- Tips on machining 2018/03/23 UP
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Tips on machining vol.17
Machining technique learned from hexahedral cutting
- Tag
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- Personnel training
- Milling
Horizontal, orthogonal and parallel are the fundamental elements to represent shape accuracy. A hexahedron is a shape that contains all of these three. So, a hexahedron is always used as basic assignments in the practical training of milling and serves as a yardstick to measure trainees’ milling skills; an accurately machined hexahedron can prove that the trainee’s milling skills have reached a certain level. In this article, we introduce practical know-how and expert knowledge to pursue machining accuracy, taking hexahedral cutting as an example.
Fig. 1 Hexahedron cutting
To conduct milling, a vise needs to be attached on the table. In fact, the pursuit of machining accuracy starts with this operation. The vise serves not only as a fixture to secure a workpiece but also as a base to achieve high machining accuracy. When attaching a vise on the table, we have to check, with our own eyes, the parallelism (in the X-axis direction) and squareness (in the Z-axis direction) of the vise mouthpiece as well as the horizontality (in the Y-axis direction) of the slideways with which a workpiece comes into contact, to make sure the vise has been attached horizontally and at a right angle. And this is something we must do ourselves.
Fig. 2 Relationship between vise and axes
When a plane surface is machined with a face mill cutter, cutting in general is performed in the X-axis direction, and do you know why? (In other words, why machining is not performed in the Y-axis direction?) A machining center employs a table that moves in the X- and Y-axis directions, and a saddle is mounted on the Y-axis to support the table, while nothing was mounted on the table. This means the X-axis can move more lightly and offer better dynamic performance than the Y-axis. That’s why milling is conducted in the X-axis direction. It also helps maintain a clean work environment as there is no chip accumulation on the operator side.
To machine a plane surface with multiple tool paths, the following three paths are conceivable: (1) reciprocating cutting, (2) one-way cutting, and (3) U-shape cutting. (See Fig. 3)
Fig. 3 Three tool paths for plane surface machining
(1) Reciprocating cutting
The advantage of reciprocating cutting is that a tool path is short, and the disadvantage is that a surface is likely to be uneven at the seams of a tool path. The direction of cutting resistance changes in accordance with the change in cutting direction, which induces a change in the spindle tilting direction and thus generates uneven surfaces. For these reasons, reciprocating cutting is suitable for roughing in which high accuracy is not required.
(2) One-way cutting
One-way cutting is suited to the finishing process thanks to its constant machining direction, which is less likely to generate uneven surfaces at tool path seams. It also has a disadvantage of a longer tool path.
(3) U-shape cutting
U-shape cutting can control places where large burrs are formed because there is only one exit for a face mill cutter to come out from the workpiece, which is the advantage of U-shape cutting. However, just like reciprocating cutting, U-shape cutting involves changes in the machining direction, so it is not suitable for finishing, a cutting process requiring high accuracy.
The important points for hexahedral cutting are outlined above. Every operation, including one that often taken for granted in our regular work inevitably has logic, such as “Where is the reference face?”, “In which direction does cutting resistance apply?” or “In which direction do cutting chips fly?” In order to further improve our machining skills, it is important for us to pursue higher machining accuracy, bearing such logic in mind. It is no exaggeration to say that high machining accuracy that can be achieved by operators’ steady efforts demonstrates their tireless efforts and strong confidence towards even higher quality and productivity.
With more and more machining centers and turning centers being numerically controlled, the range of human tasks has been narrowed, and the focus has shifted to setups, cutting conditions and tool path settings. Moreover, under the current circumstances where operators cannot view the machining point with their own eyes, they are required to increase their ability to imagine machining phenomena.
Table 1 shows a practical cutting procedure for a hexahedral workpiece. If you have any problems or questions about machining methods, procedures or conditions, please feel free to contact us. DMG MORI will support your machining process innovation by our broad experience and expertise.
Table 1. Hexahedral cutting procedure
Process 1: machining of 1st face
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[Key Points]
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Process 2: machining of 2nd face
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[Key Points]
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Process 3: machining of 4th face
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[Key Points]
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Process 4: machining of 3rd face
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[Key Points]
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Process 5: machining of 6th or 5th face
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[Key Points]
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Process 6: machining of 6th or 5th face
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[Key Points]
Attention should also be paid to the vise tightening force; if the wrench is hit with a hammer to securely tighten the vise, the mouthpiece may tilt, affecting the squareness of each face. So, if higher priority is given to accuracy, the tightening force should be at a level that can be applied by human hand. |
