Specialita Mods and Measurements

There are a few experiments I want to do which will require a more quantitative notion of grind size. To achieve this, I’ve done a few things which I’ll talk about in this post:

  • Modify the adjustment knob to allow more precise selection
  • Measure the change in burr spacing
  • Measure outfall depth

The Eureka Mignon Specialita is a solid grinder for the price, but one common complaint is that the small adjustment knob doesn’t allow for much precision in grind setting. Inspired by a post on Reddit, I created a 3D printed pointer and adhesive scale for the adjustment knob, which I’ve shared on Thingiverse.

The scale can be printed onto standard adhesive labels, then positioned in the corner of the grinder as shown.

The indicator snaps into place on the existing adjustment knob. The indicator should be positioned so that the small pointer on top aligns with one of the ticks between numbers on top of the knob. In this position, when the lower pointer lies along one of the long lines, as in the photo above, we should see a whole number indicated on the front of the dial. In the photo above, the grind setting is 5.0.

When the adjustment knob is turned, the indicator turns with it. The short lines on the adhesive scale have a resolution of 0.1 grind steps, but we can easily read to 0.05 grind steps with this setup.

If we disassemble the grinder a little, we can make a few measurements. By removing the hopper and top cover, we can measure the distance between the top burr carrier and the inside of the bottom burr, as shown here:

I added a folded Post-It note on the right to provide a reference line on the adjustment knob. Then I repeated the following steps:

  • Zero the depth gauge against a flat surface
  • Measure the depth to the bottom burr
  • Repeat three times for each grind setting

This gives the following results:

Interestingly, the depth does not change for negative grind settings, which suggests that the burrs touch very close to the zero grind setting on my grinder. In calculating the fit, I have ignored negative grind settings for this reason.

The linear fit is very good, with R^2 = 0.9980. The best-fit line has a slope of 47 μm per grind step. We can verify this by looking at the adjustment mechanism itself.

To do this, we disassemble the grinder a little further. The adjustment mechanism looks like this:

Turning the adjustment knob moves the bar on the right up and down. The spring provides a restoring force when the grind setting is increased, and also prevents backlash in the adjustment screw.

When the bar is moved up and down, it moves the lever on the bottom of the unit. The fulcrum for this lever is on the left side. The ball in the middle is attached to the motor, and also the lower burr. The distance from the fulcrum to the pivot on the right is 118.70 mm. The distance from the fulcrum to the ball is 48.60 mm. On the right edge of the motor, there is what appears to be an end stop which prevents setting the grind size larger than some limit.

The pitch of the screw is 0.80 mm, which is how far the end of the lever moves for a full turn of the adjustment knob. This means that the lower burr moves:

    \[0.80\,\text{mm} \times \frac{48.60\,\text{mm}}{118.70\,\text{mm}} = 0.33\,\text{mm}\]

There are 6 steps in a full turn, so we get 55 μm per step, which is close to the value we measured above.

Finally, while I had the grinder apart, I thought I’d try to measure outfall depth for the stock burrs. The outfall is the small gap that forms where the grooves in the face of the burr meet the edge. Outfall depth has an effect on the particle size distribution, among other things.

Measuring across the flats on the burr using a caliper, I got a height of 7.53 mm. However, when I tried to align the jaws of the caliper with an outfall, I could not get a smaller measurement. In the photo above, you can see that the outside edge of the burr is quite continuous, which seems to support the idea that these burrs have no outfall.

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