Several years ago I became convinced that to make cappuccino type milk drinks single hole steam tips were best because the extremely high exit velocity from these denatures the proteins in milk very quickly making the type of foam I like for cappuccinos. The diameter of the hole should be such that it transfers heat via steam transfer so that roughly the full power of the heating element is used. (If you make the hole too large the pressure will fall, and if too small it will take a long time to steam the milk.) The heat capacity of water is about 4.18 J/gram-degree. Milk is mostly water, so it will require about 420 J to heat 100 grams of water one degree. You need a bit more in practice because you also need to heat the container. Remember that a watt is a J/second. So a 1000 watt element puts out 1000 J/sec. A properly sized steam tip on a 1000 watt boiler should be heating the milk about 2 degrees/second.

There are engineering formulae that can be used to determine steam tip diameters to deliver this much steam. If you use them you will find that at 1.0 bar pressure the diameter of the hole in a steam tip needs to be about 1.4 millimetres to deliver this much steam. I normally use my Micro Casa a Leva for making cappuccinos, and I normally have its pressure right at 1.0 bar. It works very well with a 1.4mm hole. (It has a large enough boiler so that drawing a bit more steam than the element put out does not cause much of a drop in pressure over the time interval required to steam a drink.)

I originally had the my pressurestat 1999 La Pavoni Europiccola adjusted to about 0.9 bar with a 1.4mm steam tip hole. When I adjusted the pressure downward on my La Pavoni to 0.65-0.70 bar 1.4mm was too small. I put the steam tip in the lathe and bored it out to 1.51mm. That made it about perfect!

There are a series of drills available that decrease in diameter by a slightly varying percentage with each number of their designation. A #55 drill is 1.321 mm, a #54 is 1.397mm, and a #53 1.511mm. Below 1.0mm these numbered drills seem to almost universally used rather than "sized" ones. I used the #54 for the original tip, and bored it out with a #53. Now the La Pavoni works great again!

Great info, Robert. Thanks for sharing.

Robert: What was the improvement achieved by enlarging the hole to 1.51mm?

I got a single-hole tip from Francesco, the diameter is about 1.5mm.
I actually think I might be better served with a smaller hole, maybe 1mm, so that it would create enough microfoam while not burning the milk too fast.

Btw, on an Israeli forum, a member modified the steaming arm, by shortening the sleeve that connects to the tap and re-doing the channel that locks it in place, thus moving the steaming pipe 11mm closer to the boiler, right under the steam vent on the tap. He claims that it produces greater pressure than in a Rocket R58 machine that he got as a gift and returned after 10 days because he is too used to his La Pavoni...

For the pressure setting of the 1998 La Pavoni at 0.65 to 0.70 bar going to about 1.5mm was perfect. With it 1.4mmit took roughly twice as long to steam, and the foam was not as rigid.

With my Micro Casa a Leva with the pressure at about 1.1 bar the 1.4mm is near ideal, and I suspect 1.5mm would be too large. It is amazing that a tenth of a millimetre matters so much!

I noticed that shape of the tip matters too, I get best results with a bullet shaped tip. I have described doing this before, but I take a short piece of brass bar stock, put it in a lathe chuck and bore the small hole first. Then I turn the bar stock around and drill it with the appropriate drill for the tap. Then I tap the threads, M6x1.0 for La Pavoni, M6x0.75 for MCAL. Then I take a 3mm drill and drill from the threaded side to meet the first hole. Then I shape the outside to make a bullet shape with files and sandpaper.

can we see a picture?????

So your equation had you steaming the milk in 51 seconds correct? I can't even imagine spending that much time steaming. My steam boiler also has a 1000w element and it's about 15 seconds for a cappuccino for me.

rpavlis wrote:There are a series of drills available that decrease in diameter by a slightly varying percentage with each number of their designation. A #55 drill is 1.321 mm, a #54 is 1.397mm, and a #53 1.511mm. Below 1.0mm these numbered drills seem to almost universally used rather than "sized" ones.

For those looking to buy a set, these are called wire or wire gauge drills. Buy the best ones you can afford, because the cheap ones dull very easily and subsequently break when the drilling process doesn't go well - more force is not a cure for a dull or broken drill bit. Also, the chucks that come on many inexpensive 1/4" drills won't close down firmly enough to hold drills this small, so make sure yours will actually grip and turn it against metal.

You can do work like this with a hand held drill, but it's difficult to position the drill and work coaxially. A good vise helps to hold the tip securely, but without a lathe (or at least a drill press) it takes sharp drills, great care, and at least a modicum of skill to do things like this. If you wobble the drill at all while boring the hole, the hole will be larger than the drill size and not uniform. And you need to use the right drill speed for the metal you're cutting and the size hole you're making. The web has many drill speed charts and they're all pretty consistent - just Google it.

Typically one heats the milk for cappuccinos to about 60C from about 10C. That is 50 degrees change, so that is about 25 seconds. In practice I find it usually is closer to 20 seconds, because I apparently allowed too much for heat loss, it seems a bit faster than two degrees/second.

This morning I made a cappuccino with the La Pavoni tip bored out with the #53, 1.51 mm, drill. I put my large cap top gauge on the machine. I turned on the machine. I measured out 80 mL of milk using a finely graduated measuring cup. I poured the milk into my normal steaming pitcher. (The cream pitcher from the Frieling serving set, it is just the right size!) When the machine was up to pressure, 0.70 bar, I measured the temperature. It was 9.7C. I steamed the milk my usual way by holding the pitcher until it seems right. This took exactly 20 seconds. I measured the temperature, it was 61C. During the steaming the pressure remained between 0.68 and 0.7 bar. Then I pulled the shot and assembled the cappuccino. It was just the way I like them!!!

Success after a few broken bits. Stock on top right. Drilled acorn nuts 1.5mm. One slightly off centre. Broke all my 1.4mm drill bits