Various angles play important 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 important to know the other angles involved in the process of cutting. Let’s examine a knife and its angles.
- 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 lower facet of knife. Dependent on tilt or knife holder.
- Set between 3-8°. This is the angle that we are most concerned with, 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 tip of the blade will scrape the block and chatter will result.
- When the clearance angle is too small, the body of the blade will scrape the block and skipped sections or poor ribboning will result.
- Cutting angle: Angle between the block face and upper facet of knife. This is not an angle that can be adjusted on a microtome. It is the result of the clearance angle and the upper bevel angle of the knife or blade.
- Rake angle: 90° minus angle of upper facet of knife.