The “type” of design for flap discs specifies the angle that the abrasive sheets are aligned into the wheel.
T27 - Flat Discs
Type 27 flap discs are flat across the entire disc. This generally makes the preferred choice for smoothing/blending surfaces as well as flat sanding. Using an angle grinder, T27 flap discs should be used from a 0-15° angle from the surface.
Pros:
Better for smoothing and blending
Superior for flat sanding
Cons:
Not good for contour work or shaping
Not as effective for stock removal
T29 - Conical Discs
Type 29 flap discs have their abrasive sheets have a slight angle on the face of the disc. This design allows for more aggressive grinding and is usually preferred for contouring and shaping surfaces. Using an angle grinder, T29 flap discs should be used from a 15-25° angle from the surface.
Pros:
Superior for contouring and shaping
Better angle for heavy stock removal
Grinding and blending with one disc
More aggressive when grinding
Cons:
Inferior when used on flat surfaces
Less effective at smoothing and blending
