Drill and Tap Reference for CNC Machined Parts

Do you have a question about tapped holes on your part?  You’re not alone, luckily this guide should answer all of the questions you have about how to add tapped holes to your part.

What is the difference between drilling and tapping?

Drilling and tapping are two different actions. Drilling refers to creating a smooth hole in a material with a drill and motor. Tapping is the action that creates a thread into the side of the hole.

A variety of taps are available to match almost any screw type available, including metric and standard measurements. Information required to select the drill bit includes thread count, diameter, thread pitch, and coarse/fine threads.

Tips for Machining Threads

Successfully using thread-cutting technology requires more than just knowing how to cut threads. Here are a few design tips to help you maximize the benefits of thread-cutting technology:

  • Adding a chamfer to external thread ends will ensure a better finish.
  • The starting point of a thread must have a flat surface that is aligned to the thread’s central axis.
  • For internal threads, countersink can be used to provide a better finish.
  • Tubular parts must have increased wall thickness when undergoing thread forming or thread cutting due to the pressure.
  • Using a standard thread size is more cost-effective than using a unique thread size.
  • Lower-height threads are preferable over higher-height threads when there are no specifications for the thread size.

Tips for Tapping

  • 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.

Rules of Thumb

Regarding the proportion of tap drill to thread major diameter, for standard V threads (ISO V thread and UTS V thread), there are several rules of thumb with strong predictive power:

  • A good tap drill is 85% (± 2 pp) of major diameter for coarse threads, and
  • A good tap drill is 90% (± 2 pp) of major diameter for fine threads.
  • For metric V threads, the concept of major minus pitch (i.e., the major or widest diameter of the intended screw in millimeters minus
    the pitch of the threads of that screw in millimeters per thread) yields a good tap drill diameter.
  • The major minus pitch technique also works for inch-based threads, but you must first calculate the pitch by converting the fraction of threads-per-inch (TPI) into a decimal. For example, a screw with a pitch of 1/20 in (20 threads per inch) has a pitch of 0.050 in and a ​1⁄13 in pitch (13 threads per inch) has a pitch of 0.077 in. Your result will only land near a tap drill size (not directly on one).
  • For both of these rules of thumb (85%/90% and major minus pitch), the tap drill size yielded is not necessarily the only possible one, but it is a good one for general use.
  • The 85% and 90% rules works best in the range of 1⁄4–1 in (6.4–25.4 mm), the sizes most important on many shop floors. Some sizes outside that range have different ratios.

Tapping methods

  1. By Hand
  2. Using a Mill
  3. Using a Lathe

How do I add threads to my 3D part?

The short answer is, you don’t have to.  In most 3D CAD applications adding threading is a time consuming task that isn’t needed to add 3D taps to your part.  If you want to add threads to certain holes on your part, just follow these steps

  1. Make sure the holes match the pilot drill size required for the tap.
  2. Reference in your order notes or a reference document which holes are tapped and the thread type and pitch.
  3. Add any notes if a custom thread depth is needed.

Can my part be made 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.

What size should I make the pilot hole?

We’ve added a simple chart for the most common Standard and Metric tap sizes below.