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118
THE DISTRIBUTOR’S LINK
BRUNO MARBACHER THREAD-CUTTING SCREWS AND SELF-DRILLING SCREWS FOR SPECIAL APPLICATIONS from page 12
Type T (former Type
23): A thread cutting
screw in the fine/regular
thread series offering
maximum thread cutting area and excellent chip clearing,
with minimum tightening torques.
Type F: A thread cutting
screw with machine screw
thread with blunt tapered
point, having multi -
cutting edges and chip cavities. For heavy gauge sheet
metal, aluminum, zinc and lead die castings, cast iron,
brass and plastic.
The above point types are covered in ASME B18.6.3,
former ANSI B18.6.4, there is no DIN or ISO standard
covering these screws.
DIN 7513: For thread
cutting screws per DIN
7513, only the metric
thread profile and the
thread properties are specified. The finished internal
thread must be such so that a screw with a regular
plated thread can be used as a replacement screw in
cast iron. The spiral cutting flutes extend from the point
of the screw to its head and are designed to cut their
mating thread during assembly.
Steel thread-cutting screws are used in heavy gauge
sheet metal, aluminum, zinc and lead die castings, cast
iron, brass and plastic.
Stainless screws offer additional resistance to
corrosion, 18-8 more so than 410. When using any
thread-cutting screw, the material in which the threads
are cut should have a lower hardness by at least 10 to
20 Rockwell hardness points.
Thread-Cutting Screws For Thermosets
Special screw threads with cutting shank slot for
thermoset plastics, Thermosets, such as Bakelite,
Epoxides and Polyimides for example, are hard brittle
plastics. The material cannot be formed.
Thread Profile - Points For Thermosets
Type BT (former
types 25): Thread cutting
screws for thermosets
usually have a cutting cavity at the end of the thread.
Cutting grooves in the thread, as is found in thread
cutting screws for hard metals, would fill up immediately
and would no longer function. They are specified in ASME
B18.6.3.
Type Y: This form of
screw thread has widely
spaced threads, blunt,
slightly tapered point and
cutting flutes which extend from the point to the head.
This screw is typically used for thermosets. In a blind
hole application, the length of the hole should be such
so there is room for the chips. Thread-cutting screw type
Y are covered by BS4174.
Thread-Cutting Screw Materials And
Mechanical Properties
Thread cutting screws are fabricated from low
carbon steel, case hardened to meet the performance
equirements. The material requirements are derived from
SAE J933.
Core Hardness. Screws shall have a core hardness of
Rockwell C28-38. Case Hardness. Screws shall have a
minimum case hardness of HRC 45.
Duro PT Screws
Duro PT screw is specially
designed for thermoset, it
has a 30° asymmetrical
thread profile. This thread
form increases the stripping
torque during installation
creating a very safe assembly.
It produces a minimum of
radial stresses in the material. This specific thread form
makes these screws vibration resistant.
CONTINUED ON PAGE 172