wind it than to build one from scratch.

The fault turned out to be very simple. It is supposed to have a fusible link inside the inner chamber that melts if you overheat it (a thermal fuse). This was probably quite easy to do because the original controller was a simple simmerstat³, and I suspect leaving it at 100% would cause it to reach melting point in about 45 minutes. The thermal fuse was missing. As a temporary measure I bypassed it and the oven fired straight up.

I learned nearly everything I know about heat-treatment of steel from Hardening, Tempering & Heat Treatment by Tubal Cain from the Model Engineer’s Workshop Practice series. It was only after I’d bought my oven that I happened to be flicking through the book and saw a picture of it: the author had the same model!

Tubal Cain had replaced the simmerstat in his heat treatment oven with an early computerised temperature controller. I wanted to do the same thing, and I could have simply bought a fairly cheap Chinese PID controller like the one I used on my glue pot (though high-temperature ones seem a bit harder to find and more expensive), but for various reasons I decided to build my own instead. I used a MAX31855 thermocouple interface from Adafruit, an Arduino Nano clone, a solid state relay, a 16×2 alphanumeric LCD (HD44780 compatible), a couple of push buttons and a rotary encoder stuffed into a plastic project box. Because I already had most of the parts other than the high-temperature K-type thermocouple (which I would have had to buy anyway) and the MAX31855, the project worked out pretty cheap.

I developed the firmware using the Arduino IDE and several off-the-shelf open source libraries. This is the first time I’ve used the Arduino system, though I’ve done quite a bit of embedded programming in the past. I must admit a decent C++ compiler and a large set of libraries made it pretty easy to quickly bolt something together, though the lack of a debugger is a bit of a pain, and some of the libraries are poorly documented and/or provided with example code that doesn’t work properly out of the box.

The most difficult part of the project turned out to be tuning the PID loop parameters. Get them wrong and the oven either never reaches the desired setpoint or it overshoots and oscillates around the setpoint. One advantage of developing my own Arduino-based controller was that it was easy to log the temperatures and power level at regular intervals to a laptop over USB, then plot a graph to figure out what was happening over time. To cut a long story short, after hours of test cycles and trying many different values, I eventually found a set of parameters that perform well enough for my purposes. It overshoots by a few degrees when it first gets up to temperature or after disturbing the system by opening the door, but I don’t believe that is enough to cause a problem. In this graph, the blip at 5200 seconds is the result of me opening the door for a few seconds:

For years I have been fascinated by how punches and dies were made prior to the invention of the rotary engraving machine. I’ve read what I’ve been able to find on the subject (not very much, to be honest) and studied some antique punches to try to work out how they were made. I decided to try making a punch from my maker’s mark to see if my ideas were practical.

I think positive punches and single-line name stamps usually used at least one counterpunch per letter to form the hollow spaces (the counters), and the outside waste was cut away with saws, files and probably a selection of gravers. There were also negative punches that were probably either entirely engraved or stamped with a set of reversed positive punches, but I’m not going to cover those today.

My maker’s mark is a fairly simple symbol (an upper case A with two arms added), though I wanted to challenge myself by making a serif version with multiple stroke widths.

I made both the counterpunch and the punch from 3/8″ silver steel, which is a commonly-available high-carbon water-quenched tool steel with a little chromium in it that is supplied as accurately ground round bar. Since the counterpunch was to be smaller than the punch I first tapered the end. There are several ways to do this, but I decided to use the compound slide on my Taig lathe. I also used the lathe to face both ends to make them square, and used a file to round over the hammer ends a little so that when you strike it you don’t hit a corner.

The facing process left the end that was to become the punch square and reasonably flat but very slightly rough. Because this might effect the performance of the finished punch I decided to lap it flat.

First I clamped the blank in a Vee block, using a piece of paper as a shim to make sure the punch protruded from the block by a tiny amount:

Then I lapped it on a cheap diamond plate in a figure-eight pattern:

The lapped face of the punch blank:

Next I started cutting the counterpunch using a jeweller’s saw to make the initial grooves:

I removed the waste from the outside using a fine flat hand file:

Then I used a graver to widen the grooves (you can see in this one it’s pretty small compared to my index finger):

The nearly-finished counterpunch:

To check it was the right shape, I got it sooty in a candle flame and pressed it onto a piece of paper (this is called a smoke proof). Actually I realised at this stage that I had made a mistake, but I opted to push ahead and modify the proportions of the punch to compensate rather than starting again. I could get away with this because the design of my mark is rather fluid anyway, and doesn’t have to match the style of a particular font.

The final part of cutting the counterpunch was to bevel the corners and do a test punch into a piece of end-grain hardwood. I’ve found that a punch doesn’t work very well if you don’t bevel it at all, but it’s not entirely clear to me how steep the bevel angle should be (the old punches I’ve looked at aren’t all the same).

Next I had to harden the counterpunch before I could use it. In order to reduce scale buildup and decarburising while soaking in the heat treat oven, I coat the business end of the tool in jeweller’s borax flux. Applying it isn’t an exact science. I warm the punch to not-quite boiling point, then smear some thick borax paste onto it and wait for it to dry. Once in the oven, it will bubble up a bit at first but then it should melt and flow out across the surface (you can see the difference after heat-treating; the areas that were protected by the flux are still bright underneath).

I put the counterpunch into the oven on top of a piece of bent stainless steel that prevents it directly touching the oven floor. After waiting something like 20 minutes for it to get up to temperature, I let it soak for another 20 to ensure that it was fully austenized all the way through. From what I’ve read, not soaking for long enough will significantly reduce the hardness you can achieve.

After soaking, I pulled it out with blacksmith’s tongs and immediately quenched it in a bucket of water. The shock actually causes most of the Borax flux to fall off, which is handy because it can be difficult to remove.

As a quick check to make sure it hardened, I see if a file will cut it. Generally it will scratch a tiny bit because of the surface decarburisation effect but it will be hard enough underneath that the file just skates off. Some people recommend using a good sharp file for this test, but I find that it tends to blunt the file so I prefer using a rubbish file and just press quite hard (I have experienced a tool that didn’t harden properly and the difference was pretty obvious).

After hardening, the steel is very hard but also very brittle and highly stressed. If you’re not careful you can shatter it just by rough handling (I’ve done that with a fancy spring clip that I had just spent half an hour forging). What we do to cure this is to temper it, which means re-heating it to a lower temperature and soaking it for a while. This reduces the hardness somewhat but also reduces the internal stress and greatly increases the toughness of the steel. If you temper it lightly you end up with quite a hard tool that might be at risk of chipping. Higher tempering yields a tougher but softer tool; if you go too far the edge may tend to roll over in use. The highest levels of tempering are used to make springs.

There are several ways to heat a tool for tempering. I could even use the heat treat oven itself, but it would take a few hours to cool down sufficiently and it’s not a good idea to leave the tool in its fully hard state for that long. I decided to use a small electric deep fat fryer with sunflower oil in it. The highest temperature the thermostat will go up to is about 200C (and the oil probably wouldn’t be happy going much higher than that), which is towards the low end for tempering silver steel.

After tempering I used a rotary wire brush in the pillar drill to remove the scale and any remaining flux.

Finally I used a spirit burner to re-heat the blunt end up to a spring temper so that it can better resist the shock of hitting it with a hammer. It will probably mushroom eventually but it is easy to cure that by grinding. After it reached a blue colour I quenched it again to stop the heat travelling too far up the shank.

A quick test in a piece of scrap aluminium proved it had worked as planned:

The first stage of cutting the real punch was to strike the counterpunch into it. I think this is possibly the most difficult part of the entire process. You have to line it up perfectly, then hit it multiple times very hard to drive it as deeply as possible into the steel. Although the silver steel is supplied annealed, it is still a pretty tough material and it takes a lot of force to counterpunch it. The counterpunch bounces out of the indentations after every strike and you have to be very careful to line it back up perfectly before the next strike – I had a bit of an accident and made an extra small dent but luckily it was in a waste area. The counterpunching causes the surface to raise up a bit around each indentation, which I opted to get rid of by lapping it flat again. Here is the result after counterpunching and lapping:

I roughly removed the majority of the waste from the outside with a jeweller’s saw and files again:

Mostly done, though not yet adjusted the width of the strokes:

After a bit more fine fettling, and adding the bevels:

I didn’t bother photographing the hardening process as it was exactly the same as for the counterpunch. The final set of photos shows the result of testing it in several different materials. First end-grain beech:

Aluminium, both a light strike and a heavy one:

A light strike into side-grain pine tended to crush the soft fibres:

End-grain pine worked fairly well though. The bottom one was an experiment in soot blackening the punch first:

Brass. I had to hit it pretty hard to get a decent impression but it came out quite nicely:

Thick leather. This looks sharper in reality than the photo appears to show:

Mild steel. The top mark was a moderately hard strike onto the wooden bench and it’s barely visible. The bottom mark was a very hard strike backed by a steel anvil. I’m not sure I’d want to risk striking the punch that hard very often as I’m concerned the tiny serifs might be at risk of breaking off, though it didn’t suffer any visible damage as a result.