It is important to follow the hole diameter instructions given by the various manufacturers and standards organisations, and the data given below comes from these sources. However, the user should decide in the circumstances of a particular application which hole size, within the permitted tolerance range, to adopt as this can effect the performance of the pin considerably.
The general principle is that the inserting pin or dowel requires significantly more force at the bottom of the hole tolerance range than at the top. This has the following implications;-
- The higher insertion force required, the more difficult the assembly operation, and risk of damaging or over-stressing the area of the material surrounding the hole is greater
- The harder the pin is to insert, the more firmly it will stay in place so in applications where vibration is a consideration, it may be worthwhile to use a minimum hole size.
Where an interference fit is required (as opposed to a sliding or clearance fit) the dowel must be precision ground and the hole must be reamed.
The relationship between the diameter of the pin and the diameter of the hole must take into account the ductility of the dowel and the hole material and will vary more accordingly to this factor than according to the size of the dowel itself.
In hardened steel parts, the hole should be .005 to .008mm (5-8 microns) smaller than the dowel. In unhardened material, the hole should be 0.025mm (25 microns) smaller than the dowel.
For dowels ground to m6 tolerance (Technifast standard type), the dowel diameters are as above, and holes should be reamed to 25 microns smaller (or 5-8 microns for hardened steel) using the lower limit of the m6 range as the reference point.