The high-sized milling machines currently on the market process steel, aluminum, resins and composite materials without distinction and with top performances and excellent finishing accuracy.

Energy and heavy-industry, big industrial plants and ship-engineering,

earthmoving machines and railways, as well as aerospace, and mould-making: all fields with complex processing of high-sized and heavy mechanical components requiring the use of high-dimensioned, powerful, stiff, accurate milling machines, with very short cycle timing.

The high-sized milling machines currently on the market process steel, aluminum, resins and composite materials without distinction and with top performances and excellent finishing accuracy.

The choice of the most convenient milling machine is connected to the kind of removal to be performed, to the power of the machine itself, to the work piece morphology that, as explained below, determines the most suitable architecture.

Structure stiffness is very important because the machine’s high removal capacity, output, and processing accuracy depend on it. Some manufacturers offer cast iron crankcases, known for their rigidity, some others choose welded steel crankcases which, less stiff but lighter, facilitate movement speed; some others offer greater configuration variety with dimensions calculated through structural design software, according to the specific steel properties. The result is as follows: a mix of rigidity, inalterability, strength following the specific customer’s needs.

 

Between demand and supply

Milling high-sized work pieces involves many different factors apparently impossible to combine. Significant extension of work volumes (particularly the vertical stroke, the ram strokes, the spindle and the rotating sliding tables strokes); high powers and torques; accuracy constancy on the entire work volume throughout the whole processing time; considerable  weight of the details  on the rotating sliding tables; components’ complete processing in a single setup; machine reliability; wide range of devices and accessories for special materials processing; tools change and head change automation .

What is more, the main milling machines manufacturers provide “dedicated” solutions for fittings, morphologies and automatisms that are functional for the work piece size and the different features of its material.

There are four architectures on the market:  the horizontal movable column, the most common in Italy, with conventional sliding or hydrostatic techniques; and three vertical, in the order, with fixed portal and movable table, with movable portal and fixed table, or gantry with high portal moving on columns.

Concerning the aerospace industry for example, where the work piece has different gripping and clamping needs, the portal configuration is suggested; for shipbuilding and energy the solution with movable columns is generally used.

And again, thanks to its vertical ram – not affected by its own weight – and its general structural symmetry, the portal architecture would be more accurate than the movable one which, however, is more flexible, easy to automate, with good dynamic and better access to the work piece; and what is more, with the same work volume it results to be less expensive.

Then there is the “gantry” configuration: high-performance machine with fixed table and mobile portal, designed in order to process heavy and high-sized pieces in a single set-up, without moving the work piece. The result is as follows: significant space saving, albeit with very significant longitudinal strokes; better accuracy and less cycle time with reduction of handling costs.

The choice must be done according to the end-user’s operating need: a subcontractor, with variously different part lots, needs more flexibility, but if the end-user makes always one single type of work piece, he may need a “dedicated solution.

 

Wide range solutions

Multitasking configuration machines able to perform boring and milling, turning and grinding operations without repositioning the work piece during the two phases, are suggested for high-sized work pieces processing with the least number of set-ups. Many of the already installed machines are equipped, for example, with automatic heads, various types of sleeves or spindles for reaming and boring. Generally, this involves less processing times, quality and accuracy increase, no human mistakes, reduction of production costs and machine tooling.

Ending up, a high-sized milling machine must have static stiffness and structural dynamic, adequate for the spindle power and always constant throughout the work volume; it must have systems for dynamic compensation of the volumetric-type axis (interaction between the mappings of the single axis generated throughout the whole work volume); it must achieve dynamic performances, in terms of axis’ high speed and accelerations, able to fully exploit the latest generation tools characteristics the machine is equipped with. It must also meet cycle time and dimensional precision, now guaranteed by the accuracy of mechanical components that meet high tolerances; it must have laser and electronic systems that check and test and, where necessary, that give to the machine’s software the necessary corrections in real time.

80% of corrections come from the temperature: the one produced by the tool friction on the work piece during processing and the one transmitted in the opposite direction, as in the case of components for the nuclear energy sector, made of very strong materials. The solution lies in the coolers’ specificity, in the minimum lubrication (air /oil) at controlled and constant temperature through appropriate probes, as well as in the machines’ particular structure, which ensures the symmetry of its own primary components’ thermal expansion.

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The available architectures

Horizontal movable column

Vertical with fixed portal and movable table

Vertical with movable portal and fixed table

Gantry

 

Electronics

Given that numerical control is the most important element of all machine tools, it becomes even more essential for those that are devoted to big components processing where the risk of imprecision is proportional to the size of the piece itself. In this case the automation level is the ability to correct the processing in real time and keep it until the end of the process, through compensation systems of the variables in the working geometries.