Damaged screw heads in disassembly are usually not the result of a torque setting that was too high; the bit simply never reached the bottom of the recess. Thread engagement, meaning alignment of bit axis to screw axis plus confirmation of full engagement, is the most neglected stage of the removal process, yet it drives strip rates, head-shear rates and cosmetic damage rates. Splitting it into approach, compensation and search stages with separate parameters is far more repeatable than tuning a single press-down distance.

Stripping Comes From Engagement Failure, Not Excess Torque

The conclusion is that torque is only an amplifier in micro screw removal; engagement is the root cause. When the bit is not coaxial with the recess, torque travels along one wall, one side of the recess collapses first, and the bit then slips sideways. When the axial load is not established, the bit merely spins at the mouth of the recess, produces no effective rotation for a dozen turns, and the operator reads this as insufficient torque and adds more, destroying the recess completely. The two failures look different in data: the first shows torque rising quickly with little rotation, the second shows torque hovering at a low level. Telling them apart is more useful than raising extraction torque across the board.

Stage One: Vertical Approach and Axial Preload

The approach stage has only two goals: bring the two axes together and establish axial force. In practice the camera provides coarse XY locating, the Z axis descends at low speed, contact is declared on a displacement step or an axial force threshold, and a preload is then added. Preload supplies initial friction so the bit does not jump out of the recess at the moment of rotation, typically five to twenty newtons depending on screw size and recess depth. Tilt tolerance is the hidden specification here: most micro screws require the angle between bit and screw axis to stay within half a degree to one degree, and beyond that the bit slips out even when it has entered the recess, because the load is no longer balanced.

Stage Two: Compliance Absorbs Residual Deviation

A rigid connection converts every positional and angular residual into side load, and compliance exists to absorb that load. Three routes are common. Mechanical float uses a spring and spherical pair for passive following: low cost, limited travel, suited to consistent incoming parts. Pneumatic constant-pressure float holds up better where height varies, because pressure is adjustable and travel is long. Active compliance uses force control or a remote-centre compliance mechanism for programmable following stiffness: flexible but the most expensive. Compensation travel must cover the actual positional tolerance of incoming parts, which means reserving roughly half a millimetre to two millimetres on micro screw work; reserve too little and there is effectively no compensation. These parameters must match the machine itself, since a desktop small screw removal equipment unit is structurally more limited in float travel than an inline model, and incoming tolerance has to be part of the selection calculation.

Stage Three: Spiral Search, Engagement Confirmation and the Role of Vision

When the recess cannot be found, the correct action is a low-torque sweep in both directions, not more downward force. The method is low speed and low torque in the forward direction while monitoring whether rotation follows; if rotation stalls, reverse by a small angle and drive forward again to create a reciprocating sweep; if the sweep exceeds a set number of attempts without engagement, the screw is declared mismatched, obstructed or already stripped, and the part moves to manual work or a breakaway routine. Engagement can be confirmed by cross-checking three signals: a slope break in torque against angle, a following descent on the Z axis, and a change in vibration or sound signature, with any two together justifying release. Vision cannot replace the search, but it shortens it substantially: imaging before removal reveals recess orientation, whether the mouth has collapsed and whether debris is present, so sweep count drops from a dozen attempts to two or three, and an automatic vision screw removal machine pays off most in mixed-lot repair. One caveat: vision supplies the starting condition only, and engagement confirmation still depends on torque and displacement signals; visual judgement must not replace contact detection.

Parameters Belong on the Process Card, Not in Operator Feel

The value of a three-stage design only materialises when it is written as a parameter table. A process card should carry at least seven entries: approach speed, contact threshold, preload, compensation type and travel, search torque ceiling, sweep count and angle, engagement criteria and failure branch. The failure branch is the entry most often omitted, and it is exactly what decides whether the machine keeps damaging a part after something has gone wrong. Lines spanning M0.8 to M2.0 should maintain several parameter sets by size band and manage recipes with the segmented approach of an M0.8-M2.0 screw removal solution rather than one set for everything. Acceptance should use three challenge samples: a known stripped screw, a known misaligned screw, and one with threadlocker residue in the recess. Whether the machine detects failure without damaging the part says more than a first-pass rate.

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