Machining Steps and Squaring
Common milling operations on the vertical mill include machining steps (Figure J-50) and squaring or machining two surfaces perpendicular to each other (Figure J-51). The ends of the workpiece can be machined square and to a given length by using the peripheral teeth of an end mill. If a large amount of material has to be removed, it is best to use a roughing end mill first (Figure J-52), then finish to size with a regular end mill. On low-horsepower vertical mills, plunge cutting is an efficient method of removing material quickly
(Figure J-53). In this operation, the end mill is plunged a predetermined width and depth of cut, retracted, then advanced and plunged again repeatedly. In plunging, the maximum cutting force is in the direction in which the machine is the strongest—in the axial direction.
Plunge cutting can be an efficient method of removing material quickly. In this operation, an end mill is plunged, then fed vertically into the workpiece with the quill feed hand lever (Figure J-53). The depth of the plunge cut is limited with the depth stop. After each plunge cut the tool
is retracted and the table is advanced for the next cut. This sequence is repeated until the roughing dimension is achieved. If backlash is a problem on your machine, do not use climb milling. In plunge cutting, the highest cutting forces are in the direction of the greatest strength of
the machine, which is the axial spindle direction.
Center-cutting end mills make their own starting hole when used to mill a pocket or cavity (Figure J-54). Prior to making any mill cuts, the outline of the cavity should be laid out on the workpiece. Only when finish cuts are made should these layout lines disappear. Good milling practice is to rough out the cavity to within .030 in. of finished size before making any finish cuts.
When you are milling a cavity, the direction of the feed should be against the rotation of the cutter (Figure J-55).
This ensures positive control over the distance the cutter travels and prevents the workpiece from being pulled into the cutter because of backlash. When you reverse the direction of table travel, you will have to compensate for the backlash in the table feed mechanism.
As older machines are replaced, backlash is often eliminated by using ball screws.
If the machine you are using has no backlash, climb milling is always the preferred method for machining. This is especially true when using carbide inserts. Figure J-56 shows an end mill that uses inserts, including an insert on the bottom that allows plunge cutting.
Different insert corner radii make machining a pocket with a bottom radius easy.
It is important that the chips be removed from the cavity during the milling operation.
Chips can be blown out of the cavity with compressed air or removed with a shop vacuum cleaner.
If the chips are left to accumulate, the cutter will jam and likely break. Safety guards around the machine are necessary when using compressed air.
The size of an end mill is normally marked on the tool shank. Assuming that this marking is correct may lead you into trouble. Often, a resharpened end mill is undersize because both the diameter and the end have been reground.
When regrinding the diameter on an end mill, always grind off or remove the size marking on the shank.
End Milling a Shaft Keyseat
A common end milling operation on the vertical mill is keyseat milling.
A shaft keyseat must be both centered on the shaft and cut to the correct depth. The following procedure may be used.
Step 1 Secure the workpiece to the machine table or in an indicator aligned mill vise.
Step 2 Select the correct size of cutter either in a two-flute or center-cutting multiflute type, and preferably one that has not been reduced in diameter by resharpenings. Install the cutter in the machine spindle.
Step 3 Move the workpiece aside and lower the cutter beside the part. With the spindle motor off, insert a slip of paper between cutter and workpiece. The offset edge finder may also be used for this procedure.
Step 4 Use the saddle crank and move the workpiece toward the cutter until the paper feeler is pulled between cutter and work as you rotate the spindle by hand.
At this point the cutter is about .002 in. from the shaft (Figure J-57).
Step 5 Set the saddle micrometer collar to zero, compensating for the .002 in. of paper thickness.
Step 6 Add the diameter of the shaft and cutter, and divide by 2. This is the total distance to move the workpiece for centering.
Step 7 Raise the cutter clear of the work and move the workpiece over the correct amount in the same direction that it was moving as it approached the cutter.
Step 8 Raise the quill to its top position and lock the quill and the saddle in place.
Step 9 Move the table crank to position the cutter at the point where the keyseat is to begin.
Step 10 Start the spindle and raise the knee until the cutter makes a circular mark equal to the cutter diameter (Figure J-58).
Set the knee micrometer collar to zero. Raise the knee a distance equal to half the cutter diameter plus .005 in. and lock the knee in this position. Using correct speeds and feeds, mill the keyseat to the required length.
To machine a T-slot or a dovetail into a workpiece, two operations are performed.
First, a slot is cut with a regular end mill,and then a T-slot cutter or a single-angle milling cutter is used to finish the contour (Figures J-59 and J-60). Angular cuts on workpieces can be made by tilting the workpiece in a vise with the aid of a protractor (Figure J-61) and its built-in spirit level.
The angle can be machined with an end mill (Figure J-62) or with a shell mill (Figure J-63). Another way to machine angles to tilt the workhead (Figures J-64 and J-65). The head can be swiveled so that accurate angular holes can be drilled. These holes can be drilled by using the sensitive quill feed lever or the power feed mechanism, or in the case of vertical holes, the knee can be raised. For work involving compound angles, a universal vise (Figure J-66) is used. This vise can be swiveled 90 degrees in the vertical plane and 360 degrees in the horizontal plane.
Other Vertical Mill Operations and Accessories
The rotary table is a useful accessory for the milling machine. The rotating motion is controlled by a precision worm and worm gear assembly. Common ratios of rotary tables are and A ratio means that it takes 40 complete revolutions of the hand crank to revolve the rotary
table once. The common rotary table positions the workpiece by degrees of arc. Discrimination on rotary tables varies from 1 minute to fractions of a second. Full degrees are graduated on the circumference of the table. Fractions of a degree are read on the worm crank scale, usually with the aid of a vernier. Rotary tables are designed so that they can be mounted on a machine tool in either a vertical or horizontal
Work may be held with a three-jaw chuck fastened to a dividing head (Figure J-68).
When the rotary table is mounted in a horizontal position on the machine table, circular and linear cuts can be made without reclamping the workpiece (Figure J-69). Always have the workpiece securely fastened and supported on parallels if holes or other features go completely through the workpiece.
Every machine axis not moving during a cut, linear (machine table) or rotary (rotary table), has to be securely locked.
Remember that metal removal operations create tremendous cutting pressures, which may move your workpiece from its original location. When the rotary table is in the upright or vertical position, operations on the circumference of a workpiece are performed (FigureJ-70)—detents,ratchet teeth, engraved location marks, and other similar features.
Rotary tables are used to machine arcs, angular cuts, or circles of holes. To make accurately spaced holes on a circle, the rotary table must be precisely positioned on the machine table. Always check the surfaces of the machine table and any accessories for nicks or burrs
before mounting them on the table. The center of the rotary table also must be aligned with the centerline of the machine spindle. Using a dial test indicator mounted in the machine spindle, align the hole in the center of the rotary table with the spindle. Once the alignment is
made in both the table and saddle axes, set the machine dials or DRO on zero. Mount the workpiece on the rotary table and align it as needed. Then, move the table axis by the radius value of the desired hole circle. Lock any movable axes before starting the machining operation.
On small machines it is best to use a small-diameter cutter and take multiple passes. A large-diameter cutter requires too much horsepower and leads to undesirable small feed rates and depth of cuts. If the cutter has double positive (axial and radial) rake angles, cutting forces are reduced significantly and productivity goes up (FigureJ-71).
A differential-pitch cutter has unequally spaced teeth, which cuts down on vibration and chatter. Climb milling is always recommended, because the chip is thick as the cutter enters the workpiece and thin on exit. The thin exit chip gives extended tool life.
Optimum width of cut is two thirds of the diameter of the cutter. Centrally positioned cutters result in an alternating cutting force,
which can cause vibration. To obtain a better surface finish, exchange one of the regular inserts with a wiper insert. A wiper insert protrudes below the other inserts by approximately .002 in. The width of this insert should be approximately one third greater than the distance
the cutter travels per revolution. This wider insert then removes the tool marks left by the other inserts.
Difficult-to-machine materials such as high-alloy steels, austenitic stainless steels, titanium alloys, and heat-resistant superalloys (nickel, cobalt, and iron based) often machine easiest when round inserts are used. The round insert gives maximum edge strength and limits the chip thickness. Start with a low cutting speed and increase it if possible. The more difficult a material is to machine, the lower the cutting
speed should be. Use flood coolant. For titanium, mist coolant is preferred. Round inserts are not only effective cutting tools
for exotic materials but can be used on all materials.
Round inserts with a positive rake have an effective shearing cutting action. The large radius also provides an outstanding surface finish. For machining hardened steel, using a CBN polycrystalline insert in a face mill gives excellent results.
A high-quality surface finish and a workpiece cool to the touch can be expected.