Fine adjustment screw

The term fine adjustment screw typically refers to screws with threads from 40-100 TPI (Threads Per Inch) (0.5mm to 0.2mm pitch) and ultra fine adjustment screw has been used to refer to 100-508 TPI (0.2 mm to 0.05 mm pitch). Even though these are non-standard threads, both ISO metric screw thread designations and UNC designations have been used to call out thread dimensions and fit (class). A typical use for a fine adjustment screw is in an optical mirror mount as an adjuster. Typically 80 TPI screws are used in mirror mounts. Ultra fine adjuster screws are used in applications requiring extremely fine motion like laser alignment, fiber coupling.

Fine and ultra fine adjustment screws are often used in photonics applications as part of purchased equipment (i.e. mirror mounts) or built into lab made (home built) apparatus. Often, screws are purchased with matching bushings to be integrated into an experiment or commercial product once attained. Fine adjustment screws and mirror mounts are available as standard items from most companies that sell optics hardware.

The fine motion can most easily be calculated by using the pitch or TPI to determine how many micrometres per revolution the screw moves. The table below shows some common examples

The screw turns 360° in one revolution and with well made screws (minimal stiction) and a user with a sensitive touch, 1° movements can be achieved. Thus, with the ultra fine adjusters sub-micrometre motions are attainable.

The material of choice for producing these screws is stainless steel typically 18-8 (303). At the tip of the screw most commonly a stainless steel ball is either pressed or glued into place to provide a single point contact for the surface being moved. Common usage has shown the glue method of attaching the ball to be troublesome in many applications as some commonly used glues (Super Glue) tend to outgas and degrade over time in this application.

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Wikipedia