pre-ditch & ditch

after preparation of plaster cast,dies should be prepared to continue working,the firs step of prepare dies is determine the type of finishing line on the margin of tooth preparation.

The finished margin position and type are determined by the gingival contour, the nature of the restorative material, the presence or absence of a core margin and the choice of luting agent.Whenever possible, the margin should be supragingival following the natural gingival contours. Finish margins should ideally extend at least 1 mm past core margins to rest on sound tooth tissue

there are the several types of finish margin in prepration tooth:

  Chamfers and shoulders give definite finish margins which may be identified on preparations, temporary crowns and dies. Occasionally, knife-edge preparations may be indicated for full veneer crowns where there are deeply subgingival margins (however, periodontal surgery may be more appropriate here), bulbous teeth or pins close to the preparation margin. Metal ceramic crowns may be constructed with metal collars, especially on long preparations on posterior teeth. Lipline on smiling may indicate whether this is a practical proposition. A chamfer or knifeedge finish may avoid excessive tooth reduction in this situation. 

diagnosis and determine the finish margins and easy access to it on the die is done during the pre-ditch and ditch.there are 2 burrs for pre-ditch and ditch pear shaped burrs and round burrs that you see in images.
pre-ditch:at this stage is removed around the finish margin of gum with a pear-shaped burrs according to the images.
ditch:the following stage finish line is empty with a fine round bur and finishing line is being provided to technician clearly.