14 Shaping and Milling
SHAPING
The Shaping Machine
The shaping machine is used for making flat surfaces, key-ways and slots. Although it uses a single point tool similar to a turning tool, and cuts only on the forward stroke, it removes metal very quickly.
The machine is made of cast iron. The heavy ram on the top moves backwards and forwards in slideways and the length and speed of the stroke can be adjusted to suit the work being machined. The return or non-cutting stroke is quicker than the forward or cutting stroke. The reason is that the crank pin travels a greater distance on the cutting stroke than on the return stroke as shown in figure 1, which also shows how the length of stroke is adjusted.
The table can be raised or lowered and moved at right angles to the movement of the ram.
Figure 2 shows the shaping machine.
The work can be held in a vice or clamped on to the table as in figures 3 and 4. After the work has been positioned and the stroke adjusted so that it clears the work by about one inch both on the cutting and return stroke, the tool can be brought down to the work with the head slide handwheel. When the machine is set in motion the clapper box allows the tool to lift on the return stroke thus reducing friction.
The clapper box is so called because of the clapping sound it makes. The feed ratchet mechanism, when it is engaged, feeds the table across on each return stroke (fig. 3). When cutting a vertical face the clapper box as well as the tool must be set over so that the arc made by the tool on the return stroke clears the vertical face of the work (fig. 4)
Shaping Tools
These are similar to lathe tools but are more robust in order to withstand the shock of the intermittent cutting action. The front clearance angle should be slightly more than for a lathe tool to allow clearance on the return stroke. 10°-12° is usual. Figure 5 shows three typical shaping tools.
MILLING
The Milling Machine
In the small workshop the milling machines are either "vertical" or "horizontal". The kind of machine which can be converted either to horizontal or vertical cutting is, for reasons of economy, often used (fig. 1). The horizontal attachment is shown in position in figure 2.
In industry this machine and variations of it are used for many specialised jobs such as gear cutting, spline cutting, helical milling etc. In fact milling on its own is a specialised study and the interested reader is advised to read one of the many books entirely devoted to this subject. The milling operations discussed here are the simple ones of making flat surfaces and slots.
Milling Cutters
The cylindrical Milling Cutter. This is shown in figure 3. This is also known as a slab mill and it is used for making flat surfaces. Figure 4 shows the cutter set up on the horizontal arbor. These cutters are available in various diameters and widths.
Side and Face Cutter (fig. 5). This has teeth on the side and face and is useful for cutting grooves as shown in figure 6. These cutters are available in various diameters and up to 1 1/2 " wide.
Slitting Saw (fig. 7). The teeth are usually only on the periphery. It is used for making narrow slots and for cutting through metal. These saws are available in various diameters and widths from 1/64 " to 1/4 ".
End Mills. These have either parallel, Morse taper or screwed shanks. Figure 8 shows an end mill chuck which has been part sectioned to show how a screwed shank end mill is held. Parallel and morse taper shank end mills must be used in their proper chucks. End mills vary in length and in diameter from g" diameter to 2" and are used for cutting slots, keyways and for making flat surfaces.
Methods of Holding Work
There are scores of ways of holding work for milling, but whatever method is used the work should be as rigid as possible. Figure 9 shows three pieces of work set up. Parallel strips and the dial indicator are invaluable aids to setting up.
Milling Procedure
When using the horizontal miller always feed the work against the rotation of the cutter as shown in figure 10. This is known as "upcut milling". On large specially designed machines cutting can be done in the other direction, but it should not be tried on the school machine because the cutter tends to "grab" the work.
Use a coolant, where applicable, which is fed on to the cutter through the tube provided: but never apply coolant with a brush. This is dangerous.
When using the vertical miller the work should again be "fed" against the direction of the end mill (fig. 11). When milling out a slot (fig. 12) first drill holes to the depth of the slot but slightly smaller in diameter. These holes can be drilled on the milling machine and the micrometer dial used for obtaining the correct depth. The end mill then removes the webs between the holes. In this case the end mill cuts on both sides of the slot, i.e. upcut milling on one side and downcut milling on the other. If this slot is attempted without first drilling, the cutter will almost surely break because the cutters used are not generally designed to cut to any appreciable depth on the end.
Speeds and Feeds
The speed at which to run the cutter can be obtained by using the following formula:
revs per minute = (12 X surface speed in feet per minute)/(Pi x diameter of cutter in inches)
The surface speed at which the metal can be cut is found by referring to a table similar to that below. However, so much depends on the rigidity of the machine, the condition of the bearings and the sharpness of the cutter, that any theoretical speed obtained by using this formula must be modified accordingly.
TABLE OF CUTTING SPEEDS Cutting speeds in feet per minute
|
Aluminium |
Brass |
Bronze |
M/S |
High carbon steel |
Cast iron |
Dry |
400 |
300 |
75 |
90 |
50 |
70 |
With coolant |
500 |
- |
80 |
110 |
65 |
- |
Screw cutting |
60 |
60 |
60 |
40 |
30 |
30 |
When cutting (assuming that both the speed and feed are constant) each tooth of the milling cutter will remove a small chip of metal. The rate at which the work is fed into the cutter and the number of teeth on the cutter and its speed will alter the thickness of each chip.
For most work, using the horizontal machine, if we allow each tooth of the cutter to remove about 0.001" when cutting mild steel, we shall be on the "safe side". The rate of feed on the automatic traverse is usually stated in terms of 0.001" per revolution of the cutter. So if this stated figure is divided by the number of teeth in the cutter we arrive at how much each tooth is taking off.
For vertical milling a much finer feed must be used, particularly when small cutters are being used.
