[Manufacture of a titanium full crown by CAD/CAM--tolerance of the cutting tool].

PURPOSE We investigated the durability of the cutting tool(bar)that is used when creating a dental crown by using the CAD/CAM system of a computer-assisted NC machine tool. METHODS We used the chamfer type as the shape of the dental cervical margin of the support tooth model. We also used a mold model whose shaft surfaces on both sides had an 8-degree taper. We used the CAD/CAM system of DENTAL Cadim (Advance Inc.). First, we manufactured a nonocclusion dental crown of the resin-pattern on top of the support dental model, and measured its 3-dimensional shape. Upon measuring the 3-dimensional shape, we created a pure-titanium block by using cutting bars continuously for rough processing and for finishing processing separately. Upon completion of the pure-titanium block, we measured the time required for each processing and measured the surface roughness after each processing. RESULTS The surface roughness Ra of the rough processing became significantly worse after the fifth processing (p<0.01). Rmax became significantly worse after the sixth processing(p<0.05)and after the seventh processing (p<0.01). The surface roughness of both Ra and Rmax of the finishing processing became significantly worse after the fifth processing (p<0.05) and after the seventh processing (p<0.01). CONCLUSIONS From the viewpoint of quality of the crown's finished surface and compatibility fitness, the number of cuttings, the number of rough processing and the number of finishing processing using the same cutting tool for making a full crown of a molar tooth should be limited to four times.

目的 我们研究了在使用计算机辅助数控机床的CAD/CAM系统制作牙冠时所使用的切削刀具（刀杆）的耐用性。 方法 我们将支持牙模型的牙颈边缘形状设为倒角型，还使用了一种两侧轴面有8度锥度的模具模型。我们使用了DENTAL Cadim（Advance Inc.）的CAD/CAM系统。首先，我们在支持牙模型上制作了一个树脂模型的非咬合牙冠，并测量其三维形状。在测量三维形状之后，我们分别使用切削刀杆连续进行粗加工和精加工来制作一个纯钛块。纯钛块完成后，我们测量了每次加工所需的时间，并测量了每次加工后的表面粗糙度。 结果 粗加工的表面粗糙度Ra在第五次加工后显著变差（p<0.01），Rmax在第六次加工后（p<0.05）以及第七次加工后（p<0.01）显著变差。精加工的Ra和Rmax表面粗糙度在第五次加工后（p<0.05）以及第七次加工后（p<0.01）显著变差。 结论 从牙冠加工完成表面的质量以及适配性的角度来看，使用同一切削刀具制作磨牙全冠时，切削次数、粗加工次数和精加工次数应限制在4次。