FALL 2023

172
THE DISTRIBUTOR’S LINK
BRUNO MARBACHER THREAD-CUTTING SCREWS AND SELF-DRILLING SCREWS FOR SPECIAL APPLICATIONS from page 118
In the case of a symmetrical thread tooth with a total
angle of 30°, one flank has a 15° inclination. If the 30°
tooth is asymmetrical, the angle of the load flank is 30.
It produces more frictional resistance when tightening
the screw. The stripping torque becomes higher, and the
screw is less easily stripped out during assembly. Its
resistance to vibration is better when it is assembled.
Whenever the design is light and thin-walled, special
screws with sharper threads, such as the Duro PT screw,
are strongly recommended.
DELTA PT DS (DuroSet)
DELTA PT DS
(DuroSet) screws
are a new type of
screw for plastics,
they are furnished
with special grooves
on the thread crest,
these grooves are
pronounced at the screw point and taper off towards
the screw head.. Thus enable cutting the mating thread
into thermosets. These screws may also be used for
thermoplastic, only one screw type for thermosets and
thermoplastics. These grooves are especially apparent
at the point of the thread and taper off towards the
screw’s head. Delta PT screws DS offer a high stripping
torque.
Material and Mechanical properties - The Duro
PT screws are made from low carbon steel and are case
hardened.
The Delta PT DS screws are made from steel and are
through hardened.
Boss Geometry
The pilot hole diameter for these screws should also
be determined through application tests simulating
practical situations. Presumably, it will be in a range of
0.85x the thread diameter, the depth would be about
2-3x the thread diameter.
Since the plastic is usually very brittle, the outside
diameter of the boss must be made larger than in
thermoplastics. Boss diameters in the range of 3x the
thread diameter have been established. A counterbore,
diameter of 1.1-x d at the upper end of the boss is also
strongly recommended.
It is important that there is enough space at the base
of the hole to leave room for any chips.
Self-Drilling Screws
Self-drilling screws are technically thread-forming
screws, but because they drill the pilot hole, they
produce chips. There is a drilling point at the end of the
thread. They drill the hole for subsequent tapping of the
mating thread.
The point is either milled
during manufacturing, or
cold headed (cold formed).
Self-drilling screws are
commonly used in roofing
applications in combination
with a watertight neoprene
sealing washer, and for
attaching metal roofing and
siding panels to metal stud
framing.
When selecting the suitable drilling screw, it is
important to consider the minimum and maximum
thickness of the sheets.
The screw can only work if the drill point has fully
protruded on the backside of the sheet (sheets), before
thread forming can begin. If both sheets have to be
drilled through, the point must protrude the bottom
sheet, before thread forming can begin in the top one.
“Blind hole assemblies” are therefore not possible
(screw thread must go all the way through).
Drilling produces friction, which produces heat. A drill
point that generates a great deal of heat will fail quickly.
This can be controlled by the amount of force applied
when drilling as well as the speed (RPMs).
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