Coal mining generates significant adverse environmental impacts. Historically, coal mining has been a very dangerous activity and the list of historical coal mining disasters is a long one. Underground mining hazards include suffocation, gas poisoning, roof collapse and gas explosions.
The term 'OVD' (Optically Variable Device) is used to describe both laser generated hologram images and electron beam generated images. These optically variable devices are created in highly-secure facilities and are at the forefront of overt asset and brand protection programmes.
The number on a number-sized thread tells you the nominal outside diameter. (In practice, the OD is made a bit smaller than this, just as a 1 OD thread isn't actually 1 inch OD.) The formula for working out the OD of a number thread is: # x 0.013 + 0.060 = nominal OD in inches. In other words, multiply the number given by 0.013 and add 0.060 to it.