Rake & Cutting Angle:
If the file is sectioned perpendicular to its long axis, the rake angle (cutting angle) is the angle formed by the leading edge and the radius of the file inscribed in its circle of rotation. While using the radius as the line of reference, if the leading edge is in front of the perpendicular radius, it is referred to as a negative (scraping) cutting angle and is measured as the angle formed by the leading edge and radius. If the leading edge is behind the radius, it is referred to as a positive cutting angle.
Circumference Area & Core Area:
The circumference area (A=3.14r²), inscribed by the rotation a file, is determined by defining r as the distance from the file’s central axis of rotation to the file’s outer most boundary. The core area of the file (A=3.14r²) is the cylindrical center area of the file having its circumference outlined and bordered by the depth of its flutes. It should be noted that there are instances when the central axis of rotation is separate from the file core’s center.
FLUTES & SPIRALS:
The flute of the file is the groove in the working surface used to collect soft tissue and dentine chips removed from the wall of the canal. The surface having the greatest diameter that follows the groove (defined as where the flutes intersect), as it rotates, forms the leading (cutting) edge. The number of spirals that a file has in its working surface can be calculated by dividing the number of flutes on the working surface by the number of flutes in x-section.
The helix angle is the angle the blade makes as it intersects with central longitudinal axis of the file.
DEBRIS REMOVING AREA:
The debris removing area is the total flute space represented as a percentage of the circle area that inscribes the file.
DEFLECTION & DEFORMATION:
Deflection is measured as the file is lowered 5mm onto a 60 degree incline. If the file remains bent as it is lifted from the incline, the angle of the bend is measured as plastic deformation.
ROTATION TO FAILURE:
To determine rotation to failure, 3mm of the tip end of a file is locked into place before rotating its handle end continuously until it breaks. The degree of total rotation is measured at the point where the file fails.
PEAK TORQUE AT FAILURE:
Peak torque at failure describes the maximum torque the file undergoes as it is rotated to failure.