Drill and Tape Sizes Reference for CNC Machined Parts

Drilling is the process of creating a hole, while tapping is the technique applied to cut threads within the hole, so tap drills and tapping tools are commonly used to in turn to make CNC parts with threaded holes. When selecting their size, there are standard tap drills and tap size tables for reference.

Drill and Tap Sizes Charts

The tap size chart is a table that lists standard sizes for fractional, metric, and screw taps. For fractional sizes, these are measured in inches, while metric sizes are measured in millimeters and are denoted by an “M” before the measurement. As the screw size number increases, the diameter also increases. Thread spacing can be either coarse or fine and is denoted by a thread count, measured in threads per inch, for fractional and screw sizes. For metric sizes, the thread spacing is denoted by the thread pitch, which is the distance between threads, and is measured in millimeters.

Tap Drill Sizes- Imperial

Drill Bit Sizes- Metric

Tap Sizes- Imperial

Tap Sizes- Metric

What Is the Difference Between Drilling and Tapping?

Drilling is the process of making a hole in a material using a drill and motor to make a smooth opening. Tapping is to create a thread into the side of the hole, and it requires a type of tap that is determined by the thread count, diameter, thread pitch, and whether the thread is coarse or fine.

What Is the Difference Between Threading and Tapping?

The tool that is used as well as the surface that is worked on in both the threading and tapping operations, are what set these two processes apart from one another. In the process of threading, a die tool is used to form threads on the exterior of a hole. The tapping technique involves using a piece of tapping equipment to create internal threads inside a drilled hole.

Learn more about: Threaded Holes vs. Tapped Holes

Types of Tap Drill Sizes

The most common types of tap drills sizes are Letter, Number, Fractional, Decimal, and Metric.

1. Letter drills are typically used for threading a range of materials and come in sizes from A-Z.
2. Number drills are used for threading thicker materials and come in sizes 1-80.
3. Fractional drills are used for threading a variety of materials and come in sizes from 1/16th to 1 inch.
4. Decimal Drill Size: Write with 4 digits after the decimal point, equivalent to the drill size in fractional-inch
5. Metric drills are used for threading a variety of materials and come in sizes from 0.5mm to 12.5mm.

Tips for Tapping Operation

• Center Drills
  Before drilling and tapping, a center drill must be used to provide an exact center point for the larger bits. This is because most drill bits are not perfectly centered when mounted in a drill chuck.
• Pecking
  Pecking is a helpful method for both drilling and tapping. This involves drilling into the part, then backing out to allow the part to cool and remove chips. As the bit/tap is pulled out, chips should be removed and oil should be added to the surface and the bit/tap.
• Using Oil
  Using oil while drilling and tapping is also critical. It prevents overheating, helps remove the chips created by the bits, and assists in the cutting action. Additionally, oil prevents the bits from squealing.
• Using Tap Guides
  The tap guides, located with the taps, are essential tools for creating a straight and usable tap when machining. These guides ensure the tap is centered and held at a perfect 90 degree angle, which is often difficult to achieve by eye.
• Force
  It is important to not apply too much downward force to prevent tap breakage. Instead, use a rotational force and the tap pecking method.

Tips for Tap Drill

For standard V threads, there are two rules of thumb that have strong predictive power for determining the tap drill size:

1. For coarse threads – The tap drill should be 85% (+/- 2 pp) of the major diameter.
2. For fine threads – The tap drill should be 90% (+/- 2 pp) of the major diameter.

Additionally, for metric V threads, the major minus pitch technique (calculated by subtracting the pitch of the threads from the major diameter of the intended screw in millimeters) can also be used to determine the tap drill size. This method also works on inch-based threads, though in this case it is necessary to first convert the fraction of threads-per-inch (TPI) into a decimal.

These rules of thumb are generally applicable for sizes ranging between 1/4 and 1 inch (6.4 – 25.4 mm), though for sizes outside this range, different ratios may apply.

Can My Part Be Fabricated with Custom Threads?

Usually, yes, but it will be determined by the diameter. To make standard thread sizes, a traditional tap is often an ideal tool to create tapped holes. Or using a thread mill – a small disc to cut threads with the axes motion of the CNC machine. Thread cutters are only available for diameters greater than 1/2 inch. A thread cutter can be used to make custom threads, but in return, this may incur additional costs.

Runsom, an expert CNC machining manufacturer, is confident in meeting your custom CNC machined part fabrication needs. Our team of engineers, machinists and technicians own extensive experience in making tapped holes of any size in a variety of materials. If you have special threading or tapping needs, count on us and we will offer you the best-suited machining solutions and desirable products beyond your expectation. Request a rapid quote right now!

Learn more about our CNC Drilling Capability.

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