MECH 200 1st Edition Lecture 4Screws- Difference between bolts and screwso Bolt Externally threaded fastener tightened/ released by nuto Screw Externally threaded fastener with pre-made or self-made thread tightened/ released by the head of the screwo The main difference is the application - Threads o Squareo ACME 29 degree included angle Less backlash than a 60 degree V-threado Pitch Measured from the front of one ridge to the back of the next Pitch = 1/(Threads per inch)o Buttress A sharp forward angle w/ a shallow back angle This creates a immense strength in one direction w/ almost no strength in the other It is found in things like firearm receiverso Hemispherical/ Rounded Thread Zero Backlash Used Commonly in CNC machines- Thread Naming: 3/8 – 16 UNC -2Ao The 3/8 represents the major diametero The 16 is the threads per incho The UNC is Unified National Course Other Options are:- UNF- Unified National Fine- UNS- Unified National Special- UNEF – Unified National Extra Fineo The 2A is the Class of the screw/ Bolt All screws are either A- Externally threaded or B- Internally threaded The number is the thread percentage- 1- 50% Thread- 2- 75% Thread- 3- 100%