SAMPLING AND TESTING HIGHWAY MATERIALS. 77 



Any drill press equipped with a hollow spindle and with the table so arranged 

 that the water carrying the rock cuttings may be properly collected and carried away 

 is satisfactory for use in rock drilling. A drill press carrying a No. 2 Morse taper is 

 large enough. The speed of the drill should be in the neighborhood of 300 revolu- 

 tions per minute. 



Great care should be exercised when first using a diamond drill. A block of very 

 soft limestone or sandstone should be selected and a number of cores cut from this 

 stone until it is found that the drill is working properly, after which it may be used 



Fig. 37.— Diamond drill in use. 



on harder rock. The sample should be bedded on a bag filled with sand as shown in 

 figure 37, or in the case of very small pieces, it may be necessary to mount the samples 

 in plaster of Paris before drilling. Plenty of water should be used on the inside of the 

 drill so as to keep the space under the crown entirely free from rock cuttings, which, 

 especially in the case of soft rock, have a tendency to "gum up" the drill. After 

 one or two cores have been drilled, their diameter should be measured, and if it is 

 found that the drill is cutting cores more than 25 mm. or less than 24 mm. in diameter, 

 one or two of the diamonds must be reset. If the drill crown is turned to the dimen- 



