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Coffee grinder electric screwdriver adapter

Why

A high-end Porlex brand ceramic manual coffee grinder was obtained from Japan. These provide higher quality of coffee than high-speed electric ones, as the coffee is not subjected to an overheat.


Grinder box

Grinder box

Grinder box

Grinder box

Mill inside

Mill inside

Turning the handle is however a time consuming and laborious task.

A source of not too higher speed with sufficient torque is an electric screwdriver at slow setting. This should work comfortably enough to spare the handwork, while slow enough to not overheat the coffee.

How

The grinder has a pentagonal shaft, with matching cutout in the sheetmetal handle. This has to be adapted to a 1/4" hexagonal shape, common for screwdriver bits.


Grinder top

Grinder top

Grinder top with handle

For the hexagon, a spare lousy cheap screwdriver bit, the long kind with a groove for locking it in the chuck, was chosen. For the mating part for the shaft a M8 coupler nut was demonstrated to fit well. The diameter of the circle around the shaft pentagon is 7 millimeters.

A way to transfer the torque to at least one side of the pentagon is needed. A screw with flat tip, protruding from the side of the circular hole, was chosen as the easiest option.


Coffee grinder and precursor parts

Screwdriver bit and coupler nut

Screwdriver bit and coupler nut

Coupler nut shaft fit

The screw-facing tip of the screwdriver bit was cut off with an angle grinder.


Screwdriver bit cut off

The coupler nut was judged to be too long on its own. It was cut off at about 3/5 of its length, and faced on the lathe.

The screwdriver bit shank was mated to the hole, and pushed in with a vise. After axial aligning, the parts were welded around the entire circumference, using a 2mm electrode and 50-60 amps current. The welding was done piecewise, first on opposing sides, then in between, to mitigate warping. The slag was carefully broken off and cleaned between the welding steps. Generous amount of filler metal was used.


Bit welded to nut

Bit welded to nut

Bit welded to nut

The part was left to cool in air for a while, then was further cooled in water to save time.

The workpiece was then placed to the lathe chuck, and the excess filler metal was carefully machined away. The carbide tool tip was chipped a little during the operation, but it was chipped a little less even before. There was a generous amount of impact load on the tool, as the deposited filler metal was of fairly uneven shape, the work had to be done slowly.


Machining the part

Machining the part

Preliminary fit

Preliminary fit

When test-fitted on the grinder shaft, it shown that the hole is too shallow; there was about two-millimeter gap between the nut end and the grinder shaft flange. This was caused by excess insertion of the shank before the welding operation.

The part was placed to a drill press vise in a vertical position, then the hole was deepened using a 7mm drill bit. Test fit was now perfect. There was a concern that the screwdriver bit will be too hard to drill, but the heating and slow cooling during welding annealed the steel to soft grade.


Redrilled main hole

The part was placed again to a vise under the drill press. A 2.5mm hole was predrilled in one of the coupler nut sides, then enlarged to 4.2mm. A M5 thread was then cut using a three-piece tap set.


Side screw hole

Side screw hole

The part was then faced, an excess of length was removed and the face made perpendicular to the shaft axis.


Facing the part

Sides of the nut were then filed, to remove the welding deposits.


Finished part

Finished part

A short piece (about 10mm) of a M5 threaded rod was cut. Both sides were faced in the lathe and very slightly chamfered. Excess length was removed in this operation as well. A slit was cut to the end with a Dremel abrasive wheel, to facilitate screwdriver insertion; due to a misalignment the slit was off-center, which was not inhibiting its functionality. A flat-ended worm screw was therefore formed.

The worm screw was screwed into the M5 hole on the side of the assembly, adjusted to the proper depth using the grinder, then held in position with a lock nut from the outside.


Finished part

Finished part

Finished part with side screw

Finished part with side screw

Finished part with side screw

Final fit

The final assembly was stored in the rubber sleeve of the grinder, together with its manual handle.


Storage of handle and coupler

Storage of handle and coupler

Todo


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