6 Flask Coffee Cold Brewer





Introduction: 6 Flask Coffee Cold Brewer

About: I'm an inventor / maker / designer based in the Bay Area. My background is in residential architecture, film set design, animatronics, media arts, exhibit design, and electronics. I use digital design and fa...

Based on my Cold Coffee Brewer, this version brews six cups at once. Try six different kinds of beans for a tasting, or just make enough for your friends.

Check out Audreyobscura's Scotch Affogato Recipe instructable made with cold brewed coffee from this contraption and Glen Grant Single Malt Scotch.

Step 1: Design

Most of the large batch slow drip cold brewers I’ve seen use a large volume of water and drip into a large volume of coffee. This clearly makes economically and simplifies fabrication, but I liked the idea of being able to brew multiple kinds of beans at one time. Like wine snobs, coffee snobs have provided us with a wide variety of flavor profiles and if you pay attention, you’ll notice drastic differences in the taste of one bean from another.

I designed the piece in Fusion 360 (my go-to free 3D design and CAM software). 3D printing is a breeze with this program, making my life a lot easier with so many parts.

Fusion 360 is free for students and hobbyists, and there's a ton of educational support on it. If you want to learn to 3D model the kind of work I do, I think this is the best choice on the market. Click the links below to sign up:



I explored the idea of making a linear one (the all-to-common setup at your typical hipster coffee bar), but I thought a radial design would have a nice effect, and would also make the brewer more of a centerpiece. After all, the whole point of taking all this time to design and fabricate the thing is to draw attention to it; to make an event out of the slow process of cold brewing.

The piece is made with 39 3D printed parts (attached in this step), boiling flasks, needle valves, filter funnels, silicone stoppers, and wooden dowels.

Step 2: Parts & Materials

  1. 3D Printed Parts: I used white PLA filament on the Dremel 3D Idea Builder.
  2. Boiling Flasks (12): I used 500ml boiling flasks for this project. The neck sizing is important since the clips need to fit snugly, so I would suggest buying the ones in the link unless you want to do some 3D work.
  3. Silicone Stoppers Size 6 1/2 (12): The material of these is important. You could probably do this project with natural cork, but since the upper flasks are inverted, silicone seems like a more secure fit. Also, it’s food safe and tasteless, so it won’t taint your awesome coffee.
  4. Porcelain buchner funnel 50ml, 50mm filter size (6): These filter funnels fit well into the flask necks, and hold 50g of coffee grounds (ideal for 500ml of ice water)
  5. .50mm Paper Filters: These are used to filter the coffee grounds and keep an even distribution of water on top of the coffee grounds.
  6. Needle Valves (6): These valves allow you to set the drip speed of the cold water.
  7. Copper Tubing (18" total): This creates a connection to the stopper and a spout for the water drip.
  8. 1/8" Ø Brass Tubing: 36" total needed.
  9. 60 Second Epoxy: This is used to join all the the 3D printed and wooden parts.
  10. 1/2" Wooden Dowels: 138" total length. I used hard maple.

Step 3: 3D Printing

lf you're new to 3D printing, check out this 3D Printing Basics instructable by Printeraction. It's a great way to get started. I used a Dremel 3D Idea Builder for this project, but pretty much any FDM printed would work.

This is a large project that includes 39 3D printed parts, so be patient! Orientation is very important for getting good results. I find it's a good rule of thumb to find the largest flat area of a model and rotate it so that it's in contact with the build bed.

The printing process is basically as follows:

  1. Design the piece in Fusion 360
  2. Export the separate parts as STL files
  3. Open the STL files in Print Studio, combine them as desired with proper orientation
  4. Export to an SD card for the printer

The files types included in this step are F3D (the original design files with all the parts assembled), and STL (the exported files ready for 3D printing.

  1. Cap (1): This is the top piece to which 6 clip attach to hold the ice water flasks.
  2. Middle Ring (1): This is the middle piece to which 6 clips attach to hold the filter / coffee flasks.
  3. Lower Ring (1): This ring stabilizes the connection between the vertical posts and the diagonal posts.
  4. Clip (12): These clips hold all 12 flasks.
  5. Clip Sleeve (12): These attach to the clips in order to ensure the proper fit to the cap and middle ring.
  6. Elbow (6): These connect the 18" vertical posts to the 5" diagonal posts and are glued to the lower ring.
  7. Foot (6): These interconnect to create the base.
  8. Funnel Sleeve (6): These provide cradle for the fliter funnels that allows air to pass freely, preventing the filters from overflowing.

Step 4: Stand Assembly

  1. Cut the 1/2" Ø dowels to size. There are six at 5" long, and six at 18" long.
  2. The detailed instructions are included in the diagram for this step. Use 60 Second epoxy for all the the fitted connections as described in the diagram. It's important to hold or clamp the parts in place for over 60 seconds to ensure proper curing and prevent movement in the parts.

Step 5: Clip Assembly

The clips hold the boiling flasks in place with a small amount of pressure gripping the neck. To ensure their solid, flush, plumb connection with the ring and cap pieces, I added a sleeve that friction-fits onto each clip.

Use 60 second epoxy to join these parts to the cap and middle ring.

Step 6: Prepping the Stoppers

I borrowed these photos from the first version of this project, so they're black rubber in the photos. The process is exactly the same.


In order for the water to drip, you have to have a siphon in the upper flask, otherwise you end up with a vacuum and no water will come out. To do this I inserted a brass tube into the stopper next to the valve. Cut the brass tube to about 6" long.

Make a boring tool: sand, file, or grind the end of the off-cut tube to make it sharp around the edge. Screw the stopper down to a piece of scrap wood so it's snug, but no compressing the stopper (you don't want distortion. Chuck the tube into a drill press, and bore into the stopper. Press the 6" piece of tubing into the new hole for an airtight seal!


The needle valves need a 1 1/2" length of copper tubing clamped to each end. I used a mini tube cutter for this part. The valves have a specified flow direction, so be sure to insert the copper tube into the stopper's center hole with the correct flor direction. The valves are labeled, so it's hard to get it wrong.

Step 7: Assemble and Attach Flasks


Insert the assembled stopper / valve / siphon parts into six of the flasks. These will be filled with ice water and clipped to the upper ring of clips pointing down for brewing, so make sure you're able to get a water-tight fit.


Insert the funnel sleeves into six of the boiling flasks first, then insert the filter funnels into the funnel sleeves. The order is important, because the filter funnels aren't preciesly shaped, and may be a bit bigger than the standard size the funnels are designed for. Doing it in this order will prevent unwanted pressure on the flask neck that might cause it to shatter.

The six coffee filter flasks fit into the clips on the middle ring with the filter funnels pointing up (duh).

Step 8: Brew Some Coffee

These photos are borrowed again from the previous version of the project, but the process is exactly the same.

  1. Fill the upper flask with ice and push in the stopper with the spigot. This one goes on the upper hook pointing down (duh).
  2. Place a piece of filter paper in the funnel. Get it wet to even out the contact with the coffee grounds.
  3. Place funnel and sleeve in the lower flask. Fill the funnel with medium-fine grind coffee grounds. Don't pack it in, because it'll overflow when it's saturated- just fill to the top and wipe off any excess so the grounds are level with the top of the funnel. Prima Coffee recommends 500ml of ice water to 80g of coffee grounds, but I just eyeballed it and the result was good.
  4. Saturate the grounds slowly with cold water.
  5. Place another piece of filter paper on top of the grounds. This piece keeps the water evenly distributed through all the grounds; without it, the dripping would slowly carve a tunnel through the middle and the coffee would be very weak.
  6. Open the valve ever-so-slightly. You'll want one drop every 5 seconds or so, but the key is to sync up the dripping of the steeped coffee with the dripping of the valve. If the valve is dripping faster than the funnel, it's going to overflow (again, duh).

Step 9: Be Patient and Savor

This piece gets a lot of attention at the office (people have been calling it the coffee space ship), so it's a success as a performative centerpiece. The coffee tastes great too.



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The STL for the Middle Ring seems to be the same file as the Lower Ring (it doesn't have the grooves for the Clip)

Gorgeous work! Without a doubt, I will be making this at some point in the future. Just got my very own Lulzbot TAZ 5, so now all these 3D printed instructables seem so much more feasible! Love the scientific look the beakers give it, and as people have already pointed out- your assembly images are works of art in of themselves!

Your assembly instruction/sketches are beautiful. Great job there!

1 reply

You know you could do this with separation funnels about 5 of them. Although they would cost at least $300 for the separation funnels ALONE. Thanks for presenting this.

3 replies

Yikes! I wonder if they could be gotten used for cheaper.

Just my 2 cents on the sep funnels, I really wouldn't recommend getting used lab ware, especially if it's used in lab. I'm seeing sep funnels on amazon for about $25 bucks after S&H which is cheaper than what I get charged at school.

I never use used supplies all of them are build in germany with good glass pyrex. The sepaperation funnel is new too.

Here is the last part Safety and improvement.


We are dealing with Methanol and lye (potassium hydroxide) which could blind, burn and potentially kill you if you are reckless! THIS IS NOT FOR BEGINNERS DUE TO THE LYE AND METHANOL AND HOT OIL. I always have on all PPE protection including gloves and gloves, lab coat, apron rubber and face shield. THIS MUST BE DONE IN A WELL VENTILATED AREA DUE TO THE SMALL RISK OF AN FLASH POINT EXPLOSION. This is due to the methanol build up that is why it MUST IF (REPEATED) BE DONE OUTSIDE AND in summer.

Improvements: When pouring methanol and lye this could be done in a sealed area with salt to remove the water from the air. Thus less soap could be made in the lye process.

Always use fresh Lye (Not old lye) and fresh methanol.

The whole process including separating could be done in glass container with salt so no moisture enters the oils or reacts with the lye.

finally after making the biodiesel you need to remove impurities by filter.


when the solution is hot remove the flask quickly add the lye methanol solution to the oil and reduce the heat to around 4. Clean the flask so no liquid contacts and burns into the plate. Heat for 15 -30 min closed from air (ESSENTIAL).

Remove the flask and turn off the stir function and heat of the device. Let it cool before touching. 1 hour.

Let the solution sit in a lab for 30 min until you see two layers. A dark layer will be the glycerol which is on the bottom. The top is the biodiesel. (Impure).

Using always a clean separation funnel add the content until the two layers form. Use the separation funnel to remove the glycerol. Now add distilled water to the oil to form soap layer and biodiesel. Note: For more efficiencies you can let the methanol evaporate off (This takes a few days at normal temp).

You will see two layers ...... soap and biodiesel. The soap is usually on the bottom. so you have to carefully remove the soap and save the remaining material the fatty acid of the vegetable oil.

You can test the oil with a ph paper (Not a ph meter since the oil may damage the probe over a long period of time.) 13- 14 ph means there is too much lye and you should redo the step by adding more distilled water with oil to remove lye. 7-8 ph is good.

Now you have pure oil. However I found the efficiencies is only 25% max. Without a good method of preventing air from entering the oil with methanol and lye may be the problem. Yield is 25%.

Next Safety and better results.

Ingredients Potassium hydroxide, Methanol and Oils.

Potassium hydroxide is mixed without air with methanol which is highly toxic, flammable and corrosive.

Flasks with 500 ml with rubber stopper, Separatory funnel Polypropylene or glass pyrex will do. Separation funnel 250 ml. Polypropylene will do.

Most expensive equipment Hot plate and magnetic stirrer with stir rod. Worth 600 $!

First add 2.5 g of Potassium hydroxide to a flask and add 100 ml of pure Fresh Methanol.

Seal with a rubber stopper.

Heat up the oils in a flask with 250 ml (exact) with a stir bar set to medium speed an d heat up the oil at moderate heat value. 12 is high 1 is low so 5 or 6.


Wait Until the oil heats near 100 degrees C or is very hot to touch.

Part 1

This will continue.....

Separation funnels are known to be quite expensive any where from 60 $ for plastic or 200 to 300 $ for 500 or 1 L flasks made from glass. However they are made from high grade pyrex that can tolerate heat and corrosive chemicals too. I use plastic ones to make biodiesel from different types of vegetable oils.

1 reply

You make your own biodiesel!? Where's the instructable!? I'd love to see how you do that.

This could be a fun way to keep meetings interesting. It's something to watch while the meeting is on, and when the coffee is brewed the meeting is over!

1 reply

It already looks a bit like an hourglass doesn't it...

Wonder if you could make it into an upcycled project by using leftover wine bottles instead of the lab flasks....

1 reply

I don't see why not. You could make higher volume brews as well with a larger filter funnel.

The combination of laboratory glassware and non-laboratory parts is interesting. If you'd spoken with someone from a lab, you could have done this whole process without any custom made parts at all. Ring stands, separatory funnels, some buret clamps, etc would have done the trick much more adjustably. Those round bottom flasks on the bottom aren't even round bottom flasks - they are flat bottom flasks. So you could have skipped the lower clamps altogether, and placed these directly on the surface, which would make a much easier to work with system.

Further, how weak the coffee in these images clearly is. Cold brew is typically *stronger* than perked coffee, and the coffee here is watery. It may taste fine; that's your preference.
So what this is ultimately is a poorly considered convoluted contraption with produces extremely weak coffee through a fiddly process.
I love the work you've put into it, and your photos and planning and building skills are clearly great. But the emphasis on form over function doesn't satisfy our need for quality coffee.
If you really wanted to stick with this method, you could use fritted funnels which can be bought much taller than those buchner funnels, and you'd increase the time the water spends with the coffee on it's way down. They would also be clear, which would add to the artsy nature of your project. Harder to clean though. You could introduce some feedback mechanism between the systems so that one would cause the other to drip (probably not hard, but I haven't thought of a way to do it just yet).
Those should probably be stainless fittings instead of brass. Some brass (maybe not your brass) has enough lead to be considered unsafe. (Again, steparatory funnels, which would have teflon stopcocks would avoid this problem altogether.) (Ice could be added from the top too, so you could increase the amount of water in your system without removing parts.)

I do understand that this project isn't about doing things "easier" - it's about "turning something mundane into an exhibition..." But you could have turned the common laboratory glassware into just as much of an exhibition, and the result would be better coffee. And isn't that what we all want?

2 replies

Is that envy I hear?
You have missed the point of Instructables. To borrow a phrase from Vice President Spiro Agnew, your critique is an echo of the nattering nabobs of negativism.
JON-A-TRON's cold-brew carousel is aesthetically engaging, technically well thought out, and down right fun. It is a work of art.
That is the heart of Instructables!

Thanks for the pep talk, JohnP90!