Gun drilling is a technique that is used to make high precision deep holes in materials with a high depth: diameter ratio. As the name suggests this technique was originally used to manufacture gun barrels. For this purpose the drilled hole had to be smooth and straight while preserving the strength of the barrel. Today these drills are manufactured for a number of other uses, but many of us will get confused when choosing the right drill for our purpose.
The type of nose grind and contour
The type of nose grind and the required contour depends on many factors including the type of material you will be drilling and the required depth: diameter ratio. Usually every gun drill machine comes with a general purpose N-8 nose grind with a R1 contour. The R1 contour is the best option if you are unsure of the drilling process, when high precision is not required or when factors like fluid pressure are not optimal. However, it is always best to consult your manufacturer if you are not sure of anything.Gun drills can be used on almost any material including metals, plastics and composites. But if you want optimum results you must use specific drills for non-metal materials. The general purpose ones can be used on any material, but they produce the best results for metals.
Drilling speeds and feeds
This speed you operate the machine will decide the tool life. The drilling speed is measured in SFM (surface feet per minute). Needless to say that if you use the tool regularly at maximum speeds the tool will have to be disposed of sooner. One way to increase the tool life of the machine is to coat it. Once coated, the tool can be re-sharpened many times without having to re-coat.
During the process of durable deep hole drilling the machine should be able to effectively remove the chips formed at the tip of the drill. Usually this is achieved by pumping a high pressure coolant internally through the drill and removing the chips from a groove outside.
The coolant has a dual purpose. It cools and lubricates the machine and also carries the chips outside. It is better to use oil based coolants to lubricate the machine properly. Water based coolant are not generally good, but if you must they should be a high EP coolant. Synthetic and semi-synthetic coolants must be avoided as they will cost your tool life.
You must also consider the hole configuration. For smaller diameters you can use a single round hole one as they can handle high pressure and will ensure a steady flow. For larger diameters kidney shaped ones or those with two holes may be better. Make sure there is always an unbroken flow of coolant to the machine.
Rule of thumb is to always contact the manufacturer when in doubt.