EDIT : The original subject "When are Burrs Worn?" was a rhetorical question.

It has always been my understanding that burrs are worn when the cutting edges are no longer sharp and one could check by dragging a fingernail along the burr and if it 'slices', the burrs are still good. According to suppliers, burrs are good for several hundred kg.

I've had the Macap MC4 for about 3½ year or 182 weeks @ ~1lb / week, 80kg in round numbers. Every ½ year or so, I'd remove the burr carrier and inspect the burrs. Using the finger nail test, the breaking edges (green arrow) are still as sharp as the new set I'd ordered with the grinder. Dragging a fingernail along the flat cutting surface (red arrow), there is still very little difference in between old and new.

As of late, the MC4 had been getting more and more difficult to dial in. Since I had the set of new burrs, I decided to replace them. By chance, the sun was pouring through the window. As I examined the old vs. new, the sunlight caught the edge of the flat cutting edges (red arrow). On the old burrs, the flat cutting edges glint much more. Under normal room light, it was almost impossible to tell.

Examining the burrs in the sunlight with a 10x loupe, the edges of flat cutting area are clearly worn. On the new burrs, the edges of the flat cutting area (red arrow) are just like the edges of the bean breaking area (green arrow).

Gently running a finger, not the nail, against the flat cutting area (red arrow), it's easy to detect the wear by comparison.


Worn MC4 burrs

Didn't check shots before reassembly, so lost new burr photos. Don't want Ken yelling at me for taking my grinder apart too often.

It would be nice if there was some way of telling by taste or some other espresso making factor when to change the burrs. The edges actually don't cut, crush or grind, they act like scissoring channels, forcing the beans from the inside to the outside of the burrs. Presumably, when they get dull, grinding speed will slow down, since the beans will not get forced through as efficiently; and really fine grinding may become impossible.

Also, if the beans are getting nibbled in tiny bites, rather than chewed, as they go through the grinder, there will be more fines. More fines means coarser grinds for the same flow rate, and a rougher taste.

However, people who write about changing burrs always talk as if the burrs actually slice beans, which can only happen in a Kung Fu movie. So I haven't read anything credible on exactly why burrs need to be changed.

The useful life of a set of grinder burrs is going to depend on several factors, even if we don't know exactly how many pounds or kg of coffee should be run through a particular set of burrs before they are changed.

The most obvious factor is the amount of cutting surface, which for planar grinders will equate to the burr diameter. It is well known that small planar grinders such as the Rocky need to have their burrs changed often, every 1-2 years, if I can believe what I read on these forums. As the diameter of the burrs and the size of the grinders increase, the need to change burrs is reduced. At 64mm diameter and beyond one is really dealing with a commercial grinder intended for a business, and these will have hundreds of pounds of beans run through them before even a conscientious establishment will change the burrs.

The original freezing study http://www.home-barista.com/store-coffee-in-freezer-details.html had one small "side experiment" having to do with comparing two identical 64mm diameter (burrs) Cimbali Junior grinders, one of which had brand new burrs and the other of which had 100 to 150 lbs of coffee already run through it. The hypothesis we were testing was that changing grinder burrs more frequently than usually recommended would have an impact on taste, and at least in this blind tasting experiment, this hypothesis was not proven in 64 paired shots tasted blind by 4 different tasters.

Large conical and hybrid grinders have large grind surfaces and I believe that in home use it is going to be well more than a decade before any consideration would need to be given to changing the burrs on such a grinder in home use.

Another factor that might deserve consideration is the type of beans that one runs through the grinder. Some of the green beans that are regularly available, such as some dry processed Africans, come with a "free bonus," chunks of rocks and concrete mixed in with the beans. If these beans are commercially roasted, virtually any high end roaster will have apparatus for "destoning" the roasted beans so that what you receive in your bag of roasted coffee will not include these impurities. Home roasters do not have destoners and other equipment for eliminating these undesirable things, and unless one is careful at examining the roasted beans, it is always possible to have some of them run through the grinder, which can't possibly do the burrs any good. So, a home roaster might want to change his burrs out more frequently.

ken

The beans will remain in the high pressure grind zone for a longer time, so the local heating will be higher. This could denature some of the flavors as well. Slower feed, higher temp, lesser taste kinda thing.

Ken Fox wrote:The useful life of a set of grinder burrs is going to depend on several factors...
It is well known that small planar grinders such as the Rocky need to have their burrs changed often, every 1-2 years...
Cimbali Junior grinders, one of which had brand new burrs and the other of which had 100 to 150 lbs of coffee already run through it.

From the above we can see that the grinding capacity is directly related to the burr diameter, all other things being equal. For a Rocky @ 1lb / week, taking the 80kg through the MC4, we'd expect about 120lb or about two years as is oft mentioned herein. For the Jr. and its 64mm burrs we could expect about 220lb to reach the same level of wear. The blind test between the two Jr's may not have shown any difference simply because the 'old' Jr. was not yet worn. 18 months ago, the MC4 would probably have shown no difference either. Additionally, a test on 1 SO coffee @ 1 roast @ 1 dose doth not a maxim make.

Before the change, I'd been drinking Supreme Bean Ring of Fire for several weeks. After the burrs were changed, almost no coffee exited the grinder at the same setting. The hard and soft zero points are close enough to be identical, but the grind point is about 35° coarser to achieve ~10g 30ml in 30s.

The coffee has gone from a bucket of mud to Marshall's 'Mo' Better Clarity'

In searching for something on my wandering initial readjustment MC4 puzzle, I found this thread. I am experiencing very "hard to dial in a grind" shot pour time. This has been occurring for the last year. The grinder is about 4 years old (2 doubles a day), and the espresso machine currently is an Alex Duetto set at 9bar. The puzzle is that once I get a setting with a batch of coffee dialed, the pour times are very consistent. But when a new batch is introduced just moving the wheel/screw slightly will do nothing or jump the grind by 5 plus seconds....so it is a back and forth kind of approximation and sneak up...
The stepless adjustment mechanism is adjusted closely to the gears, so the threads move as the knob is turned, but I get this initial readjustment kind of sloppy back and forth manifestation in shot pour times. A year ago I inspected the burrs, and there were some chipouts, but no real visible wear. The motor axis was very solid and burr screws tight. Calling Chris's, I was told that minor chipouts were not unusual for even new burrs....But I can't help maybe some form of wear might have an effect. CafeIkes pictures lead me down the path of thinking replacement might be a solution, but I cannot understand how slight wear would create the wandering initial adjustment issue. Have there been any other similar experiences, and am I missing something?
Any thoughts
Thanks

I just changed my mazzer mini burrs that were about 3-4 years old, with about 60 lbs of coffee per year through them. I concluded i needed new burrs when i was having trouble getting non-bitter espressos with certain blends that i used to love (Ambrosia), as well as when i started seeing lots of super fine sludge in my drip coffee filter.

In removing the burrs, they looked to be in pretty good condition, but rubbing the skin side of my finger on them i could feel a big difference.

I've only pulled 2-3 shots, but the Ambrosia is much improved in flavor. I didn't have a chance to brew some drip this morning to see if hte sludge has decreased, but i expect it will have. I'm also having to grind a little bit coarser. At my previous espresso setting, it made what felt like talc fineness

I did order new burrs and changed out the old ones, and cannot say I can tell a significant improvement in flavor , however the grind extraction timing (dialing in extraction time with new beans) seems to be a bit more precise. I was expecting after 4 years use wear might be a factor, but looking at the pictures it does not seem the old burrs are significantly worn. Doing the fingernail drag test, the new burrs seem very slightly sharper both on the inside cutting surfaces, and outside. It is more like the old burrs cutting surfaces were "polished" slightly. The interesting thing is there are several (maybe 3) dings total on the two new burrs. I was told to expect that (my old original ones had several also). The replacement burrs dings I dressed with a diamond stone, but in retrospect I could have left them, since dressing them eased the edge of the adjacent cutting edge. This is a case of VERY small dimensions. It is also interesting to note the bottom burr has about a 0.004-0.009" wobble to the top after installation where the screws were carefully alternately screwed down to allow natural centering. I did not take note if the original installation was perfectly centered, but suspect not, since the burr would settle similarly, unless the holes have a margin of error in the new burrs (macap logo stampings are the same on both sets). I havent tried to mark and recenter the lower burr since offset is so small, and since the outer flat burrs should mitigate that small a wobble. Any offcenter might though, make more of a particle size distribution.
So what is the takeaway? Little wear over 4 years, maybe some improvement, the grinder was thoroughly cleaned, and it is very hard to take pics that show a difference between old and new. In any case the project felt good!