Drill Sizes for Tapping

Tapping is a process of thread cutting in a material automatically by machine or manually by hand. For automatic process we may use lathe, tapping machine or milling machine, etc. But in manual process we use a tool called tap wrench. But before tapping (thread cutting) we should drill a hole in the material. Only then we can do tapping. We say I need to do tapping for M5 thread (5 mm metric thread) . It's OK. But what is the drill size for M5 bolt? Is it 5 mm hole? Or less than that? Please refer the table given below.

Drill Sizes for Tapping

Here, We have given drill sizes for tapping for metric thread and for B.S.P. thread.

In the first table you can find the drill size for metric thread.

Here, "M" in the M1 means metric thread. So M1 means metric thread of 1 mm. 

Here in the metric thread table, Pitch means the axial distance between one  thread and the next one thread in mm. 

Drill size means the hole first to be drilled in the material before tapping.

You can get some more useful information at the bottom of last table.

DRILL SIZES FOR TAPPING FOR METRIC THREAD

(60⁰ Included Angle)

Metric Thread

Pitch (mm)

Drill Size (mm)

M1

0.25

0.75

M2

0.40

1.60

M3

0.50

2.50

M4

0.70

3.30

M5

0.80

4.20

M6

1.00

5.00

M8

1.25

6.80

M10

1.50

8.50

M12

1.75

10.20

M16

2.00

14.00

M20

2.50

17.50

M24

3.00

21

M30

3.50

26.50

In the second table you can find the drill size for B.S.P. thread.

Here, "B.S.P. thread" means British Standard Pipe thread. So we say 1/8" thread as one by eight inch B.S.P. thread.

Here in the B.S.P. thread table, unlike metric thread TPI (Threads per Inch) means the number of threads cut in one inch. 

Drill size means the hole first to be drilled in the material before tapping. Note that drill size is in mm.

 DRILL SIZES FOR TAPPING FOR B.S.P THREAD

(55⁰ Included Angle)

B.S.P.

Threads per inch

Drill Size (mm)

1/8

28

8.80

1/4

19

11.80

3/8

19

15.25

1/2

14

19

3/4

14

24.50

1

11

30.75

1 1/4”

11

39.50

1 1/2”

11

45

1 3/4”

11

51

2

11

57

Metric Thread Table:

Let us understand this table more deeply using an example.

Let's take an example of M10 metric thread, 1.5 pitch and 8.50 mm, from the metric thread table. 

Here,

a) Metric thread:   As we already stated M10 means Metric thread of 10 mm. So we say bolt as M10 bolt and nut as M10 nut.  Commonly, we mention the bolt with the pitch size. For instance, M10 x 1.5. Because there are many pitch sizes in M10 like coarse thread and fine thread. Here, 1.5 means pitch size . For example, M10 x 1.25 and this thread is mainly used for standard cylinders.

b) Pitch:   1.5 pitch means the axial distance between any one of the thread and the next one thread is 1.5 mm. See the figure below.

c) Drill Size:   8.50 mm is the hole size to be drilled before tapping. How it is arrived. The formula is Major diameter - Pitch.

For M10 bolt major diameter is obviously 10 mm. Pitch is 1.5 mm.

Drill size = 10 - 1.5 = 8.5 mm.

Then, you may ask why M8 and M12 don't follow the formula.

For M8,   8 - 1.25 = 6.75 ~ 6.80.  To eliminate accuracy from 0.05 mm to 0.1 mm we round of the value as 6.80. And so for M12, drill size is 10.25 ~ 10.20.

B.S.P. Thread:

Let us try to understand this table more deeply using an example.

Let's take an example of 1" B.S.P. thread, 11 threads per inch and 30.75 mm, from the B.S.P. thread table. 

a) B.S.P. Thread: Here we say 1" thread as one inch B.S.P thread. Unlike metric thread here 1" do not means that the major diameter of thread. 

Let us go back to history and understand this. Earlier days, as B.S.P. stands for British Standard Pipe, for piping application they will generally say the internal diameter of the pipe as 1 inch or 1/2 inch.

So the internal diameter of the pipe of 1 inch will have 25.4 mm internal diameter (1 inch = 25.4 mm). So to connect these pipes we may need to cut thread in the outer diameter. For that we need wall thickness. Those days as the materials properties are not improved we used 3.8 mm wall thickness and the outer diameter is 33 mm. And so Major diameter for 1 inch thread is taken as 33 mm.

But modern days as the properties of materials improved we don't need 3.8 mm thickness. So if you reduce thickness the outer  diameter will be reduced and vary according to each and every materials. It that's true then there will be confusions. So the major diameter is standardised (33 mm for 1 inch) and the internal diameter is increased.

Note that nowadays if you take a one inch pipe, the outer diameter will be constant as 33 mm, but the inner diameter will vary. It may be more than 25.4 mm.

b) TPI:  In inch bolt we can see the number of threads in one inch as 11. See the figure below.

Unlike pitch in metric thread here we say TPI (Threads Per Inch).

c) Drill Size: 30.75 mm is the hole size to be drilled before tapping. How it is arrived. The formula is Major diameter - Pitch. 

How to calculate pitch for B.S.P. thread from TPI?   

Pitch = 1/TPI in inch.

Pitch = 1/11 = 0.0909 inch

         = 0.0909 x 25.4 mm 

[since,1 inch = 25.4 mm]

Pitch = 2.3088 mm.

For 1" bolt major diameter is 33 mm. Pitch is 2.3088.

Drill size = 33 - 2.3088

               = 30.6911 mm ~ 30.75 mm.

Then, you may ask why 30.69 mm goes to 30.75 mm. Here in B.S.P. thread, using the formula we can arrive only at the closest value. But the exact value is the standardised one. 

If you have any quries or doubt, please comment below.

1) If you are interested to know about the metric bolt head sizes then follow the link below:

Metric Bolt Head Sizes

2) If you are interested to know about spanner / wrench and allen Key or hex key sizes of the Metric Bolts then follow the link below

Spanner / Wrench and Allen Key or Hex Key Sizes of the Metric Bolts

 

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