AndyS wrote:1. The rate of grind varies very little with burr speed (~5g/sec regardless).
2. Increasing the burr speed results in a coarser grind.

Interesting findings. If the grind rate is invariant with speed, then the burrs have a mechanical capacity limit per unit time. As it appears that Andy's robur works better at a slower speed, one could speculate that many grinders are not configured to run at their optimal speed. It's simply cheaper to manufacture grinders with "standard" speed motors. So if you have a VFD or can change the motor/gearing, running at a lower speed may minimize noise and grind heating without reducing grind quality.

Not certain why increased burr speed produces a coarser grind. Could be reduced compressive load, mechanical inefficiencies e.g. slippage, burr chatter, size distribution, etc. As I can't think of a reason why frequent speed adjustment would be desirable, this likely isn't of practical concern but more useful for understanding how burrs work.

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another_jim wrote:the grind setting needs to be finer when grinding few beans, either when single dosing or when the hopper is near empty. This occurs because with fewer beans in the burrs at any one time, there is less bean to bean crushing action. To get the same fineness, the grindstones have to pick up the slack; and they do this by being closer together.

I like that explanation, thanks.

wookie wrote:If the grind rate is invariant with speed, then the burrs have a mechanical capacity limit per unit time. As it appears that Andy's robur works better at a slower speed, one could speculate that many grinders are not configured to run at their optimal speed. It's simply cheaper to manufacture grinders with "standard" speed motors.

I wasn't really clear about this. It's not exactly the case that grind rate is invariant with speed. Running faster rpms produces coarser grinds, so one adjusts the burrs closer together. Closer burrs slow down the grinding rate...so the one adjustment more or less compensates for the other, in my limited experience.

As far as "optimal speed" goes, it always seemed to me that the Italian grinders are engineered to run at 50 Hz. If you bring them to N America, they generally run at 60 Hz, so the burrs spin 20% faster. Is that enough of a change to be significant? I don't know, but it seems likely that they'd heat up more in commercial use.

Also, with the Versalab M3 grinder, one can adjust the rotational speed with the little potentiometer on the DC circuit board. Running too fast seems to generate some off, metallic flavors.

It occurs to me that the specific geometry of the burr set causes the description of the burr set performance to be correspondingly different with each different set. Any set that has progressively smaller geometric openings would eventually plug or get very hot at the point of compression because once the volume between the tooth forms are full, this would result in compression of the incompressible solid coffee grounds with little or no relief.
The relative rotation between stationary and rotating burr would not assist in advancing the ground material to the discharge end of the burr set without the push of the tooth form's helix angle. About the only way to visualize this is to have a set of burrs that you can inspect in your hand while thinking your way through the internal flow of coffee beans and fragments while simultaneously rotating the appropriate burr of your set.
It is easier to focus on just one geometry set such as the Italmill 68mm burr set which is found in several Titan grinders, while trying to get these issues straight in your mind.
It appears that the beans are crushed and fragmented in the relieved entry zone to be small enough for the pieces to enter the Helical part of the burrs. The actual cross-section size of the tooth form is essentially the same along the length of the twisting tooth profile. However, since the teeth are on a conical shaped surface, the volume of the space between teeth must actually remain constant until it reaches the end of the tooth space at the discharge of the burr set. The teeth can never touch or mesh, all they can do is fill the tooth form clearances and the gap between the two cones determined by the grinder setting gap. It is important to note that this gap once filled remains filled and it appears that the larger fragments partially protruding out of the tooth pocket can interfere with one another and result in fragment to fragment crushing in the gap. Continued fragment travel seems only driven by gravity and friction of the material in the helix on the protruding material on the opposing burr. On this burr set I have observed the ground material exiting the burr-set during grinding on a conical shaped fan trajectory in pulsing bursts. At the end of a grind it slows to a drizzle of coarser particles released because the cones can become temporarily misaligned to one side by pushing the outer burr to one side and let bigger fragments out up to the size of the maximum diametrical clearance due to the partial fill. I find it hard to effectively describe these observations in words, but I think you will get a much better understanding with the real parts in your hands using your own observation skills. I hope this description is clear enough to be useful.

hbvd2 wrote: ... the volume of the space between teeth must actually remain constant until it reaches the end of the tooth space at the discharge of the burr set.

Thanks for the excellent post; but I'm either misunderstanding this part, or I think it's wrong. Imagine the grindstones without teeth -- the actual surfaces are getting closer together as the bean gets ground, which is how they are crushed into finer pieces. The scissoring action of the burrs forces the beans through the narrowing burr, but the crushing is done by the surface, not the burrs (think of "ghost teeth" grinders or the way flat burr grinders get higher towards the outer part)

No need to apologize for any lack of words to describe what you see....seems like you pretty much verbally covered it . Very difficult description and you made it quite clear.

I think what you may be seeing with the noted pulse and fan shape discharge can be best analyzed as you say, with the burr set in hand. When turned upside down (teeth down) it is not possible to engage the final faces of the inner and outer at all 360 degrees around the circle. There is a plane at 180 degrees all around that one can feel "wobble" of the inner on the outer burr so if you have touching of the cutters or burr lower edge there is a small opening at 90 degrees from the two touching areas. This slight asymmetry allows the coffee to exit at grind speed. The same can be sensed when adjusting the burr tighter on a running machine...the burr will only touch at one spot, but is perceived as a BZZZZ sound of metal to metal only. One can not tell how much of the burr is engaged in this rubbing but it is NOT all 360 degrees.

This wobble is necessary for proper grind but some people interpret it as misalignment. It must be there or as you said, the burr would simply plug with coffee and jam, or it would become more and more like a grist burr with no blades or simply two nearly smooth conical surfaces.

I am taking a Titan burr at the machine shop to have cut in half. I would like to see it the small cutters are parallel or form a wedge shape narrowing to the exit. I do not have to sacrifice a good burr to do this since I dropped an inner burr on the concrete floor and found out that yes, they are not exactly fragile but can have the edges of the metal chipped and broken much like cast iron .

Doug, could you please check your inbox?

Thanks!

Hi Jim,
What I was trying to point out there, was that along the tooth opening, when there is open volume,that is " a partial fill" there is room for more detritus to fill in those voids. If the geometry is is such that the free volume is constantly diminishing and it fills in these open spaces such that there is rapid compression, that it can stall the material flow out of the grinder. I have experienced this by setting the burr gap to a very tight setting. Although I know this observation is somewhat anecdotal and to be sure, one cannot actually see inside or measure this phenomena, but that condition seems to induce what I describe as a plug or stall. Under these conditions you can maintain grinding speed, but no material discharges.
I'd be the first to admit my observations may not be interpreted or reported correctly by me. As to the other types of grinders that you mention, I have insufficient experience with them to comment. Best Regards!

The height of the burr over the grind stone source is constant. But in conical burrs, the angle of the inner burr is less vertical than the outer burr, and it approaches it more closely at the bottom. The shape of flat burrs creates the same effect. Since the diameter of the burrs increases as you move downwards or outwards, the total volume may stay the same, but the distance between the two burrs decreases. if it dod not, the coffee would no longer get crushed into finer pieces.

In effect, a bean starts as roughly spherical at the inside/top, and exits as a pancake of particles at the outside/bottom. The volume is the same, but the thickness is much reduced. This pancaking is achieved becuase the grindstones get closer as the bean particles move through. The easiest place to see this is in a roller mill, where two cylindrical grindstones nearly touch at their closest.

Doug, I don't think you need to cut the burr in half. Take a piece of clay, play dough or plumber's gunk and press it into the burr. You'll get the relief reverse that might tell you what you want to know.

Andy, why only 36hz? Wierd electrical noises or not enough torque? For the former there should be frequency setting somewhere in the vfd menu that can put it out of the audible range. For the latter you can get nearly full torque at like 6 hz with some of the newer (vectorless?) Drives.