Taps are cutting tools used by machinists to cut or form screw threads. The first recorded use of taps in metalworking is placed sometime in the 18th century, while the British engineer and industrialist Joseph Clement popularised the tool in the mid-19th century. We then saw standards emerge in threading with the tap drill chart. As a consequence the development of taps corresponded to this standardisation ensuring enhanced compatibility.
The Threading Process
As you would expect, taps come in all forms and sizes in order to accommodate standardised screw threads. Therefore the tap drill chart is extremely important. To create a thread properly, one would need a set of three taps that correspond to the same bit size; namely the bottoming, the intermediate, and the taper tap. The intermediate, or plug tap, is used first to start cutting out material from a non-threaded hole. That is why it features a tapered bit, allowing the machinist to align it correctly.
Unfortunately, due to the shape it can’t create a thread all the way to the end of the hole. So next comes the bottoming tap that has the capacity to do this. However, the bottoming tap cannot start the threading on an uncut hole, so it can only take part in the cutting process as the second or third tap. The taper tap is only used in certain situations such as when the material is very hard or the hole is too small. The taper tap features a smooth diameter transition that allows for a less aggressive cutting action. This minimises the risk of damaging the drilled hole. If the taper needs to go first, the intermediate tap follows second and the thread is finished with the bottoming tap.
Thread Geometry And Drilling The Hole
As is evident from the above, to create the thread we first need to drill a hole on the workpiece. Having determined the bolt that we want to use in the specific case, we already have a technical specification for the thread which will host it. Bolts are classified by specification codes such as:-
- M4-0.7 x 20
The “M” indicates the “metric diameter” of its thread (see tap drill chart below), which in our case is 4 mm. The second number (0.7) is the pitch which corresponds to the distance between two adjacent threads in millimeters. Finally, 20 is the length of the bolt in mm, measured from the tip of the thread to the bottom of the head (with the exception of oval headed bolts).
So, if we wanted to drill a hole for our example bolt, we would need to use a drill that is smaller than 4 mm. The material will have to be cut and pushed out in order to create a thread in that hole. If that is the case, can we simply use the next smallest available drill size. That would be a 3.7 mm drill bit, but using that drill would leave an inadequate margin for the creation of the appropriate thread depth. A rule of thumb is that the right tap drill is around 85% for coarse threads and 90% for fine threads which feature a higher number of threads per axial distance. In our case, the correct tap drill would be the closest to 3.4 mm from below, and that is the 3.3 mm or 0.13-inch tap drill. Lower percentages such as “75% of the thread depth” are also widely used and considered safe.
To help you get the grasp of the fine-coarse difference, the M4 classification comprises of the:-
- Coarse 0.7 mm thread
- Fine 0.5 mm thread
- Extra-fine 0.35 mm thread pitch
The smaller the pitch value, the less deep the threading needs to go. So for:-
- M4-07 we use the 3.3 mm tap drill
- M4-05 we use the 3.5 mm drill
- M4-035 we use the 3.6 mm drill bit
As per the ISO metric screw thread standard, the thread depth should be 0.614 x pitch, which is why a smaller pitch number also results in a lower thread depth value. Now engineers can sit down and manually calculate the right tap drill. Or they can consult a tap drill chart like those found in the next section.
Some prefer to use somewhat arbitrary but reliable formulas like: “Dtap = Nominal Diameter – Thread Pitch”. This formula works for both metric and imperial units, and for all 60-degree threads (all ISO, DIN, UTS, UNC, UNS, and UNF bolts). The first tap drill chart below is in mm and the next in inches. Both are available as a printable tap drill chart PDF as well.