Rok Coffee Grinder Reviews
This review of the Rok Coffee Grinder was sent in by Jurien Dekter and is too good not to share in full.
The ROK coffee grinder is a very good grinder with a few quirks. You probably can't buy a better grinder at this price. With a small manufacturing adjustment, this can be one of the very best grinders at any price. Definitely better than the majority of electric grinders which retail for far higher prices. Huge amount better than the ceramic grinders. Sans the manufacturing fault it would get 5 stars.
0. Short Summary
Quite large, occupying a big space. Nice looks, though does not quite match the ROK espresso maker's visual style. High quality steel conical burrs, but mine has a manufacturing fault. Static is a problem but this is not unique to this grinder, and ameliorated by putting drops of water in with the beans before grinding. Hopper needs a cover. Be very very careful of the tacky rubber pad on the bottom that it does not glue itself to your benchtop. If you can get one without the manufacturing fault, you can't buy a better grinder at this price. Streets ahead of 2 well-known ceramic grinders I had first - absolutely no comparison. Do yourself a favour and don't buy a ceramic-burred grinder instead of the ROK.
1. Unpacking and first impressions
The ROK grinder being quite a large piece of equipment necessarily comes in a large box. The grinder crank arm does not come attached to the main works; it is attached by threading it onto the crank spindle on a M6 right-handed thread. A shoulder on the spindle positively stops the crank arm from threading beyond a certain point and effectively locks it on the spindle.
The rest of the grinder is ready to use, nothing further to assemble or attach. A simple manual shows how to adjust the grind setting and do grinding. A separate sheet informs the user that the grinder has been pre-assembled with an infinite grind setting ability as opposed to a setting with detent stops (more on this later). There was also a spare plastic washer in the box.
The grinder is quite a striking piece of equipment. Shiny steel and polished aluminium complement the looks of the ROK espresso maker. It is roughly the same overall size as the espresso make but different in shape. The base is rectangular as opposed to oval; the vertical struts are steel bent tubing as opposed to shaped cast aluminium. For my personal taste I find it a bit strange that the two pieces of equipment aren't more similar in looks and presentation. Perhaps economics and design parameters determined the shape. Just a small issue that doesn't detract from performance, of course.
Attached to the bottom of the base is a clear plastic sheet protecting an inch-wide rubber strip all around the perimeter of the base, meant for preventing the base from sliding around. More on this later.
A black numbered wheel right at the bottom of the hopper/grinding works is used for adjusting grind fineness.
An aluminium cup catches the grinds as they exit the burrs.
2. First grind
I jumped into grinding a single dose straight away. Beans are tipped into the hopper and you begin cranking away while holding the top stable. A little vane near the bottom of the hopper churns the beans around to ensure they fall into the burrs. I found that the burrs tend to shoot beans out the top, making the occasional bean fragment jump right out of the hopper.
Lots of static! Grinds stick to the adjustment wheel and outside of the body.
I didn't adjust the grind but the first dose came out feeling quite similar to my current grinder, a Rosco Mini Hand Grinder. Grinding was easy due to the long crank arm and wide base providing a lot of stability. I was immediately struck by how quickly the grinder worked through the dose; previous ceramic burr grinders I had took much longer for the same amount, several times longer in fact. The ROK grinds about as fast as my Rosco.
Grinding makes a bit of noise but generally much less noise than an electric grinder.
The espresso shot made with this grind was quite good but grind fineness needed a small adjustment. I guess the grinder was set up at the factory to yield an espresso grind.
3. Grinding mechanism
The crank handle rotational movement is horizontal, and is translated to the vertical shaft by a set of conical nylon (self-lubricating) gears. This works well enough and I don't see this ever giving problems.
The grinding is done by a set of hardened steel conical burrs. I found these to be of the same size as the ones in my Rosco, explaining why they grind at similar speed. The bottom burr is mounted on a driver which is locked to the vertical shaft by fitting on a flat portion of the screw thread on the bottom end of the shaft. So as the shaft turns, the flat portion forces the burr driver around, which in turn forces the burr to rotate. The burr driver drives the burr by a couple of pins that engage a matching set of holes in the bottom of the burr.
Expect the steel burrs to last a lifetime. While commercial setups may need burr replacements, this unit for home use is unlikely to ever get to the point where replacement burrs are needed. I have been using my Rosco Mini for about 4 years now and the burrs seem as sharp as when new.
4. Issues found during settled use, and compared to my Rosco
4.1 Static: The pamphlet mentions using a few drops of water to combat static. Just add a few drops of water on top of the beans after tipping a dose into the hopper. I found this works well enough; after the grind is finished, a few bangs on the backtop with the whole grinder tend to shake loose most grinds from the works into the cup. Static is also a problem with my Rosco Mini when grinding a single dose; but due to the fact that everything is closed on the Rosco, you don't get grinds flying everywhere. Also, when grinding for 2 cups, the screw-on cup of the Rosco becomes completely full and somehow this dissipates the static charge.
4.2 Grind setting adjustment: Adjustment is done by holding the crank arm and turning the adjustment wheel. 12 numbers on the wheel tell you how far you are adjusting. I had the nagging feeling that the infinite grind setting tended to back off during use; I often tightened it up a bit. I wasn't rigourously checking this by noting numbers, just the shots became a bit gushy, prompting me to tighten the wheel. There is nothing positively locking the adjustment wheel in infinite mode.
After a few weeks I decided to try the original design of using the detent stopped adjustment. This is achieved by fully unscrewing the adjustment wheel and removing the washers between it and the bottom of the burr driver. The washers' function is to present the adjuster wheel with a smoothly turning adjustment; removing those washers make small bumps on the wheel fall into the matching detents on the burr driver. Adjusting the grind, I felt that this adjustment was too notchy; I had the feeling that after a short while, the bumps would be too worn down to provide adequate stops; I ended up sticking a layer of duct tape over the bumps to provide some protection. Adjustment felt smoother but still firm enough to lock the assembly at a given stop.
I understand ROK provided the infinite adjustment capability after user requests; not sure this is not splitting hairs, as the thread pitch is 1mm, and there are 12 stops, so this means each stop gives an adjustment of 83µm (0.083mm). Does it go beyond reason to want finer adjustment than that? Besides, here is another glaring issue that renders this point quite moot:
4.3 Burr eccentricity!! At one point while looking down into the hopper while turning the handle, it seemed to me the burr was not centered exactly. As it rotated, I could see the burr wobbling. I unscrewed the adjuster and checked the burr driver tolerances but it was reasonably tight and I couldn't make a difference. This is a big issue; coffee grinds need to be of consistent size to yield the best extraction. It seems to me that the uneven gap between top and bottom burrs will cause grinds to be of uneven size.
To track down the reason for the eccentricity of the rotating burr, I disassembled the entire grinding mechanism. This involved gently tapping out locking pins from both nylon gears, freeing the top shaft to be pulled out, and tapping out the locking pin of the little vane on the vertical shaft, freeing that shaft also to be removed completely.
My first thought was that maybe the vertical shaft was bent. This was easily checked by rolling it on my kitchen bench top. It rolled completely smoothly, eliminating the bent shaft theory. Next I scrutinised a slightly narrower section on the shaft bottom: The shaft itself is made from stock 8mm steel rod, and the bottom bit where the burr driver slides on, is turned down to 7mm. On this 7mm section, there is a thread cut for the adjuster wheel, and a flat side for engaging the burr driver.
Using a feeler gauge, it found that this 7mm section was eccentric by 50µm, and that is the cause of the burr side-to-side wobble as it rotates. Effectively, this means that if the bottom burr cone is touching one side of the top burr, the opposite side has a gap of 0.1mm. So you can see that the 83µm adjustment stop size is ruined a bit by this 0.1mm eccentricity.
It is not clear to me if mine is the only one doing this wobble; it is clearly a manufacturing fault - either the lathe spindle was out by this amount, or a piece of metal swarf on one of the lathe head clamp jaws caused an offset, ruining the eccentricity. But what I do know, is that this is easily avoided and really an inexcusable error. I tried to compensate for the error by packing it to one side with thin tape but this did not yield better results - as mentioned before, the burr driver has tight tolerances, not allowing this big an error to be compensated.
I found that my Rosco grinder which uses exactly the same type of burr, gave consistently better shots than the ROK, and I am convinced this is the reason. It is a pity because the ROK could easily have yielded the exact same grind quality as the Rosco Mini at a much lower price point. Again, I am not sure how many ROK grinders are out there with this problem, because it is clearly a manufacturing issue, not a design issue.
I would recommend that ROK sends users a replacement shaft. It is a bit of a finicky procedure to replace the vertical shaft though - I found it easy enough but it may not be for everybody.
4.4 Manual grinding gets old: Anybody who has tried a Rosco Mini will attest to the fact that it's hard work - very hard. Manual grinding with quality steel burrs is VASTLY different to grinding with a ceramic burred mill. The grinding rate is much faster with steel but this necessarily takes more torque to crush those beans. The ROK is much easier due to its longer crank arm - more leverage. Nevertheless, it gets old. So pretty early on, I unscrewed the crank arm from my ROK and threaded two M6 nuts on, and locked these against each other. I use a battery drill with 13mm socket attached to drive the spindle - easy as! The battery drill must be on its high torque setting to cut the beans, and consequently turns quite slowly so it's only marginally faster than hand cranking, but a lot easier on the shoulder especially if you have guests! Battery tools are dirt cheap these days so you can easily buy a nice small drill for just this function. Just make sure it has a low speed, high torque setting. A spare battery pack on the charger makes life easier as well.
4.5 It's different for lefties: Because of the right-handed thread of the crank arm, left-handed people who want to operate the grinder must do so backwards, i.e. turn the handle over the top towards you to prevent it from unscrewing and more importantly, getting the burr to turn in the clockwise direction. Not a biggie, I think.
4.6 Open-topped hopper: The little vane tends to churn beans a bit too vigorously when grinding, especially when using a drill, making beans escape the hopper. In addition, bean fragments occasionally shoot up and out of the hopper, so the hopper really needs a cover. I cut a round fast food plastic container's bottom off, that was an exact fit. A slot to the middle enables the make-shift lid to slide around the shaft.
4.7 Stepped hopper chute: The hopper does not descend smoothly down to the burrs; instead there is a little shelf. It is not clear to me why this shelf was included in the design. The shelf makes beans get stuck, so this makes the little churning vane necessary. Without the vane the beans tend not to continuously drop on the burrs - I tried it without the little vane, and was forced to put it back in after I found the beans don't descend. The little vane is also slightly too high as often a bean or two will still lie undisturbed on the shelf as the vane passes above them. Maybe that was done for a reason, like avoiding the vane getting stuck against a bean. Not a big problem.
4.8 Aluminium cup: The little cup that catches the grinds is quite fiddly. Static makes the grinds stick to it. It is a bit small to grind two double shots' worth of beans into it. It is very slippery; this, combined with the tapered shape, caused me to drop it a few times while going through the motions of making shots, so eventually I defaulted to my slightly larger, straight-sided plastic cup which does not pose a static problem either. The cup needs to be high enough to it is higher than the adjustment wheel, but not so high so that removal becomes a problem.
4.9 Silicone rubber stability pad on the bottom: After about a week of use, I decided to finally remove the protective plastic from my ROK's stability pad as it was sliding a bit on my kitchen benchtop as I cranked it. So I pulled the plastic off, placed the grinder on my benchtop and proceeded to grind a batch. I tested the stability by not holding the grinder as firmly as I usually do, and yep, it was stable.
I got a bit of a shock when trying to put the grinder away afterwards. It was VERY FIRMLY glued to my benchtop! I could absolutely NOT lift it. I pushed quite hard against the top to lever it off but no go. I was concerned that I would actually break the top assembly loose from the base, or delaminate my benchtop. So in the end I got a big flat screwdriver and levered the base carefully away from my benchtop. Even so it took quite a bit of effort. I checked the silicone rubber when I finally got it off; it is the type that stays tacky. My benchtop is smooth & shiny so you can see that the large tacky rubber area was completely effective at firmly gluing the ROK to my smooth benchtop! I put the protective plastic back on. Maybe much smaller pads would be good, or a less tacky substance.
The ROK grinder is a quality grinder with a few small design quirks, and a manufacturing fault of non-eccentric burr. Despite this it still delivers good grinding; if ROK fixed the manufacturing fault (and I have no idea how many units would be affected) then you can't buy a better grinder for the price. Even with the fault, it is an excellent choice, way better than ceramic grinders such as the Porlex.