Machine Shop Tools: Twist Drills
When a machine shop quotes a job for a potential customer, they need to take into account even the smallest details that can affect the time to complete the job and the perishable tooling cost. One important consideration is the type of twist drill used. The construction of the twist drill, the steel it is made from, and the point style can drastically change the number of drills used and the speed with which the job can be completed.
Originally, twist drills were manufactured by twisting a forged flat strip or forged round bar with two grooves on opposite sides. Hence, the name twist drills. However, these manufacturing methods have been replaced by either milling a round bar or rotating the round bar while moving it past a grinding wheel to cut the flutes, similar to cutting helical gears.
The twist drill can be essentially divided into three predominant parts - the point, the body, and the shank. The point is the cone shaped surface at the cutting end of the drill, while the part held by the chuck is referred as the shank. The point consists of a cutting lip, which performs material removal. Drill bit shanks are of two types - straight and tapered. Morse taper shanks are the most popular versions today. The remaining part of the drill bit is known as the body and consists of the flute - the twisting groove running around the bit. The flute is used for removing the cut material from the drilled hole. These features are common for most forms of twist drills, though in many cases, manufacturers produce special customized versions of twist drills for particular applications.
Factors Governing Twist Drill Performance
Various factors and parameters govern the function of a twist drill. The sharpness of the cutting edges, rate of the twists, point angle, and lip angle are some of them. Twist drills are produced in various sizes and chosen based on the required application. The most common point angle found on twist drills is 118 degrees, suited for drilling operations in a wide range of applications. However, for drilling steel or hard materials, tools with shallower angles of about 150 degrees are typically used. Another type of drill, one with a zero point angle, is used for drilling holes with flat bottoms. Maintaining twist drill bits is of vital importance, and machine shops need to keep the drill bits sharp in order to maintain drill performance.
Twist Drill Materials and Coatings
As a work piece harder than the drill bit can cause it to catastrophically fail, care needs to be taken while choosing the twist drill material. Twist drill bits are mainly made from steel, while other material bits are adopted for certain special applications.
Twist drill bits are often manufactured from carbon steel, which is considered an inexpensive option. These types of twist drills are soft and the type of materials on which they can be used is limited. Carbon steel bits require low RPM drilling to prevent excessive wear or bit failure. Low carbon steel drills are generally used with wood, as they do not hold the cutting edge well. Though high carbon steel bits can be used on both wood and metal, they are susceptible to the heat generated while drilling.
Another material commonly used for drills is high speed steel (HSS), which is compatible with most materials. These drill bits have predominantly replaced carbon steel bits due to their versatility and ability to cut at higher speed. Twist drills made of high speed steels can withstand temperatures of up to 1000 degrees Fahrenheit. In addition, coating the cutting edges with wear-resistant materials may enhance the HSS drill cutting speed from 25 to 90 percent.
A variation of the HSS twist drill is the cobalt high-speed steel or cobalt twist drill, which is a combination of cobalt and high-speed steel. These twist drills are generally used for working with ferrous metals, armored plate, and tough stainless steel. Another extremely hard material used for twist drills is tungsten carbide, which can drill into virtually all materials and hold the cutting edge longer. Typically, the tungsten carbide drills consist of a carbide tip brazed or screwed on top of a bit made of another material. This is done due to the high cost and brittleness of tungsten carbide. However, solid tungsten carbide drills are available as well. Though insert drills are less expensive than the solid drills, they can perform well on ferrous and non-ferrous materials such as glass bonded plastic, brass, aluminum and bronze.
To extend their lives and improve performance, twist drills are coated with a number of wear-resistant coatings. Some common coatings are titanium nitride, aluminum oxide, black oxide, titanium carbon nitride, zirconium nitride, and titanium carbide. These wear-resistant materials are capable of extending the life of the tool. In addition, black oxide coating aids in providing heat resistance as well as corrosion resistance. Drill bits are also coated with diamond power, making them ideal for cutting tile, stone, and other hard materials. One major disadvantage of diamond power coated bits is that they require water cooling often to prevent damage.
In conclusion, a twist drill is one of the most common tools used in the machine shop, however, the proper research must be done before the job is started in order to avoid unnecessary expenses and wasted time due to choosing the wrong tool. Furthermore, an improperly maintained or dull twist drill can increase the machine time required to complete the part, thus affecting the bottom line.
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