The ST tap, also known as a thread insert special tap, is a dedicated tool used for machining installation holes for stainless steel wire thread inserts. “ST” stands for “Screw Thread.” These taps are manufactured in accordance with national standards such as GB3464-83 and can be used manually or with machines.
Classification of ST Taps
Depending on the material being machined and the method of operation, ST taps are generally categorized into the following three types:
1. Taps for Light Alloy (Machine or Hand Use)
These are used for tapping internal threads in non-ferrous metal alloys like aluminum, magnesium, and copper.
Single-tap design is used when the pitch is less than 2.5mm.
Double-tap design is used when the pitch is 2.5mm or more.
Marking example: ST8×1.25 indicates a special tap for creating bottom threads in an M8×1.25 hole for steel wire inserts in light alloy materials.
2. Taps for General Steel (Machine or Hand Use)
These are suitable for steel, cast iron, and cast steel. Typically, they feature a double-tap design for durability and precision.
3. Special-Purpose Taps
Custom-designed taps made for specific applications or materials such as:
Soft aluminum or copper
Machine or hand extrusion taps
Flat-bottom taps for blind holes
Spiral flute taps
Thread-forming taps
Taps with cutting tips, etc.
Common ST Tap Types and Their Features
● Straight Flute Taps
Versatile and cost-effective, ideal for both through and blind holes. Applicable to a wide range of ferrous and non-ferrous metals.
● Spiral Flute Taps
Ideal for blind holes as chips are evacuated upwards.
For ferrous metals, a low helix angle (~30°) provides strength.
For non-ferrous metals, a higher helix angle (~45°) ensures sharper cutting performance.
● Forming Taps (Roll Taps)
Designed for non-cutting thread forming in materials like aluminum. These taps form threads by cold deformation rather than cutting, resulting in threads with high strength and smooth finish.
Note: Proper hole size is critical—too small can break the tap; too large may weaken thread strength.
ST Tap Operation Process
To ensure high-quality threading, follow these four essential steps, keeping precision and cleanliness in mind throughout:
1. Drilling the Hole
Use a standard twist drill to create a hole deeper than the wire insert length. Avoid creating a tapered hole, and ensure chips do not fall inside.
Countersinking should not exceed 0.4 pitch depth, as excessive chamfering hinders insert installation.
2. Tapping the Thread
Use the designated ST tap according to thread size.
The tap must exceed the insert’s length.
For blind holes, apply steady force to avoid breakage.
After tapping, clean the thread thoroughly—preferably with compressed air or specialized spray tools.
For critical applications, inspect the thread with a gauge for thread insert holes.
3. Installing the Thread Insert
Use a dedicated installation wrench. Place the insert on the tool, align the driving tang with the guide slot, and screw it into the hole.
Leave 0.25 to 0.75 turns of the insert below the surface.
For M14×2 and larger inserts or small batch jobs, a simple T-slot or threaded-head tool may be used.
Avoid excessive axial force to prevent misalignment or insert damage.
4. Breaking the Tang
The final step is removing the tang (tail handle) of the insert, which is crucial for allowing proper function and longevity of the installed insert.
Common Tapping Issues and Troubleshooting
| Problem | Solution |
|---|---|
| Bottom hole drill too small | Use a properly sized bit for the base material |
| Worn drill leads to poor hole quality | Replace with a new drill |
| Conical hole shape | Use a standard twist drill to correct |
| Distorted or angled hole | Re-machine the bottom hole |
| Tap incompatible with material | Select a tap suited to the material |
| Worn-out tap | Inspect with plug gauge; replace if worn |
| Direct use of forming cone | Start with a working tap first |
| Tap breaks during blind hole tapping | Apply proper tapping force and correct tool feed |
| Off-center holes | Maintain correct setup and tapping technique |
