Bunnzilla Goes Stepless - Smoothing Out the Bunn G Series Grinder

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TomC
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#1: Post by TomC »

I've been nagged by a retention issue unique to this modified Bunn G1 + Ditting machined burrs, whereas other stock units I've owned in the past didn't exhibit this problem nearly as bad. It's what led me to decide that I wanted to crack it back open again and see what I might be able to do to fix the 1-3g retention I am intermittently plagued with. While I was in there, I decided to go ahead and make it truly stepless and now it works perfectly.

To do this, it's a lot easier to start with the Trifecta detent plate vs the original one that used to come with older models. I imagine the older detent could be used, but it would take a ton of time to fill and flatten out the face of the plate with epoxy.


Then, mixed up about an erasers head amount of Water Seal epoxy and filled the 27 small pockets in the plate and smoothed it out as best I could without peeling it right back out. I let it dry for about 2 hours but it's ready to sand in only 1.



Sanded smooth and ready to reinstall ( I realize now that these pictures suck, but I just included them to show how it's done and give an idea of the thickness I used).



All reassembled and now it holds its spot perfectly without slipping and is smooth to adjust.

As far as the retention, my night began with the teardown and grinding and sanding the exit chute. It started out showing the rough casting.



And down to nearly a mirror finish. Part of the goal here was to make it easier to clean as well.



The retention dropped to essentially nil, but not consistently. I am still plagued with this irritating thing that happens intermittently where one bean fragment that doesn't get ground with the initial dose, seems to finally grind once the burrs are spinning down to a stop. It bugs me because my previous 2 G1's didn't do this, so I am left wondering how a Ditting burr set could cause this. But the answer doesn't lay in the burrs, it's something particular to this specific grinder from what I can tell. I've considered using the same putty and going in there and filling the small upper corners and nooks/crannies and smoothing things out. I might tackle that some other time. But I'm thinking it might be because of the very chipped and worn auger screw. Maybe a new one would do a better job moving every bit of coffee towards the burrs.

Until I get around to meeting up with Dan Remer and chopping the height of the case a bit, I'll stick with deactivating the bag switch temporarily with a piece of electrical tape on the front. But I did notice one nice added bonus of doing this. The slight rubbery nature of the electrical tape makes the catch cup sit in place nicely without having to force it hard up into the bag chute. It stays there without slipping during grinding too which is even nicer. When I chop the case, I'll bypass the switch, but I think I might leave the bit of tape there, since it can't be seen and it does a wonderful job holding my catch cup.



All told, this whole thing took me about 5 hours, but only an hour of that was the stepless modification. The rest was the slow sanding and cleaning, disassembly and reassembly.
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Chabeau
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#2: Post by Chabeau »

Thanks for doing this Tom.

I thought about doing something like this (with a 3D printer), but I worry the grind setting will shift while grinding or even during startup. Have you notice if your grind setting had wandered at all?

Also, my Bunning behaves the same way yours does when spinning down. There is always that last little bit that gets ground when the burrs approach 0rpm. One bump of the motor takes care of it. I agree; it is irritating. Since it effectively doubles the grind time for an ~18g dose.

Chad
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-C.M. Burns, c.1996

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JohnB.
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#3: Post by JohnB. »

The detent spring tension is adjustable. Properly set the knob should hold any setting between the detents effectively making the adjustment stepless. Tom's mod will make the adjustment feel smoother but you would need to have the spring tension adjusted to hold the setting.

As far as that last bean chip thing remember that these grinders were designed to grind 2 lbs a minute, not single dosing 18g at a time. There are bound to be some trade offs when used in a home environment.
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TomC (original poster)
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#4: Post by TomC (original poster) »

I'd think if the setting is shifting then either the spring is losing tension or maybe the detent pin is worn. But no, it doesn't shift, but I'll monitor it. The Water Seal certainly appears and feels a great deal harder than the plastic. You can sorta see on the picture of the unmodified detent plate, the ball had already started to wear and dig into the plate, so it's got a pretty decent bite into the whole thing.

I remember when I got the Trifecta detent plate, I assumed it set the burr gap from each individual detent pocket (each having a slightly greater depth as it progressed). It wasn't till I really bothered to look at it closely that I realized that the gap is set by the gradually increasing edge thickness instead. I imagine if someone were able to make a material hard enough to stand up to the abuse with a 3D system, they could print an improved plate that has the two stops set just slightly wider apart thru the grind range and therefore enable a slightly more gradual edge thickness to be designed as well, without losing the safety of the stop that will prevent burr rubbing at the fine end. You'd get a more discrete choice of burr gap.
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ira
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#5: Post by ira »

As far as I can tell, the detent plate does nothing but provide detents. You can remove it and the spring plunger that goes into the detents and still adjust the grind. As long as the bearing on the inside end of the knob is free there should be very little force from the grinder trying to move the knob. The adjustment is made by the screw that the knob attaches to.

Ira

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#6: Post by TomC (original poster) replying to ira »

That's what I used to think too. But I think there's more to it. First, objectively, the detent plate creates the stops. Not so important on the coarse side, but it prevents the burrs from touching at the fine side. The next part is certainly up for debate.

I can't speak for every version of the Bunn G series, but I'm more inclined to believe they never changed the basic design of the adjustment mechanisms. The detent plate is needed for the stops actually, but to what degree the rest I'm sure is debatable. The big spring in the back pushes the free spinning burr forward at the same time the threaded shaft pushes in the opposite direction. The detent plate splits the gap and acts like an adjustable wedge between these two points of force. Without it you'll still have the center screw setting the desired grind and the big spring in the back will be pushing the spinning burr forward, but you won't have anything "trapping" the burr in a fixed place from moving back. Forces could still push the spinning burr back towards the fixed burr and create consistency issues.

Now with coffee being forced up between the two, and the big spring pushing, the opportunity for the spinning burr to move rearward is rather small if at all, but there's the potential for it. I'd rather have it locked in place with force from two opposing directions.
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JohnB.
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#7: Post by JohnB. »

ira wrote:As far as I can tell, the detent plate does nothing but provide detents. You can remove it and the spring plunger that goes into the detents and still adjust the grind. As long as the bearing on the inside end of the knob is free there should be very little force from the grinder trying to move the knob. The adjustment is made by the screw that the knob attaches to.
This is correct. The original knob/adjuster assy. that I converted to stepless when I built the first Bunnzilla doesn't use the detent plate or spring/plunger assy. I installed a couple of springs which provide the tension to hold the grind setting when the metal knob is installed. Also changed the location on the stop.

The two lock screws in the side of the knob tighten down on the threaded adjustment pin so when you turn the knob you are screwing that pin/bearing assy. in or out against the spring loaded inner burr carrier assy.



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ira
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#8: Post by ira »

TomC wrote: but to what degree the rest I'm sure is debatable. The big spring in the back pushes the free spinning burr forward at the same time the threaded shaft and pin push in the opposite direction. The detent plate splits the gap and acts like an adjustable wedge between these two points of force.
I don't want to seem like I'm trying to cause trouble, but you're just not looking at it properly. You can remove the knob and use a big screwdriver to adjust the grind with the slot in the end of the shaft. It will still work perfectly though the adjustment might tend to change because the bearing might transfer enough force to turn the screw. In reality, I'd guess the big spring is also unnecessary as the beans will accomplish the same thing but a spring is expected.

Ira

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TomC (original poster)
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#9: Post by TomC (original poster) »

ira wrote:I don't want to seem like I'm trying to cause trouble, but you're just not looking at it properly. You can remove the knob and use a big screwdriver to adjust the grind with the slot in the end of the shaft. It will still work perfectly though the adjustment might tend to change because the bearing might transfer enough force to turn the screw. In reality, I'd guess the big spring is also unnecessary as the beans will accomplish the same thing but a spring is expected.

Ira

With all due respect, for the spring, you're plain and simply completely wrong. Here's some facts as it pertains to my grinder. Unless this grinder is an outlier on all design specs and tolerances, other Bunn G series grinders will share similar, if not all these traits too.

1) On my grinder the spring is entirely necessary. Without it, the spinning burr assembly sits slightly unsupported and poorly centered. The tolerances between the inner sleeve of the auger and the rotating shaft of the motor is not precise enough for it to grind properly without the spring helping hold things in place. So there's no way that it would grind "perfectly" either. Once you back off the screw the whole assembly will just sit there, unsupported and you can slightly wiggle it like a loose tooth. Yes you can set the maximum gap allowed but that is all. And you cannot call it adjusting the grind when you can't even adjust it coarser. Please show me how you can adjust it to grind coarser without the spring? Backing off on the adjustment screw accomplishes nothing when going coarser if the spring isn't in place. Are you going to use a screwdriver and reach up under the chute and try to pry the burrs apart? I took the chute off to easily visualize the burr gap and I've tested it all just to prove the point. You can start out with a desired burr gap and turn the grinder on and watch the burr dance back and forth slightly on the shaft. Never mind that the grinds look like pepper that I crushed under a saucepan. You're welcome to grind all you want at a finer setting without a spring too, at risk of damaging the burrs but I'm not going to try it.

2) The main screw can only set a safe burr gap with the spring in place.

3) Turning that screw will only control the maximum gap that is allowed between the burrs. It will do nothing to ensure even grind size on it's own.

4) The detent plate must be used (and is therefore essential like I alluded to previously), unless you're willing to add things like John did to come up with new stops on either end of the grind range and springs with enough force to hold your grind setting in one spot during use. If you don't want to use the detent plate and spring loaded detent pin, you have to use something else that does the exact same job as the both of them, rendering the point irrelevant.

5) Not doing the above will result in burrs you cannot adjust with any kind of precise control. There's a reason the engineers designed it using the parts they did; they're necessary. The knob can spin in 360° rotation and you'd have no idea where your grind is set if you didn't do what John did to sidestep the detent plate and detent pin.
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ira
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#10: Post by ira »

I did point out the grind would likely change. I didn't point out that it would be hard to repeat adjustments, but it would be hard at best. The only purpose of the knob and detents is to make it easy to repeat adjustments, it has no effect on the stability of the burrs, only on their location.

I do understand that it feels necessary that the spring hold the burrs apart, but I think the beans being ground exert far more force on the burrs than the spring ever will. The auger would feed the first bean into the burrs and that would push the burrs fully apart no matter where they were before that.

My auger fits tight enough on the motor shaft that there is no wobble, if yours doesn't, then it's likely the bushing in the auger or the motor shaft is worn enough to be a problem. The spring should have no bearing on the wobbling of the burrs.

Ira

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