1. Classification of Toolmarks
Tool Mark Analysis
Classification of Toolmarks

2. 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

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

4. Tools – Flat Action
Flat action tools work in parallel to the working surface. These types of tools include screwdrivers, pry bars and tire irons.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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
Manufacturing Process – We will discuss the manufacturing later in the lecture.
Use – As tools are used, the metal-on-metal contact will cause minor damage to the edge of the tool. This will result in defects or “dings.”
Abuse – When a tool is used in a manner other than it is designed, abuse can occur. An example of this is using a screwdriver as a chisel.
Rust – Oxidation on the surface of a tool which has not been maintained. This oxidation changes the surface of the tool and will ultimately change the marks it leaves when it comes in contact with an item.

10. 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.
The individual characteristics can be seen at higher magnification. The defects along the edge are examples of individual characteristics.
Screwdriver blade tip at 20X magnification
Lineman’s pliers blade at 20X magnification

11. 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.

12. 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.
Barstock – solid uniform bars of steel used in manufacturing.

13. 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.

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

15. 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.

16. 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.
Lubricant is sprayed on the work piece during the machining process to keep the tools cool. Keeping the cutter cooler will lengthen the life of the cutter. The lubricant also removes chips from the cutting process.
The first segment of the video shows the main bevel of the blade being made. The second segment of the video shows the cutting edge being machined on the blade.
The picture in the bottom right corner can be used to show the two different cuts.

17. 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.

18. 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.

19. 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

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

21. 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.
Demonstrate to the students the correspondence between the toolmark and the contours of the chisel blade.

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

23. 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.

24. These are marks on two different pieces of metal from the same set of bolt cutters. The individual characteristics within the tool mark can be seen clearly.

25. 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.
It is important that students understand the concept of the impressed marks made by a compression action.
The defects on the surface of the tool are it’s individual characteristics. They will examine compressed marks and individual characteristics in a laboratory exercise.
Stamped Impression
Stamp

26. 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

27. 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.

28. 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.

29. 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.
Class characteristics are measurable features of a specimen which indicate a restricted group source. They result from design factors, and are therefore determined prior to manufacture.*

30. 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.
Individual characteristics are marks produced by the random imperfections or irregularities of tool surfaces. These random imperfections or irregularities are produced incidental to manufacture and/or caused by use, corrosion, or damage. They are unique to that tool and distinguish it from all other tools.*

31. 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.
Teachers, elaborate on damage for lab

32. 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.

33. 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.

34. 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.

35. 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.

36. Toolmarks - Comparisons
The casting of the toolmark is a mirror image, therefore a casting of the test marks must be made for comparison.
If the casting was compared directly to the test marks, then a raised area of the test mark would correspond to a lowered area of the casting and vice-versa.

37. Reference
Precision Forensic Testing