In most mechanical products, screw fastening is the commonest way to join parts.
TV sets, mobile phones, cameras and other electronic devices use large quantities of screws whose nominal diameter is below 5 mm, and “screw float” (the head stands off the surface) is a frequent defect.
Float can irreversibly damage the threaded boss or the PCB, so lock-quality control must prevent it.

What is screw float?

The screw reaches the target torque (e.g. 0.7 N·m) before it has reached the programmed depth; the driver stops and reports “OK”, but the joint is not actually clamped.
A manual re-tightening curve shows that the 0.7 N·m peak was only an abnormal spike caused by burrs on the hole, burrs on the screw, a local raised spot on the part, etc.

Limitations of conventional drivers

Standard electric screwdrivers cannot detect float; they only flag an error when the gap is already > 2 mm.
There is still no universal float standard.
Existing methods are:

• Torque audit: compare the recorded peak with a reference—indirect and unable to measure actual height.

• Laser distance sensor: accurate but adds an extra station and cost.

Servo-driver solution

A servo screwdriver measures both torque and spindle displacement in real time with its built-in encoder and linear scale.
Software compares the achieved position with the programmed seating depth and decides instantly whether the screw is:

• properly seated, or

• floating.

By closing the torque–position loop inside the same tool, the servo driver gives a direct, quantitative float check without additional sensors, keeping takt time short and quality high.