This article needs additional citations for verification. Drilling drilling machine operations pdf a cutting process that uses a drill bit to cut a hole of circular cross-section in solid materials. The drill bit is usually a rotary cutting tool, often multi-point.
In rock drilling, the hole is usually not made through a circular cutting motion, though the bit is usually rotated. Instead, the hole is usually made by hammering a drill bit into the hole with quickly repeated short movements. Also, the inside of the hole usually has helical feed marks. Drilling may affect the mechanical properties of the workpiece by creating low residual stresses around the hole opening and a very thin layer of highly stressed and disturbed material on the newly formed surface. For fluted drill bits, any chips are removed via the flutes. Chips may form long spirals or small flakes, depending on the material, and process parameters. When possible drilled holes should be located perpendicular to the workpiece surface.
This minimizes the drill bit’s tendency to “walk”, that is, to be deflected from the intended center-line of the bore, causing the hole to be misplaced. The higher the length-to-diameter ratio of the drill bit, the greater the tendency to walk. Spot facing, which is machining a certain area on a casting or forging to establish an accurately located face on an otherwise rough surface. Surface finish produced by drilling may range from 32 to 500 microinches. Finish cuts will generate surfaces near 32 microinches, and roughing will be near 500 microinches.
Cutting fluid is commonly used to cool the drill bit, increase tool life, increase speeds and feeds, increase the surface finish, and aid in ejecting chips. Application of these fluids is usually done by flooding the workpiece with coolant and lubricant or by applying a spray mist. There are a variety of drill styles that each serve a different purpose. The subland drill is capable of drilling more than one diameter.
The spade drill is used to drill larger hole sizes. The indexable drill is useful in managing chips. The purpose of spot drilling is to drill a hole that will act as a guide for drilling the final hole. The hole is only drilled part way into the workpiece because it is only used to guide the beginning of the next drilling process. Deep hole drilling is defined as a hole depth greater than ten times the diameter of the hole. These types of holes require special equipment to maintain the straightness and tolerances.
Other considerations are roundness and surface finish. Deep hole drilling is generally achievable with a few tooling methods, usually gun drilling or BTA drilling. Using methods such as a rotating tool and counter-rotating workpiece are common techniques to achieve required straightness tolerances. A high tech monitoring system is used to control force, torque, vibrations, and acoustic emission. Vibration is considered a major defect in deep hole drilling which can often cause the drill to break. A special coolant is usually used to aid in this type of drilling.
Gun drilling was originally developed to drill out gun barrels and is used commonly for drilling smaller diameter deep holes. The depth-to-diameter ratio can be even greater than 300:1. Trepanning removes the desired diameter by cutting out a solid disk similar to the workings of a drafting compass. Microdrilling refers to the drilling of holes less than 0. Drilling of holes at this small diameter presents greater problems since coolant fed drills cannot be used and high spindle speeds are required. High spindle speeds that exceed 10,000 RPM also require the use of balanced tool holders.