 Diamond knife

A diamond knife uses a sharpened diamond edge to cut ultrathin sections in ultramicrotomy. The edge's geometry and condition affect the force applied to the specimen and the resulting section. JEOL describes its use for hard inorganic materials and for obtaining uniform sections from resin specimens.^[ref]

The knife wedge angle is the angle between the blade faces. It differs from the knife clearance angle, which concerns how the blade is oriented relative to the freshly cut surface. Manufacturer descriptions include $35^{\circ }$ and $45^{\circ }$ knife families, with the lower-angle design promoted for reduced section compression and a higher-angle design discussed for hard materials.^[ref]

For an original ideal symmetric wedge of included angle $\beta$, the blade width a distance $h$ behind the apex isAt $h=1\mu \mathrm{m}$, this gives $w\simeq 0.631\mu \mathrm{m}$ for $\beta =35^{\circ }$ and $w\simeq 0.828\mu \mathrm{m}$ for $\beta =45^{\circ }$. The wider ideal wedge has about 31% more material behind the edge at that distance. This illustrates a geometry tradeoff; it does not predict a 31% difference in cutting force or assert the exact profile of a commercial knife.

A useful materials experiment compares otherwise matched cuts while recording knife orientation, feed and specimen condition, then measures section dimensions and surface defects. A cut stripe that repeats at the same lateral position suggests an edge-related artifact worth checking. Diamond hardness alone does not guarantee an undamaged edge or an undistorted section.