Milling after turning can be considered the most commonly used machining method. Milling machines do not have as long a history as lathes. The first milling machine was developed and built by the American engineer Eli Whitney in 1818. This milling machine performed the rotary movement of the milling cutter and automatic movement of the working table. More than 40 years later, in 1862, another American, Joseph Rogers Brown, launched the production of a universal milling machine. During this period, the designs of copier milling machines were created, which enable the processing of an object according to the spatial outline of a pattern. In 1953, also in the USA, the first electronically controlled automatic milling machine was implemented. However, conventional milling machines have been and are still used for over 200 years. In this article, I present the basics of the construction of a conventional horizontal and vertical milling machine and the possible machining on them.
Regardless of whether we are dealing with a conventional horizontal or vertical milling machine, the main movement is transferred to the tool (cutter), and additionally, the movement of the working table is carried out, depending on the design, either along the X or X and Y axes. Designs that provide 5-axis machining are distinguished in CNC milling centers. In addition to the movement in three axes, rotations are performed around two axes (X and Y). In this article, I focus on conventional milling machines. The work table is used for fixing and clamping technological equipment (chucks) and workpieces.
Conventional vertical milling machine – construction
Figures 1 and 2 show a conventional vertical knee-type milling machine with a sliding spindle head. Conventional knee-type milling machines are used to machine small and medium-sized workpieces. The workpieces are fixed and clamped on the work table. The work table rests on a vertically movable support, also known as the bracket. Within the knee-type milling machines, there are milling machines: light, simplified, production and universal.
Figure 3 shows the spindle in the moving head. In the case of the presented milling machine, the head only moves along the Z axis (vertically). In order to fix the tool (mill) in the spindle, it is necessary to use a tool holder with a defined cone in line with the cone in the spindle. In the discussed conventional milling machine, it should be assumed that the spindle tip was made in accordance with the PN-76/M-55081 standard. The cone SK was used according to DIN 69871 with a taper of 7:24 with the front block drivers. The task of the tapered seat is to center the position of the tools to be clamped. The task of the block drivers is to transfer the torque.
Tool holders (tapered shanks) using this type of cone are balanced on demand and are dedicated to conventional and high-speed machining (HSM – High Speed Machining). As a rule, this type of holder is made of chrome-manganese steel, carburized to a surface layer depth of 0.7 mm. The cone surface is hardened and precision ground. The surface hardness of the cone is 58± 2 HRC. Conical tool holders are pressed against the spindle taper seat by a screw passing through the spindle bore. Other types of cones are also used in the construction of milling spindles.
How the tool is clamped to the spindle tip shown depends on its size and design. Shank cutters and large diameter cutter heads sit directly in the spindle nose. On the other hand, end mills with smaller diameters and cylindrical shanks require clamping or reduction holders. Milling arbors are used for cutter arbors.
Fixing the workpiece
The workpiece is fixed and clamped either directly on the working table or in technological equipment, e.g. in a machine vice (fig. 4). Special machining holders are used in series production (fig. 5). In the special machining fixture shown in Fig. 5, the workpiece is fixed on the abutment surface as the main contact base, and the angular fixation is carried out by means of a full bolt and a truncated bolt. The workpiece is fixed by means of two clamps. Before machining the groove, the tool (cutter) was set using the so-called set block (in figure 5 under the left clamp). The fixture itself was fixed on the worktable and fastened with screws and T-slots. The slots for the screws are shown on both sides of the chuck body in figure 5.
Clamps of various designs are used to fix the workpiece, as well as technological equipment on the machining table with T-slots. Figure 6 shows the contemporary design of clamps used on CNC machine tools.
Tool setting requires manual setting using spark, i.e. getting the tool into contact with a specific surface. The holders for serial production (fig. 5) used the so-called set blocks, which significantly helped in setting the tool for a given milling cut. Thanks to the set blocks, an acceptable repeatability of the treatment was obtained. In the case of the presented conventional moving head knee-type milling machine, the tool mounted in the spindle has limited vertical movement. Setting the tool in relation to the workpiece in the Z axis, and in principle setting the object in relation to the tool in the Z axis, requires the vertical movement of the support and the head when it is moving. The setting of the cutting width depends on the positioning of the workpiece on the work table and the position of the table in the transverse direction (cross slide – fig. 1). The design of the presented milling machine provides the possibility of short transverse feed and long longitudinal feed.
Values of spindle speed and feed rate
As in the case of a conventional universal lathe, setting the spindle rotational speed and the feed rate consists in setting the values from the available range of constant values - e.g. for the rotational speed of the main drive, these can be the following values: 500; 640; 760; 955; 1200; 1500. The values of the gradual change of parameters, including the rotational speed of the main drive, were defined by the PN-62 / M-03150 standard Metal machine tools, which was finally withdrawn in 2001. The values within the aforementioned standard are derived from the Renard R20 series. The selection of a certain value is made by setting the position of the gears in the gearboxes. Having specific gears and possible combinations of them in 2 or 3 gears at our disposal, we can obtain a gradual change of values. Fig. 7 shows the dials indicating the selected rotational speed values of the main drive along with the selected value of the feed rate (fig. 8) and the lever for switching the direction of rotation.
Figure 8 shows the view of the discs indicating the values of the rotational speed of the main drive and the feed rate from the operator’s side.
The main drive speed value setting and the feed rate value are set using the electric control box (fig. 9). The value of the feed rate is available within the range of specified values [mm/min]. Often in the case of milling, the value of the feed rate is given in mm/tooth, so it is necessary to convert the value to [mm/min] and select the closest available value, but lower. I wrote about the machining parameters in milling in the article Milling – selection of machining parameters.
In the first step, the selection of machining parameters consists in selecting the cutting speed for a given material and blade material. In the case of conventional milling machines and lathes, we deal with graded rotational speeds of the main drives. After determining the cutting speed from the classical formula for cutting speed vc, the value of the rotational speed n of the main drive should be determined. Then the value of the rotational speed n is then compared with the available values on the given machine tool. Usually, the determined value is not consistent with the available ones, and then the value of the speed n from the machine tool closest to the determined n, but lower as far as the value, should be selected. This, in turn, requires the cutting speed to be recalculated.
The graded ranges of rotational speeds and feed rate in the case of conventional milling machines and lathes are the consequence of technological capabilities from decades ago, when numerical control was at most predictable. Nowadays, such grading of values in relation to the stepless selection of values on CNC machine tools can be considered a disadvantage. Here, however, it should be emphasized that in conventional lathes and milling machines, only solutions using kinematic chains (toothed gears) were able to provide any possibility of selecting the rotational and feed rate. In the presented conventional milling machine, switching between individual values of both the rotational speed of the main drive and the working feed takes place with the use of electric drives changing the position of the gears. By pressing the button (fig. 9) the operator carries out single switching operations until the required value of a given parameter is obtained.
In the case of a conventional lathe described in the article Universal lathe, construction and possible machining – basics, switching between the values of individual parameters took place by manually turning the lever, which resulted in the mechanical shift of the change gears in the machine gears.
Figures 10 and 11 explain other selected parts of the steering and set-up.
Examples of possible machining on a vertical knee-type milling machine
Figure 12 shows typical machining operations that can be performed on this conventional vertical knee-type milling machine:
- milling a plane using a milling head or end mill (Figure 14) – Figure 12.a;
- milling a T-slot with a dedicated cutter – Figure 12.b;
- milling a groove (also a keyway) using an end mill (Figure 13) – Figure 12.c;
- milling of the side surface with the end mill – Fig. 12.d;
- machining a chamfer at a specific angle with an end mill – Figure 12.e.
The chamfer machining at a specific angle, shown in Figure 12.e, is possible when the rotation of the movable milling head around the Y axis is structurally provided.
Figures 13 and 14 show examples of holders and tools used on conventional milling machines.
The design and kinematics of the presented conventional vertical cantilever milling machine do not allow threading. Machining of holes is possible, but limited by the distances of the spindle tip from the work table. The technological capabilities of conventional milling machines in relation to modern CNC vertical milling machining centers are small. Despite this sequence, conventional milling machines are still effective in the field of basic surface machining and preparation of blank (workpiece).
Modernization and revitalization
An interesting possibility is the modernization or revitalization of a conventional machine tool. Thanks to this, it is possible not only to extend the service life of a given machine tool, but above all to improve the precision of positioning and machining. Revitalization includes a general renovation, often the modernization of guides, replacement of drives and a new control system. As part of the revitalization process, a conventional machine tool is transformed into an NC machine tool. An example of an upgrade is a conventional vertical milling machine equipped with digital readout rulers (Figures 15 and 16).
Figure 17 shows a conventional lathe, deeply modernized by SIEMENS (new drives, measuring rulers, CNC control). In 2016, at the CAMdivision stand at international fair MACH TOOL 2016 in Poznań, you could see how to modernize a lathe.
The English version was introduced on July 14, 2022.
- Paderewski K., Obrabiarki, WSiP 1993
- Erbel J. (red.), Encyklopedia technik wytwarzania stosowanych w przemyśle maszynowym, tom II. Obróbka skrawaniem. Montaż, OWPW 1993
- Author’s own notes.
- Wsparcie ZAiOS Instytut Technik Wytwarzania na Wydziale Inżynierii Produkcji, Politechnika Warszawska
- Kunstetter S., Narzędzia skrawające do metali. Konstrukcja, WNT 1973