Tap drill charts are a machinist’s best cheat sheet and how-to guide all in one. But to a newcomer, they might not appear to make a whole lot of sense. Add in some potentially complicated sizing systems, and even a simple tap drill chart might require some explanation.
That’s what we’re here to do – give you a tap drill chart and teach you how to use it. Let’s start with the basics.
Drilling and tapping
Creating a threaded hole is one of the most common machine shop tasks. There are two basic steps to the process – drilling the hole with the correct drill bit size and then threading or “tapping” the hole.
Each step requires precise measurements. Drill a hole too large, and your screw, bolt, or pipe thread will be too loose. Too small, and you won’t be able to make a connection at all – you’ll have to re-drill the hole.
The correct threading is also vital. Whether it’s a coarse thread or fine thread, matching the correct thread to your intended fastener is key to a good fit. Thread it incorrectly, and you’ll find that inserting the fastener is a much harder job than anticipated. You’ll also ruin the threads on both the fastener and in the threaded hole.
Drilling the correct hole size and tapping the right thread pitch is essential for fasteners that stay tight and keep parts performing as required.
Pre-drill a hole with the correct tap drill size, and you’ll ensure that all of your screws and threaded bolts will have a tight, close fit. But how to tell the correct size? You’ll find a number of different sizing systems in common use for measuring drill bits. In fact, that’s where tap drill charts come in – they help machinists navigate the different terms and find the right size.
There are at least five common sizing systems in use. Any given drill size chart might use several of the sizes at once. All of these systems refer to the diameter of the drill bit.
Metric – The diameter of the bit given in standard metric sizes, typically expressed in millimeters.
Letter – A-Z, with A being the smallest and Z the largest.
Number – Twist bits (a particular kind of drill bit) are the most likely to use the letter and number systems. Size 80 is the smallest, size 1 the largest.
Fractional – For standard English measurements (inches), drill bit diameters are expressed as a fraction of an inch – say, 1/16 inches.
Decimal – Decimal sizing is used to express fractional equivalents in the metric system, given out to four places.
A good tap drill chart doesn’t just give the diameter of the drill bit. Machinists also use them to calculate the correct threading.
Screw thread sizes are expressed differently depending on the sizing system used. The English system expresses a thread count – the number of screw threads per inch, also known as TPI. Metric measurements use thread pitch – the distance between threads, expressed in millimeters.
You may also see charts that refer to major diameters and minor diameters. Major diameters refer roughly to the height of the thread itself, while minor diameters calculate the overall number of threads. The more threads, the higher the minor diameter.
A good tap drill chart will also suggest the correct bit size for the initial drilled hole. Once that hole is created, it can be tapped with the right thread count or pitch. Some charts will also include a standard size of thread depth, depending on the application.
Your average tap drill chart might appear confusing at first. Once you understand what’s being expressed, things become a bit clearer. You’ll always want to know the intended depth of the hole and the screw thread information for the fastener.
* These Sizes Are Spark Plug Taps