General Cutting Tools offers a large selection of drills and taps in high speed steel (HSS), cobalt, carbide tipped and solid carbide from the most trusted manufacturers in the world. We also have our own, in house tool shop, tool makers with decades of experience, and 7-axis CNC tool cutter grinders to put a special grind, taper, relief and recondition to any drill.

CNC Machined Parts Drill and Tap Reference

Is there anything you need to know about tapped holes on your part?  It’s okay, this guide should answer all of your questions about adding tapped holes to your part.

Drilling vs. tapping: what’s the difference?

There is a difference between drilling and tapping. Drilling is the process of creating a smooth hole in a material using a drill and motor. Taping is the act of creating a thread inside a hole.

Taps are available to match almost any screw type, including metric and standard measurements. There are several factors to consider when selecting a drill bit, including thread count, diameter, thread pitch, and coarse/fine threads.

Guides for taps

Tap guides are crucial to creating a straight and usable tap. Taps are automatically centered and straight on machines such as mills and lathes. You should be careful when aligning a tap manually since the human eye isn’t as accurate as a perfectly 90-degree tap guide.

Using Oil

While drilling and tapping, oil is a necessity. Overheating is prevented, chips are cleaned out, the cutting action is assisted, and squealing is prevented.

Center Drills

The first step in drilling and tapping a hole is to use a center drill. As most drill bits aren’t perfectly centered when mounted in a drill chuck, this gives the larger bits a center to align with. By doing this, the larger bits will drill exactly where they are supposed to and won’t walk across the part.

Pecking

When drilling or tapping, pecking prevents the bits from overheating or breaking. A pecking process involves drilling into a part, then backing out to remove the chips and allow the part to cool. Typically, the handle is rotated about a full turn, then half-turned back. Whenever a bit/tap is pulled out, as many chips as possible should be removed and oil should be added to the surface and to the bit/tap.

Force

Don’t apply too much downward force.  To prevent tap breakage, use rotational force and tap pecking.

A few general rules

Several rules of thumb can be used to predict the proportion of tap drill to thread major diameter, for standard V threads (ISO V threads and UTS V threads):

• A good tap drill for coarse threads should drill about 85% (2pp) of the major diameter, and

• Fine threads require a tap drill that is 90% (20% 2-pp) of the major diameter.

• In metric V threads, the principle of major minus pitch (i.e., the widest diameter of the screw in millimeters minus the pitch of its threads in millimeters per thread) yields a good drill diameter for tapping.

• Major minus pitch can also be applied to inch-based threads, but first you must convert the fraction of threads-per-inch (TPI) into a decimal. An screw with a pitch of 1/20 in (20 threads per inch) has a pitch of 0.050 in, while one with a pitch of 13 threads per inch has a pitch of 0.077 in. There will only be a tap drill size near your result (not directly on it).

• According to these two rules of thumb (85%/90% and major minus pitch), the tap drill size yielded is not necessarily the only possible size, but it is a good one for general use.

• There are several trade-offs between 85% and 90% rules, however the rules work best in 14-in (4.6-25.4mm), the sizes most used on a shop floor. There are some sizes outside that range that have different ratios.

Tapping methods

1. By Hand

2. Using a Mill

3. Using a Lathe

What is the best way to add threads to my 3D model?

You don’t have to.  To add 3D taps to your part, you don’t need to add threading, which is a time-consuming task in most 3D CAD applications.  If you want to add threads to certain holes on your part, just follow these steps

1. Ensure that the holes match the pilot drill size required for the tap.

2. In your order notes or in a reference document, specify which holes are tapped and what thread type and pitch are used.

3. If a custom thread depth is needed, add a note.

Is it possible to have my part made with custom threads?

It depends on the diameter, usually.  Tapped holes can be made with a traditional tap, which is a tool made for a standard thread size, or with a thread cutter, which is a small disk that cuts threads using the CNC’s XYZ movement.  The thread cutter can only cut threads larger than 1/2 inch in diameter.  It is possible to make custom threads with a thread cutter, but there may be an additional charge.

How big should the pilot hole be?

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

Download the Chart

Check out the video below to learn more: “Drill and Tap Tips and Tricks – Kevin Caron”