![Picture](/uploads/2/7/6/5/27655795/6420202.jpg?303)
(Above Image on Left is a 3D Topography of a Punch Toolmark) http://phys.org/news/2013-12-pml-validity-toolmark-identification-forensics.html
(Above Image on Right Are The Class Characteristics of a Bullet) http://www.dtic.mil/ndia/2011ballistics/Keynote_Hernandez.pdf
How The Evidence is Analyzed
Before evidence is passed on to professionals, it is important that the crime scene investigator is thorough in their search for fired evidence. Tool marks are most often observed at crime scenes in or on a wide variety of places like; doorknobs, wires, chains, bolts, locks sections of sheet metal, human bone or cartilage, doors, window frames and safe-deposit boxes. As most evidence at a crime scene, tool marks are photographed before they are examined further. Photographs show where the tool marks were located in relation to the rest of the crime scene; however, they are of no value for identification purposes because they lack detail. Attempts at individualization of tool marks are made in a lab using preserved tool mark casts. With that in mind, whenever it is possible, investigators will remove the entire surface (such as a doorknob or lock) containing a tool mark because the original surface is easier and more accurate for identification and comparison purposes. If the tool mark cannot be removed easily (such as a wall), a silicone rubber cast is made of the tool mark. Rubber casts are also often made because tool marks can be small, using an extruder gun (see photograph above on left). When the trigger of an extruder gun is squeezed, it dispenses a small amount of silicone directly onto the tool mark. After a short time, the silicone dries, creating a cast that is then removed and analyzed. (Refer to Cast Making Video)
(Above Image on Right Are The Class Characteristics of a Bullet) http://www.dtic.mil/ndia/2011ballistics/Keynote_Hernandez.pdf
How The Evidence is Analyzed
Before evidence is passed on to professionals, it is important that the crime scene investigator is thorough in their search for fired evidence. Tool marks are most often observed at crime scenes in or on a wide variety of places like; doorknobs, wires, chains, bolts, locks sections of sheet metal, human bone or cartilage, doors, window frames and safe-deposit boxes. As most evidence at a crime scene, tool marks are photographed before they are examined further. Photographs show where the tool marks were located in relation to the rest of the crime scene; however, they are of no value for identification purposes because they lack detail. Attempts at individualization of tool marks are made in a lab using preserved tool mark casts. With that in mind, whenever it is possible, investigators will remove the entire surface (such as a doorknob or lock) containing a tool mark because the original surface is easier and more accurate for identification and comparison purposes. If the tool mark cannot be removed easily (such as a wall), a silicone rubber cast is made of the tool mark. Rubber casts are also often made because tool marks can be small, using an extruder gun (see photograph above on left). When the trigger of an extruder gun is squeezed, it dispenses a small amount of silicone directly onto the tool mark. After a short time, the silicone dries, creating a cast that is then removed and analyzed. (Refer to Cast Making Video)
Bonus: Learn how Toolmark Impressions are Collected In the Video Below!
http://www.dailymotion.com/video/xg5m01_csi-and-impressions-how-are-tool-marks-collected-and-analyzed-in-csi_lifestyle
With respect to tools and tool marks, when the surface of a harder object (the tool) comes into contact with a softer object (the work piece), the harder object will impart its unique marks or features on the softer object. By this definition, a well-trained examiner should be able to identify the source of a tool mark by comparing known tool marks produced by that tool to the one in question. It is also important to understand how tools are manufactured and the mechanics by which tool marks are made. The analysis process involves three major steps. In the first step, class characteristics are eliminated, but individualization does not always occur. Tool mark examiners typically look for “unique” features that can be used to produce a positive match between the bone and the tool in question by using pattern matching. Pattern matching is when topographical features such as the relative height or depth, width, curvature, and spatial relationship of the features are defined for one tool mark and are then compared to the corresponding topographical features in the other tool mark. (See the picture above on the left of 3D topography of a punch tool mark.)
![Picture](/uploads/2/7/6/5/27655795/1428601.jpg?365)
In the second step, microscope comparisons are made and individualization occurs during this step. (See microscope picture to left). During microscope comparisons, two bullets or casings can be observed at the same time, by placing them in the same field of view. In order for there to be a match, longitudinal striations between two bullets as well as the breech face impressions must coincide. Generally, microscopic observations are conducted of striations on a tool, to be compared to the evidence marked by that tool recovered from the suspect or the crime scene.
In the third step, the examiner would come to one of three conclusions; an identification, elimination, or inconclusive. Saw mark analysis for example does not identify a specific weapon, but a class of tool, which would be considered a class characteristic. Since manufacturing operations create microscopic random imperfections on/in work pieces individual characteristics on bullets and cartridge casings can be created. For this reason, sometimes an additional step may occur, where there is identification with unique marks. These types of irregularities may also be caused by scratches, nicks, breaks, or wear. These tools would be considered individual and on top of manufacturing differences, are often custom-made or altered in some way, such as a bent knife. Once these three steps are completed, a tool manufacturer, model and size can often be
determined, providing leads about who may have bought or used the tool.
In the third step, the examiner would come to one of three conclusions; an identification, elimination, or inconclusive. Saw mark analysis for example does not identify a specific weapon, but a class of tool, which would be considered a class characteristic. Since manufacturing operations create microscopic random imperfections on/in work pieces individual characteristics on bullets and cartridge casings can be created. For this reason, sometimes an additional step may occur, where there is identification with unique marks. These types of irregularities may also be caused by scratches, nicks, breaks, or wear. These tools would be considered individual and on top of manufacturing differences, are often custom-made or altered in some way, such as a bent knife. Once these three steps are completed, a tool manufacturer, model and size can often be
determined, providing leads about who may have bought or used the tool.