木材の超音波振動切削における切込過程の特徴

Abstract

木材の超音波振動切削において認められる切込促進効果を検討するために, 勾配切削方法を用いて, 微小切込における切込量と切削力の関係, 切削力の変化過程の特徴等を明らかにした。結果は次のように要約できる。1) 切刃が被削材の傾斜表面に接触すると背分力は立ち上り始め, 切込量の増大とともに増加し, 切屑発生点において最大となり, 以後漸減し, ある程度切込量が大きくなると負になる。一方, 主分力は一般に切込量の増大とともに増加する。2) 振動切削における切屑発生点の切込量t_pは無振動切削におけるそれよりも小さく, かつ振幅が大きいほど小さく, 明らかに切込促進効果が認められる。またt_pはその切刃による切削距離Lの増加とともに大きくなる。切屑発生点の背分力および主分力についても以上と同様の傾向が認められる。ただし, 主分力はLの増加に関係なくほぼ一定の値になる。3) 無振動切削において, 切屑発生点の主分力は背分力に匹敵する大きさを示す。ところが, 振動切削におけるこの点の主分力は背分力よりも小さく, さらに無振動切削の主分力に比較して極端に小さく, 場合によっては負の値になる特徴的現象が認められる。なお, 平削りの振動切削においても微小切込量領域では主分力が負になる可能性がある。木材の超音波振動切削における切込促進効果は主分力のこのような特性と密接に関係しているものと考えられる。4) 勾配切削における切削面のうねり曲線から, 微小切込段階における切削過程の特徴をある程度明らかにすることができる。5) 平削りにおいて切屑が発生するために必要な最小限の切込量は勾配切削におけるそれとほぼ一致するものと推定される。また平削りにおける臨界切込量 (背分力が0となる切込量) およびこの切込量における主分力は勾配切削におけるそれらとほぼ一致する。

In order to examine the incision effect (promotion effect of the penetration of cutting edge into work material) in the ultrasonic vibratory cutting of wood, relation between depth of cut and tool forces, and characteristic tool of force patterns, etc. in the region of infinitesimal depth of cut were clarified using an inclined cutting method in which original work surface was inclined by a given angle toward the cutting direction and the depth of cut increased linearly during cutting. The results obtained are summarized as follows; 1) The vertical tool force in the inclined cutting rises at the point of cutting edge-work material contact and increases with increasing depth of cut, and before long takes a certain maximum at chip initiation point (at this point the cutting edge penetrates first into work material and the chip moves up the tool face), and then the vertical tool force decreases with increasing depth of cut and finally becomes negative. On the other hand, the horizontal tool force in general increases monotonously with increasing depth of cut. 2) The depth of cut tp at the chip initiation point in the vibratory cutting is smaller than that in the conventional cutting (cutting without vibration), and the greater the amplitude of vibration becomes, the smaller tp becomes, so that there is the incision effect undoubtedly. Furthermore, tp becomes greater with increasing cutting length L. The same tendency is recognized concerning the vertical and horizontal tool forces with the exception that the horizontal tool force is almost constant independently of the increase of L. 3) In the conventional cutting the magnitude of horizontal tool force at the chip initiation point is comparable to that of vertical tool force. However, in the vibratory cutting the characteristic phenomenon is recognized that at this point the horizontal tool force is smaller than the vertical tool force and is an extremely small positive value or is equal to zero and, in certain circumstance, takes a small negative value. In the vibratory cutting with constant depth of cut, there is the possibility that the horizontal tool force also takes a negative value in the region of infinitesimal depth of cut. The appearance of incision effect in the vibratory cutting of wood is due probably to this characteristic phenomenon. 4) Based on the analysis of the undulation pattern of machined surface, characteristic of cutting process in the region of infinitesimal depth of cut in the inclined cutting can be clarified to some extent. 5) It is assumed that the minimum depth of cut essential for chip formation in the inclined cutting coincides almost with that in the orthogonal cutting with constant depth of cut. The critical depth of cut (in this case the vertical tool force is equal to zero) and the horizontal tool force at this depth of cut in the inclined cutting coincide almost with these in the orthogonal cutting.