1st group body prototype

The prototype of the group body is done. Although there are a few minor surface blemishes, the overall finish looks to be on par or better than the two vintage groups that I have. Certainly, the lines are a little crisper. A closer inspection of the photos indicates that there may be an issue or two to address with the secondary machining processes. I've requested a few more photos and some QC measurements. The part may have to go back for a little more machining before it is shipped.

Cold Aurora in February

One particularly cold February morning, I got up to discover that the auto-fill circuit on my Brugnetti Aurora had failed. As I opened up the Gicar controller a few days-without-proper-morning-coffee later to figure out what was wrong (an old and faded capacitor), I realized that I had now either fixed or maintained every system on the machine except the lever group. So I took that apart too and replaced the spring and the gaskets. As I was putting it back together I started wondering about what it would take, given that the original Brugnetti company no longer exists, to make more of these fantastic machines.

A few days ago, after nearly three months of research, drawing, discussion and negotiation, I received these photos:



... a wax positive of the group body which, over the next few days, is going to be encased in plaster and replaced with molten brass.

The photos are also a sign, at least to me, that this project is now well and truly underway and that it is time to share it with the generous people on this forum, whose advice I have greatly benefited from since my obsession with coffee began. I am a long, long way from a working machine, but you have to start somewhere.

For the shrink that fits - Alice's adventures in tolerance land

More thoughts about the group body casting.

Having stared at the casting after looking directly at sun for a few minutes I am now pretty sure that it is an assembly of two parts: the bulk plus a small internal sleeve that holds the dispersion screen. Putting these two parts together could be done in a number of ways, but to avoid the possible consequences of heating the casting to soldering or brazing temperatures, I think that a shrink fit is probably the best bet.

From the engineeringtoolbox:

Shrink-fits are assembled by heating them to temperatures where the expansion exceeds the interference. Required temperature heating can be calculated as

dt = δ / α d_i         (1)

where

dt = temperature heating (^oC, ^oF)

δ = diametric interference (mm, in)

α = coefficient of linear expansion (m/m^oK, in/in^oF)

d_i = initial diameter of hole before expansion (mm, in)

Diametric interference can be calculated as

δ = dt α d_i         (2)

So assuming the sleeve is at room temperature and the body is heated in boiling water:

Nominal hole diameter d_{i =} 58mm
Temperature heating dt = 80^oC
Coefficient of linear expansion α for C23000 brass = 18.7x10^-6 (°C)^-1
Coefficient of linear expansion α for C87850 brass = 18.5x10-6 PER °C (20-100 C)

Therefore:

for C23000
δ (80^oC) = 80^oC x 0.0000187 (°C)^-1 x 58mm = 0.086768mm

for C87850
δ = 0.08584mm

...or more or less the same.


So over-sizing the sleeve diameter by 0.08mm (which is pretty close to 0.003") will allow it to be shrink fit into place with minimal force.

And after drinking the potion mark "drink me" Alice can now go down the rabbit hole to tolerance land. 

Interference H7/s6 S7/h6  - Medium drive fit for ordinary steel parts or shrink fits on light sections, the tightest fit usable with cast iron. 

The tolerance for the sleeve and the hole diameters is important as the two add together. So a symmetrical tolerance of +/- t will possibly result in a total tolerance of 2t.

So allowing for a minimum of 0.05 interference which should still provide a 'medium drive' shrink fit, the diameter of the hole should be 58 +0.015/-0 and the sleeve diameter should be 58.08 +0/-0.015

So in the worst case scenario, the hole is 58.015 and the sleeve is 58.065 which still results in 0.05 interference.

Contemplating building a lever group - that way madness lies

The Brugnetti Aurora is my favorite single group vintage espresso machine: minimalist design, bold colors and a fantastic logo in a cousin of the 1962 Eurostyle font. The one in the picture, an HX version, was restored by Orphan Espresso. It was likely assembled and sold in the US by Termozona, an importer from Connecticut who added, what is in my opinion, the criminally design-inappropriate Europa badge on the front.  

Everyone who I have spoken to agrees that they are great machines: simple, well built, small enough for home use, and above all else, capable of producing an incredible shot of espresso. They are, however, rare. The original Sr. Brugnetti closed down and liquidated his factory before the brand was relaunched under new management after a five year hiatus. Brugnetti no longer makes lever machines and I presume that the original group head molds, manufactured by them or a supplier (Tortorelli Meccanica in Sienna?), either no longer exist or would be impossible to track down with my hundred words of Italian.

The heart of the matter is the lever group and the heart of the group is the group body which is a complicated part. So the million dollar question is what would it take to reproduce it?

No, I will weep no more. In such a night
To shut me out? Pour on; I will endure.
In such a night as this? O Regan, Goneril!
Your old kind father, whose frank heart gave all—
O, that way madness lies; let me shun that;
No more of that.


King Lear Act 3, scene 4, 17–22

References

A discussion of building a lever machine from scratch with reference to the aurora. A thread discussing the origins, shrouded in myth, of the machine.
Another couple of restorations of the European Aurora from kaffee-netz in Germany are here and here.

A restoration of a two group model - surely not an original case?