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Tri-clamp biped build
Anot
Hi! I've been lurking around this forum since april-may when I was looking into building another roaster and I wonder why I had not stumbled upon it earlier. The information gathered in this forum has been invaluable to me and I now want to give back something by presenting my design. It is very similar to other tri-clamp fluid beds but naturally has a few unique design choices.

1. Introduction
I started roasting roughly 2 years ago. Previous experiments were mainly with a breadmaker and heatgun which gave varying results. I managed to get some great cups out of this which spurred my interest further. I later moved to an apartment without a balcony and got a gene so I could roast indoors under the kitchen fan. I was never really happy with it and the resulting coffee often tasted burnt despite lots of testing and varying variables. It also had a problem with chaff not collecting properly (instead staying in the roast chamber) so I suspected a bad fan but opening it up did not present any obvious flaw. I probably could have troubleshot the machine better but the lack of good coffee made me fed up, which was exacerbated by all the fiddling with the roasting chamber, limited data output and lack of automatic logging. So I decided it was time for another roaster.

2. Goals
1. Batch capacity of 200 g
2. Minimal indoor smoke
3. Good chaff collection
4. Software controlled
5. Relatively compact. Should be able to fit on my kitchen windowsill while still being able to open one of the windows
6. Easy, preferably in-line, cooling
7. Easy bean dump

I decided to make a fluid bed as it seemed a good choice overall, although goal 7 posed a challenge

3. Main construction

I went for the tri clamp system as a base for its ease of assembly. See picture below. The right box (25 cm * 19 cm * 20 cm) contains all electronics. The walls of the box were 3D-printed while the bottom consists of a plank onto which the electronics and steel construction are fastened. I had originally planned the roaster to be 15 cm shorter, but had to add a pipe before the roast chamber in order to get a good thermocouple placement. To balance the left side I added a small black drawer found in a thrift shop, which incidentally allows for storing spare parts and such.

i.imgur.com/C75zgfh.jpg

Roast chamber:
Starts with a cone (1.5"--> 3"), then a 3" inch sight glass and ending in a 3" T-pipe. The top of the T-pipe allows for loading and dumping of beans and alleviating back pressure early in the roast (more about this under fan). The T-pipe is connected to the cyclone separator via another cone (3"-->1.5").

Cyclone separator
https://www.bangg...rehouse=CN
This was "adopted" to the tri-clamp family by adding a 1.5" wedge on the inlet, and a 2" wedge on the bottom. The wedges were fastened by screws and nuts. The gaps were filled with fire clay. Drilling through the wedges required a lot of effort, partly because they are made of stainless steel but also because they are rounded making initial threading harder. I therefore had problems getting the holes exactly where I wanted them, but this could easily be compensated for when drilling the aluminium separator which was like butter in comparison.

Exhaust

On the cyclone separator outlet, I have fastened a flexible exhaust pipe (36 mm diameter, https://www.bilte...2000017821). I would have prefered a smaller diameter as the separator outlet is only 25 mm but could not find one. The exhaust pipe was cut to 50 cm which suited my setup. It is fastened with 3 screws/nuts only. It was slightly tilted over the separator outlet to minimize air leaks. It is of course not completely airtight but good enough. I can easily bend the pipe out my window during operation, without needing any other support for the pipe. I have not seen this solution before and consider it one of the highlights of my build. Note that it was quite challenging to get the screws in, the first one was relatively easy but the rest had to be guided in through the bottom of the separator. With a chopstick, some tape to adhere the screws too (and some cursing) I could get it into place. The screw length needs to be chosen carefully: should ofc be long enough to fasten the exhaust pipe (accounting for diameter difference of the parts), but also it should not be longer than the diameter of the separator outlet (including thickness of screwhead and chopstick) in order to get them in. One other solution could be to simply drill through both sides and use longer screws, though that might have some implications for back pressure.

Chaff collection / left leg
I'm using a 2" sight glass as a chaff collector and a ball valve to empty it. I still need to fabricate a funnel to guide the chaff into the container below. It is also necessary to poke around in the collector (using a chopstick atm) to get it to empty properly. So the idea is to make funnel with an inbuilt "poker". We'll see how that goes. The underside of the ball valve doesn't get hot so using a plastic like PETG here will not be an issue. The reason for having a ball valve was to use this route as both the chaff and bean dump but this didn't work out (read more about this under "bean dump"). In hindsight I would skip the ball valve and 2" sight glass and just fasten a mason jar to the cyclone separator.

I had originally planned to just let the chaff collector hang free, but the whole left "leg" felt a bit too heavy for this. I therefore made it connect with the underlying surface. It is connected to a 3D-printed "foot" with a place for a container into which the chaff is dumped. The base was fastened to the ball valve by threaded rods. The threaded rods were screwed into extended nuts (that I had swapped to from regular nuts) on the bottom side of the ball valve. On the foot the threaded rods are inserted into holes (but without touching the bottom of the holes), the weight is instead supported by the nuts which rest over the holes. This allows for making minor adjustments (+/-1 cm) to the height by turning the nuts up or down which makes precise height adjustment possible.

i.imgur.com/Xr9zGPQ.jpg

4. Electronic components
Schematics
i.imgur.com/ilOM2FI.png
Notes:
- My TC4 has a 5V pin beside the gnd on the IO headers, so all orange lines are actually connected to the IO3 pin-cluster but didn't bother enough to change it on the schematics.
- Also not shown is the on/off switch which is connected to the live and neutral wire of the power cord before it is split to SSR/heating element and DC psu. In hindsight I guess it would have been smarter to connect the switch only to the heating element.

Fan
WS7040-24-V200 : 65 W, 280 LPM and 6.5 kPA of pressure: http://www.wonsma...17949.html
I took inspiration from Sinobi for my fan choice (see here http://www.sinobi.dk/henrik/coffeeroaster1). His roasting chamber and inlet diameter are similar to mine so I figured his testing would be relevant for my system. This fan seemed to have a good balance between pressure and airflow and on paper (compared to the mattress pump 300 LPM and 3.5 kPA) the fan was easily gonna be sufficient for my intended capacity of 200 g. This turned out to be only partly true, while it can loft 200 g of lesser dense beans it is not sufficient for high density beans. For dense beans the capacity seems to be at least 170 g which I am OK with, but in hindsight I would choose a slightly more powerful fan. The top cap on the T-pipe needs to be open in the first minute or so to alleviate back pressure limitation in my system. As there is no smoke and minimal debris at this point this isn't an issue. I consider this another, albeit small, highlight of this build. This principle can be used to somewhat increase batch size in systems limited by backpressure as is often the case with the common aluminium separator.

I don't have a perfect spouting action with my fan/setup. It starts out spouting but gradually turns into making a wave motion going up along one wall and down the opposite wall before going back to spouting (and some bouncing) when approaching first crack. But the roast development is even so I am content.

Note: according to the instructions my fan should not be turned off by power but instead by the EN-pin, either by connecting EN to GND or to pin in a low state. I don't really know why, and if it is required when using PWM and a flyback diode as I am. If anyone knows more about this please let me know. Anyway, it was easy to comply with this by connecting EN to an arudinopin (12 in my example) and adding the below code to the arduino (I am using arduino uno + TC4 with pwm control). The IO3 cutoff value of 30 was chosen as my fan will never be below this value during roasting and because it will be apparent if the pin state-switching is working.

if (levelIO3 > 30) {
digitalWrite(12, HIGH);
}
else {
digitalWrite(12,LOW);
}

Heater and SSR
2000 W heat gun element with mica paper. In line with a thermal fuse. The SSR is some random chinese variant that specs a very questionable 25 AMPs, but should be OK for my load of 8,3 AMPs cross fingers

5. Software control and operation
Software control/roasting
I am a novice when it comes to microcontrollers and went for an arduino uno with the TC4+ to make it easy. The TC4+ might have been a bit overkill in my case but the sleekness of just adding it onto the arduino is worth it either way for a compact system imo.
I run the arduino with USB connected to a laptop running artisan. I experimented a lot with thermocouple-placement which I will not delve into here. I am now only using an "ET" probe. This probe is below the roast chamber (about 2.5 cm from the bottom of the bed). The readings are quite heavily influenced by bean circulation. To achieve reasonable precision I am continuously lowering the fan duty to keep the bed at a steady agitation rate throughout the roast. For the heater I have ended up using PID-control with a background profile that I design as I see fit. I originally tried to control both the heater and fan manually but this was too difficult for me. Leaving the heater on PID control both did a better job at following the target temperature and, equally as important if not more so, allowed me to solely focus on controlling fan duty.

As explained under the fan section, I keep the top of the T-pipe open during the first minute or so to reduce back pressure. I then add the top cap. To simplify adding and removing the cap I have "glued" the PTFE-gasket onto the cap with high temperature silicone as adherent. Another thing I do to make it easier is to secure the cap in place by putting a small weight onto it rather than clamping it down. Towards first crack, when the fan is turned down considerably, I remove the weight. The top cap sits secure without it at this point but some tightness between the gasket and metal parts are relieved and this helps tremendously in hearing first crack. Without this "relaxation" it is barely possible to hear the cracks even when I put my ear right next to the machine.

Cooling
Just turning the fan to maximum for 3 minutes before dumping. For reference, when I had a bean probe the temperature got below 100 ?C within 60-90 seconds.

Bean dump
My initial plan was to use the same route for beans as the chaff. The idea was to:

Roast --> cool --> dump the chaff --> turn the fan to maximum to blow beans into the chaff collector --> dump beans

To use the same mechanism for dumping chaff and beans was not possible as my fan of choice does not have the extra capacity to make this possible. The back pressure probably also hard limits it. Furthermore, the cyclone separator does not seem that sanitary. So I realised I had to get the beans out some other way. As I was gonna want to use my vacuum after roasting anyway I made a contraption to blow up my beans utilizing it. It consists of some silicone tube (28 mm diameter), a 3D printed elbow and a T-connection to which the silicone tube, a PET bottle and the vacuum nozzle is connected (with a mesh to prevent beans from entering the vacuum). It is not perfect and occasionally requires some fidgeting to loosen beans that get stuck in the tube (mainly if blowing them up too fast). But overall it works okay and definitely beats disassembling, though I'm probably gonna make a better design for this at some point.

i.imgur.com/MMuUeBv.jpg

6. Conclusion
Goals accomplished, although with a parenthesis around batch size and bean dump.

Ending with a staged pic (from before adding the 15 cm pipe).

i.imgur.com/GR9TKGl.jpg

 
CK
Nicely done, and well put together write up.

 
jkoll42
Looks great!

Thanks for the detailed build walkthrough. Like you said there is a wealth of knowledge here and it is because people like yourself take the time to detail builds.

I also appreciated the use of a chopstick and cursing. While I never had to use a chopstick there was much cursing trying to get all the rivet/TC holes drilled through relatively thicc curved stainless on my last build!


-Jon
Honey badger 1k, Bunn LPG-2E, Technivorm, Cimbali Max Hybrid, Vibiemme Double Domo V3
 
cdrake39
Very nice build! I went a similar route with a compact tri-clamp design - also using Henrik's (sinobi) recommendation on the blower. Although I ended up switching to a different model with higher static pressure (13 kPa).

I like the idea of the additional sight glass/valve dump for the chaff collector. I also had issues with too much back pressure generated by the cyclone and opted for a different chaff collection arrangement (see photo). Maybe this would help you reach the target batch size? For smoke control I place 4" ducting over the top of the bowl reducer.

Interesting note about the flyback diode, that's probably something I should add to my setup as well. How did you go about sizing it?I haven't connected the EN wire (or the 5V) on mine (just the GND and the SV to the respective IO3 pins). It has been running fine thus far, but I'm only 2 roasts in...

cdrake39 attached the following image:
roasterv1.jpg
 
greencardigan
Nice build and thanks for the detailed descriptions.

What fan driver board are you using?

 
cdrake39

Quote

greencardigan wrote:

Nice build and thanks for the detailed descriptions.

What fan driver board are you using?


I believe it would be this blower/driver combo. Anot, feel free to correct me if I'm wrong! :)

https://www.aliex...67367.html

 
Anot
Thanks all for the feedback and kind words! ThumbsUp

Quote

jkoll42 wrote:
I also appreciated the use of a chopstick and cursing. While I never had to use a chopstick there was much cursing trying to get all the rivet/TC holes drilled through relatively thicc curved stainless on my last build!


I can certainly understand that! The cursing was not isolated to this event ;)

Quote

cdrake39 wrote:
Very nice build! I went a similar route with a compact tri-clamp design - also using Henrik's (sinobi) recommendation on the blower. Although I ended up switching to a different model with higher static pressure (13 kPa).


Thanks! It will be interesting to see if more projects involve this type of blower in the future. They certainly have some advantages.

I have actually been following your project and think it is really neat! I like the use of a computer case as a scaffold. Your detailed and illustrated questions about electrical wiring have helped me a lot, you might see some direct inspiration in the upper left part of my wire diagram ;) I also really like the idea behind your bean dump. Very curious to hear more about it! I might try something similair in the future and have some questions, but will post them in your thread.

Quote

cdrake39 wrote:
I like the idea of the additional sight glass/valve dump for the chaff collector. I also had issues with too much back pressure generated by the cyclone and opted for a different chaff collection arrangement (see photo). Maybe this would help you reach the target batch size? For smoke control I place 4" ducting over the top of the bowl reducer.


That's a clever solution! Although I have come around the back pressure by opening the top cap in the beginning of the roast, after that there is no issue. So to increase capacity I think the way to go should be a more powerful fan. So far I have only had problems with one bean when trying 200 g. I think it was because of high density while still being large. I went down to 167 g and it worked fine. I only want to use batch sizes that can be evenly divided by 1 kg, so I have not tested the range in between 167 and 200 g. For now I'll continue with 167 g batches for all beans, at least until I learn the machine better. If the blower fails sometime down the line I'll get something with a bit more punch!

Quote

cdrake39 wrote:
Interesting note about the flyback diode, that's probably something I should add to my setup as well. How did you go about sizing it?I haven't connected the EN wire (or the 5V) on mine (just the GND and the SV to the respective IO3 pins). It has been running fine thus far, but I'm only 2 roasts in...

Yes, as I understand the flyback diode is very important to protect the motor. I didn't make any formal calculations, I just made some google-searches to get an idea of diode-requirement. Ultimately I went for a 1N4007 diode, this was overkill for my fan but the price difference compared to "lesser" diodes was insignificant. Regarding the 5V-voltage supply I realize I should not have connected it to the arduino. I guess it's only needed when connecting it to a pot as, unlike the arduino, it doesn't have a voltage to reduce. But I guess it shouldn't be a problem if the voltage does not exceed 5 V? It could be a handy way to power the arduino if I want to implement BT connection instead of USB, given that I can trust the voltage rating.

Quote

greencardigan wrote:
What fan driver board are you using?

Actually not completely sure and it's fastened in a very tight spot so can't check the exact model. It should be this one as the seller stated this would be shipped with the fan. Mine is blue though, but the layout matches at least:
https://www.aliex...f14c6fb3ab


Edited by Anot on 10/17/2020 8:14 AM
 
cdrake39
Glad to hear you got some benefit from those posts! That's the best part about forums like this :)
I'll post some more info on the bean dump mechanism in the coming days


Good point about connecting the 5V if you end up using the Bluetooth feature.

As for the diode, are you able to post a photo on how you installed it for the blower you're using? I'll likely do the same soon to prolong the life as those blowers are too expensive to replace! Haha

 
renatoa
I am a bit confused about the flyback diode requirement for the aliexpress blower pointed by the link some posts above.
As I see the PCB components, I suspect the motor is a brushless type, kind of three phase, controlled by pulses sequence, not PWM. Also, the driver board, is sold as a combo with the motor, should contain all necessary components, if well designed, no need for external additions.

 
Anot
Renatoa, you are right. It is indeed a brushless DC motor with driver board. I actually don't know how it operates, just assumed a flyback diode was required as with brushed DC motors. It was also shipped ready to use and I agree that it would be unreasonable to ship it without adequate circuit protection. So it should not be neccesary. I'll leave mine in as it is so cheap and not worth removing, and even if there is no risk I'd rather be safe than sorry.

cdrake39, Great! Ill be sure to check your thread in the coming days! I guess you're no longer thinking about installing a diode. But just in case, I have attached a picture that I found on my phone. Sorry for the poor quality - I couldn't take a new picture of it, as mentioned before, it is in a very tight spot. I just crammed it in the JST connector so that it touched the metal. I also made a continuity test from the solder joints to the diode to make sure it was in contact.

I read some more about this 5V-pin which I guess may be obvious (but not for an electrics noob like me). While it is possible to power the arduino this way it is not recommended as it bypasses the voltage regulator and risk damaging the ardunio (and TC4 as well I assume?). If the 5V is well regulated on the BLDC driver board I guess it should be fine but I dont think its worth the risk. If I want to use a BT module I can use cables from my DC PSU. I feel kinda stupid for connecting it in the first place but am glad it was brought to light!

 
renatoa
I use brushless motors a lot in another hobby, model flying, and these controllers/drivers are quite complex, microprocessor driving FET bridges as outputs, and every FET has embedded its own protection diode by design.

You have attached the typical scheme of the final driving stage, the diodes pictured near each MOSFET are on same silicon as the transistor.

renatoa attached the following image:
bldc_drive.gif
 
Anot
New bean dump mechanism
I have now made a bean dump mechanism using a diverging pipe and homefabricated gate valve. All in all, it works good but there are some kinks I want to work out. Heres a video of it in action:

https://i.imgur.c...sRshHK.mp4

I have wanted to implement a bean dump utilizing a diverging pipe as used in many other tri clamp builds but was unsure how well I'd be able to reliably fasten a mesh. I cant weld and the only other way I could think of was to drill and fasten it with screws, though that would require quite a lot of screws for a mesh. I got the idea that instead of adding a mesh, I could cut an aluminium sheet and fasting it as a slope and fasten it with 2 screws, and rely otherwise on its rigidity. I decided it was feasible and decided to try to find a lateral 1.5" wye.

Diverging pipe with slope
The lateral wyes seem hard to come by, at least in Sweden/EU. I eventually found one for standard 1.5" tri clamp on aliexpress after long browsing https://www.aliex...G&mp=1). I considered buying it but eventually decided against it for two reasons. Firstly the tax and import fees from China are a bit hefty in my country. Secondly I found another, more accesible, variant that I could use instead. See picture. It is a straight tee but the diverging connection is very close to the main pipe, so it seemed reasonable to make a slope going out of it. The black represents where I will be adding the screws. An advantage of this design is that it will be easier to drill a hole on the underside of the divering pipe, compared to the steep angle of the lateral wye. The main downside is that the beans can loose momentum and that the exit circumference is a bit reduced. This worked out fine at first and the beans easily slid out. But after reattaching the slope some beans stay on the botter portion, but with some air flow and a poke the "stuck" beans easily come out. I will try to angle the slope better and see if I can resolve this issue. This problem may also have been introduced because I tested the mechanism with a larger bean load, so the beans may have get slightly stuck together on their way out. If so, maybe it is enough to slide the gate valve open more slowly. A note, I could not get a straight path for the "bottom" screw with my drill bit, so I have only attached the "upper" screw, and have pushed the slide in position on the lower side. However the bottom lip can slide up and beans can get in under it. I actually got 3 beans down to the heating element after rigourous testing, so it is evident I need to fasten this better. Not much force is required to hold the bottom lip in place so I might add some kapton tape (and keeping close check) until I can manage to drill a hole.

Schematic
i.imgur.com/JJmzlqc.png

"Finished" construction

slope from above
i.imgur.com/eBSr9ZB.jpg

slope from exit point
i.imgur.com/dA2Eozt.jpg

Gate valve
I considered using a ball or butterfly valve here, but have heard people having problems with beans getting stuck, so instead I wanted a gate valve. This is very difficuilt to order, and single unit prices for the ones found would be very high. So instead I made my own gate valve using the great principle by jetroaster (https://homeroast...ad_id=4961). See picture. I have not made the line of silicone, but might try to seal some with kapton tape. I used an aluminium sheet (0.5 mm). The saw blade had a thickness of 0.65 mm. To get a clean centered cut in the clamp was a challenge and required two attempts. It is, as expected, not completly airtight. I have been able to clamp it down just enough to reduce most of the leakage, while still easily being able to slide out the gate valve. However, the construction gets a bit askew above the gate valve, as the downclamping where the gasket is cut puts the metal parts closer together. To alleviate this I will try with a more rigid gasket (something like this https://www.aliex...1c38bC0jlA). I am also considering buying a mesh design (and removing the mesh). As this gasket design is open where the mesh is inserted, it may be possible to cut only the outer rim where the gate will be inserted, and sliding it/sandwhiching it in where the mesh used to be.

gate valve schematic
i.imgur.com/6iqRKtZ.png

gate valve "finished" construction (approx 32 % open area)
i.imgur.com/mjUbNQk.jpg

Perforation design for the gate valve and slide
My main concern was how all this would affect air dynamics as it basically replace one air obstacle with two bigger air obstacles (I used a 20 mesh, so about 50 % open area, which is hard to match). This also turned out to hold true. I tried my best to come up with a good perf design, I wanted about 3 mm holes and minimum 1.5 mm spacing, eventually reduced to 1 mm. See below. Inner hole is 4 mm in both design, in the right design the rest of the holes are 3 mm, and in the left the holes between the outermost holes and center hole are 3.5 mm. 29 and 34 % open area respectively. I went with the left one (34 % open area) I also added 3 x 2 mm holes to improve air flow (but this was really hard, and I accidentally cut out adjacent holes). I think its hard to increase the open area for the already fabricated gate from here. What I can see from the design is that there is some unutilized space towards the edges. Also the gate was more rigid after drilling then I thought it would be. So I think about 4 mm holes with slightly less spacing all around is feasible, which would give 48 % open area (37 x 4 mm holes, 35.1 mm total diameter).

i.imgur.com/5s9nlrE.png

For the slope, I had kinda lost patience so I just improvised the holes. Since it was on an angle, I figured the open area will be larger, though im not completly sure how the angle affects how the air flows through the holes (I guess flow is reduced compared to the same open area for a flat stream going to a flat plate but have no idea of the magnitude). I also eventually added slits in the edges to improve air flow.

At first I had problem with my already measly batch size of 167 g, and could only manage 150 g so that is why I added more holes/slids to both the gate and slope. Based on some initial testing 167 g now seems feasible again (though I need to test more beans to be certain). In hindsight, I think adding a mesh for the slope mechanism, using a bit more screws would be a better idea in my case, to allow for more air flow, and I might do that at some point.

From here I am considering adding an elbow with DN32 thread (something like this https://www.aliex...web201603_) and screw cap, that way I can place a container below and open it with one hand while simultaneously opening the gate valve. For this to work I will need to improve the slope mechanism. If all else fails, I think that just blowing out the stuck beans should be enough. I could even add a closed gate to slide into the valve in order to aid this if needed.


Edited by Anot on 04/18/2021 4:26 AM
 
Anot
So I did a roast today with 167 g, there were some small movements in the beginning of the roast so I thought it would be OK, but it took a bit too long before the bean bed started moving properly so I need a bit more airflow still.

I also realised that the gate valve was leaking more than I thought, which was easier to notice when the air was hot. I have now ordered a 1.5" PTFE gasket with mesh, I will remove the mesh and cut at the rim where the valve will be inserted. You could cut a non mesh gasket all the way, but it is convenient that all but the rim is already cut for the mesh. I also doubt I could cut the rest with the same precision. My hope is that this will reduce leakage when clamped down and that the the airflow / pressure through the beans will increase enough for my batch size. I will probably have to loosen up the clamp somewhat towards bean dumping to be able to slide out the gate valve.

I had an already butchered mesh-gasket (with bigger diameter) where I cut the rim, so I can show the basic concept.

i.imgur.com/e6x93yM.jpg

 
Anot
Small update. The idea above (with the mesh-gasket) worked quite well and reduced some air leakage but did not notably increase batch capacity (I have now replaced the gate valve with another mechanism but will present that as a separate post). I also switched the slope from two posts above to a mesh to increase air flow. It is fastened with two screws and now held in place securely, but that have not completely resolved the batch capacity either.

 
Anot
Yet another bean dump mechanism! I think this is a suitable design someone seeking a simple and easy way to implement a bed-valve that doesn’t requiring any heavy tools – not even drilling is necessarily required (note however that drilling may be required depending on clamp design).

I considered a mechanism involving a machine screw attached to a perf plate that can be closed and opened by turning the screw. One challenge would be to make it lock in a horizontal position for the roast. I considered having small snippets that would touch the side when turned into the horizontal position. However, I came up with another solution that also holds the screw in place.
This solution is basically to attach a nut and attach the perf plate/mesh to the screw so that is horizontal when the screw is turned in all the way. I considered finding a way to weld the nut onto the outside, but I came up with another idea - simply wedging a nut between the grooves of the tri clamp connection. This works great with the nut shown below (specs are in mm, I think this is a common size for M2 nuts) and a Teflon gasket where the slimmest part (the rim) is about 3 mm thick. I couldn't find screws as long as I wanted so had to settle on 20 mm length for now. With a thin mesh like I’m currently using I think there is some sag when loaded, but still seems to work for my setup.

i.imgur.com/NMSMhzP.png

I (happily) found that the tightness of the clamp affects the torque and thereby enables minor adjustments of the plane of the "fixed" (fully screwed in) position. Here is a picture of how it is placed in the clamp groove and video demonstrating opening and closing:

i.imgur.com/Jg4Kqmm.jpg
vid:https://imgur.com...

Air leakage is also very low if cutting the gasket with precision (I can’t feel any air streaming out during roasts). See picture for my cut.

i.imgur.com/Kemb7q9.jpg

Another aspect is how to attach the perf plate to the screw. I would prefer welding it in place but currently I have just bent down a snippet on the mesh on one side and wedged it between 2 nuts (which you can see in the picture above). I suspect that it will get loose and require tightening occasionally. Finally, depending on clamp design you might be able to access the screw without drilling through it. See picture. I have clamps of another design where drilling would be required though. The next step is to either glue or solder a small, angled screwdriver onto the screw head.

i.imgur.com/TT5M5KM.jpg

This works better than my previous design as it is easier to handle and leak less air. It also allows using meshes and a larger selection of perforated metals (limited earlier by thickness) making it easier to source a good ratio of open area. Tests so far indicate that I can go back to batch size of 167 g.

 
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