MILLING

A milling machine rotates a multitooth cutter into the work. A wide variety of cutting operations can be performed on milling machines. They are capable of machining flat or contoured surfaces, slots, grooves, recesses, threads, gears, spirals, and other configurations.

There are more variations of milling machines available than any other family of machine tools. milling machines are well suited for computer controlled operations. As mentioned, metal is removed in milling by means of turning a multitooth cutter into the work, Fig. 12-2. Each tooth of the cutter removes a small individual chip of material.

Work may be clamped directly to the machine table, held in a fixture, or mounted in or on one of the numerous work- holding devices available for milling machines.

SAFETY NOTE! Never attempt to operate a milling machine while your senses are impaired by medication or other substances.

WARNING! Never activate rapid traverse with the cutter in a cut.

MILLING SAFETY PRACTICES

Milling machines, like all machine tools, should be cleaned after each work session. A medium width paint brush may be used to remove accumulated chips.

CHIPS are RAZOR SHARP; do NOT use your hand to remove them. NEVER remove chips with compressed air. The flying chips may injure you or a nearby person.

If cutting oil was used, the oily mist produced by the compressed air is highly flammable. If ignited by an open flame, it can produce explosive results. Finish by wiping down the machine with a soft cloth.

The following procedures are suggested for the safe operation of a milling machine.
 

1. Become thoroughly familiar with the milling machine before attempting to operate it. When in doubt, obtain additional instructions.
2. Wear appropriate clothing and approved safety glasses!
3. Stop the machine before attempting to make adjustments or measurements!
4. Get help to move any heavy machine attachments, such as a vise, dividing head, rotary table, or large work.
5. Stop the machine before trying to remove accumulated chips.
6. Never reach over or near a rotating cutter!
7. Be sure the work holding device is mounted solidly to the table, and the work is held firmly. Spring or vibration in the work can cause thin cutters to jam and shatter!
8. Avoid talking with anyone while operating a machine tool, nor allow anyone to turn your machine on for you.
9. Keep the floor around your machine clear of chips and wipe up spilled fluid immediately! Place sawdust or special oil absorbing compound on slippery floors.
10. Be thoroughly familiar with the placement of the machine's STOP switch or lever.
11. Treat any small cuts and skin punctures as potential infections! Clean them thoroughly. Apply antiseptic and cover injury with a bandage. Report any injury, no matter how minor, to your instructor or supervisor.
12. Never "fool around" when operating a milling machine! Keep your mind on the job and be ready for any emergency!

MILLING OPERATIONS

There are two main categories of milling operations:

1. Face milling is done when the surface being machined is PARALLEL with the CUTTER FACE, Fig.12-27. Large flat surfaces are machined with this technique.
2. Peripheral milling is done when the surface being machined is PARALLEL with the PERIPHERY of the cutter, Fig. 12-29.

MILLING CUTTERS

The typical milling cutter is circular in shape with a number of cutting edges (teeth) located around the circumference.

There are two general types of milling cutters:

1. A solid cutter has the shank and body made in one piece.
2. The inserted tooth cutter has teeth made of special material which are brazed or clamped in place. Worn and broken teeth can be replaced easily instead of discarding the entire cutter.

TYPES AND USES OF MILLING CUTTERS

The following are the more commonly used milling cutters with a summary of the work to which they are best suited.

End mills
End milling cutters are designed for machining slots, keyways, pockets, and similar work. The cutting edges are on the circumference and end. Solid end mills may have straight or helical flutes, and straight or taper shanks. straight shank end mills are available in single and double end styles.

Several end mill styles are available:

1. A two flute end mill can be feed into the work like a drill. There are two cutting edges on the circumference with the end teeth cut to the center.
2. The multi-flute end mill can be run at the same speed and feed as a comparable two lip end mill, but it has longer cutting life and will produce a better finish. It is recommended for conventional milling where plunge cutting (going into work -Like a twist drill) is NOT necessary.
3. A shell end mill has teeth similar to the multi-flute end mill but is mounted on a stub arbor. The cutter is designed for both face and end milling. shell end mills are made with right-hand cut, right-hand helix, or with left-hand cut, left-hand helix.

CARE OF MILLING CUTTERS:

Milling cutters are expensive and easily damaged if care is not taken in use and storage. The following recommendations will help extend cutter life:

1. Use sharp cutting tools! Machining with dull tools results in low quality work and it eventually damages the cutting edges beyond salvage by grinding.
2. Tools must be properly supported and the work held rigidly.
3. Use the correct cutting speed and feed for the material being machined.
4. An ample supply of cutting fluid is essential.
5. Employ the correct cutter for the job.
6. Store cutters in individual compartments.
7. Clean cutters before storing them.

METHODS OF MILLING

Milling operations can be classified into one of two distinct methods:

1. With conventional or up-milling, the work is fed INTO the rotation of the cutter, the chip is at minimum thickness at the start of the cut. The cut is so light that the cutter has a tendency to slide over the work until sufficient pressure is built up to cause the teeth to bite into the material. This alternative sliding to start, followed by the sudden breakthrough as the tooth completes the cut, leaves marks so familiar on many milled surfaces. The marks and ridges can be kept to minimum by keeping the table gibs properly adjusted.
2. With climb or down-milling, the work moves in the same direction as cutter rotation. Full engagement of the tooth is instantaneous. The sliding action of conventional milling is eliminated, resulting in a better finish and tool life.

Climb milling is NOT recommended on LIGHT MACHINES nor on large OLDER MACHINES that are NOT in top condition.

VERTICAL MILLING MACHINE

The vertical milling machine is capable of performing milling, drilling, boring, and reaming operations. it differs from the horizontal mill in that the spindle is normally in a vertical position.

The spindle head swivels 90 deg. left or right for machining at any angle. The ram, on which it is mounted, can be adjusted in and out. On many vertical mills, it revolves 180 deg. on a horizontal plane.

Cutters for vertical milling machine:
Although adapters are available that permit the use of side and angle cutters, face mills and end mills are the cutters normally used. Straight shank end mills are held in a spring collet, or in an end mill adapter. Small drills, reamers, and similar tools are held in a standard Jacobs chuck.

VERTICAL MILLING MACHINE OPERATIONS

• In addition to the usual precautions that must be observed when getting a machine ready for a job, the spindle head alignment must be checked. Make sure that the spindle head is at right angles to the work table. Otherwise, it is NOT possible to machine a flat surface.
• If a vise is utilized to mount the work, wipe the vise base and work table clean. Inspect for burrs and nicks. They prevent the vise from seating properly on the table. Bolt it firmly to the machine.
• The next step is to align the vise with a dial indicator if extreme accuracy is required.
• Wipe the vise jaws and bottom clean of chips and dirt. Place clean parallels in the vise and place the work on them.
• Tighten the jaws and tap the work onto the parallels with a mallet or soft face hammer.
• Never strike the vise handle with a hammer to put additional holding pressure on the jaws.

1. Machine the first surface. Remove the burrs and place the first machined surface against the FIXED vise jaw. Insert a section of soft metal rod between the work and MOVABLE jaw if that portion of the work is rough or NOT square. Fig. 12-128.
2. Machine the second surface.
3. Remove the burrs and reposition the work in the vise, C, Fig. 12-128. Then machine the third side. This side must be machined to dimension. Take a light cut and "mike" for size. the difference between this measurement and the required thickness is the amount of material that must be removed.
4. Repeat the above operation to machine the fourth side.
5. If the piece is short enough, the ends may be machined by placing it in a vertical position with the aid of a square, Fig. 12-129. Otherwise, it may be machined as shown in Fig. 12-130.

Milling a keyseat or slot
An end mill may be used to cut a keyseat or slot. After aligning the vise with a dial indicator, the work is clamped in the vise, or to the machine table. If mounted to the table, a section of paper between the table and the work will seat the work more solidly and prevent slippage.

A sharp cutter, equal in diameter to the keyseat or slot, is selected. A two-flute end mill is employed when a blind keyseat or slot is to be machined. Figs. 12-137 and 12-138.

Machining internal openings
Internal openings are easily machined on a vertical milling machine, Fig. 12-140. A two-flute end mill must be utilized if the cutter must make the initial opening. It can be fed directly into the material in much the same manner as a drill.

When the slot is wider than the cutter diameter, it is important that the direction of feed, in relation to cutter rotation, be observed. Feed direction is normally AGAINST CUTTER ROTATION, Fig. 12-141. This applies ONLY when the cutter is removing metal from one side of the opening.

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MILLING MACHINE CARE

1. Check and lubricate the machine with the recommended lubricants.
2. Clean the machine thoroughly after each job. Use a brush to remove chips. Never attempt to clean the machine while it is running.
3. Keep the machine clear of tools.
4. Check each setup for adequate clearance between the work and the various parts of the machine.
5. NEVER force a cutter into a collet or holder. Check to see why it does not fit properly.
6. Use a sharp cutter. Protect your hands when mounting it.
7. Have ALL guards in place before attempting to operate a milling machine.
8. Start the machining operation only after you are sure that everything is in satisfactory working condition. It may be necessary to make special fixtures to hold odd shapes and difficult to mount work.
9. Use attachments designed for the machine.

PRECAUTIONS WHEN OPERATING A MILLING MACHINE

1. Avoid performing a machining operation on the milling machine until you are thoroughly familiar with how it should be done.
2. Some materials that are machined produce chips, dust-, and fumes that are dangerous to your health. NEVER machine materials that contain asbestos, Fiberglass, beryllium, and beryllium copper unless you are fully aware of the precautions that must be taken.
3. Maintain cutting fluids properly. Discard them when they become rancid or contaminated.
4. Be sure the cutter rotates in the proper direction. Expensive cutters can be quickly ruined.
5. Carefully store milling cutters, arbors, collets, adapters, etc., after each use. They can be damaged if not stored properly.
6. Never start a cut unless you are sure there is adequate clearance on all moving parts!
7. Exercise care when handling long sections of metal. Accidentally contacting a light fixture or busbar can cause severe electrical burns and even electrocution!
8. Carefully read instructions when using the new synthetic oils, solvents, and adhesives. Many of them dangerous if NOT handled correctly.
9. Use adequate ventilation for jobs where dust and fumes are a hazard. Return oils and solvents to proper storage. Wipe up spilled fluids. Do NOT pour used coolants, oil, solvents, etc., down a drain.