Tools can include the usual instruments such as screwdrivers, drill bits, hummer, pry bars and punch or other substances such as rocks. Different tools will create different impressions or marks but each tool creates an impression that is unique to itself.
Tools and their impressions are largely defined by the manufacturing process that was used in production and thus lab scientists should be familiar with the various manufacturing processes including casting, forging, extrusion, machining and others. Also certain manufacturing process produce marks that change with time due to wearing of the cutting tool when in contact with the work piece (Monturo, 2010, p 8). Ductile materials may also develop built up edges during machining when the material cold welds onto the tool cutting edge due to high pressures resulting from the process. As more pressure is applied, the built up edge breaks off and the new material replaces the edge and thus the change in the cutting surface is used to examine the manufactured tools and their marks.
The main types of marks that are created by different tools including impressed marks such as hammer strikes and striated marks such as the marks made when a screwdriver slides over a soft surface (Association of Firearms and Tool Mark Examiners, 2007).The toolmarks are further classified according to their class and subclass properties. Tools also have individual characteristics including random imperfections resulting from the manufacturing process, corrosion or damage (Association of Firearms and Tool Mark Examiners, 2007). Class characteristic features are defined before the manufacture of the tool and include its shape and size and the type of impression that is imparted by the tool including shearing, slicing, compression and crimping (Association of Firearms and Tool Mark Examiners, 2007, p 8). Subclass properties are in most situations confused with the individual characteristics. They are the