TOOL MARK IDENTIFICATION

Tool

An object used to gain mechanical advantage. Also thought of as the harder of two objects which when brought into contact with each other, results in the softer one being marked.

Tool MarksAny impression, abrasion, or cut

made when contact occurs between a tool and an object

Even mass-produced tools have minor differences that can differentiate one tool from another (Recall the differences between class and individual characteristics)

Types of Tool MarksIndentation Marks: made

when a tool is pressed against a softer surface; this mark is a negative impression of the tool; ex: screwdriver on wood

Abrasion Marks: made when surfaces slide across one another; ex: pliers or knives; the harder surface leaves scratch marks or striations on the softer surface

Cutting Marks: produced along the edge as a surface is cut; ex: saw or wire cutters

Tools – Classification

Tools can be classified according to type of action it imparts on the work piece. The action affects the marks left on the work piece.

Examples of different types of actions include:

Compression Flat Action Gripping Shearing Slicing

Tools - Compression

Compression tools are designed to compress by pressure or impact. These tools include hammers, chisels, and die stamps.

Tools – Flat ActionFlat action tools work

in parallel to the working surface. These types of tools include screwdrivers, pry bars and tire irons.

Tools – Gripping Action These tools use

opposing jaws to squeeze or hold an object. Tools in this category include vise grips, tongue and groove pliers, and pipe wrenches. They are also called serrated jaw gripping tools.

Tools - Pinching Action

These tools use opposed jaws which are designed to cut using a pinching action. Examples of these types of tools include bolt cutters and diagonal cutters.

Tools – Shearing Action

Shearing tools use two blades on adjacent planes that pass by each other and are designed to cut. These types of tools include scissors, tin snips, wire cutters, and pruning shears.

Tools – Slicing Action

Slicing tools cut materials by moving a sharp blade along the direction in which it is traveling. These tools include knives and razors.

Documenting Tool Mark Evidence—PhotographyPhotograph all the evidence with

a measuring device to show appropriate scale for reference

Use oblique lighting because it casts shadows and highlights details that are not easily visible under direct light

Documenting Tool Mark Evidence—Casting

If possible, the piece of wood, metal, etc. that has the tool mark evidence should be removed and kept as evidence

If not possible, a cast impression should be made using silicone or rubber-based casting materials such as Mikrosil

Be sure to dust for prints before making a cast

Test MarksSeveral types of tools are tested

to see if they can make a similar mark

By comparing the crime scene tool marks with the test marks, the kind of tool used in the crime can be determined