Making a pigment grinder

Luthier based in Villeneuvette, France

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Traditionally, luthiers grind pigments for varnish by hand, using a glass muller on a piece of frosted glass. This is a job that takes a long time, and is sometimes made more difficult because some recipes call for pigments that are harder to grind. For these reasons I developed a machine to do the work, based on equipment designed for polishing gems. I find that using this device gives better results than grinding by hand.

The machine requires two pieces of plywood, 10mm and 2.5mm thick; a wooden dowel of 8mm diameter; a 12V transformer; a 12V rotating motor wiTha speed of 100rpm; and a number of screws. All these parts are easy to source; I found the motor and transformer online.

Dimensions of the grinder

1 Using the 10mm-thick plywood, I cut the different pieces of the box according to the diagram shown here. The four sides of the grinder are 150mm by 160mm, while the top and bottom are 170mm by 150mm.

I cut two windows, boTh35mm by 86mm, on opposite walls, one for air ventilation and the other to fix the motor. Then I glue or screw it all together apart from the roof. Dimensions can be adjusted according to the size of the motor.

Making a wide nick in the motor shaft

2 Using a Dremel router fitted wiTha grinding disc, I make a large, wide cut on the shaft of the motor.

A piece of wood protects the motor

Welding the transformer

3 I cut a piece of the 2.5mm plywood as shown in the above ' picture (60mm by 100mm) wiTha small hole for the motor axis. Then I use screws to fix it in place on top of the motor.

4 I weld the 12V transformer to the motor.

a

b

ALL PHOTOS AND DIAGRAMS NICOLAS GILLES

5 Next, I screw the combined motor-transformer into one of the windows of the plywood box. As can be seen in picture 5a, the other window on the back will act as an air vent for the motor.

a

b

c

6 From the lOmm-thick plywood, I cut out two discs of 150mm diameter (6a). Then I drill the 6mm centre hole for the axis and the nine 8mm holes according to the plan (6b).

These nine holes make up the points of three equilateral triangles, respectively 40mm, 50mm and 60mm from the centre. This will allow the grinder to hold different-sized pots.

Then I make a channel on the first disc, in which I insert a nail going through the cut in the axis, locking it to the disc (6c). Finally I screw the second disc on to the first one.

The rotating disc is attached to the box

The grinder wiTha pot in place

7 Now it’s time to screw on the roof and paint the box. While it’s drying, I cut the wooden dowel into three 20cm sticks. Then I insert them into the 8mm holes. These will hold the pot.

I have also added two screws at the front of the machine’s underside, which operate as legs. It means I can adjust their height to change the horizontal level of the pot, if I feel it needs to rise up a little.

The pot can be secured wiThsticky tape and the machine placed on a plastic tray, if you feel there’s a danger of breakage.

Using the grinder

8 I place five 18mm metal bearings inside the pot wiThdry pigments. They can also be mixed wiThturpentine or thin varnish. While the pot is turning, the metal balls will go up and down and grind the pigments.

The grinding time varies depending on the hardness of the pigments. For a madder-root lake it takes around six hours to make them very finely ground. I prefer to grind a good quantity that I can then store for later use, wiThlinseed oil and turpentine inside to prevent the oil from drying in the pot. A speed regulator can be used to improve the machine’s efficiency.