I applaud your efforts in sourcing brass with low levels lead. This has been a concern of mine and one of the main reasons I have not proceeded with a vintage commercial lever purchase.

Why not use stainless steel in place of brass ? Could it be cost, machinability or heat management ?

redbone wrote:Why not use stainless steel in place of brass ? Could it be cost, machinability or heat management ?

The results of the spectroscopy that I had done (based on a sample size far too small (i.e. one) to draw any larger conclusions) showed that lead levels in my antique Brugnetti group (circa mid 80s) were actually low; similar to the levels mandated by California and EU law. The results indicated an alloy with high corrosion and dezincification resistance - commensurate with the operating conditions. However, until quite recently, among the most common additions to bronze used to promote corrosion-resistance was arsenic. Now, if there is one thing other than lead that you don't want in your coffee it is arsenic.

U.S. EPA current maximum contaminant levels (MCL):

arsenic - 10 micrograms per liter (parts per billion)
lead - 15 micrograms per liter (parts per billion)

Of course, the amount of arsenic present in the alloy is proportionally small, what proportion of that would actually leach into the fluid medium is not known and no one (I assume) drinks gallons of espresso every day. On the other hand, the guidelines represent maximum levels and in this case none is definitely better than any. After the laws on heavy metal content in potable water fittings in California, the EU and other places were passed in the 90s, the industry developed new alloys to meet the requirements. From the reading that I have done, silicon-based alloys seem to be the winner for potable water applications as they promote both machinability and corrosion resistance.

Why not just use stainless? Basically for all of the reasons that you give. Stainless is more difficult to work and it is more costly. Also, it conducts heat roughly 1/20th as well as bronze. Building a group entirely from stainless would, I am sure, be feasible. It would, however, have to based on radically different principles.

^
Good info re arsenic as I was not aware of the potential in water contact metals. Cadmium in skin contact items such as costume jewellery has garnered some recent media attention.

Following your good work.

BTW, have this shown below if of interest to you.

Hi Rob,

I had heard about the cadmium in costume jewelry marketed to kids. Nice.
Up there with melamine in milk formula. Caveat emptor. Sigh.

Many thanks for the super offer of the heat controller. That is a very nice unit.
At the moment, the temperature management is being take care of with the PID routine running on the micro-controller. I will very happily take you up on your offer if the need arises (hmmm - powder coat oven?).

Best,

Thomas

Sure, anytime.

Skakerato is finished!

I've been away for a couple of weeks and I haven't had time to post the progress. For starters, the new castings are coming along. Unlike the brew reservoir (which was sand cast) the yoke is a precision part, so this time the foundry is using the lost wax method.

A three part aluminum mould (mold? depends who you ask about the spelling of that word) for making a pair of wax positives. The third part (not shown in the picture), is a bar that runs through the fork and seals the lower parts of the mould edges.



The wax positive with a tapered in-gate to provide a reservoir of molten metal for the casting to draw from as it solidifies and shrinks.

Funny how I thought the Skakerato was a drink shaking machine as opposed to it's repurpose here as a parts tumbler polisher.

Thirty days hath September,
April, June, and November;
All the rest have thirty-one,
Except orders from my foundry which have about 90 so far.

The weeds behind the long grass just past the bog at the end of the swamp.
Sheesh.

So, first test results: pressure profiling.

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.