FIGURE 7. - Bit attack angles: A, Side view of cut with continuous mining machine; B, side view of linear test cut.

coal sample moving through it by the traversing bed. Figure 8 shows a closeup of the minibob bit cutting coal.

During the test, horizontal, normal, and lateral forces, airborne dust, and test time were automatically recorded in the data acquisition system. After each test, the cut coal was collected in filter material with a nominal pore size of 3.0 um for the collection bag of the vacuum cleaner. Subtracting the empty bag's weight gave the total cut weight for determining the respirable airborne dust volume per gram of coal cut. The material was then stored for further screening and Coulter sizing to determine the nonairborne respirable fraction. The screening and Coulter sizing techniques are completely described in references 3 and 15.

COAL SAMPLES AND PREPARATION

The types of coal along with their proximate analysis are shown in table 1 with the sample orientation shown in figure 9.

Freshly mined samples of Illinois No. 6 and Pittsburgh seam coal from pillars, in approximately 30-cm cubes, were received at the laboratory within 72 hours after acquisition. relative humidity environment until they were cut to size and encased in a neutral gypsum cement that neither expands nor contracts upon setting. This procedure totally encased and preserved each sample prior to testing and provided support during testing. Attempts at encasement using another cement were stopped because contraction upon setting produced unknown triaxial loads and led to extensive internal fracturing. Just before testing, the encased block was cut in half with a wire saw and the front face of gypsum cut away to expose the coal to the cutting tool. A prepared coal block, ready for testing, is shown in figure 10. plane. All cuts were perpendicular to the bedding

The blocks were then stored in a 95-percent