Engineering · Manufacturing
CNC Spring Hook Forming: Setup, Geometry and Programmed Motion
A step-by-step explanation of forming a centred hook on a tension spring using a main arbor, auxiliary hook-forming arbor, pitch tool and programmed coiling-point motion.
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. Hook geometry in the source process
The source machine forms a hook on one end of a closely wound spring by creating two initial coils, moving them partially off the main arbor, forming a straight leg, rotating the loop into position and then bending that leg around a separate auxiliary arbor. The final hook sits approximately on the spring centreline and includes a straight leg on one side of the spring body.
The sequence is important because the wire must remain supported at every stage. The main arbor supports the spring body, the pitch tool moves the initial coils and helps support the loop, the auxiliary arbor provides the bending form, and the coiling point both guides and bends the wire. If the wire leaves the coiling-point groove during bending, hook angle and centring become unstable.
2. Auxiliary hook-forming arbor geometry
The hook-forming arbor is shaped specifically for the operation. The source tool has a relief that lets the hook loop clear the body of the tool and a working edge around which the straight leg is bent. Its lower portion extends far enough to hold the spring coil in place during bending. A later source update emphasises that the bottom of the hook arbor must extend below the lower edge of the hook loop so the forming tool cannot wedge between the first two coils.
Front position
The front edge should be close to the main-arbor centreline so the hook bends around the intended axis.
Vertical position
Low enough to retain the spring coil, but high enough that the hook loop clears the tool relief.
Clearance
The auxiliary arbor must not touch the main arbor when fully down. Mechanical clearance is checked before any automatic program is run.
Relief
The relieved zone gives the hook loop space to rotate and prevents the tool from trapping the coil.
3. Setup philosophy: small changes and slow speed
The source repeatedly warns that successful hook forming depends on small changes. The program moves multiple axes around a confined tool stack, so large edits can create both malformed hooks and collisions. The operator first runs at slow speed and deliberately stops the program at selected positions to inspect clearance and wire location.
The source control supports wait commands and empty program rows as a practical way to slow the sequence between tool moves. At production speed, a command can be issued before a previous servo has completed its motion. The hook may look correct at slow speed but fail dynamically if timing margin is insufficient.
4. Step 1 — create the hook loop
The program begins by feeding enough wire to create approximately two closely wound coils. These initial turns will become the hook loop. They must be round, not distorted by a previous cutoff, and correctly seated on the main arbor. The source advises generating a short blank section if the end of the wire has been disturbed during setup, because a deformed starting coil can make the hook open between the first and second turns.
5. Step 2 — push the loop from the arbor and create the leg
The pitch tool moves the two initial coils toward the end of the main arbor. Wire is then fed while the loop is held in this offset position, creating a straight section. This straight section becomes the hook leg. Its length is controlled by the programmed wire feed during this stage.
Leg length is not only a finished-part dimension. It also affects whether the loop can rotate without striking the block-guide hardware. A later source troubleshooting note describes hooks that failed to bend because an excessively long leg caused the rotating loop to hit a guide screw before reaching the correct forming position.
6. Step 3 — rotate the leg into the bending position
The pitch and torsion axes return toward their normal positions while wire is fed, allowing the loop and leg to rotate around the main arbor. The source then feeds additional wire to rotate the hook approximately another half-turn in preparation for bending. Exact feed values and line numbers vary from program to program; the manual explicitly says its line numbers are examples only.
During this rotation, clearance around the block guide, pitch tool and coiling point must be checked. A loop that strikes a tool corner can overlap or twist before the bending step.
7. Step 4 — lower the auxiliary arbor
Once the loop is in position, the auxiliary hook-forming arbor extends downward. Before this automatic move is enabled, the operator checks that the tool clears the main arbor through its full travel. The bottom of the hook arbor supports the coil so the coiling point can apply bending force without forcing the forming arbor between the hook turns.
8. Step 5 — align wire with the coiling-point groove
The torsion axis moves to bring the hook leg into the lateral groove of the coiling-point tool. This groove is a temporary guide during bending. It controls the wire vertically and helps set the perpendicularity of the finished leg. If the wire sits outside the groove, the hook can angle upward at production speed even when it looked correct during slow setup.
9. Step 6 — create the initial bend
The coiling point first moves away in the diameter direction to clear the spring, then moves forward using the coiling-point axis to create the initial bend around the auxiliary arbor. The source emphasises that the wire remains in the coiling-point groove during this motion. This initial bend positions the leg against the bending form without forcing the tool directly into the spring body.
10. Step 7 — complete the bend
The coiling point then moves in the diameter direction to wrap the leg fully around the hook-forming arbor. While the wire remains captured in the groove, small changes to torsion position change the perpendicularity and vertical attitude of the hook leg. This is the fine adjustment used when a hook is otherwise correctly formed but sits slightly high, low or out of square.
11. Step 8 — return to production and form the spring body
After the hook is complete, the forming axes return to their production positions, the auxiliary arbor retracts and the machine continues coiling the spring body with its required pretension. This return is a critical transition. If the coiling point returns all the way to zero and misses the wire, a gap can appear in the coils immediately behind the hook. The source later recommends returning the coiling point to an intermediate position that reliably picks up the wire before moving to the normal pretension setting.
12. Key programming safeguards
| Safeguard | Why it matters |
|---|---|
| Check auxiliary arbor clearance mechanically | Prevents tool-to-tool collision before any program timing is involved. |
| Run at reduced speed during development | Makes wire location and interference visible and reduces crash severity. |
| Use wait commands / deliberate spacing | Allows one axis to reach position before the next move begins. |
| Keep the wire in the coiling-point groove during bend | Controls hook angle and prevents high-speed slippage. |
| Use small coordinate changes | Hook geometry is highly coupled; large moves can create interference rapidly. |
| Inspect coiling-point position before homing after E-stop | The tool may be in front of the main arbor and can collide during an automatic return. |
13. Geometry tuning guide
Hook leg too long/short
Adjust the feed amount used while the two hook coils are held off the arbor. Check that the loop still clears the guide hardware during rotation.
Hook not perpendicular
Fine-tune the torsion position while the leg remains in the coiling-point groove during bending.
Hook sits off centre
Check pitch-tool support and zero position, then check auxiliary-arbor centreline rather than immediately altering final bend coordinates.
Gap behind hook
Program the coiling point to return to an intermediate pickup position before moving to the pretension coordinate.
14. First-off acceptance
Inspect the first hook at slow speed for loop roundness, leg length, centring, perpendicularity, coil spacing between the first two turns and clearance from the spring body. Then increase production speed and repeat the inspection. The source contains several defects that appear only at higher speed because the wire slips from the groove or tool motions overlap in time. A hook is not proven until it is stable at the intended production speed.