Tool Mark Analysis

Tool Mark Analysis Classification of Toolmarks

Tools – Classification • Tools can also 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 Action • Flat 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.

Individual Characteristics • Within the class characteristics are the individual characteristics. These are microscopic imperfections on the edge of the cutting surface. • They are caused by: • The manufacturing process • Use • Abuse • Rust

Individual Characteristics • Because individual characteristics are unique on the surface of the tool, they are essentially fingerprints. • These unique marks will be transferred on any object they come in contact with. Screwdriver blade tip at 20X magnification Lineman’s pliers blade at 20X magnification

Tools – Manufacturing • By understanding how tools are made, we can understand how the class and individual characteristics are made. • Tools are typically made using a combination of machining methods. • The initial shape of the tool may be stamped from a solid piece of metal. • Another way to make the basic shape of tools is by forging. • Forging involves shaping the metal into the desired shape under high pressure. This is typically done by a hammering action. • Tools can have the final shape and cutting edge made by milling or grinding.

Stamping • Some blades of tools are made from steel barstock. • The barstock is fed into a hydraulic press that acts as a “cookie cutter” that stamps out the basic shape of the tool blade.

Grinding • Grinding is a method used to put an edge on a blade. • A grinding wheel is composed of small stone fragments embedded in a matrix. The surface of the grinding wheel is always changing as it comes in contact with metal. This causes the marks it makes to be individual.

Grinding • Surfaces that are ground typically have parallel marks • The individual characteristics of a ground surface can be seen below.

Milling • Milling machines remove metal using a rotating carbide cutter. • Unlike a drill, mills can cut using the side or the end of the bit.

Milling • The raw stamping of the blade is placed in a milling machine to cut the contour of the cutting blade into the stamping. • This will commonly require two passes. The first pass cuts the basic shape of the blade. The next pass cuts the secondary cutting angle.

Milling • The milling process leaves marks in the direction of the travel of the mill. In this example, the mill cut parallel to the edge. • The individual characteristics can be seen in the picture below.

Toolmarks • Two types of toolmarks that can be made when a tool comes in contact with an object. They are impressed and striated. • These marks are reproducible and can be used for comparison and identification of a toolmark to a particular tool.

Toolmarks - Impressed • Impressed Toolmark - The mark produced when a tool is placed against an object and enough pressure is applied to the tool so that it leaves an impression in the object. The class characteristics (shape) can suggest the type of tool used to produce the mark. The individual characteristics can be used to identify the tool with the mark. Also called Compression Mark.* * Association of Firearm and Tool Mark Examiners Glossary 4th Edition

Toolmarks - Impressed • Impressed marks can be left by a variety of tools. The most common are bolt cutters, pry bars, chisels, and hammers.

Toolmarks - Impressed • Impressed marks are mirror images of the tool that left the impression. • Any imperfections in the tools surface will be imparted to the work piece and can be used for comparison and identification.

Toolmarks - Impressed • Bolt cutters are another tool that leave impressed marks as they squeeze the material in the cutting process.

Toolmark - Comparisons • Compressed marks from bolt cutters can be used for comparisons. • The individual characteristics of the bolt cutter blades reproduce on the surfaces they come in contact with.

Tools - Comparisons • Toolmarks made by compression tools yield mirror images of the tool. • Any defects on the surface of the tool will be transposed onto the surface of the item it comes in contact with. • The overall size and shape of the imprint is measurable and a class characteristic of the tool. Stamped Impression Stamp

Toolmarks - Striated • Marks produced when a tool is placed against another object and with pressure applied, the tool is moved across the object producing a striated mark. Friction marks, abrasion marks and scratch marks are terms commonly used when referring to striated marks.* * Association of Firearm and Tool Mark Examiners Glossary 4th Edition

Toolmarks - Striated • The most common tool that leaves striated marks encountered in crime laboratories is a screwdriver. Criminals typically use screwdrivers to try to pry open cash drawers, windows, doors, and car locks.

Toolmarks - Comparisons • The striated toolmarks created by flat action tools are reproducible and can be used for comparison between the mark and test marks made by that tool. • When making test marks a softer metal is used, typically lead. Lead will not damage the blade during the course of making test marks, yet is hard enough accept the marks.

Toolmark - Comparisons • The first step in performing a comparison of striated toolmarks is to establish the correspondence of class characteristics. • This will typically be blade width or spacing of the blades if a tool such as a pry bar is used.

Toolmarks - Comparisons • After correspondence of the class characteristics has been established, the individual characteristics can be evaluated. • We can use these individual characteristics to conclude that the same tool made both marks.

Toolmarks - Striated • Another tool that leaves striated marks when contacting a surface is a drill bit. Drill bits cut as they rotate, therefore the striated marks are circular. These striated marks are caused by imperfections on the cutting edge of the blade.

Toolmarks - Striated • As with striated toolmarks from bladed tools, striated marks from drill bits can be compared to each other. • This is an example of an evidence mark on the left and a test mark made by the suspect drill on the right.

Toolmarks - Comparisons • There are times when the toolmark can not be removed from the location for submission to a crime laboratory. When this occurs the toolmark must be cast. • Casting makes a negative impression of the toolmark and is typically done with Mikrosil.

Toolmarks - Comparisons • Mikrosil is a two part system that must be mixed prior to using. • After it has been completely mixed, the Mikrosil can be applied to the area of the toolmark.

Toolmarks - Comparisons • The casting is documented while in place as part of the crime scene photos and documentation process. • After it has hardened, it can be peeled from the surface. • The casting will be submitted to the crime laboratory for comparison to the tool.

Toolmarks - Comparisons • The casting of the toolmark is a mirror image, therefore a casting of the test marks must be made for comparison.

Precision Forensic Testing www.precisionforensictesting.com Reference

Tool Mark Analysis