Milling

Milling refers to the chip removing machining of metals, wood or plastics by means of a milling tool. It is done on special purpose machine tools, usually on a milling machine or a machining center.

In contrast to turning is necessary for chipping cutting movement is produced by rotation of the cutting tool relative to the fixed in the machine table clamped workpiece. The other hand, necessary for forming the feed movement is achieved depending on the design, either by displacement of the machine table or by movement of the milling tool to the workpiece around. Feed movements may vary according to design - are in the X -, Y - and Z-axis or along the respective axis of rotation - in combination.

In the classification of the manufacturing method according to DIN 8580 milling counts as separation processes.

• 4.1 Manual or mechanically controlled milling
• 4.2 CNC milling

Distinction according to the direction

A basic distinction between milling:

Conventional milling

In conventional milling moves the cutting edge of the rotating tool in the engagement area opposite to the feed direction of the workpiece and forms from the entry point to the exit point of the blade thickening tension (comma span). Before the onset of cutting in the material to slide on the work surface and solidifies the existing structure. This only creates a high friction and then cutting through the solidified material must penetrate. Due to the increasing chip thickness, the machine is loaded differently and tends to vibrate (See also: regenerative effect). Also, is due to the effort slowly rising. With cutting edge admission he is low because not much material must be removed, but then grows during the milling and reaches just before cutting discharge its maximum value before the comma chip is finally separated.

The by the compression produced during the milling process, high pressure causes severe wear on the cutting edges of the open spaces, thus increasing the service life reduced. Because of this drawback, the conventional milling is therefore only makes economic sense when workpieces hard and abrasive liquids border zones have (lower hardening effect) or if the table drive is not without play. Although the milled in the mating surfaces have one by the sliding action of the cutting edge smooth but wavy texture.

For with play table drive, which occurs usually only in older or defective milling or boring, the conventional milling is recommended, because with this the cutter pushes the driven workpiece table with a uniform surface load distribution against the drive screw. In this way, backlash can be achieved for the feed and exclude unwanted carriage movements.

Climb milling

Climb milling, the cutting edge of the rotating tool moves in the contact area in the direction of the vector of the workpiece feed direction. Degrades the conventional milling the power up slowly, so it is climb milling directly at cutting admission greatest, but then decreases continuously. The chip is used for cutting exit towards becoming thinner, and finally peeled off, which, in proportion to conventional milling smoother surface is obtained (again, the chip is comma-shaped, but is initially removed a lot of material in this case and at the end a little).

The force- induced directional inclination of the cutter, bobbing to tear the workpiece in the feed direction, favored unwanted changes in the carriage or workpiece position. Therefore, the feed mechanism of the carriage of the machine tool must have zero backlash and high stiffness. Due to the low chatter, the achievable surface qualities when climb milling compared with the conventional milling are better under the same machining conditions - provided they do not favor the formation of built-up edges.

Because of the lower cut - and flank wear can be increased by 50 % with the same service life compared with the conventional milling the feed rate.

Distinction according to the milling process

Furthermore, after the milling process as specified in DIN 8589, Part 3, are distinguished:

Distinctions according to the materials to be processed

Wood is milled without coolant (KSS ) and with high cutting speed. Since wood is a poor conductor of heat, replace the tool surfaces to high temperatures. Therefore, feed and depth of cut must be chosen so that the surface has no burn marks. Plastics are processed with medium cutting speed, depending on the nature of the material ( hardness and plastic behavior ) is water or alcohol used for cooling.

The average speed for metals include, depending on type ( brittle or tough) a broad spectrum. On machines like those found in workshops or training facilities, is approximately 20-300 m / min. worked. In series production, or at fräsintensiven workpieces, for example, in aircraft frames, all the parameters are perfectly matched because of the economy, and it speeds up to 10,000 m / min. In most cases, this cooling lubricants are used, but there is a clear tendency towards dry machining. The percentage of KSS costs in the total cost of a machined workpiece is between 10 and 20%.

The tools used also differ greatly in cutting material and shape. Wood and plastic cutters always, in contrast to metal burs on a positive rake angle and larger cutting chambers.

Distinction based on the machine equipment

When milling is mainly distinguished between conventional milling and CNC milling.

Hand or mechanically controlled milling

In conventional milling, the feed rates are selectively controlled by cranks or simple mechanical feed devices. The dimensions are traversed on scales with verniers, which are attached to the cranks, read. Meanwhile, almost all milling machines productively used with opto- electronic distance measuring systems are retrofitted. The tool change is done by hand.

CNC milling

In CNC milling up to 5-axis machines can be programmed via a machine control. The axes are controlled individually or simultaneously with feed rates. The paths are measured by glass measuring strips. A distinction is incremental and absolute measuring systems. CNC milling machines run with sometimes extreme feed rates of 100 m / min and speeds of 100.000/min, however, the values ​​are usually much lower. The tools are stored in a tool change magazine called automatically when needed and is replaced by a tool changer. The CNC technology allows 3D milling can be generated with the complicated 3D contours. Often, the workpiece is traversed in many small adjacent rows. In 5- axis milling machine can position the router at any angle on the workpiece and process whereby the production is made possible by extremely complex 3D contours. Characteristic data of a milling machine, the working space coordinates, that is, which travels in the coordinates X, Y and Z are possible which driving power and which are available speed ranges.

Recently, there are machines with up to 8 axes for milling (Chiron FZ 08S ) or up to 15 axes ( longitudinal turning ).

Lathes

Looking for ways to complete processing in as little setups also turning machines have been developed which have driven tools and allow for easy drilling and milling on the lathe.

Distribution of forces

Each cutting edge of the cutting tool exerts on the workpiece from the cutting force Ftot. It consists of the main cutting force Fc (cutting force) in the direction of cutting motion (ie, tangential), the average normal force FcN in direction normal to the cutting motion and the passive force Fp in the direction of the rotational axis of the tool together. The cutting force Ftot is the vector addition of the forces Fc, FcN and mp