Key takeaways
- In-die tapping pays when the thread is small, the volume is steady and the part can be held flat while the tap engages.
- A thread in thin strip needs enough material around the hole, or the first full thread sits in the die rollover and never develops full strength.
- Countersinking is usually a forming job, not a cutting job, and a formed countersink changes the flat pattern and the strip layout.
- Compare the two routes on total cost per good part, including handling, thread inspection and the scrap risk of a tap broken in the die.
What in-die tapping involves
In-die tapping puts a tap or a thread-forming tool inside the press tool. The strip is still located on pilots when the tap advances, so the thread is cut or formed in the same cycle as the pierce and the form. The tap unit is usually a self-reversing head driven by a lead screw, a cam or a servo, and it needs space in the die, a drive and a control interlock.
The attraction is obvious. The part leaves the press with the thread already in it, so there is no second machine, no second handling step and no queue between stamping and tapping. For a part with one or two small threads and a stable volume, that can remove a whole operation from the route.
The cost is equally obvious. The die gets taller and more complex, the tap needs a window in the strip, and a broken tap can stop the press and damage the tool. In-die tapping is a process decision, not a feature that can be added to any drawing at the end of the project.
When in-die tapping pays
The first condition is volume. A tap unit has a capital cost and a setup cost, and those are recovered over parts. At a few thousand pieces a year, a secondary tapping cell is almost always cheaper. At hundreds of thousands, the handling saved by tapping in the press can dominate.
The second condition is thread size and position. Small threads in thin material are well suited to a compact tap unit. A large thread needs torque, space and a strip that can carry the load, and the tap unit may be too large for the die. A thread close to a bend or an edge is harder because the strip must support the material while the tap cuts.
The third condition is part geometry. A flat, well supported area around the hole gives the tap a clean entry. A thread on a formed wall or a narrow tab is difficult to tap in the die because the strip moves and the tap can wander. If the thread has to be perpendicular to a formed face, secondary tapping in a fixture is often the only reliable route.
When to tap after the press
Secondary tapping is more flexible. A dedicated tapping machine or a CNC cell can handle several thread sizes, reach threads on formed faces and stop to clear chips. The part can be inspected before tapping, so a bad stamping is not wasted on a thread operation. Changeover is faster when the thread changes.
Secondary tapping also suits hard or abrasive materials, where tap life in a die is short and a broken tap is expensive. It suits large threads, deep threads and threads that need a specific lubrication or chip control. And it suits low volume, prototypes and parts with several different threads.
The trade-off is handling. Every part is picked up, oriented, tapped, cleaned and inspected. That handling adds cost, and it adds the risk of a part being tapped on the wrong face or missing the operation entirely. The route needs a poka-yoke or a traceability step to prevent that.
| Question | In-die tapping | Secondary tapping |
|---|---|---|
| Economic volume | Steady, high volume | Low to medium volume |
| Capital cost | Tap unit and die space | Tapping machine or CNC cell |
| Handling | None after the press | Every part handled and oriented |
| Thread size | Small to medium | Small to large |
| Thread on a formed face | Difficult | Standard with a fixture |
| Chip control | Chips stay in the strip | Can be cleared at the station |
| Failure mode | Broken tap stops the press | A missed part can escape |
Many parts use both routes: a small thread in the die and a larger thread in a secondary cell.
Drawing calls for a stamped thread
A thread needs material. The rule of thumb is that the hole should be at least one material thickness from the edge, and the material around the hole should be thick enough to develop at least two full threads. In 1 mm strip, an M3 thread has very little full thread to work with. An extruded or coined boss around the hole can add the height that the strip does not have.
Specify the thread standard, the class and the depth. A callout such as M3 x 0.5, 6H, through thread is clearer than a note that says tapped. State whether the thread is cut or formed. A thread-forming tap moves material rather than removing it, which changes the hole size, the torque and the inspection.
Say which face the thread enters from. A part can be tapped from the burr side or the clean side, and the choice affects the tap access and the thread start. If the thread must be perpendicular to a datum, put that on the drawing as a geometric control, not as a note.
The first thread is not always a full thread
The die rollover at the hole edge can consume part of the first thread. If the drawing asks for full threads to the surface, the process may need a countersink, a coin or a secondary operation to meet it.
Countersinking and the flat pattern
A countersink for a flat head screw is usually formed by coining or extruding the material around the hole, not by cutting a cone. Forming preserves the strip and gives a clean seat. Cutting a countersink in the die creates a slug and a burr, and it removes the material that carries the clamp load.
A formed countersink changes the flat pattern. The material that becomes the cone is drawn from the surrounding area, so the blank needs more material around the hole. That can change the strip width or the pitch, and it can pull the part out of flat if the form is not balanced. The die design should show the countersink station and the material it consumes.
The alternative is to machine or deburr the countersink after stamping. That adds an operation but gives a precise cone and a clean seat. For a short run or a large countersink, secondary machining is often the practical choice. For a high-volume part with a small countersink, forming in the die is usually faster and cheaper.
Building the cost comparison
Compare the routes on cost per good part, not on the price of the tap unit. The in-die route carries the tap unit, the die space, the drive, the control, the setup time and the scrap risk when a tap breaks. The secondary route carries the machine, the fixture, the handling, the extra inspection and the cost of the floor space between the two operations.
An illustrative example: a part with one M4 thread at 200,000 pieces a year. Suppose secondary tapping adds £0.09 per part in handling and machine time, while the in-die tap unit adds £18,000 in capital and about £0.02 per part in cycle time and tap consumption. The in-die route saves roughly £14,000 in the first year before the cost of downtime, and the gap widens in year two. Change the volume to 20,000 and the same capital takes years to recover. These figures are illustrative and must be replaced with real quotes.
Add the cost of quality. A thread that is tapped in the die can be checked on the press, but a broken tap can damage a batch before it is found. A secondary cell can gauge every part or sample it. The inspection plan should match the failure mode, and its cost belongs in the comparison.
Inspecting threads and countersinks
A thread is inspected with a go and no-go gauge, and the depth is checked with a thread depth gauge or a comparator. For a formed thread, the minor diameter and the lobe shape matter as well as the gauge fit. A thread that gauges correctly can still fail under torque if the engagement length is short.
Countersinks are checked for diameter, angle, concentricity with the hole and seat contact. A flat head screw should sit below or flush with the surface as the drawing requires, and the seat should not rock. A blue check or a marker check on the screw head shows contact without a destructive test.
For both features, the first article should include the gauge results, the thread depth, the countersink angle and a section through the thread or the cone. A section is the only way to see the full thread form and the material under the countersink. Add a torque test if the joint is structural.
Frequently asked questions
Can any stamping be tapped in the die?
No. The part needs a flat, supported area around the hole, enough material for the thread, a thread size the tap unit can handle and a volume that justifies the capital. Threads on formed faces or in very thin strip are usually better tapped after the press.
Is a formed countersink as strong as a machined one?
A formed countersink keeps the grain flow and avoids cutting the load path, which can be an advantage. A machined countersink gives a more precise angle and seat. The right choice depends on the screw, the clamp load and the volume.
How many full threads should a stamped thread have?
As a working rule, aim for at least two full threads in the engaged material. If the strip is too thin, an extruded boss or a thicker material may be needed. The joint design, not the tap, should set the requirement.
What happens if a tap breaks in the die?
The press should stop on a torque or position fault, but the broken tap can mark the strip or damage the die before the stop. The die needs a detection method and a recovery procedure, and the cost of that downtime belongs in the in-die case.
Sources and further reading
- ISO 68-1, ISO general purpose screw threads, basic profile
- ISO 965-1, ISO general purpose metric screw threads, tolerances
- Qumek stamping tooling and secondary operation review notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Deciding between an in-die thread and a second operation?
Send the drawing, the thread callout and the annual volume. The engineering desk will compare the two routes on cost per good part.
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