Chapter (6)

MILLING FIXTURESA milling fixture is located on the machine table and bolted in position; the w.p. is, in turn, located and clamped to the fixture. A fixture does not controldoes not control the cutting tools during the actual cutting, but the tools can be positioned before cutting is commenced.

6.1. Milling Methods

The fixture design depends upon the milling methods to be employed.

1. Straddle milling

In this method two cutters are mounted on the arbor so that two faces are machined simultaneously. The m/c. table is positioned relative to one of the cutters using a setting block.

2. Gang milling

Three or more cutters can be mounted on the arbor so that several faces can be machined at once.

Straddle

Two w.ps.

3. String or line milling

In this method several w.ps. are mounted along the length of the m/c. table so that they can be machined in one pass. One cutter or a number of cutters can be used, and the w.ps. Can be arranged in a single or a double line.

4. Pendulum milling

Cutting takes place when the table moves to the right and also when it moves to the left. In Fig. 6.4 one w.p. is machined at a time, and is indexed between passes.

Another version of pendulum milling uses two fixtures, so that when one w.p. is being machined the other fixture can be unloaded and reloaded.

5. Rotary-table milling

A rotary table is mounted on the m/c. table, and which rotates continuously past a facing cutter. Loading and unloading done when each fixture in turn passes the operator.

6. Profile milling

6.2. The Location and Clamping of the Fixture on the Machine Table

- The machine table must be clean before the fixture is placed upon it.

- To locate the milling fixture, a rectangular tenon, secured to the underside of the fixture, locates in the narrow portion of the ‘tee’ slot.

- Two tenons, positioned as far away as possible to minimize errors, are used for this location; the location must be from only one slot, otherwise there will be redundant location.

- The fixture is bolted to the table using ‘tee bolts’ in the tee slots.

6.3. Workpiece Location

- The w.p. must be located on the fixture base.

- The location system must be positioned relative to the tenons.

- Milling is often the first operation to be done upon a casting; when this is the case, some of the seating location points must be adjustable for height.

6.4. Workpiece Clamping

- The cutting forces during milling are very high and are interrupted.

- The clamps and bolts must be sturdy, and usually hexagonal nuts are used instead of hand nuts (as in drilling jigs).

6.5. Tool Setting

- The fixture is positioned relative to the cutter with the aid of case-hardened steel setting block.

- A feelerfeeler is introduced between the cutter and the setting faces to ensure that the setting block will not be damaged by the cutter during the machining that follows.

6.6. General Features of Milling Fixtures

1- A milling fixture must be strong and very rigid.

2- A milling fixture is secured to the m/c. table and not moved until the completion of the batch: and so lightness is relatively unimportant.

3- Milling is an operation that removes a large quantity of swarf, and so it is necessary to provide adequate swarf-clearance ports that enable swarf to be removed without having to invert the fixture.

4- Adequate clearance must therefore be provided so that the operator can easily load and unload the w.p. even when it is oversized (the w.p. comes from casting).

5- The w.p. must be given adequate support during cutting, but because of the uneven surfaces of a rough casting these support points must be adjustable.

6- The body (or base) of a milling fixture is usually of grey cast iron (grey cast iron is used because it has good vibration-damping properties).

6.7. Special Vice Jaws

The machine vice should be regarded as a simple but effective milling fixture: by using special jaws it can be adapted to accommodate w.ps. of awkward shape, or carry a location system.

6.8. Milling Fixtures

Fig. 6.18 Line or String Milling Fixture