Unfortunately brass acorn nuts, otherwise known as dome nuts and cap nuts, are expensive and hard to get hold of. I got a couple of un-drilled ones from BitsFromBytes.
My plan was to start with the smallest hole I could drill and expand upwards to see what effect it had and then drill the other to the size I found to be the best. Stupidly, I overestimated how thick the dome was and put a centre drill right through the first one. So now I have one with a 0.3mm hole and the other is about 1.1mm.
This is the 0.3mm bit I used :-
If you use a drill or a drill press it is easy to snap drill bits this small but it is actually very easy to drill 0.3mm holes with a lathe. I spin the nut in the chuck and hold the drill in my fingers. I drill from the inside of the dome. The drill finds its own centre and then I apply light pressure. I expect the same could be done by spinning the nut in a drill chuck.
The RepRap design for the heater barrel is just a flat ended threaded brass tube made from an M6 bolt. This is easy to make but not the ideal shape. Brass acorn nuts seem to be machined from a solid piece of brass. The internal thread is made by drilling and tapping. Because it is a blind hole that means that the thread does not go all the way to the end. If you screw a flat ended barrel into it then it stops short of the end, leaving a void that will fill with molten plastic as can be seen here. Molten thermoplastics compress under pressure, so ideally the amount of molten plastic in the extruder should be kept as small as possible to make the start stop response as fast as possible.
I decided to sacrifice my over drilled nut to find out the inside profile by cross sectioning it :-
Not surprisingly, the inside profile matches a 5mm drill as that is the correct size for tapping an M5x1 thread.
This is how far a flat ended barrel can enter :-
This is my attempt to match the profile :-
And this is the improved fit :-
The chamfer at the end is not quite right. My DeWalt bits have a 110° angle but the standard appears to be 118°.
I decided to take a look at steel acorn nuts :-
These are a completely different animal. Rather than being machined out of one piece they consist of a nut with a dome pressed into it.
They have some advantages and disadvantages : -
- They are cheaper and more commonly available.
- They are smaller so less thermal mass.
- The dome is much thinner, about 0.4mm rather than 1mm, so it is easier to get a short hole.
- Steel has a much lower thermal conductivity than brass so plastic may cool down in the nozzle.
- Steel has a different thermal expansion rate than brass. Fortunately it is less so it should get tighter as the extruder warms up.
- The steel dome might spring out under the pressure of extrusion.
- The flat ended heater barrel goes in further but leaves voids at the side.
These are just the nuts I have managed to buy. I have no idea how much they vary from one supplier to another.
The heat insulator in the picture above is an experimental one turned from a bar of soapstone.