Roasting Module: New Bowl Arrives

2012 March 15
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We are now beginning the production of the roaster as mentioned in the post “Moving to the Building Process“. The methodology is DBTM (Design-Build-Test-Modify) and the project is broken down in to sections which are perfected before moving on to the next section. The first section is the roasting bowl and drive system.

Our prototype roasting bowl and drive worked rather well, but we felt that the semi-elliptical shape of the bowl (in cross-section) created slowing of the beans at the edge since the bowl is essentially vertical at that point. The forces tend to push the beans against the edge of the bowl instead of up the edge and into the return vanes. Our solution is to start with an oversized hemispherical bowl and cut it down to approximately half of its original depth. This results in an edge with a generous slope. The forces on the beans are then both  pushing the beans against the bowl and upward into the return vanes. This is also important for effective bowl evacuation when the roasting is completed. We plan on spinning the beans out of the bowl and edge stalling slows their departure, taking longer to complete the evacuation.

Our new “rough” bowl has arrived from a manufacturer in Ohio. It has an outside diameter of 40.5″ and a nominal 1/4″ thickness. The bowl is made of carbon steel, which has excellent thermal characteristics for roasting.

This modified photo shows the basic machining we plan to do, after heat treating the bowl to relax any existing stresses. The bowl will be cut down to 36″ in diameter with a corresponding depth of approximately 11″. We will machine a 2″ locating hole for the drive hub along with six smaller holes for the bolt down studs to secure the bowl to the hub.

We’ve also decided that instead of moving the return vane ring upward to open up a passageway for the beans to escape at the end of the roasting cycle, we will move the bowl downward. The diagram below shows the new methodology.

The bowl will move down approximately 3″ (all while spinning), uncovering ports that the beans flow into. Note that since the bowl now moves up an down within a cylinder, the edge needs to be flat vertically rather than horizontally and very concentric to keep from scraping the sides of the cylinder. This means the bowl will need to be precisely machined and the lift mechanism must be very stable and accurate.

Our next step is to begin machining the new bowl. Simultaneously, I will be finishing the design of the structural frame for the roasting module and the aluminum carrier for the bowl and its motor.

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3 Responses
  1. March 29, 2012

    This thing is so amazing. Joe, you are a freakin genius! (Thanks for all the birthday food btw : ))

  2. Scott B permalink
    February 15, 2014

    Very nice! My wife’s family has a coffee export business overseas and so I found this page while browsing around. I’d like to make a fire-pit from two hemispherical bowls tacked together into a sphere. I’d like to cut a design into the sphere with a plasma torch. Can you provide the bowl manufacturer’s contact details?

    Thanks

  3. rob hartley permalink
    March 11, 2014

    Hey there
    Great project…

    just wondering who did u get the 1/4″ bowl from?? Just a fab shop or somewhere else??

    Cheers

    Rob

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