TOOL MARK ANALYSIS

badwaterreflectiveUrban and Civil

Nov 29, 2013 (3 years and 7 months ago)

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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.

Lineman’s pliers blade at 20X
magnification

Screwdriver blade tip 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.

Stamp

Stamped Impression

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.

Reference

Precision Forensic Testing

www.precisionforensictesting.com