Milling machine

From Wikipedia, the free encyclopedia

Example of a CNC vertical milling center

A milling machine is a machine tool used for the shaping of metal and other solid materials. Its basic form is that of a rotating cutter which rotates about the spindle axis (similar to a drill), and a table to which the workpiece is affixed. In contrast to drilling, where the drill is moved exclusively along its axis, the milling operation involves movement of the rotating cutter sideways as well as 'in and out'. The cutter and workpiece move relative to each other, generating a toolpath along which material is removed. The movement is precisely controlled, usually with slides and leadscrews or analogous technology. Often the movement is achieved by moving the table while the cutter rotates in one place, but regardless of how the parts of the machine slide, the result that matters is the relative motion between cutter and workpiece. Milling machines may be operated manually or by CNC (computer numerical control).

Milling machines can perform a vast number of operations, some of them with quite complex toolpaths, such as slot cutting, planing, drilling, diesinking, rebating, routing, etc.

Cutting fluid is often pumped to the cutting site to cool and lubricate the cut, and to sluice away the resulting swarf.

Contents

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• 1 Types of milling machines
  • 1.1 Milling machine variants
• 2 Computer numerical control
  • 2.1 Milling machine tooling
• 3 History
  • 3.1 1810s-1830s
  • 3.2 1840s-1860
  • 3.3 1860s
  • 3.4 1870s-1940s
  • 3.5 1950s-1960s
  • 3.6 1970s-1980s
• 4 References
• 5 Further reading
• 6 See also

How Manual Transmissions Work

by Marshall Brain

Inside this Article

1. Introduction to How Manual Transmissions Work
2. A Very Simple Transmission
3. First Gear

4. A Real Transmission
5. Lots More Information
6. See all Transmissions & Drivetrain articles

If you drive a stick-shift car, then you may have several questions floating in your head:

• How does the funny "H" pattern that I am moving this shift knob through have any relation to the gears inside the transmission? What is moving inside the transmission when I move the shifter?
• When I mess up and hear that horrible grinding sound, what is actually grinding?
• What would happen if I were to accidentally shift into reverse while I am speeding down the freeway? Would the entire transmission explode?
  

In this article, we'll answer all of these questions and more as we explore the interior of a manual transmission.

Cars need transmissions because of the physics of the gasoline engine. First, any engine has a redline -- a maximum rpm value above which the engine cannot go without exploding. Second, if you have read How Horsepower Works, then you know that engines have narrow rpm ranges where horsepower and torque are at their maximum. For example, an engine might produce its maximum horsepower at 5,500 rpm. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You shift gears so the engine can stay below the redline and near the rpm band of its best performance.

Photo courtesy DaimlerChrysler Mercedes-Benz Actros, manual transmission

Ideally, the transmission would be so flexible in its ratios that the engine could always run at its single, best-performance rpm value. That is the idea behind the continuously variable transmission (CVT).

A CVT has a nearly infinite range of gear ratios. In the past, CVTs could not compete with four-speed and five-speed transmissions in terms of cost, size and reliability, so you didn't see them in production automobiles. These days, improvements in design have made CVTs more common. The Toyota Prius is a hybrid car that uses a CVT.

The transmission is connected to the engine through the clutch. The input shaft of the transmission therefore turns at the same rpm as the engine.

Photo courtesy DaimlerChrysler Mercedes-Benz C-class sport coupe, six-speed manual transmission, graphic illustration

A five-speed transmission applies one of five different gear ratios to the input shaft to produce a different rpm value at the output shaft. Here are some typical gear ratios:

Gear	Ratio	RPM at Transmission Output Shaft with Engine at 3,000 rpm
1st	2.315:1	1,295
2nd	1.568:1	1,913
3rd	1.195:1	2,510
4th	1.000:1	3,000
5th	0.915:1	3,278

You can read How CVTs Work for even more information on how continuously variable transmissions work. Now let's look at a simple transmission.

Details on Involute Gear Profiles

On an involute profile gear tooth, the contact point starts closer to one gear, and as the gear spins, the contact point moves away from that gear and toward the other. If you were to follow the contact point, it would describe a straight line that starts near one gear and ends up near the other. This means that the radius of the contact point gets larger as the teeth engage.
Figure 10. Animation of involute gear

The pitch diameter is the effective contact diameter. Since the contact diameter is not constant, the pitch diameter is really the average contact distance. As the teeth first start to engage, the top gear tooth contacts the bottom gear tooth inside the pitch diameter. But notice that the part of the top gear tooth that contacts the bottom gear tooth is very skinny at this point. As the gears turn, the contact point slides up onto the thicker part of the top gear tooth. This pushes the top gear ahead, so it compensates for the slightly smaller contact diameter. As the teeth continue to rotate, the contact point moves even further away, going outside the pitch diameter -- but the profile of the bottom tooth compensates for this movement. The contact point starts to slide onto the skinny part of the bottom tooth, subtracting a little bit of velocity from the top gear to compensate for the increased diameter of contact. The end result is that even though the contact point diameter changes continually, the speed remains the same. So an involute profile gear tooth produces a constant ratio of rotational speed.

Rack and Pinion Gears

Rack and pinion gears are used to convert rotation into linear motion. A perfect example of this is the steering system on many cars. The steering wheel rotates a gear which engages the rack. As the gear turns, it slides the rack either to the right or left, depending on which way you turn the wheel.

Figure 9. Rack and pinion gears from a household scale

Rack and pinion gears are also used in some scales to turn the dial that displays your weight.

How Gears Work

by Karim Nice

Inside this Article

1. Introduction to How Gears Work
2. Basics
3. Spur Gears

4. Helical Gears
5. Bevel Gears
6. Worm Gears
7. See more »

Worm Gears

Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater.

Photo courtesy Emerson Power Transmission Corp. Figure 8. Worm gear

Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.

This feature is useful for machines such as conveyor systems, in which the locking feature can act as a brake for the conveyor when the motor is not turning. One other very interesting usage of worm gears is in the Torsen differential, which is used on some high-performance cars and trucks.

How Gears Work

by Karim Nice

Inside this Article

1. Introduction to How Gears Work
2. Basics
3. Spur Gears

4. Helical Gears
5. Bevel Gears
6. Worm Gears
7. See more »

   7. Rack and Pinion Gears
   8. Planetary Gearsets & Gear Ratios
   9. Details on Involute Gear Profiles
   10. Lots More Information
   11. See all Internal Combustion articles
       

Bevel Gears

Bevel gears are useful when the direction of a shaft's rotation needs to be changed. They are usually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well.

The teeth on bevel gears can be straight, spiral or hypoid. Straight bevel gear teeth actually have the same problem as straight spur gear teeth -- as each tooth engages, it impacts the corresponding tooth all at once.

Photo courtesy Emerson Power Transmission Corp. Figure 5. Bevel gears

Just like with spur gears, the solution to this problem is to curve the gear teeth. These spiral teeth engage just like helical teeth: the contact starts at one end of the gear and progressively spreads across the whole tooth.

Photo courtesy Emerson Power Transmission Corp. Figure 6. Spiral bevel gears

On straight and spiral bevel gears, the shafts must be perpendicular to each other, but they must also be in the same plane. If you were to extend the two shafts past the gears, they would intersect. The hypoid gear, on the other hand, can engage with the axes in different planes.

Figure 7. Hypoid bevel gears in a car differential

This feature is used in many car differentials. The ring gear of the differential and the input pinion gear are both hypoid. This allows the input pinion to be mounted lower than the axis of the ring gear. Figure 7 shows the input pinion engaging the ring gear of the differential. Since the driveshaft of the car is connected to the input pinion, this also lowers the driveshaft. This means that the driveshaft doesn't intrude into the passenger compartment of the car as much, making more room for people and cargo.

How Gears Work

by Karim Nice

Inside this Article

1. Introduction to How Gears Work
2. Basics
3. Spur Gears

4. Helical Gears
5. Bevel Gears
6. Worm Gears
7. See more »

Helical Gears

The teeth on helical gears are cut at an angle to the face of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in full engagement.

Photo courtesy Emerson Power Transmission Corp. Figure 3. Helical gears

This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. For this reason, helical gears are used in almost all car transmissions.

Because of the angle of the teeth on helical gears, they create a thrust load on the gear when they mesh. Devices that use helical gears have bearings that can support this thrust load.

One interesting thing about helical gears is that if the angles of the gear teeth are correct, they can be mounted on perpendicular shafts, adjusting the rotation angle by 90 degrees.

Photo courtesy Emerson Power Transmission Corp.
Figure 4. Crossed helical gears