The milling fixtures should be designed such that the work in fixture is located as close as possible to the milling machine table. This avoids the possibilities of chatter or of springing the work. The base of the fixture should be rigid enough not to deflect during upset milling, and made of cast iron to absorb the forces tending to set up vibrational effects of chatter.
Milling fixtures are usually provided with keys for locating the fixture properly in the T-slot of the milling machine table. Base is provided with lugs on each side for fixing the base to the machine table.
Lug surfaces are slightly raised up and slot faced. In some instances an extension of the base of the fixture can be provided for the application of clamps so that the fixtures can be properly secured to the milling machine table.
In designing a milling fixture suitably, complete data about the milling machine, such as the dimensions of the milling machine table on which the fixture is to be used, dimensions of T-slots and distance between centre lines of T-slots, maximum height to which a fixture can be designed, the width, the amount of horizontal traverse of the table, the amount of cross feed, etc. should be available.
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Clamping devices employed for milling fixtures should be very strong and should be provided with springs on the stud so that the clamp itself will spring away from the work when the clamp nut is loosened. In milling work, it is very important that work be held rigidly, for which clamps must be located properly and the work located against definite stops.
All parts of the fixture subject to wear should made of material that can be hardened. Design of fixture should be as simple as possible in order to make the work of operator easier.
Milling machine vise is the simplest form of milling fixture. Many types of jobs can be clamped on vise by inserting jaws of various shapes. For continuous production, two vises may be mounted on the milling machine table so that when milling operation is going on one job, other job can be in other vise.
For components which are complicated in shape and require special adjustment of the jack screws or spring loaded rest pins individually for each component, the plain milling fixtures can be designed. These are specially designed for only one component and do not serve the purpose for other shapes.
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Fixtures for straddle milling are used to mill a work piece on both sides at one time. Two cutters are mounted on the milling machine arbor so that both surfaces or sides can be milled at one time. A fixture for this application is designed with a cast-iron base in which means like stop pin, rest pin etc. are provided for locating the work.
In designing a straddle-milling fixture, it is necessary to see that proper clearances are provided so that the cutters will not interfere with the fixture. Usually in the straddle milling fixture, several components are held in a line so that continuous milling operation is possible. As the cutting load in this type of milling is very heavy, it is necessary to design very rigid fixtures.
Milling fixture arranged for lateral and angular adjustment. A job shown in Fig. 28.74 is required to be machined on a milling machine. In this job four surfaces, i.e. two parallel surfaces P and two angular surfaces A1 and A2 are to be milled. It is proposed to carry out these operations in one setting of the work.
For this job, the rectangular black is first cut off from bar stock and drilled and counter bored. These holes are provided to receive two fillister-head screws which hold this part in place on the machine of which if forms a part. The use of these holes is made for holding the block in position on the fixture.
Three operations are required to mill this job; in one operation, two parallel surfaces P can be milled by straddle milling, and then sides A1 and A2 can be milled in subsequent operations.
The fixture for this job consists of an upper plate U which is pivoted on studs S. (Fig. 28.75). This stud is mounted in the cross-slide C which operates on the base B. The upper plate U is provided with two tapped steel bushings which are a forced fit in holes drilled and counter bored for the purpose.
The job is secured in position on the fixture by two screws which screw in to the tapped steel bushings. The purpose of providing tapped bushings and not to tap threads directly in the cast iron plate U is to facilitate easy replacement of worn threads by continual use. The fixture is shown in Fig. 28.75 for milling the parallel surfaces P. Taper pins T1 and T2 are used to locate the work in the required position.
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For milling the parallel side the pin T2 is inserted in the hole H to locate the cross-slide C in the required position; and pin is located in the central hole L to locate the upper swivel plate U in position. It may be noted that the pins are merely used to locate the fixture, but the fixture is secured and clamped by bolts R1 and R2.
For milling the side A1, the pin T1 is inserted in the hole L1 and the pin T2 in hole H1. This sets the swivel plate U at the required angle and also locates the cross-slide C at the required distance off centre to enable the work to be milled by the outer edge of the cutter. For milling the face A2, the swivel plate U is swung over to enable the pin T1 to enter the hole L2; and the cross-slide C is moved so that the pin T2 will enter the hole H2.