What is the difference between a cutter, a plotter and an engraving machine? Different people use these terms interchangeably, frequently to the same machine. Indeed, the tasks of these machines often overlap, but this is not the rule. We usually define a milling machine as a heavy cast iron machine with a coordinate table and a spindle speed range from several dozen to several thousand revolutions per minute, which allows milling with milling heads of considerable size. The torques achieved through the gearing (usually toothed) are several hundred newton metres (N-m). These gear units are one of the reasons for the relatively low speed of maximum milling spindles. Milling machines and milling machining centers are used primarily for milling with multi flute heads with diameters up to 300 mm, milling with finger type milling cutters with diameters: 3-50mm, drilling with drills with diameters: 3-40mm, boring, dredging. They are used mainly for rough and semi accurate processing of bodies, moulds, dies, die boards, matrices.
A milling-engraving machine is a lightweight milling machine with spindle speeds reaching 24,000 or 40,000 rpm. For engraving works there is a need to use tools with much smaller diameters (from 0.05mm) and which require much higher spindle speeds. A conventional spindle motor-spindle system with gear is not used here as a spindle, but there are electro-spindles integrated in one housing and there is a spindle and a drive motor, usually inductive one, on one axis. The advantage of this solution is smooth and quiet operation at high speeds and the lack of brushes, and the disadvantage is the relatively low torque, which is due to lack of gears. Therefore, the diameters of the tools used in milling-engraving machines do not exceed 16mm in metals and 100mm in other materials. The solution to the disadvantages associated with low torque of the electro-spindle is to use a considerable excess of its rated power (at least 7kW with the cutter 10 mm in steel), which may often cause a potential buyer's amazement. Still the matter must be thoroughly explained.
Induction electro-spindles, in addition to many undeniable advantages, have one serious drawback - a constant torque. If we need to reduce speed (e.g. for a large head) in spindles with gear, we change a gear ratio to a lower one, which simultaneously increases the torque. In inductive electro-spindles the rotation is regulated by the inverter. The u/f characteristics of the engine shows that torque is almost independent of the rotation (there are inverters which allow to boost torque at low speeds, but not more than 200% of the nominal value). Therefore, the electro-spindle with the value of 7kW on its identification plate may achieve this power at a certain speed (not necessarily maximum speed noted on the plate) which is called the rated speed. Let us say that our rated speed is 18.000 rpm. The power is torque times speed, which means that if you set the speed to 9000 rpm, the power which is available equals 3.5kW, and at 1800rpm the power is only 700W.
However, comforting is the fact that during operation the average power consumed by the sample electro-spindle does not exceed a few hundred watts. Because high revolutions which allow giving high power are used only for tools with small diameters, they are not able to get such a power, whereas at low speeds and large tools the power which is reached by the spindle is small, which results in low power consumption. Summarizing the above observations, you should always choose the electro-spindle with a rated power much greater than it results from the calculated power demand for a specific tool.
Milling-engraving machines which are equipped with electro-spindles are increasingly applicable in the production of injection moulds, dies, die boards, matrices, punches, copper and graphite electrodes, thanks to the availability of a wide range of tools made of monolithic sintered carbide. These tools can machined at speeds many times greater than existing HSS tools, which combined with the possibility of obtaining high speeds on machines equipped with inductive electro-spindles, becomes the ideal tool for many applications in each workshop dealing with machining.
Computer controlled engraving machines are even lighter than milling-engraving machines. They are used in particular to cope with very small tools, usually with conical geometry of the footer with a diameter of 0.02mm. Due to the very small diameter of the working part of the engraving tool the power of the spindle does not need to be high (up to 2.2kW). But stability, stiffness and the resolution of positioning and the properties of dynamic control system should be at the highest level. In this case the resolution of positioning is absolutely important because engraving involves works which are not visible with the naked eye. The smallest unitary increase of the position, which is called the resolution of positioning, should not exceed 0.001mm with the backlash of the entire drive system not exceeding 0.005mm. Otherwise, you will see the "teeth" which often disqualify the possibility of using such a machine for more precise works.
In computer controlled engraving, there is also a function of the control system which is rather unavailable in other machines, but which is indispensable at work. It refers to the possibility of automatic cutting corners with an engraving cutter. This is of particular importance if we apply raised lettering which appears to be 'raised out' of the material, produced by the background surface being removed leaving the lettering that protrudes above the surface. If there is no such a function, each sharp internal corner will be cut only on the bottom, while the arc of the tool radius on the peak height of the sign will not be cut on the surface.
Milling plotters often called routing tables probably owes its name to the first constructions that were built on ordinary tables. But that is history. Today milling plotters are modern and multi-urpose CNC machines which are most relevant in the advertising industry, for cutting, drilling, engraving plates in plastic, aluminum or composites, for cutting foil, cardboard or paper. In 3D versions plotters are used for modelling reliefs, making spatial mockups and three-dimensional presentations. These are the cheapest of computer-controlled machine tools on the market. They are produced mostly in the version with a mobile gateway, which is optimal in relation to the use of the surface with relatively large working areas (up to 3 x 8m). Milling plotters tables are usually covered with T-slots, which gives great versatility either by underpressure or vacuum system to keep the material, which reduces versatility but greatly facilitates the attachment of large and thin boards. There is also a hybrid version, a table covered with T-slots additionally equipped with underpressure system, depending on the treatment we can use one or the other mounting system. In milling plotters the 0.8k - 18 kW spindle is used. Optionally, you can equip them with basing system, a laser or touch scanner, the axis of rotation, additional cutting, drawing or pouring heads, anglular aggregates for cutters and circular saws, etc.
Light gantry milling machines are devices similar to milling plotters, but are much more rigid, heavier and more accurate. They are primarily used in industrial environments, or where there is a need for processing large areas, with large tools and with high efficiency. The capacities of spindles installed in light gantry milling machines begin with 7 kW.
