Engineering · Manufacturing
Hook Forming Troubleshooting and High-Speed Stability
A dedicated defect guide for open hook coils, off-centre hooks, overlapping loops, gaps behind the hook and parts that form correctly at slow speed but fail when the CNC coiler is accelerated.
Source fidelity and use of values
This handbook page is derived from an anonymised machine-specific operating and setup manual. Numerical settings are retained only where they carry practical technical meaning. They are marked as source examples and are not universal specifications. Machine builders, tooling geometry, wire condition and site safety procedures can require different values.
1. Why hook defects deserve a separate troubleshooting method
Hook forming combines wire feed, rotation, pitch movement, torsion alignment, a descending auxiliary arbor and two-direction coiling-point movement in a very small space. A body-spring problem can often be reduced to diameter, guide pressure or tool condition. Hook defects additionally depend on timing, temporary wire support and clearances that exist for only part of the cycle.
The source contains multiple updates added over several years, showing that some hook faults were only understood after repeated production experience. Those observations are consolidated here as a fault-isolation guide.
2. Gap between the first and second hook coils
When the first two hook turns open apart during forming, check the condition of the spring end first. A direct cut after a highly pretensioned section can distort the end that will become the next hook. The source recommends generating a short blank section so the hook starts from undisturbed wire.
Next check the relationship between the coiling point and the auxiliary hook arbor. If the coiling point comes too close, it can force the first turns apart and allow the hook arbor to enter between them. The bottom of the hook arbor must extend below the hook loop so the coil remains fully supported during bending.
3. Hook loop twisted before bending
A gap can also appear because the loop twists before it reaches the bending position. The source observed this more often at high speed, where the rotating loop could strike the arbor. Slightly increasing leg length helped in that specific setup by changing the loop’s rotational clearance. A later update adds another strategy: avoid rotating the hook excessively just to make it sit down; instead, use a modest change in the diameter-tool bend position.
These are examples of coupled geometry. If one feature is wrong, changing the closest-looking coordinate is not always the best correction. First identify where the loop first leaves the correct path.
4. Hook perfect at slow speed, deformed at production speed
This is one of the clearest dynamic faults in the manual. The machine can form an acceptable hook during setup, then continue coiling the spring body before the coiling point has returned from the hook-forming position when speed is increased. The program is advancing faster than the physical tool movement.
The source resolves this by adding timing margin between commands. In one later revision, a large group of blank program rows was inserted between relevant moves. The exact count is controller-specific. The important diagnostic is: if geometry is correct at slow speed but wrong only at high speed, inspect motion completion and timing before changing the final coordinates.
5. Hook loops form but the hook does not bend
The source reports a case where the loop existed but the hook was not bent around the auxiliary arbor. The cause was excessive leg length. As the hook rotated, the loop hit a screw associated with the block guide and could not reach the correct bend position. This defect can be mistaken for an arbor or coiling-point problem because the failure occurs at the bending stage.
Check the full rotational envelope of the loop with the machine stopped at setup positions. The loop must clear guide screws, the pitch tool, the block guide and the coiling point before the auxiliary arbor descends.
6. Gap in the spring body immediately after the hook
After the hook is bent, the wire tends to point forward on the main arbor. If the coiling point returns directly to its normal zero, the groove may pass behind or beside the wire rather than capture it. The next body coil then forms without the normal support, creating a visible gap.
The source’s solution is to return the coiling point first to an intermediate pickup position between zero and the normal pretension setting. Source example: with pretension around 0.77 mm, a temporary position around 0.4 mm was used so the groove reliably picked up the wire before moving to the full pretension coordinate.
7. Overlapping loop in the hook
If the two hook turns overlap or become axially offset, the loop may be striking the underside/corner of the coiling-point tool while the straight leg is being generated. The source notes that a small audible click may accompany the contact. The practical correction is to create a small radius or relief on the offending outside corner, provided that the functional groove depth and load-bearing land are not damaged.
This fault illustrates the importance of tool relief outside the primary forming surface. A corner that appears harmless when the spring body is formed can become an interference point during the larger hook-loop motion.
8. Hook unexpectedly moves off centre
An off-centre hook can be caused by the pitch tool being too far back. During bending, the pitch tool should support the bottom of the spring coil so the loop stays on the intended centreline. The source reports one instance after a power outage where the pitch-tool zero appeared to have shifted. This is a reminder to check zero integrity after abnormal power events.
However, moving the pitch tool too far forward is also undesirable because it can contact the spring during normal coiling and reduce pretension. The correct position is one that supports the hook-forming state while clearing the body-forming state.
9. Hook angles upward at high speed
The source compares a correct hook at slow speed with a hook that tilts upward at higher speed. The cause was the wire slipping out of the coiling-point groove during the bending operation. The final coordinate was not fundamentally wrong; the wire was no longer constrained by the intended groove when the dynamic load increased.
The correction was a small change to the torsion setting so the wire sat more deeply and securely in the groove during the bend. Source example: one setup improved when a torsion value changed from about -0.5 to -0.3. This is not a standard setting; it demonstrates the sensitivity of groove engagement to torsion height.
10. Noisy hook in the ejector ramp
If the hook leg is not parallel with the spring body, the hook can strike the ejector ramp as the spring rotates down the chute. The result may be a rattling noise, local waviness and a reduction in pretension. Correct the hook’s perpendicularity using the bending/torsion setup before modifying the ramp simply to make more clearance.
11. Fault matrix
| Defect | First likely cause from source | First check |
|---|---|---|
| First two hook coils open | Distorted starting wire, hook arbor insufficiently supporting loop, tool too close. | Generate blank start, inspect support below loop, check clearance. |
| Hook good slow / bad fast | Servo timing overlap or wire leaving the groove dynamically. | Slow down, add timing margin, inspect groove engagement. |
| Hook not bent | Loop hits guide hardware because leg is too long. | Check rotational envelope and leg feed. |
| Gap behind hook | Coiling point returns to zero and misses forward-pointing wire. | Add/interrogate intermediate pickup position. |
| Overlapping loop | Loop contacts a sharp coiling-point corner. | Inspect for witness marks and add controlled relief if appropriate. |
| Hook off centre | Pitch tool not supporting coil or zero shifted. | Check pitch support and zero reference. |
| Hook tilts upward at speed | Wire slips from coiling-point groove during bend. | Fine-tune torsion/groove alignment. |
12. High-speed qualification sequence
- Form several hooks at slow speed and verify geometry.
- Use wait points to inspect the loop immediately before auxiliary-arbor descent and before the final bend.
- Confirm the wire is physically in the coiling-point groove at both bend stages.
- Increase speed one step and compare hook angle, loop spacing and body transition.
- If the defect appears only with speed, restore the last stable speed and inspect timing.
- Add controller-appropriate dwell or spacing rather than increasing bend travel to compensate.
- Repeat until the intended production speed is stable for multiple consecutive parts.
13. Preventive checks before chasing program values
Inspect the coiling-point groove for chipping, the hook arbor for correct vertical projection, the pitch tool for support, the loop clearance around guide screws and the zero positions after any power interruption or crash. A program that was previously stable is unlikely to “forget” its geometry; a changed tool, shifted zero or new interference is often more probable than a suddenly wrong coordinate.
14. Record the dynamic process window
A final hook setup should record not only the coordinate values but also the validated run speed, any required waits/blank rows, the hook-arbor mechanical position, torsion value at the bend, leg-feed value and the intermediate coiling-point pickup position. This creates a repeatable process window and makes later high-speed drift much easier to diagnose.