Knee and column milling machines

Short presentation on knee and column milling machines and peripheral and face milling operations.

1. Knee and column milling machines. Types of peripheral milling and face milling Gokulanand.P (16M213 ) Mechanical Engineer, PSG College of Technology

2. Introduction The milling machine removes metal with a fast moving multi tooth cutter. The multi tooth cutter is called milling cutter. The feed can be longitudinal, vertical or transversal.

3. Column and Knee type milling machine Most commonly used machine for shop work. Used for working on small workpieces . For bigger pieces bed type machine is used. It has 4 types.

4. Column Column is a main supporting frame for the knee,worktable,over arm etc..and houses all the driving mechanisms for spindle and table feed. Column has guideways on which knee can be moved up or down. Knee The knee gives supports the saddle and gives the table vertical movement so that the depth of cut can be adjusted.

5. Hand miller It is the simplest and a small machine having only hand feed for table and power rotation of the spindle. It is used for making small jobs like making slots, keyways and grooves. It has a horizontal arbor for holding the tool and the table usually has 3 movements.

6. Plain milling Machine Similar to hand miller but sturdier in construction. Table has power feed in horizontal and vertical directions. It is intended for heavier milling operations. It also has a horizontal arbor and cutting tool having hole is mounted on this arbor.

7. Universal Milling Machine Has the ability to rotate the cutter vertically or horizontally. Increased flexibility and can be used for a range of applications Spur, helical, spiral, level gears, twist drills, reamers , milling cutters can be machined along with other milling operations. The table can be swiveled horizontally and fed at an angle.

8. Vertical milling machine Spindle is in vertical direction. Small axial spindle travel permits step milling. End mill cutters and face mill cutters are the ones commonly used. They are used to produce grooves,slots and flat surfaces. Auxillary operations like drilling and boring are also performed.

9. Peripheral milling Also known as plain or slab milling. Plain and horizontal surface are milled. It results in production of a machined surface parallel to the axis of rotation of the cutter. The cutting tool is usually steel and has a number of teeth along its circumference.

10. Factors in peripheral milling In this process, the cutting force is not uniform throughout the length of the cut by each tooth The quality of the surface generated and the shape of the chip formed depend on the rotation of the cutter relative to the direction of feed movement of the work . Due to these factors, peripheral milling results in vibrations during machining.

11. Cutting tool in peripheral milling Cutters have straight or helical teeth. Helical teeth results in oblique cutting action. Straight teeth results in orthogonal cutting action. Helical teeth are preferred because load on teeth is lower resulting in smooth operations and reducing tool forces and chatter.

12. Up Milling This is the process of metal removing by a milling cutter which is rotated against the direction of feeding of work of the cutter. The chip thickness is minimum at the beginning of the cut and reaches maximum at the end. Because of this, the cutting force in up milling increases from zero to the maximum value per tooth movement of the cutter. As the cutting progresses and there is difficulty in pouring coolant on the cutting rdge of the cutter to flush out chips, there is accumulation of chips at the cutting zone and when the chips are carried with the cutter they spoil the work surface. Since the cutter teeth do not begin cutting as a soon as they touch the work suface , in the first instance, sliding of cutter teeth takes place for small distance on the workpiece which results in waviness of the resulting machined surface.

13. Conventional or Up milling The maximum chip thickness is at the end of the cut. The advantages are that tooth engagement is not a function of workpiece surface characteristics. Contamination or scale does not affect the tool life. There is tendency for the tool to chatter. The workpiece has a tendency to climb upwards necessitating proper clamping.

14. Down milling It is the process of removing metal by a milling cutter which is rotated in the same direction as the travel of the workpiece . The thickness of the chip is maximum when the cutter tooth begins its cut and minimum when the cut terminates. The cutter tooth starts removing the material as soon as it touches the surface and without sliding. The cutting force is maximum when the tooth begins the cut.

15. Since the cutting force is directed downwards it tends to seat the work surface firmly on the fixture which suits more for thinner jobs being easily and firmly clamped in the fixtures. Coolant is more easily provide in this process thereby avoiding overheating and accumulation of chips in the cutting area. Better surface finish is obtained in down milling as the cutter takes a chip of zero thickness at the end of the cut. There is tendency of the cutter to pull the work forwards.

16. Backlash in down milling It cannot be conducted in old machines having backlash error between table and feed screw and the nut. The backlash causes the work to be pulled below the cutter when the cut begins and leaves the work free when terminated. As this action is repeated during maching , vibrations in the machine are set up damaging the work piece considerably. It is because of this that down milling is performed only on those machines that are rigid and have backlash elimination arrangement.

17. Face milling In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the workpiece surface. The cutter rotates at a speed N and the workpiece moves along a straight path at linear speed v.

18. Effect of Lead angle The lead angle of the insert in face milling has a direct influence on the undeformed chip. As the lead angle increases the undeformed chip thickness decreases and the length of contact increases . The range of lead angles for face milling cutters is from0-45 degrees. As the lead angle decreases, there is a smaller and smaller vertical force component. In typical face milling operation, the ratio of the cutter diameter to with of cut should be no less than 3:2.

19. Effect of Rake Angle Cutting pressures are smaller with positive rake cutters than with negative rake cutters under the same conditions. Zero or negative rake angles are strong and will give good service under heavy impact . Negative rake angles force the workpiece away from the cutter.

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21. Profile milling Profile milling is used to duplicate the profile of a tool on the workpiece . Profile milling covers multi axis milling of convex and concave shapes in two and three dimensions.

22. Pocket milling In pocket milling the material inside an arbitrarily closed boundary on a flat surface of a work piece is removed to a fixed depth.  Generally flat bottom end mills are used for pocket milling. Firstly roughing operation is done to remove the bulk of material and then the pocket is finished by a finish end mill . It is widely used in aerospace and shipyard industries.

23. Operating Guidelines for milling Chamfers should be used instead of radii because of the difficulty of milling various intersecting surfaces. Internal cavities and pockets with sharp corners should be avoided due to difficulty in milling them because of finite edge radius Work pieces should be sufficiently rigid to minimize any deflections resulting from cutting and clamping forces. Cutters should be mounted as close to the spindle as possible to reduce deflections. Tool holders and fixturing should be as rigid as possible. For less vibrations cuuters with fewer cutting teeth or with random tooth spacing should be used.

24. References Machine tool and Manufacturing Technology-Steve F.Krar , Maria Rapisarda , Albert F.Check Manufacturing Processes and Systems-Phillip F.Ostwald , Jairo Munoz Manufacturing Engineering and Technology- Serope Kalpakjian , Steven R.Scmid Manufacturing Processes- J.P.Kaushish www.sandwick.coromant.com www.mitsubishicarbide.net