Minor Diameter

• A UNM screw's thread diameter determines the integrity of the grip.screw image by Zbigniew Nowak from Fotolia.com
  
  The root of a screw (minor diameter) is the core determinate of a screw's strength. The larger the core, the more torque the screw can handle. A trade-off exists between a larger minor diameter size and the screw's tap drill diameter. A screw needs to grip the material it is fastened to; the larger the tap drill diameter, the better this grip.
  
  

Thread Fit

• UNM screws cannot conform to the Uniform Screw Thread tolerances because their micro-size would result in a structurally unsound fastener. UNM screws are designed to allow the flank (the angled thread) of a screw to fully engage a material without weakening the crest (the outer edge of a thread). Precise machining is necessary during UNM screw manufacturing and can only produce a structurally sound fastener as a result of a complicated balance of the individual components of a screw.
  
  

Torque Failure

• The full assembly strength of a screw is an important specifcation to know while fastening a UNM screw.New Wood Screw image by Jim Mills from Fotolia.com
  
  Torque specifications for UNM screws are measured in two ways, depending on the application. The first torque specification is the torsional strength of the screw. This is the maximum amount of rotational force that can be applied to the UNM screw without the head or shaft failing. The second torque specification of UNM screw failure represents the maximum amount of downward-force that a screw can withstand against a material. This is referred to as the full assembly strength.
  
  

Torque Specification

• Torque failure values for UNM screws are measured in Newton-centimeters or inch-ounces. The factors to convert between these units are 1.416 Ncm/1 in-oz. or .7062 in-oz./1 Ncm. It is important to fasten a UNM screw to no more than 60 percent of the maximum calculated torque failure value. This is important because a screw is subjected to increased dynamic torque forces while actually fastening two materials.
  
  

Torque Specification Values

• As a general guide, UNM screws up to 0.060 inches can handle up to 35 in-oz of torque, depending on the application. Don't torque a fastener this tight if won't be subjected to this much torque during its application. UNM screws up to 0.086 inches can handle up to 192 in-oz. Remember to reduce this figure by at least 60 percent when applying the fastener.