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Small Programmable Kiln

A small kiln with big expectations: Fusing PMC, Solder PCBs, Harden/Temper/Anneal/Carburize Steel, Pottery, Glass, Melting Precious Metals

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Required Max Temperatures:

  • PMC: 900C
  • PCB Solder: 260C
  • Harden/Temper/Carburize/Anneal Steel: 900C/550C/260C
  • Pottery (Low fire): 1100C
  • Glass: 800C
  • Melting Precious Metals (Silver/Gold): 1064C

Max ramp rate required is 1C/s under 260C. Hitting 3C/s is a nice to have.

In-field firmware reprogramming.

Safe to run for 8-10 hours.

20200503_080356135_iOS.jpg

Sample UI shot (protective plastic causing blur)

JPEG Image - 95.33 kB - 05/03/2020 at 11:53

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20200503_080144528_iOS-1.jpg

Wrapped in Foil and controller box.

JPEG Image - 162.52 kB - 05/03/2020 at 11:51

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Portable Network Graphics (PNG) - 149.49 kB - 04/27/2020 at 13:07

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  • 2 × Arduino Nano One for the LCD GPU and another for the PID controller.
  • 1 × ESP8266 Weemos D1 Lots of RAM and Flash to store and orchestrate UI forms and firing schedules. Wifi is tempting for UI in future.
  • 1 × Kanthal A1 Resistance Wire 24AWG/0.5mm 30M
  • 1 × SSR Fotek 40DA Only need 10A, but not much more for good margin.
  • 1 × PSU 9V 2A From old parts bin.

View all 11 components

  • Tweaking oven design

    mybura05/26/2020 at 23:33 0 comments

    After the last test, decided to add some fiber padding to the kiln inside. Faster ramp times and better sealing keeping more heat in (is the theory). The blanket is 25mm thick and I'm trying to work out how to mount the element now as it used to be stapled to the brick walls. The fiber is not rigid enough to support the staple approach and it will be coated with water glass which in theory will be molten at the element operating temperatures..

    As a result I created some water glass as a side project to seal the fibers and keep them out of lungs.

    Currently the frame is pickling in vinegar to get rid of the mill scale. That stuff is evil :)

    Once the pickling is done, I'll finish up some final welding needed to get some handles and feet of the oven attached.

    Painting then comes next. I managed to get 1093C resistant paint from the auto store. It needs to bake at 200C before it is ready and I don't have anything large enough to bake it in. Thinking I'll just blast it with a flame for a bit and hope the oven at full speed will do the rest on first launch.

  • More integration testing

    mybura05/03/2020 at 10:38 0 comments

    Tweaked PID controller to stabilize target swings. Still needs further tweaking, maybe even adding Auto Tune as the oven behaves differently at different temperatures. Increased the PWM from 25% max to 100% max to do a full load test.

    Wrapped most of the oven in foil, shiny side inward. Made a big difference to perceived heat emission and it completely blinded the laser temp sensor (was reading room temp all over the surface)

    Ran up to 800C to check the thermal stability of everything hooked up. Some findings:

    • Max external oven temperature measured was 350C and seems to mostly range between 77C and 123C.
    •  Between 1400W and 1600W needed to keep oven at 800C. Calculations for ideal oven suggest 817W.
    •  Controller box was cold to touch with the 80mm fan on. The SSR heat sink reading was max 29C at ambient 24C.

    Overall the construction seems sound enough and electronics/electrics can handle the design well.

    Think there is a massive loss to convection caused by the gaps between bricks. Also noticed the max power the coils are willing to draw peaked at about 1650W suggesting a 3rd coil might be needed to reduce the resistance. Trying to keep the max current consumption under 10A to avoid needing a special wall socket.

    Some other random notes:

    • Mains fluctuated between 245V and 226V during the day. Suppose as people start cooking in the complex towards the evening the grid takes a bit of a knock.
    •  Guesstimating it took about 3 hours effort to get to 800C from 24C room. Was tweaking the PID controller a lot during the day, pausing the temperature on its way to 800C for extended periods. According to my calculations it can take about 72 minutes min.
    • Aluminum foil has a little dance on its surface when exposed to convection at ~200C. Suppose its a bit of contraction/expansion competition across the surface as the hot air flows under it trying to escape to room temperature.
    • Power meter for the day suggested 6kWh spent on the experiment.

    Next potential steps:

    • Investigate a layer of Kao wool and brick interleaving to reduce convection and thermal conductivity.
    • Reflect on the coil layout to see if a 3rd might fit.
    • Start uploading some code to see if the community can identify any glaring improvements.

  • First Integration Test

    mybura05/01/2020 at 14:34 0 comments

    Great news! First integration test went okay. Hooked one set of coils up to the assembled controller box and fired it up to 108 deg C. Held it for about 1H.

    Only two things got hot: The inside of the oven and the 9V PSU in the controller box. Didn't have the 80mm fan on so hoping it will keep the PSU temperature in check. It was hovering around 50 degrees with ambient at 18.

    Had some teething issues with noise in the I2C bus and the GPU controller ran out of RAM causing random things to show on the display. Don't think it is all fixed but getting close.

    Before running the test I limited the PWM to 25% of its max capacity. This translates to the coil only ever getting power 25% of the time max.

    Some measurements while cruising at 108C (18C ambient):

    - Coil at 65 Ohms

    - Max Temp 110.1C/Min Temp 106.7C

    - Max Power 500W / Min Power 270W

    - Power meter power factor of 50

    - Power meter 4A max

    - Multi meter AC Voltage over coil Min 40V Max 80V

    - Clamp meter Amps at 0.3A - 0.45A

    To the untrained eye these numbers don't add up. The trained eye doesn't care to do the math to recon why not :)

    Not sure what's next. Really want to check the controller box can handle full on state for 8 hours. That needs a couple of small problems solved before it can be executed (like the stuck terminal block screw for the second coil, lugs and insulation for the remaining coil wires and more schedules to ramp the temperature to various intermediate levels).

    The PID needs some serious tuning but 2.5 degrees swing around the target is not bad for an initial guess.

    Wonder how one can calculate the efficiency of the insulation and oven design using the results from above?

  • Captain's log... First Entry.

    mybura04/27/2020 at 13:24 0 comments

    Thought I'd add some of the journey towards the final product.

    Where is it up to?

    - Main kiln body manufactured and fired up to 700C.

    - PID Controller Software Developed (Alpha)

    - LCD Touch Screen Controller Developed (Alpha)

    - Master Controller Software Developed (Alpha)

    What's still left?

    - Controller Box Manufacturing

    - Electronics mounting

    - Final Component Integration

    - Tweaking body design

    Kiln body design in links. Will add code to GitHub as soon as initial integration testing is successful.

View all 4 project logs

  • 1
    Design

    Decide on kiln size. Some factors to consider:

    • Max temperature needed
    • Mains socket available (limits available amps)
    • Max weight
    • Min ramp rate
    • Availability of material and components
    • Top load/front load

    These will drive the max wattage needed, internal size and the type of materials that can be used as the max temperature limits the range.

    K26 bricks matched my max temperature requirements and angle iron was most economical to purchase for the frame build. The min. ramp rate suggested the oven should have some fiber lining to reach temperature quicker. Kanthal A1 wire could handle the temperature and weight was not much of a problem for me.

    With a 10A/240V socket available, I opted for around 2000W as I could get an SSR, fuse holder/fuse, high temperature silicon wire and some terminal blocks that could handle the amperage fairly easily.

  • 2
    Calculate heater coil parameters

    Calculate what size wire and how much is needed, used https://kruegerpottery.com/products/element_240v_16amp

    The shape of the coil, width between windings and internal layout of coil are impacted by:

    • Dept of grooves
    • Number of mounting points in grooves
    • Number of coils in parallel/series to limit the wattage
    • Diameter of the wire
    • Even heat distribution (assuming no coil on the door)

    Used Fusion 360 to model the K26 bricks, layout the easiest manufacturing pattern and then play with coil layout on the model until it fit with minimum installation interference.

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Discussions

Dan Maloney wrote 04/30/2020 at 00:07 point

Seems like a huge range of temperatures for one machine. How do you plan to hit your target temperatures on both ends of the scale?

  Are you sure? yes | no

mybura wrote 04/30/2020 at 08:13 point

Simples, PID does the heavy lifting. Have a K-type (1200deg) probe hooked up to a Max31855 amplifier hooked up to a  Arduino Nano which in turn is controlling an SSR using PWM on a custom Kanthal coil. The Arduino Nano receives it's target temperature/ramp rate from a master controller that lives on a another MCU which is responsible for schedules and translating user input into target temperatures.

  Are you sure? yes | no

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