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 
m 
M r 
1 1^30"- ~ 
ft 
K&4 '' Hm 
- " l ' ; <C"~' ' m\"- * * 2b 
Si ■ Mil 
«:[■"« 
« 
wWmm$ m* ill 
lH!L-- lira 
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. Alter 
one or two cores have been drilled, their diameter should be measured, and it 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- 
