Self Tapping Screws
Recommended hole and drill sizes for Type AB and B self tapping screws - in thermo (pliable) palsticsNote: because of the vast differences in these plastics, the following table is only intended as a guideline. It may be necessary to increase or decrease the recommended hole size to obtain optimum fastening conditions.
- Firstly make a hole to suit the diameter of the screw being used. The hole can be clean punched or drilled as shown above in Fig 1 or plunged as shown in Fig 2.
- Fig 5 for light sheet assembly, no clearance hole is necessary in the top sheet.
- Fig 6 and 7 When fastening to solid section of casting or die-casting, or to heavy guage sheet metal, a clearance hole in the top sheet is required. If a clearance hole is not provided, the total thickness of material must be taken into account when deciding on the tapping hole size. Remember that the thickness of material increases, it becomes more difficult to pull the two sheets together tightly.
Recommended Hole and Drill Sizes
Type B self tapping screws (Drilled or cored holes) in non ferrous castings, aluminium, zinc, brass, bronze etc.
- Cored holes. A side taper of 1°11' is permissible. The diameter of a cored hole should equal the nominal hole size shown in the table to the right at one half the screw penetration depth.
- Porous castings may require the use of a smaller hole and/or increased depth of engagement.
Type AB, B Self-Tapping Screws
18-8 Stainless Steel, mild steel, monel metal, brass and aluminium alloy sheet
- Because conditions differ widely it may be necessary to vary the hole size to suit a particular application.
- 18-8 quality stainless steel self-tapping screws are much softer than case hardened steel screws and therefore case must be exercised in using them. They cannot be used in very hard material. Also due to the galling tendency of stainless steel they should not be used in stainless steel sheet
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