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Toolmark Identification (Basic Toolmark Identification (Tool Actions…
Toolmark Identification
Tool Manufacture
Cutting Operations
Introduction
use tool harder than object focused on thin point so force of tool is harder than force holding crystalline grain structure of metal together
Chip types
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discontinuous chips - segmented; break into particles, brittle metals, coarser result
continuous chip with built-up edge - accumulate small particles that weld to object; get coarse surface on object; often from dull tool
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Lathe Operations
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turning - rotates exterior of object against single-point tool; facing - tool used on surface perpendicular to rotating spindle; boring - single-point tool enlarges drill hole
Milling
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main categories
face - object surface perpendicular to cutter face at end of spindle; cutters on face and edge of rotating cutter
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Other cutting operations
shaping - single-point cutter, cuts in straight line with parallel strokes across flat surface
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broaching - tool moves against stationary object; removes metal on both interior and exterior; each tooth cuts deeper than previous; used for rifling
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abrasive machining - thousands of small cutters made up of abrasive grains; finishing operation; each grain removes very small amount of material (ie. grinding, tumbling, etc.)
Metal Forming Techniques
metal forming - use high heat and/or pressure to squeeze metal into die; most require finishing operations; forms subclass characteristics in objects that don't go through finishing
cold forging (cold heading) - no heat, just large pressure
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casting - pour molten metal into mold; possible subclass characteristics; sand or investment casting common
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Toolmark Examination
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Microscopic Comparisons
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for comparison, select appropriate magnification for both objective lenses
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Inconclusive Results
possible reasons: different tools; damage occurred that changed microscopic detail in toolmark; tool changed due to damage or use; numerous subclass characteristics present
potential remedies - remount and reexamine; smoke evidence with magnesium to enhance detail; prepare different test marks with different material; change method of making test marks
Intercomparison
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examine toolmarks on both evidentiary items separately; select one with best quality individual marks
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place selected evidence item on right stage and orient it so best marks are in center of field of view; mark area
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if class characteristics match sufficiently, search for and align individual characteristics
if area of sufficient agreement is found, index mark
if there is not sufficient agreement, document reason
document area of best agreement, preferably with camera
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Source: Savage K, Freed G. Firearm Examiner Training. National Forensic Science Technology Center. [accessed 2019 Jan 25].