Manufacturing Process
Tapping
攻丝
Cutting internal threads into a pre-drilled or punched hole so the part accepts screws or fittings in final assembly.
Process Steps
- Hole position and size checked
- Tap cuts thread
- Thread gauge test
- Chip removal and inspection
What is tapping?
Direct answer: tapping cuts an internal thread into a hole. In a stamping it is done either as a secondary operation on a drilling machine, or in the die with an in-die tapping unit that threads rotating holes while the strip is still in the tool.
In-die tapping versus secondary tapping
| In-die tapping | Secondary operation | |
|---|---|---|
| Cost | High tooling, very low piece price | Low tooling, higher piece price |
| Position accuracy | Set by the tool, no re-fixturing | Depends on the fixture |
| Best volume | High — tens of thousands and up | Low and medium volume |
| Flexibility | Fixed thread and position | Any thread, any position, any time |
What decides the thread you can have
- Material thickness. A thread needs enough engagement to hold torque. As a rule, thread depth should be at least one diameter in steel, and more in soft materials such as aluminium or brass.
- Hole size. The tap drill size sets the minor diameter of the thread; a pierced hole that is coaxially correct but out of size will gauge badly.
- Distance to the edge and to other holes. A tapped hole needs wall material around it; too close to an edge and the thread distorts or tears out.
- Material. Stainless work-hardens and needs the right lubricant and a sharp tap; aluminium tends to smear and needs more clearance.
Threads and how they are checked
Metric (M), unified (UNC/UNF) and pipe (BSP) threads are all standard, cut to class 6H unless the drawing says otherwise. Inspection is by go/no-go gauge rather than by measuring the minor diameter: the go gauge must enter by hand, the no-go gauge must not. After plating, threads are gauged again, because plating grows into the thread and a coated part can fail a gauge it passed before coating.
Design rules for tapped holes in sheet metal
- Leave at least 1× the thread diameter of material between the hole and any edge or adjacent hole.
- Avoid tapping sheet thinner than about 0.8 mm for anything structural — use a flanged hole, a weld nut or a clinch fastener instead.
- Specify the thread and the depth, plus whether the thread may be cut by tapping or must be roll-formed.
- Keep tapped holes clear of bend lines: forming distorts a thread that is too close.
Frequently asked
What is in-die tapping?
A tapping unit mounted in a stamping die that threads holes while the strip is still located in the tool. It removes a secondary operation entirely, at the cost of a much more expensive die.
What hole size do I need for an M4 thread?
A 3.3 mm hole for a standard M4 coarse thread. The drawing normally specifies the thread and lets the toolmaker set the hole; where the hole is pierced in the die, its size becomes a tooling tolerance.
Why does my tapped hole fail the gauge after plating?
Plating adds material to the thread flanks. Threads that must gauge after coating need the hole sized for the plated condition, or the thread protected during plating. It is a route decision, not a plating-shop problem.
Can you tap stainless steel stamped parts?
Yes. Stainless work-hardens, so it needs a sharp tap, the correct lubricant and a hole size at the upper end of the tolerance band. Reworking a work-hardened thread is difficult.
Related: piercing · hole flanging · countersinking · surface treatment · deep drawing