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Posted on Aug 5, 2020
Unlike lathes, which have thousands of years of history, milling machines are less than 200 years old. Because they require more power than manual lathes, their introduction has to wait for the invention of industrial water and steam power. Similarly, all of its mechanical components must be provided first, such as precisely mounted slide rails, large castings that resist cutting forces, calibrated screws and hardened steel cutting tools.
Eli Whitney is credited with the invention of the first milling machine around 1818, but the knee and column support system of the universal milling machine of Joseph A. Brown (later Brown and Sharpe) dates from 1862 and represents an important step in machine development. In the second half of the nineteenth century, milling machines gradually replaced milling machines and planers that have turning, single-point bits of tools that move after work in a straight line and scrape the metal in one stroke. Milling machines, with their continuous cutting action, not only remove metal faster than milling and planning machines, perform additional operations such as cutting screws for gears and twist drills. Currently, milling machines far outweigh the shaping and planning machines. Americans in New England and later in the Midwest were constantly adding functions leading to a modern milling machine.
Another important development came in the 1930s, when Rudolph Bannow and Magnus Wahlstrom introduced a Bridgeport style vertical milling machine. This design offers versatility and economy instead of higher metal removal rates from traditional horizontal milling machines. Because of this versatility, there are more Bridgeport cutters today than any other milling machine design. Horizontal mills are usually reserved for production applications where high metal removal rates from identical parts are needed, rather than prototyping and short runs. Bridgeport style machines are also called knee and column machines and turret mills. The key features of these machines are:
● Support for knees and columns of the milling table, which ensures vertical movement of the work relative to the tool.
● Saddle that supports the table to ensure inlet and outlet movement from the vertical column.
● One-piece tool head that holds the motor, pulleys and spindle.
● Sliding arms or cylinders were finally added to allow the tool head to be moved or extended relative to the vertical column. Some machines are designed to tilt the tool head sideways or from back to front.
● The biggest advantage is the pen's ability to easily move and retract the cutter without unscrewing to raise and lower the milling table. This speeds up production and reduces operator fatigue. The extendable pen allows the operator to quickly remove the tool to remove chips from the hole or check its progress. Tactile feedback via the pen feed handle or handwheel also informs the mechanic how it cuts the tool and allows it to optimize manual feed with less risk of tool breakage. Vertical table movement is still available for fine depth adjustment or when more force is required on the tool.
● The second biggest advantage is the Bridgeport-style machine's ability to make angled cuts. In the case of a horizontal milling machine, the milling cutter is carried out at an angle or the work must be set at an angle to the spindle axis. On a Bridgeport-style machine, the operator only needs to tilt the spindle to make an angle cut. Of course, Bridgeport can also use an angle knife or mount work at an angle.
● Vertical milling machines must use smaller cutting tools than horizontal milling cutters because they have less rigid, less massive castings and motors with less power. Still, they can achieve the same end results as a horizontal cutter, only slower.
● Vertical milling machines are less complicated than horizontal machines because the one-piece tool head eliminates the need for complicated toothing inside the vertical column.
● Bridgeport milling machines usually have 1 to 5 motors with less power and smaller castings than most horizontal mills.
There are 15 to 36 designs or styles of milling machines, depending on who counts, but the main purpose of this study is a Bridgeport-style vertical knee mill, because it is most commonly used in prototype and R&D stores. Together, they outweigh all other projects. This project has so much to offer that it has been copied in every industrialized country. At one time there were no less than thirteen separate Spanish Bridgeport mill companies. Practical knowledge of the Bridgeport vertical milling machine also provides a good start to operate any milling machine of a different type.
Lathes and mills are complementary machines. While lathes rotate the workpiece and produce a cylindrical cut, milling machines transfer work to the rotating knife and make the cut in a straight line. Both lathes and mills are capable of drilling large-diameter holes, but the mills are better suited for placing holes anywhere on the work surface. Although sometimes you can settle for just a lathe or a mill, a well-equipped store must have both machines.
When cutting with a lathe, the tool is in constant contact with the workpiece and performs continuous cutting. Milling machines are just the opposite. They use multi-tooth cutting tools, and their cutting action is interrupted when each tooth bites. The metal is removed in small individual chips. Unlike turning tools, end mills, the most popular cutting tool for Bridgeport mills, you cannot sharpen by hand because they must be perfectly symmetrical. Sharpening them requires special fixtures and shaped grinding wheels. Smaller shops send their cutters out for sharpening.
Adding digital readout (DRO) is a great convenience for any milling machine. This reduces the need to stop the mill multiple times for measurements and reduces the risk of errors. After resetting to zero, DRO displays the exact offset from the reference point on the workpiece, enabling the operator to work directly with the dimensions in his work drawing.
For production applications there are large, expensive milling machines with at least three computer controlled axes. Some machines perform all operations, including automatic tool change. However, today there is an intermediate stage between the manual and fully automated mill. This is done by adding a computer, digital readings and actuators to the Bridgeport-style mill X and Y axes. This improved machine not only can tirelessly perform all its repetitive functions, but also added new possibilities. Now the mill can engrave (drive a tool for cutting numbers and letters in various sizes and fonts), cut out radii and angles without a rotary table, create islands, pockets and cut out ellipses and frames. Entering the position, diameter and number of holes automates the cutting of bolt holes; the system does math. The computer can also automatically compensate for the reduced diameter of sharpened cutters, saving time and money. The system can be manually programmed using the control panel, use saved programs, "learn" new tasks, remembering a series of manual operations when the operator performs the first part, or accept files from CAD programs.
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