Getting Started

Safety Considerations

Collection and/or containment of the dust is very important to reduce exposure. The permissible exposure limit for soapstone as set by OSHA is 20 million particles per cubic foot, and according to NIOSH has a recommended exposure limit of 3mg/m³. Good ventilation and/or a respirator should be used for protection. It is also very important to get soapstone from a trusted source because soapstone often forms alongside asbestos containing materials. Many companies selling soapstone test for asbestos, it’s recommended you find as source that provides the results to consumers.

Standard safety measures for the CNC and kiln will suffice for this experiment. Have the kiln properly ventilated and away from any flammable material and be careful around an operating CNC.

Acquiring Stock

There is a large variation in natural soapstone, the kind used for this application is high in talc content. This softer variety is typically found marketed for carving. The more homogenous the stock, the better, reducing the potential to warp or fracture along the veins of the stone. Brazilian soapstone was used and there were only a few minor issues with cracking along the grain of the stone. Stonebridge Imports in the USA and Canada has 3x3x5” and 4x4x6” blocks that work well for a desktop scale, and they have asbestos reports available on the item page.

Set Up

This experiment used a Genmitsu 3018 and a Sterilite 1497- 64 Qt latching box. For other CNC machines, the box should be large enough to place the CNC inside. Milling of the soapstone should occur with the CNC inside the box to limit the dust that becomes airborne. Included in the files are two mounts used to anchor the stock. These fit the bed of the Genmitsu machine and were found to be very useful in securing the soapstone. Once the stock is secured and the CNC covered, machining can begin.

Machining

For machining soapstone, setting spindle rate of 2000 rpm and having a feed rate of 250 with a maximum step down of 1 worked well for the Genmitsu. These settings are conservative, so milling times are rather slow. The CNC began to struggle at feed rates of 400 and a step down of 3. Other than a high spindle rate and low feed rate and step down, no other changes are needed for soapstone verses other stock.

A note on the stock:

Most soapstone stock does not come in very precise dimensions. A difference of up to 5% from the dimensions listed is not unusual. Because of this, leveling one face of the stone and using that face as the base when machining was helpful to ensure consistency.

In addition to this, the grain of the stone can affect how the dimensions may change during firing. The largest dimensional change seen between the unfired and fired pieces was a 1.2% increase in length along the grain of the stone. The same piece had a .71% increase in width and a .08% decrease in height.

Firing

When being fired, the soapstone needs to reach a temperature of 1,000-1,200 C (1,832-2192 F). Settings for the Tabletop Furnace Company Muffle Furnace used were as follows:

C01 50

T01 10

C02 1900

T02 100

C02 1900

T02 50

C03 1000

T03 30

C04 500

T04 30

C05 500

T05 -121

Which translates to:

50 F for 10 min

Ramp up to 1900 F over 100 min

Held at 1900 F for 50 min

Cool to 1000 F and held for 30 min

Cool to 500 F and held for 30 min

Shut off furnace and allow to cool