a web designer welding!

Taps, dies and drills

taps and tap holder

Tap and drill sizes

To find the correct drill size for tapping a thread, simply subtract the core diameter from the thread size. The most common thread sizes are given in the table.

Metric Course Threads
Diam. core tapping drill clearance drill
4.0 3.14 3.3 4.1
5.0 4.02 4.2 5.1
6.0 4.77 5.0 6.1
8.0 6.47 6.8 8.2
10.0 8.16 8.5 10.2
12.0 9.85 10.2 12.2
14.0 11.55 12.0 14.25
16.0 13.55 14.0 16.25
20.0 16.93 17.5 20.25

For more tables I recommend buying the Zeus book. I also use the Presto book.

Zeus and Presto books, well used


Drilling speeds

It is important to select a suitable speed when drilling. Softer materials are drilled at higher speeds, deeper holes should be drilled at lower speed.

The recommended cutting speed for mild steel is 30 to 36 m per minute. To find the suitable RPM setting for a particular drill bit size use the formula R =s/ π D where R is the RPM, s is the cutting speed and D is the drill diameter in metres.

As 10π is within the range for mild steel, an aproximation of this formula is given by 10/d =R, where d=bit size in mm and R=1000 RPM for example a 5 mm drill bit 10/5=2 so the speed should be 2000RPM. A slightly higher speed would be acceptable for shallow holes with the metal firmly held in a pillar drill. This formula also indicates whether to use a high or low speed setting on a hand drill.


Drill bit sharpening

Drill bits need to be kept sharp to ensure efficient cutting. The angle should be about 30 degrees