Spruing a large bridge
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When spruing a large bridge you would think you need a lot of sprues to keep it secure, but that is not the case. If you put a sprue on every unit, this makes it difficult for the burr to get in between to get the the job done safely.
In the photo above, there is a 14 unit abutment bridge. This spruing would work perfectly in the mill. If there was a sprue on every unit along the lingual, it would be unsafe for the .3mm and 1mm burr to get in between and could collide with something and maybe even break the tool.
In this second photo, you can see a good spot to place the sprues on the units. You want to stay as close to the middle or height of contour of the units. Also, you want to make sure the sprues are perpendicular to the unit and the sintering connector and not at an angle.
In the photo below, the red is showing finishing. You can see how close the sprues are to where the burr is trying to finish, and with all the sprues around you can run into problems like the flute of the burr rubbing into the sides of your bride. This can cause the bridge to chip or fracture or your burr to break.