Toolmark identification

for test 2

What is toolmark identification

the discipline of forensic science that as its primary concern to determine if a toolmark was produced by a particular tool

toolmark

• forcible contact of two objects - softer surface receiving the mark

• markings: striated or impressed

striated

• when a tool is moved across an object

• also referred to as friction marks, abrasion marks and scratch marks 

impressed (compression)

• A tool against an object with enough pressure to leave an impression

Class characteristics

• features that indicate a restricted group source. (suggest type of fool). Result from deisgn factors - determined prior to manufacture

Individual characteristics

produced by random imperfections or irregularities tool surfaces. caused by the manufacturing process, use, misuse, and corrosion. they are unique to that tool- specify a particular tool

Factors that affect tool marks

• relative hardness of each

• build-up on the tool and or receiving surface

• action of the tool

• orientation and movement of the tool: speed, angle, pressure

Design - tool examples

• screwdrivers/chisels/pry bars

• pliers

• pipe wrenches

• diagonal cutters

• bolt cutters

• wire cutters/strippers

• tin snips/scissors/shears

• hammers/punches

• circular cutting tools

• knives/axes

• saws

• files

Tool mark action type

• flat action (scraping)

• gripping/pinching

• shearing

• compression

• slicing

Flat action/scraping screwdrivers 

Often used as prying tools

characteristic of the blade:

• tip design (flathead, Philips)

• shape

• width

• thickness

• finish - grinding, casting, stamping

Shearing - tin snips

• serrated blades

• typically a drop off on a side of the test wire unlike pinching

impressed/compression stamp/punch

• made by compression tools yield mirror images of the tool

• any defects on the surface of the tool will be transposed onto object

tools - manufacturing 

• determines class and some individual characteristics

• The manufacturing process may involve stamping, forging, grinding, and milling

Stamping

• stamping out the basic shape of the blade

• Forging - shaping metal under high pressure, typically done by a hammering action

Grinding

• putting an edge on a blade

• the grinding wheel (stone) - changes as it contacts metal, causing individual markings

Milling

• rotating carbide bit cuts contour of cutting edge 

• mills can cut on side or the end of bit