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Goldilocks part 2

Illustration by Pervprude

It is high time for more porridge!

Several days after I did the first tests, it occurred to me that the horseshoe tube that I had made when I built the boiler was not the same as the one in the Brugnetti boiler. I wasn’t really thinking that hard about how a heat-exchanger actual works while I was making it. So I took apart the boiler (again). This is the original tube, nicely blackened after a few months of service:

I measured the interior volume of the ‘Papa Bear’ version and compared it with the volume of the vintage HX horseshoe (modeled in CAD): 115ml for the vintage and 135ml for Papa Bear. So I put Papa Bear to the bandsaw, and cut him down to make Mama Bear.

Blue – Boiler
Red – Brew reservoir
Purple – Group neck

(The data for the neck was very noisy for some reason that day – the rapid deviations do not reflect what is actually going on.)

Despite having the same volume as the vintage tube, ‘Mama Bear’ doesn’t behave the same way. The peaks in the brew reservoir temperature are similar to Papa Bear, i.e. around 5 degrees, and recovery times are about the same. It should be noted however, that with adequate recovery time, the neck temperature shows a high degree of stability.

I then modified the horseshoe again, lowering the volume to around 100ml (Auntie Bear??) – but the results didn’t change significantly. So I decided that drastic measures were called for: ‘Baby Bear’ – roughly 60ml volume which is about half of the vintage.

Though the results are better, there are still spikes in the brew reservoir temperature.

Time to alter the Goldilocks plot line.

Introducing ‘No Bear’! Partly to make sure that I wasn’t entirely out to lunch but also to measure the other extreme, I connected the supply to the brew reservoir directly to the mains, bypassing the HX and injecting room temperature water into the reservoir. Unsurprisingly the results are dramatic!

Very cold porridge indeed.

Interestingly however, while the neck temperature rises by a few degrees initially, it recovers quickly (less than a minute) and then drops to 2.5 degrees below idle.

Where does this leave us? The concept of the HX is of course to inject (fresh i.e. non-boiler) cold water through the hot water in the boiler in order to raise it to the ‘correct’ temperature to feed the reservoir. If the machine has been idle for any length of time, the water in the HX will be at the same temperature as the boiler. Subsequent shots will draw cold water into the HX, which, depending on how it is designed, will consistently bring room temperature water up to a specific temperature (either boiler temperature or slightly lower), as long as the heating element can keep up with the demand. So some not very earth-shattering conclusions:

  • Changing the design of the HX will determine the temperature ‘profile’ of the water delivered to the brew reservoir.
  • Somewhere between 60ml and 0ml of HX volume, the water delivered to the reservoir will offset the heat gain and result in equilibrium.

There is one additional question that results from the three bears test: why do identical horseshoes in the vintage and new boilers not exhibit the same thermodynamic behavior? My hypotheses is that materials used for the brew reservoir and boiler are playing a much bigger role than I first thought. Both of these parts on the prototype are made from stainless which is roughly 20 times less thermally conductive than copper and bronze. To test my theory, I put the prototype group onto the vintage machine: compared to the all-stainless boiler assembly, the new group runs around 12 degrees hotter on the copper/bronze boiler. The stainless brew reservoir is slow to acquire heat from the boiler and the water in the reservoir and reluctant to relinquish it to the air or pass it on to the group.

SO…. two rather more significant conclusions:
1 – There should be (or rather, spoiler alert, there is, as will be seen in an upcoming episode) an HX design that meets the requirements of stainless boiler and group combination.
2 – It is time to make a new boiler – using the bronze brew reservoirs that I received a few months ago.

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Goldilocks part 1

Illustration: Arthur Rackham

[Ed: Re-post from May 12 2018]

I imagine everyone remembers the story of Goldilocks and the Three Bears?

I love the painterly references on the wall – a Degas ballerina on the left, an old master on the right, an enameled reproduction of a Greek vase in blue? But I digress. Apart from carrying a wee bit of Freudian baggage, it is also perhaps not the most pertinent of fairy tales for our times (how would you react if you came home and found someone sleeping in your kid’s bed?). Anyways, what I wanted from it for today’s post is the porridge. You know, the three kinds of porridge that are too hot, too cold and just right? You see where I’m going with this… Except that in my story there are four bears, or rather three bears and no bear, but neither of those versions scan very well, so we are going to stick with the original title. First up is Papa bear’s porridge.

This is one of the first temperature profiles that I took of the prototype machine. The methodology is very similar to the tests on the Brugnetti [Auroras] that I posted previously. As always, the Scace-values are not terribly useful except for comparison.
A reminder of the colors:
Blue – boiler wall
Red – brew reservoir
Purple – group neck
Green – PF receiver on the group
Yellow(ish) – Scace puck

Observations:

  • The boiler temperature curve is sinusoidal as opposed to saw-tooth and has a delta of 0.7 C between minima and maxima. This performance was obtained using only the P term of the [PID] algorithm. It could probably be improved with some additional tuning, but I haven’t bothered because it is already pretty good.
  • Brew reservoir temperature is stable at idle and shows no trace of the boiler temperature variation.
  • Recovery times for a shot are about 2 minutes per degree for the group and 1 minute per degree for the brew reservoir.
  • Group temperature at idle (~70 C) is quite a bit lower at idle than the Auroras (~80 C and ~83 C for the horseshoe and diagonal versions respectively).
  • Don’t touch the probes.

Conclusions:

  • Job (pretty much) done on the PID boiler control.
  • Plenty of room for improvement on the brew reservoir temperature stability which is gaining too much heat after a pull.
  • The whole shebang has to be hotter.

Next up: Mama bear and No bear.

Goldilocks Part II

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temperature profile diagonal HX

1987 Porsche 911 Turbo Cabriolet

[ED: Re-post from May 4 2018]

This is a longer test (over an hour) on a 1987 diagonal HX machine.

Methodology:

The same methods as were used for the horseshoe HX machine were used with a few minor differences.

  • This machine is on 24/7 so there is no warm-up period.
  • It had been at idle for at least a couple of hours.
  • The same pseudo-Scace device and needle-valve was used to simulate shots.
  • The period between simulated shots is generally longer and I did not repeat the faster than commercial usage of the previous test, rather, I waited for the group to recover between shots.
  • At minute 3 and 32 there are cleaning flushes – i.e. pulls that were significantly larger in volume than an actual shot.
  • At minute 23 a PF filled with real coffee was locked into the group and a shot pulled.
  • At minute 25, the steam wand was used to foam milk for a cappuccino.
  • The additional orange line that starts at minute 46 is another K-type thermocouple that was placed on top of a puck of coffee before the PF was locked into the group.

Observations:

  • This machine has the same pressurestat as the horseshoe HX machine from the previous test and consequently we see the same saw-tooth wave for the boiler temperature.
  • With the exception of the cleaning flushes, the brew reservoir is [b]remarkably[/b] stable – exhibiting almost no discernable trace of the fluctuation in boiler temperature. The temperature drop for the outside of the reservoir after a shot is pulled is less than one degree and the recovery time is between two and three minutes.
  • The group as a whole exhibits the same tendency as the horseshoe HX machine to gain heat with each shot.
  • The recovery time for the group is a little over a minute per degree C of heat gain i.e. essentially the same as the horseshoe machine. As the groups are identical this is unsurprising.
  • The shot test with real coffee plus the additional thermocouple starting at minute 46 shows a 90 C peak shot temperature and is quite likely to be accurate.
  • At minute 40 I removed the PF to prepare for the next shot and the preparation time is longer than usual because I was fussing with the thermocouple. Removing the PF has a fairly significant effect on overall group temperature as the neck falls to the same 83 C that it was when the machine was at idle.

Conclusions:

  • The diagonal HX is about as good as it gets as far as temperature stability of the brew water reservoir goes. The reservoir recovers from what little variation there is in less than half the time that the group takes to recover between shots.
  • The slower, more realistic, pace of shot pulls in this test is illuminating. The actual temperature gain seen at the group is between 4 and 5 C per shot. This translates to a group recovery time of five to six minutes – possibly a little slow for a commercial setting, but definitely fast enough for home use and entirely manageable with a cooling strategy such as a cooling flush of a known volume.
  • 1987 was better than 1982.
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temperature profile horseshoe HX

[Ed: Re-post from April 29 2018]

So, if a pressure profile is a cat among pigeons around here, here’s a fox in the hen house:

What are we looking at? This is a multi-channel temperature log of my early 80s [Brugnetti Aurora] horseshoe HX machine over a period of about half an hour.

Methodology:

Five K-type thermocouples were installed on the boiler, the side of the brew reservoir, the group neck (i.e. extension between the bolt flange and the body of the group), the outside of the portafilter holder on the group and the puck inside the pseudo-Scace. The machine was turned on at roughly 10am and was at idle for at least one hour before the test. The pressurestat is set so that the average boiler pressure is in middle of the recommended band (the green area on the Brugnetti gauge) at 0.9 bar.
At 5:30pm the portafilter was inserted cold into the holder and 6 simulated shots were pulled with the needle valve set to give 20-30 seconds of flow per shot. The spacing of the shots is very quick – about 30 seconds apart – much faster than a ‘worst case’ scenario of constant use in a commercial setting.
At 5:34pm the machine was left to idle for 3 minutes then a further 5 shots were pulled.
At 5:40pm the machine idled for 5 minutes and 3 shots were pulled.
At 5:59pm the portafilter was removed.

Limitations of the methodology:

A) The pseudo-Scace is by no means a perfect analog to actual coffee for a number of reasons:
First, the Acetal puck, although vaguely similar, does not conduct heat in the same way as coffee grounds.
Second, the puck gains heat from shot to shot unlike coffee, which will always be at (close to) room temperature.
Thirdly, the flow rate through the needle valve is not very repeatable.
Fourthly, fluid flow through real coffee is unlikely to be linear, and this non-linearity is not modeled with the needle valve.
So temperature readings “at the puck” should be taken with a grain of salt.
That being said, for comparative study of different machines, Scace-like instruments are a valuable tool.

B) Also, with the except of the pseudo-Scace, these measurements are surface temperatures not actual water temperature.

Observations:

  • The roughly triangular wave of the boiler (blue line) is from the pressurestat controller with roughly 0.2 bar of hysteresis (dead band). Total variation is approximately 4 degrees C.
  • The (red) brew reservoir temperature (and by inference also the water inside it) correlates closely with the boiler temperature but the variation is considerable damped. Total brew reservoir variation is less than 1.5 C.
  • Average (or baseline) temperature of the brew reservoir is affected only very slightly by continual use rising from 100.5 C at idle to 102.5 C after intense activity.
  • Neck temperature (purple) seems to be a better analog to puck temperature than the outside of the portafilter holder.
  • The group gains heat with each shot if not left enough time to recover and (based on a small sample of only two points on the group) the heat gain is uniform throughout the group.
  • With the exception of the walk-up shot with a cold PF, the temperature gain of the group is between 1.5 C and 2.5 C per shot.
  • Group recovery time – that is, the time it takes for the group to cool from any given temperature gain – is about 67 seconds per degree C of temperature gain.
  • Puck temperature (bearing in mind the caveats above) is lower than expected, especially for the walk-up shot – but the results in the cup are known to be good with this machine at these settings.

Conclusions:

  • Brew reservoir temperature demonstrates a relatively high degree of stability even with a dead band of more than 20% of the operating pressure setpoint.
  • Group temperature should be stable if correctly managed. If the machine is left to idle between shots for a reasonable amount of time (between 2 and 3 minutes – i.e. not inconsistent with the time it takes to prepare a shot), it will return to its initial temperature and provide repeatable shot temperatures.
  • The methodology should be changed for future tests to reflect a more realistic real-world usage with a sufficiently long period of time between shots.
  • Not bad for 1982.
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pressure profiling 1

[Ed: Re-post from April 28 2018]

I did pressure profiling partly as an idiot check, but mostly out of curiosity. As I am using the same springs in the old machines and the new group the performance should be the same. The only thing that really needs to be verified is initial/peak shot pressure which depends on the length of the compressed spring with the lever in the down position (and any variation between the actual springs themselves). The method used to do the profiling was laborious. I made my own Scace-type device with an Acetal disk to fill in for the puck, an analog pressure gauge, a K-type thermocouple and, something not featured in the Scace, a needle valve to control the flow.

Then I filmed some simulated shots and logged the pressure readings on the video every five or ten frames. Boooooooooooring. There are many better ways to do this, but, as I said before, I don’t really need to check the variation over time, just the steady states. I tested three machines: the 1987 diagonal HX, the slightly earlier horseshoe HX and the new prototype group. However, the results between the two antique machines are essentially the same, so I only bothered logging one of them.

Conclusions:

It is hard to adjust the needle valve to get repeatable flow rates and thus repeatable simulated shot times, but you can see that the spring performance is essentially a straight line from peak to about 6 bar. At 6 bar, the lever reaches the end of its travel (i.e. the spring is completely extended) and the rest of the curve is just residual pressure.

When the lever is lowered, the line plateaus at the pre-infusion pressure: just over 2 bar for the antique machine which has a pressure regulated water supply and a little over 3 bar for the prototype which was directly connected to the city supply.

Peak pressure for the antique groups was just above 12 bar.
Peak pressure for the prototype is about 11 bar – this was adjusted to 12 bar by shimming the spring by about 2mm.

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When last we left our heroes…

I have an indelible childhood memory of watching reruns of serialized television from the 50s in black and white. At the end of each episode the heroes always seem to be in an entirely intractable position: hurtling towards certain death as the car / rocket ship plunges to the ground or tied up as the bad guys abandon them to their fate as the building burns / volcano erupts. How do they escape? “Find out next week” on …

During the interlude since the last episode (during which I can assure you that the heroes have been frantically filing away at their shackles) I took stock. Here is the state of the production:

Frame – complete & painted. 100%

Cable harness – all modules of the cable harness are complete. A few remain to install with the bodywork. 100%

Controller – four out of five boards designed, tested and in production. One board under design revision. Machining and labeling of enclosure, assembly and installation remain. 75%

Firmware – functional with a few small problems to resolve before it is “good enough” as firmware is never “finished”. 90%

Hydraulics – the most complicated part of the plumbing including all tubing runs connected to the four lower boiler ports, the HX, solenoid, manifold and drain are complete. Six upper ports remain. Of these six, three require fabrication of tubing runs. 70%

Boiler – complete & installed. 100%

Group – The main casting is complete (no small milestone), machined and honed. All the fixed components are complete and on the shelf. The spring is out for quotes as are the parts for the piston assembly. One part remains to fabricate in house and then, once all the parts are in, the group can be assembled. 70%.

Bodywork – Two pieces remains to fabricate and one may have to be revisited. 70%

Millwork – LRFs are complete and installed. All of the cup rail parts are fabircated and finished and are waiting for installation. Tap handles are machined but need to be assembled and finished. Stock has been prepared for making the lever and portafilter handles. 85%

After final assembly is complete there remains testing and packing… in short, there are still a few episodes of this particular series left. How many? Find out next week on ….

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In praise of LRF

1

Left Ring Finger? Long Range Forecasting? Low Resolution Fox? Nope, definitely not the last one (look it up (yet another minor moral quandary about whether something sexist can also be amusing; probably not allowed, but I digress)). No, rather, I offer some small observations on Little Rubber Feet!

LRF are perhaps something that you might not have spent a lot of time thinking about, but they are ubiquitous and surprisingly important. They are a crucial component of almost every single contemporary household object: from the chair I’m sitting in, to the computer monitor in front of me and even the keyboard I writing this with. The underside of your mouse (if you still have one)? LRF, albeit very small and not at all rubbery. You might say that LRF, if one were to stretch the definition just slightly, are the industrial design equivalent of building foundations: the point(s) at which objects touch the surface they rest upon, negotiating the transfer of the load, evening out imperfections and keeping delicate surfaces away from harder ones. They come in a myriad of shapes and sizes. There are hard ones that slide, soft ones that grip and everything in between. In addition to being made from every kind of natural and artificial rubber and plastic imaginable, they also are made from wood, glass, felt, cork and occasionally even metal. The most special LRF are the orphans that turn up on the floor or being chewed on by your pet and or child. They only reveal their origins six months later when you finally find the lamp that now both wobbles and scratches the table. These are also the same kind that, origins revealed, you are guaranteed not to be able to find again or to just have finally thrown away. Not me though. I have a special LRF drawer.

The scale of a single group lever machine comes with a few challenges. Both the porta filter and the lever itself require the user to apply relatively large amounts of force to the device. Ideally, it should resist these forces without moving when they are applied. Although single group machines are quite large and heavy compared to most items that might sit on your counter, they are featherweights compared to multi-group machines. These are just in, custom made from a low-durometer self-adhesive backed 3mm silicone rubber. The weight of the machine forces the soft material to conform to minor imperfections in the supporting surface vastly increasing the contact area and friction. Result? It grips like a barnacle to a rock.

One more tiny detail closer to finishing.

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1 – Image: Brik Pixel Art Designs by BRIK.

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The rite of spring

IGOR STRAVINSKY – ERICH AUERBACH / GETTY IMAGES

So Stravinsky’s ballet caused a riot… perhaps it was the shocking juxtaposition of pagan and modern Weltanschauungen, maybe the audience just didn’t like the music. What can I say? Springs are contentious. With that warning, I shall begin trying to unravel the (minor) mysteries of one particular spring.

A little more than a year ago, it was brought to my attention that replacement springs for the Brugnetti Aurora lever group were no longer available. I checked a number of suppliers and looked for substitutes without any success and I immediately went to my local parts dealer and bought the rest of their stock – a shockingly large number: four.

This is a problem if you are me and want to make new groups, or, if you are not me, repair old ones. So I took the dive into spring design and started to think a bit more carefully about a part that I had presumed was going to be “off the shelf”.

I have since come to the conclusion, for a number of reasons that will further elucidated, that the spring that was sold to me as a replacement for part number A.29 for the Aurora group, may not be, well, a replacement for part number A.29.

When I changed the spring on my first machine (well before I embarked on this voyage deep into the jungle down the Congo River) I noticed that the new one was a little taller than the original, making it quite a bit more difficult to install. Installed it was however, and I thought no more about it. That particular machine dates from 1987 and was rebuilt by a local dealer in the mid 90s – but may well have kept its original spring given how much of a hassle they are to change.

Under normal operation, springs will deliver an amount of force that is directly proportional to the distance they are compressed: F = kx (where F is Force, x displacement and k is the spring constant). Put another way the spring constant is simply how much force the spring imparts per unit of compression. So determining the spring constant can be done by measuring force versus displacement. This is the setup used – the force gauge (i.e. bathroom scale) isn’t ideal because of the built-in “intelligence” which automatically tares (zeros out) small readings and shuts off the display, but it does measure up to 175Kg. Displacement is measured using a digital height gauge (not shown) that (as so long as the same datums are used and the gauge isn’t re-zeroed) provides more accuracy than required with enough precision (I can never keep those straight).

I believe that I have identified three different springs that were installed in the Aurora group at various stages:

The replacement “after-market” spring from the local dealer which has an uncompressed length of 133mm.
Antique spring #1 – which has an uncompressed length of 128mm.
Antique spring #2 – which has an uncompressed length of 116mm.

Here are the compression versus force profiles of those three plus a fourth new prototype spring.

The two trials of the after-market 133mm spring suffer from some measurement error – i.e. if you extend the lines back towards the X axis, they don’t intersect the origin coordinates; which they should, as zero spring displacement results in zero force. If they were to be normalized (i.e. shifted down until they would intersect the origin if extended) you can see that they correlate closely with the all of the other trials except the Antique 128mm. In fact the spring constants (calculated using the average slope of all the data points in each trial) for all but the 128mm spring are around 60 N/mm whereas the 128mm is significantly lower at 44 N/mm.

However, the fact that the springs are different lengths is not a minor detail. As the geometry of the piston assembly remains the same, the springs are all compressed to the same size when they are in use i.e. they have an installed length of 96.25mm (corresponding to the lever in the up position) and a fully compressed length (when the lever is in the down position) of 75.5mm.

Installing a springs of different lengths will mean they are operating over different force ranges. The corresponding pressures that the piston will deliver can be easily obtained from the equation P=F/A (where P is pressure, F is force and A is the surface area [19.63cm sq for the 50mm diameter piston in the Aurora group]).

Spring length(mm)Spring constant (N/mm)Force @ fully compressed (N)Pressure (bar)Force @ installed length (N)Pressure (bar)
aftermarket13362.80361118.4230811.8
antique #112844.39233011.914097.2
antique #211659.21239812.211696.0
lapera11661.99251112.812246.2

Presuming that you find tables at all interesting, some interesting points can be drawn from the one above (although now we are getting into subjective and therefore contentious territory). The first line shows that the replacement after-market springs from the local dealer are very likely incorrect as their theoretical operating range is 18-12 bar. I think most people would agree that this is too high. The case for these springs being incorrect is strengthened by the fact that a design analysis of that spring configuration, (i.e. the spring constant, wire size, number of turns, end conditions etc) results in a non-compliant design when used in this application (i.e. the installed length and travel) meaning that it is likely to fail to perform as expected or simply to fail over time.

The second point of interest is that the two antique springs, despite their different properties, yield very similar pressures in the installed configuration – approximately 12bar maximum and 6-7bar minimum. Without knowing more about the provenance of these particular parts, it is hard to know whether they have changed over time or whether they are still operating as designed. However, based on the subjective results of the quality of the coffee that the machine produces when it is operated over this pressure range, I believe that this was the design intent. Further testing of other old springs of known provenance would be helpful to confirm this hypothesis.

For the prototype I chose to use the shorter ~116mm format because it is significantly easier to install and opted for a similar 12-6bar range at the installed configuration. The new spring should be a drop-in replacement for the old Aurora groups.

Prototype Lapera spring on the left, 18bar monster on the right. We fervently hope this little one doesn’t dance herself to death.

Pina Bausch – The Rite of Spring

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Love in the time of cholera

Don’t know about you, but I think the penguin looks worried.

In these strangest of times, I hope that you are all well.

I’m not sure where to begin really. Writing about small goings on in my tiny corner of the vast tableau of human experience seems rather pointless given what the whole of humanity is currently enduring. But in the hope that some may find these musings mildly amusing and possibly a welcome diversion from the cataclysmic car crash of out there, we humbly soldier on.

I took delivery last autumn of the bulk of the sheet metal work from a new Canadian supplier. Since then, the stainless steel covers have been sitting on the shelf waiting for the completion of the frames so that all the parts that need to be painted can go to the shop at the same time. As those of you who follow the posts on that photo site owned by the social media company with the reputation for (among other things) being extremely cavalier about their users’ privacy will know, the frames were finished some time ago. So now it’s time to put the finishing touches on the covers. A long, long time ago, (you know, in 2020BC) I discovered that the straight folds on two front flanges of the cover didn’t have quite enough relief on the interior flanges (gasp) to allow the requisite over-bend during fabrication. The upshot of this is that the front panels are only nearly, not exactly, at 90 degrees to the sides (re-gasp). Always seeking to shave off another minute source of imperfection, adding a tab that connects the front flanges to the sides will resolve this and have the additional benefit of increasing the overall stiffness.

This of course requires a new tool. One strangely reminiscent of an earwig.

As always, you get what you pay for and this thing is neither terribly expensive nor terribly good. But it will do (some of) the job. As the following test shows, it can produce a mechanically sound, if slightly less than perfect, spot weld.

Bunch o’ tabs cut from some scrap stainless.

A tab spot-welded into place.

BUT… without radical modification, the spot welder can’t be positioned to make the second weld on the front flange. So, it’s back to the old fashioned (i.e. long and slow) way – TIG welding by hand – with a simple jig to get things all orthogonal-like.

Welding thin material is really easy to get wrong. We managed to pull these off without blowing any holes (or at least ones that I will show you pictures of).

Rinse, repeat:

Drive to nearest big-box store for, well, boxes so that they can be shipped to the painters :D

… hmmm, wonder how they turn out.

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Lenovo Weldcenter

The control components and the wiring for the welding turntable are too delicate to leave exposed to the dust, metal chips and occasional flying tool around the shop. They have to go in a box. I could have drawn one up and had it fabricated along with the rest of the sheet metal parts for the bodywork, but (a) fabrication shops hate/love one-offs and charge accordingly and (b) a recently deceased Lenovo PC (born 2006, died 2019, R.I.P.) seemed like it might fit the bill.

At first glance, just a simple box.

Its apparent simplicity belies an extremely clever design that a lot of people thought long and hard about. It is also a masterclass in metal folding: the sophisticated locking(!) clamshell and double-pivot mechanisms are assembled from stamped and folded parts using only four screws. Very swish.

Having extolled the virtues of the Lenovo case I no longer need to feel guilty about cutting it up. Possibly voiding the warranty?

Time to start putting a few new things inside the now empty box. The heart of the controller is the stepper motor driver: a Geckodrive g203V. The literature states that the V stands for Vampire, as in unkillable. I have thus far failed to find the correct mixture of garlic and silver bullets required to prove them wrong. Other than an issue with making them play nicely with some common motion control boards that have a different electronic setup for their step and direction signals, these things are great, if not particularly cheap.

The heat sink from the GPU on the Lenovo motherboard is just the ticket for the stepper motor driver. It has a convenient spring-steel mounting clip that makes it easy to attach it to the opposing face of the motor mount.

A healthy application of thermal grease on all of the mating surfaces ensures the efficient transfer of heat from the driver to the sink.

Next, the new Weldcenter needs a face plate for the interface which will be made from a small scrap of 1/4″ acrylic. As usual, drawing up the cutouts and programming the CNC takes substantially longer than the actual cutting. Back surface first: a bunch of holes plus a relief pocket for the rotary pots that are designed for thinner material…

…and the front with engraved text for all of the various buttons, switches, sockets and dials.

Test fit of all the pre-wired controls including the LCD interface.

The front side, with residual laser-engraved tennis racket – the acrylic stock is a left-over from a sign project.

There were, of course, a couple of small, but in some cases mildly baffling, errors. The text at the bottom interferes with the mounting screws because I neglected to model them in CAD. Rather more inexplicable is the engraving of the “Gas” text which appears to have been outline, as opposed to single-line, engraved. Don’t know why, nor, as this is strictly a one-off will I spend any more time thinking about it. Much.

All the various bits and pieces stuffed into the box. Top left: the main cooling fan that used to be at the front of the PC. (For the ultra-observant, now you see why the fins of the heat sink below it are not oriented vertically the way they would be normally). Below the stepper controller is a solenoid for controlling the purge gas. Below that the microcontroller. The middle column is a DIN rail with a small (5 watt) 24 volt DC power supply and a bunch of terminal blocks for connecting everything together. The last column on the right is the other two power supplies. This is a bit of a kluge. There were only supposed to be two flavors of DC in the design: 5 volts for the microprocessor and 24 volts for the solenoid and motor and no fan. However, the 24 volt supply was very bulky so I swapped it for a much more compact 60 volt switching supply. The black box is a dual voltage (5 & 12) supply for an external hard drive of which I have many. So… four flavors: microprocessor 5 volt, fan 12 volt, solenoid 24 volt and stepper motor 60 volt. So much for simplicity. As always, the box is about 25% too small for all the stuff. There should really be cable tracks between each column and along the top and bottom. There aren’t so the resulting wiring is not exactly as neat as it might be.

The final piece of the puzzle is a foot control made from a couple of robust momentary switches (the kind used for guitar pedals), a nice metal case (ditto) and some three conductor wire (the extra green wire was spliced on).

The switches are wired in parallel with their corresponding button on the front panel of the interface. Pressing either the button or the foot switch triggers the control.

Finished foot controls. The throttle pedal, which is for an electric scooter and cost less than $10 (Canadian, including shipping), is extremely well made.

The Hall effect sensor inside it however, (the white stuff is silicon and isn’t as disgusting as it looks), is not. Either I killed it by miss-wiring it briefly while I was hooking everything up, or it was lousy to begin with (I lean towards the latter). Either way, a broken sensor means no throttle pedal which means the entire machine is about as useful as a third shoe. So I replaced the sensor with a brand-name version over-nighted from MagicKy (aka Digi-Key) – (which cost more than the entire pedal after shipping). Hall effect sensors work by measuring the flux of the magnetic field that passes through them. The stronger the magnetic field – i.e. the closer the magnet is to the sensor, the stronger the signal. The silver lining of replacing the sensor with a high quality part is that it has a bigger range – it detects lower strengths of the magnetic field and outputs lower minimum and higher maximum signal voltages, so the pedal response is far more sensitive. Coupled with a pseudo-logarithmic curve which is applied to the output from the sensor in software, the throttle pedal now allows very precise control at the lower end of the speed range where it is most important.

Et voila, the finished LenovoⒸ Weldcenter – running Windows Vista (ok, no it doesn’t).

The LED display shows the low and high RPM settings that are mapped to the throttle pedal output. The rewind amount is a fraction of a single rotation at high speed – a tap on the rewind button and the turntable reverses by that amount. In retrospect this turns out to possibly not have been the best way to do this as in practice the amount of rewind required changes too often. At some point I may (or possibly may not) add a mode where the rewind is active while the button/pedal switch is depressed. It also occurred to me too late that I could combine the two foot switches in a single cable and XLR jack, which means that I now have a spare jack for the ion cannon accessory.

A quick test with some scrap tubing before all the kinks with the code were worked out.