STI threading tools for the receiving thread
A wire thread insert needs a thread that is larger than the thread you want to end up with. That is exactly what an STI tap cuts. STI stands for Screw Thread Insert. VÖLKEL makes 212 versions to ISO 529: short machine taps in form D from HSS-G, hand taps, machine taps in form B and form C, and forming taps. Metric from M 2 x 0.4 to M 48 x 5, plus UNC, UNF, UN, BSW, BSF, G (BSP) and BA.
Why a dedicated tool is needed
The insert is a coil that sits in the receiving thread and forms the nominal thread on the inside. The receiving thread therefore has the same pitch but a larger diameter. A normal tap of the nominal size would be too small; one of the next size up would have the wrong pitch. So an STI tap for M 8 x 1.25 cuts a thread with a pitch of 1.25, but at a diameter matched to the insert.
Form D for both hole types
Form D with a four to five thread lead is the universal version for through and blind holes, which is why it is the most common in the STI range. HSS-G covers the materials where thread repairs occur: aluminium, magnesium, cast iron, structural steel. Where the work happens on a machine there are machine taps in form B and in form C with a 35° spiral flute from HSSE, and forming taps for ductile materials.
The sequence
Drill out with the STI twist drill, cut the receiving thread with the STI tap, wind in the insert with the installation tool, break off the tang. The drills and installation tools are among the thread repair tools, complete sets under thread repair kits, the cordless-driver version under V-COIL rapid.
Checking
The finished receiving thread cannot be checked with a normal gauge, it is too large. That is what the STI thread gauges are for; the reason is explained in the article checking STI threads correctly.
