Various angles play essential roles in obtaining the perfect tissue sections. Although the clearance angle is the ONLY knife angle that can be adjusted on a microtome using the knife tilt, it is crucial to know the other angles involved in the process of cutting.
- Wedge angle: Angle of the edges of the wedge knife. This is the body of the knife or blade; normally 15 degrees.
- Bevel angle: Angle of the very tip of knife or blade, between cutting facets; normally 27–32°.
- Clearance angle: Angle between the block face and the lower facet of the knife. Dependent on tilt or knife holder.
- Set between 3–8°. We are most concerned with this angle, which can be adjusted easily to suit the blade and block for better sectioning. Once the proper clearance angle is found for a particular blade, it rarely needs to be changed. Most microtomes using low-profile blades cut best when the clearance angle is set at 5°.
- When the clearance angle is too wide, the blade tip will scrape the block, and chatter will result.
- When the clearance angle is too small, the blade's body will scrape the block, and skipped sections or poor ribboning will result.
- Cutting angle: Angle between the knife's block face and upper facet. This is not an angle that can be adjusted on a microtome. It results from the clearance angle and the upper bevel angle of the knife or blade.
- Rake angle: 90° minus the angle of the upper facet of the knife.
