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Sheet Metal Tapping

Metal tapping is also called extension tapping, flanging tapping, and stretch tapping. It is mainly used in sheet metal parts. Since the sheet metal parts are thin, the thread is short and easy to release if the thread is tapped directly. Therefore, punch a bottom hole (determined according to the thread specification) on the sheet metal parts, and then use a special stretching tool. The die (usually on an ordinary punching machine) is punched out. Simply put, the hole is punched, the height (wall thickness) of the hole is lengthened, and then the thread is tapped to achieve the purpose of increasing the length of the thread.

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Metal Tapping

Under What Circumstances Is The Hole Flanged And Tapped?

To explain under what circumstances flanging and tapping are required, a concept must be introduced: the minimum number of thread turns, generally the number of mechanical thread turns is 8 to 10 turns, and the minimum number of thread turns is 3 turns. That is to say, the effective number of turns of the thread is 8~10 turns to ensure that the connection of the thread is strong, and the thread will not be damaged during the stress process. Sheet metal fabrication threaded connections are low-strength connections, and at least the minimum number of thread turns must be guaranteed. When the wall of the sheet metal part is very thin, it is necessary to re-tap the thread if the minimum number of thread turns is not met, mainly to increase the connection strength, such as the aluminum alloy casing of some electronic products.

How Big Is The Bottom Hole For The Tapping Of Sheet Metal Parts? What Is The Height Of Sheet Metal Flanging?

The flange height depends on the size of the bottom hole. Generally, the pre- punching hole of the small threaded bottom hole is 0.45 times (ie: the pre-punching hole with a diameter of 4.2mm is 1.8~ 2mm in diameter) —- In this case, the . flanging height is equal to about 3 times the material Thick (3.6~ 4mm). (It is certain that the flanging part becomes thinner, otherwise there is no limit to the height) .

The Precautions For Sheet Metal Tapping

Under normal circumstances, the connecting threaded holes with no force or the self-weight of the sheet metal less than 5KG are directly tapped on the sheet metal parts, and the nuts are welded when there is a large force requirement.
Plate Bottom Hole Size1.52.535
M3 (tapping)2.5OKOKKok
M4 (tapping)3.3oKoEOKOE
M5 (tapping)4.28KKOEOE
MB (tapping)5DBCE8E
MB (tapping)6.8aR
M1o (tapping)8.5
M12 (tapping)10.2
M14 (tapping)12
M6 (T-type nut)8OKOK0KOR
MB (T-type nut)10OROKOK0R
M1o (T-type nut)12OKOK0R0K
M12 (T-type nut)14OK0KOKOK
M14 (T-type nut)16OKOKOKOK
M5 (hexagonal nut)6OKOK0R
MG (hexagonal nut)7OR0KOKOK
M male (hexagonal nut)gOK0KOKOR
Bottom Hole Size For Tapped And Weld Nuts
The steel plate with a thickness of 2mm is suitable for flanging tapping, and the corresponding thick plate is not suitable for flanging tapping
Tapping has strict requirements on material hardness, plate thickness and bottom hole size. Thread Size Minimum Plate Thickness

Standard Bottom Hole

  • M2.5 1.5mm Φ2.1
  • M3 1.5mm Φ2.5
  • M4 2mm Φ3.3
  • M5 2.5mm Φ4.3
In order to ensure the quality of the thread, it is generally not recommended to tap directly on the aluminum plate. If it is necessary to tap on the aluminum material, it is necessary to ensure a sufficient thread depth (generally more than 3mm).
Note: The parts that are directly tapped on the plate should not be cut by laser, so as not to increase the hardness of the material due to the heating at the cutting place, which will lead to difficulty in tapping.

Sheet Metal Tapping–The Relationship Between Pitch And Plate Thickness

Sheet metal tapping has the following principles:

  1. The thin plate is generally only suitable for fine thread
    Thin plates below 2.3mm are generally not suitable for structural load-bearing use, most of which are decorative plates or small brackets
  2. Theoretical minimum number of threads is 1.5 turns
  3. The ideal number of threads is more than 2 turns
  4. Very thin sheet metal, not easy to tap, use self-tapping screws

Then the corresponding minimum plate thickness is 1.5 times the pitch and most of the thin plate materials are low carbon steel, Q series, or common materials such as SPCC, SPHC, DC01, and their own strength is not a problem.

The following table is a simple illustration I made, listing some common specifications. X means not applicable, O means applicable, triangle means it can be used according to the situation, but it is not recommended (exactly between 1.5 times the distance and 2 times the distance). I have specially added two specifications of T1.2 and T1.6 here. It is rarely used in China, but the Japanese like to use it, and the reason is not difficult to draw from the table below.


In order to reduce the use of non-recommended situations, the Japanese generally use the boss method in a large number to achieve the method of increasing the wall thickness. For high-quality materials, this is not a problem, but because domestic steel materials are generally of poor quality and prone to slippage, it is not recommended. If you are really worried, you should still use the rivet nut method (the Japanese don’t like to use it because it costs a lot).At the same time, the table of thread pitch of each model is attached.

Millimeter Coarse FangsFine TeethNo. <1/4″Inches InBom mm Coarse Fangs UNC Fine Tooth UNF
M1.60.350.22 common0.0862.1845664
M40.70.56 common0.1383.5053240
M50.80.58 roses0.1644.1663236
M81.25112 even0.2165.4862428
Nominal Diameter Tooth Distance mm Tooth Distance mm Nominal DiameterDiameter SizeDiameter SizeTooth pitch = teeth per inchTooth pitch = teeth per inch

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