renatoa wrote:Ok, I see they go forward with this ludicrous claim and goal, so it's time to demolish these false claims.
Beware, the following are not guesses, but simple math and logic.

If this 3 microns per click would be true, then this means that the whole range of particle size able to be dialed will be 3 * 180 = 540 microns.
This is simply not enough for a grinder to be capable to switch from espresso to brew. You need at least 800 microns range, from 200 to 1000, for this goal.
This is the first fact.

You are perhaps right on the account of needing a coffee particle size range from 200 to 1000 microns to go from espresso to brew.

Where you are wrong, however, is in assuming that the burr spacing translates directly to the particle size.
It doesn't, and you can check that by measuring the burr spacing and the resulting grinds size distribution.
Furthermore, the relation is not only not direct 1:1, but it is also not linear: +3 microns of burr spacing doesn't always mean +3n microns of resulting particles. The differences in grind size get bigger, as the spacing gets bigger.
It very much depends on beans flexibility - that's why different coffees, even same roast batch stored differently - can grind visibly differently on the same grinder, same settings.

However - before you say 'AHA! So this 3 micron resolution bullsh** is worthless, exactly as I said!" - it is obvious, that the more incremental steps in burr spacing, the more incremental changes to the resulting grind size, even if this relation is not direct nor linear. Thus giving you more precise dial-in opportunities.

You mention that 3 micron increment is comparable with the bearing play, thus meaningless. But it isn't. The bearing play is a 'constant variable' factor - it is mostly same in any grind setting, its +/- influence on actual burr spacing linearly increasing with the vertical burr spacing, as the distance of the inner burr from the lower bearing increases. That's another reason why grind distribution tends to be tighter with finer grind settings than with coarser ones, an effect very visible with cheaper, more wobbly grinders.
So from one grind setting to the very next, the bearing play's +/- influence on burr spacing remains almost exactly the same, making the difference between average minimal burr spacing 3 microns still.

renatoa wrote:Now the second, that demonstrate the 6 microns claim wasn't true, thus even more the 3 microns claim.
Measured the burr cone and it's slope is 73 degrees at base, so 17 degrees ar top.
(...)
A last note, the burrs required for a 3/6 microns particle resolution should have, according to basic geometry, 80 degrees slope:
sin (10 deg) = 0.17 = 1mm / 30 clicks / 6 microns
So almost cylindrical. Definitely not a typical Italmill burr !

Peter himself stated, that the 80 degrees slope applies to the lowest few mm of the burr - the so-called calibration band. This is the part of the burr which defines the narrowest gap, and this part is relevant for measuring burr spacing changes as the burrs move vertically and making any kind of resolution comparisons of different burrsets/grinders.

Reviewed again the video, and there is no mention about the degrees slant of the calibration band. Nothing related to 80 or eighty. Maybe elsewhere...
Actually, it is exactly viceversa, at the base the burr has a lower angle than the 73 degrees average, it is in the 6x degrees range.
Check the burr image and you will see what I mean.
userpix/29507_arco_burrs_hb5266TE.jpg

Actually, this angle change is exactly the detail that make possible your explanations, i.e. resolution increasing for coarser grind. Else, with a perfect cone, would be more difficult to explain it.

Except this, your post is very good, exactly the arguing I expect to read here.
However, with three turns I still feel limited at the coarse side of grinding.
Ok, I understand that adding a new engraving on the grinder means additional costs, I agree to not have the engraving for the fourth turn visible, but please don't limit mechanically the choice of users to rotate the ring beyond the third turn mark.

renatoa wrote:Reviewed again the video, and there is no mention about the degrees slant of the calibration band. Nothing related to 80 or eighty. Maybe elsewhere...

The 80° (or 10° measured the other way) is from post #23

PeterTheGoat wrote:The angle of the "calibrating band" part of the burr (the lowest few mm on the cone of the burr) is 10°.

k0nfuse is quite right in his comment
and yes; as previously stated; the calibration zone of the ARCO grinder is at 10° while the the rest of the burr is not as steep. Compared to the Comandante where the calibration zone is about 15° again with the rest of the burr not as steep. Neither is better or worse - you just have to take that into account when deciding how steep you want the adjustment thread to be.

I just did a quick sifting test for a couple guys that requested it on the campaign page with our light roasted Colombia Marinela Microlot (we have a small roastery as a part of our office).
The purpose of this test is to show that 540 micron burr spacing produces much coarser grinds than 540 microns.

We currently don't have sieves bigger than 1100 microns at the office on our motorized Kruve sifter, but those will suffice to prove this point.
I opened the ARCO fully to what would be a calculated 540 microns and ground up 40g of coffee; here is what I got after sifting:

above 1100μm: 35.8g

above 1000μm: 1.0g

below 1000μm :2.6g

Optically I can say that the particles above 1100μm extended into the 2000μm range
Obviously it would be better if I could do the sift up to 1600μm, but we just don't have those sieves at hand right now.

But we will be publishing the lab test results done with a Laser Diffraction Particle Size Analyzer today or tomorrow anyway and one of those will be for the coarse side of the grind spectrum. They will be available on the KS page.

Peter

So the conclusion is the resolution is variable, ranging from 3 in espresso area to whatever 6-12 in the brew area.

Still... if Commandante crowd is happy with 12 clicks per turn, I don't get why you should get trapped in this resolution race, when is obvious that 30 clicks were more than enough...
There are already complaints that now is too difficult to switch from a setting to other and remember values due to the increased number of steps.
The Kinu guys, with stepless adjustment and 50 divisions per turn are rarely using the third digit when talking about a grinder setting. So they are mainly using 10 divisions per turn and are happy... keep in account that 10 divisions per turn of Kinu equates to 20 "clicks" for Arco, due to their thread pitch, half as fine compared to Arco.
Not always more is better

Again; the clicks and turns are marked with numbers on the ARCO
This makes it no harder to adjust with 60 clicks per turn than it would be with 30 or 5 clicks per turn.
You don't have to zero the burrs out and count from there. You just go to position 23 on rotation 3 for example.
There is no drawback to having more clicks in a certain range.

Agreed with Peter there. On my 1z JX I have a piece of paper handy that notes all my desired clicks for various coffees and brews. If you're switching coffees and styles frequently, no doubt you have a similar solution, and you'll never forget your spot.

@peter, thanks for all the info here, I think the HB community is all the better for your transparency, and for sure that only fuels the conversation, but we all get to learn a lot more for it! We are an analytical crowd, but everyone here is passionate to a fault when it comes to smashing up beans and drinking their blood.

@everyone thanks for teaching me more than I ever realised about calibration bands, grind sizes and burr geometry

Hi Peter,

I was going to post this on kickstarter but it seems that the interesting conversation is taking place here.

I have a question about the motor which is specified as a 15000rpm, AC motor. I agree that 15000rpm would mean a small motor size but its not clear how this is achieved. I am wondering if this is a mistake and the motor is actually an asynchronous motor which operates at 1500rpm at line frequency.

15krpm is more typical for a synchronous AC machine which needs a drive, or a DC motor which also needs a drive but is more common in appliances as it tends to be cheaper. Based on the video demonstration the grind speed is variable (speed ramp when grinding finishes) which implies that there is no speed control.

Also, the gear ratio for stepping down from 15000 to 360 is 40+. It was mentioned in the campaign that ARCO uses a belt drive rather than gears, but that would mean that the ratio of pulley diameters is 1:40, which is quite big.

Can we get a bit more info about the motor? are the specs correct?

renatoa wrote:The Kinu guys, with stepless adjustment and 50 divisions per turn are rarely using the third digit when talking about a grinder setting.

Just as a side note, we "Kinu guys" rarely mention we use the third digit because it is pointless to tell precise settings to other users because of variability in production of the grinders, different beans, water, machines and what have you. I usually mention my "espresso range" etc but even that is by and large pointless. I do however use the in-between settings regularly and am happy to have them available.

_sawas_ wrote: ...
Also, the gear ratio for stepping down from 15000 to 360 is 40+. It was mentioned in the campaign that ARCO uses a belt drive rather than gears, but that would mean that the ratio of pulley diameters is 1:40, which is quite big.

Can we get a bit more info about the motor? are the specs correct?

Yes, the specs seems right for a DC motor followed by a planetary gearbox, 40-60 ratio is quite common.
A similar solution solution is used by a Turkish company to drive Commandante grinder, 24V DC, 1:100 reductor, check here:
https://www.motordante.com/
The gearbox ratio seems more than double than Arco, but also RPM is less than half, 80 to 96.

My guess about the pulley... it is 1:1, not involved in reduction, used just to obtain a more compact shape of the whole assembly.