Introduction to Drills
When selecting a drill bit, there are many variables to consider. This includes: the material being cut, the depth, accuracy, number of holes, functionality, and much more. This article is here to help you make a decision and give you a better understanding of drill bits. Machinists use drills on a daily basis, so it is worth the time to learn the terms and cutting angles. Below we will be discussing the different types of bits, a diagram showing the anatomy of a drill bit, cutting angles, materials the bits are made of, coatings, feeds and speeds, and lastly we will conclude with lubing your bit.
The Different Types of Drills
Drills are broken down in three different sizes when it refers to the diameter.
- Number sizes- no. 80(.0135 inch) – no. 1 (.228 inch)
- Letter sizes- A (.234 inch) – Z (.413 inch)
- Fractional sizes- 1/64 (.0156 inch) upwards by sixty-fourths of an inch
Twist– A shaft with helical flutes. This drill bit is the most commonly made.
Jobber– Standard lengths for twist drills up to 11/16 inch in diameter.
Stub– Shorter length for twist drills, made for rigidity.
Morse tapered– A bit that is self-holding by the Morse taper angle. This allows the operator to secure the tool in the machine by the use of the taper.
Center Drill– A tool used for spot drilling but is used for Centers on a lathe! Spot drills are the main source for spotting a hole. This ensures the hole does not start off drilling at an angle. (Stub drills do not need a center drill)
Spade– The spade is made up primarily of three parts, the cutting blade or insert, the body also called shank, and the fastening screw that holds the insert to the body. This tool can be used for a variety of different operations. The main use is for long holes and diameters over 1 inch.
Step– A tool bit with different sized diameters.
There are many more but these are the most commonly used in a machine shop.
Diagram
There are three main parts to a drill: the shank, the body, and point. The Shank is the area of the bit that inserts into the chuck. The body is the area containing the flutes. The point or tip is the main cutting area of the drill; the most important feature of a drill is ground here. Here is a diagram showing the terms and areas of a drill.
Cutting Angles
There is much debate as to the best cutting angles for specific materials. By researching these angles, there is an obvious pattern. Below is a quick interpretation:
60 degrees- Used for soft materials such as wood, fiber, and plastic.
100 degrees- Used for copper, and material such as medium to hard brass
118 degrees- This angle, being the most common, is used for softer steels and other materials such as aluminum.
135 degrees- This angle is used for harder steels such as stainless.
Materials
There are various types of materials that drills are made of and each has its own purpose for different applications.
High speed steel– Most commonly used. It is a form of tool steel, which makes it hard.
Cobalt– Although it is more brittle than HSS, cobalt drill bits are used for harder materials such as stainless steel.
High and low carbon– DO NOT BUY
Tungsten carbide– More expensive, but it cuts hard metal like a champ.
Coatings
Coatings work with the drill bit to make it more durable, hard, and long-lasting.
Black oxide– A black coating that improves heat resistance.
Titanium– A hard ceramic material that can be used to coat HSS. Brittle but extends tool life 2 to 3 times.
Diamond– This coating is used for tile, stone, or really hard material. Use water on the drill to prevent damage to the work piece or tool bit.
Feeds and Speeds
Aluminum- 200-300
Bakelite- 100-150
Plastics- 100-150
Brass and bronze- 200-300
Bronze, high tensile 70-100
Cast iron, chilled 30-40
Cast iron hard 70-100
Cast iron soft 100-150
Magnesium 250-400
Malleable iron 80-90
Monel, metal 40-50
Nickel 40-60
Steel annealed 60-70
Forged steel 50-60
Stainless, easy 60-70
Stainless, hard 30-40
Tool steel 50-60
Slate marble and stone 15-25
Wrought Iron 50-600
V= velocity or cutting speed in feet per minute (spfm)
D= Diameter of drill
N= revolutions per minute (rpm)
V= 3.1416 DN/12
Example
Drill- ½ Material-Aluminum
200*4= 800
800/.5= 1600 RPMs
A basic rule to follow- Hard material= Slower Spindle speed. Smaller drill= Faster spindle speeds
Lubricants
Aluminum- Kerosene, soluble oil
Brass- Dry, soluble oil
Bronze- Dry, soluble oil
Cast iron- Dry, air jet, soluble oil
Cast steel- soluble oil
Copper- Dry, kerosene, soluble oil
Malleable iron- Dry, soda water
Monel metal- soluble oil
Steel mild- soluble oil
Forged steel- soluble oil
Tool steel- soluble oil
Magnesium- Dry
Wrought iron- soluble oil
Conclusion
Although the topic of drill bits is vast, there are a few simple rules to follow when getting started:
- Buy HSS bits
- Go slow with the spindle speed on big drills and fast with little drills.
- Go fast on soft material and slow with hard material
- Use lubrication to prevent damage to your bit or work piece.
These guidelines are good starting points for any beginner. Do not feel overwhelmed with all of the different types of drills. 90 percent of parts require a standard twist drill bit. Other bits are required for a special job that most bosses won’t even think about giving to a beginner. Experience will bring confidence in this trade. Knowing the terms and angles comes with the territory as a machinist. Having a good understanding of drills can make your life easier for those days that require getting the job finished with little time to spare.
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