Thread forming screws per DIN 7500 produce a chip-free, gage correct metric internal thread into ductile metals up to 160HV hardness. Thread forming screws may be reused up to 20 times, but care must be taken when re-starting screws to avoid cross threading. They are not intended for use in brittle metals such as gray cast iron; however, they can be used in non-ferrous metals and light metals, as well as steel.
Screw Hole Preparation
Hole preparation and hole size are key to trouble free assemblies with thread forming screws. In general, the material thickness should be 1xd minimum (for example, a 6mm plate for a M6 screw). Punching, drilling or laser cutting are common methods for creating through holes for thread forming screws. Different hole sizes may be required for each method based on the break-out of a punched hole, or the size of the heat affected zone of a laser cut hole, which makes the surface harder and more difficult to thread. Different material composition and thickness will also require different hole sizes.
Good design practice requires a countersunk hole as thread forming will slightly raise the surface of the mating part without the chamfer. This can cause mating parts to not sit flush against the tapped component creating a gap.
Assembly Torque
Recommended assembly torque is 80% of the breaking torque of the screw, and half way between the driving torque and the breaking torque. Drive torque and strip/break torque is recommended to select the optimum hole size for your application.
Check out www.bossard.com for more recommendations for thread forming screws, or contact us directly at ProvenProductivity@Bossard.com to perform a drive/strip torque test for your application.
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Doug Jones
Applications Engineer
djones@bossard.com
