4.3 Feeds and Speeds


• Milling is somewhat different than drilling and turning,



• Typical speeds are, [Krar]



• Typical feed per tooth values for HSS cutters, [Krar]



• Typical feed per tooth values for cemented carbide (tipped) cutters, [Krar]




4.3.1 The mrr for Milling


• considering the parameters defined in the discussion of speeds and feeds, etc, the mrr is given below,




4.3.2 Process Planning for Prismatic Parts


• The basic steps are,

1. Cut off the stock slightly larger than required.

2. Cut the basic outside diameter to size using a milling machine.

3. Lay out the basic features of the parts (in manual setups, this involves coating the surface with a blue stain, this is then cut and marked).

4. Use a bandsaw to rough cut the work.

5. On the mill, cut steps, radii, angles, grooves, etc.

6. Lay out the holes to be drilled, and then drill them.

7. Ream holes as required

8. Grind any surfaces that require it. Ground surfaces should generally have 0.010”



4.3.3 Indexing


• It may sometimes become necessary to rotate parts on a milling machine, beyond the rotation offered in some beds (e.g. Universal Milling Machine).


• Some of the applications that require this capability are milling of,

- polygons,

- splines

- gears,

- cams

- spirals


• This method can be done with a dividing head. This is basically a worm gear unit. As the crank is turned, the cylindrical gear will drive the round gear. This will result in an apparatus that takes large motions in the crank, and results in small rotations of the work. When coupled with a scale of some description this becomes very accurate.


• If a worm wheel has 40 teeth, each rotation of the crank will result in a rotation of 40/360 degrees, or 1/40th of a rotation. This means the rotation is 40:1.


****************************** INCLUDE FIGURES OF INDEXING HEAD



• There are two methods of indexing,

- Direct Indexing - A notched plate is located so that the crank shaft can be fixed at set positions (notches).

- Simple Indexing - Work is rotated by turning a crank. The crank is finally positioned using a plate with holes, and a sector arm. (The sector arm is used to count off the divisions on the plates)


• An example of the calculations involved is,



• Another example of indexing considers a rotation of 50 degrees,



• Differential indexing - is sometimes required to move plates both forward and backward part of a turn to obtain correct spacing. i.e., output shaft through gear train drives the index plate. XXXXXXXXXXXXXXXXXX


• Helical milling - the machine table is rotated through a helix angle. The machine lead screw drives the dividing head. Work is rotated while the machine table feeds. XXXXXXXXXXXXXXXX


• CAM Milling - requires a milling machine with a rotating vertical head. The dividing head is driven by the machine lead screw.