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New Doosan horizontal HMC machines challenge conventional thinking

New Doosan horizontal HMC machines challenge conventional thinking

Added to MTDCNC by Mills CNC Ltd on 08 October 2012

Mills CNC, the exclusive distributor of Doosan machine tools in the UK and Ireland, has announced the launch of a new series of Doosan linear guideway (NHP) and box guideway (NHM) horizontal machining centres.

These new machines developed by Doosan, one of the world’s leading and largest machine tool builders and suppliers, represent an important step-change in horizontal machine tool design and construction, and demonstrate that a combined linear/box guideway technology platform, a global first incidentally, delivers improved performance for both types of machines.

The traditional view

Traditionally, certainly up to the early 1990’s, box guideways were the preferred route and the technology-of-choice for horizontal machine tool manufacturers, and their customers. Over the last 20 years or so, this situation has changed with machines, equipped with linear guideways, making their mark and becoming increasingly popular.

When comparing traditionally-built horizontal linear guideway machines with box guideway machines there are a number of differences.

Linear guideways are bolted to the machine's cast base and are then adjusted, as opposed to being machined, meaning that a single-piece base casting can be used. Box guideway horizontal machines by comparison have a two-piece base casting because it is not possible to grind box guideways 90° to each other.

In a traditional box guideway machine, the column moves in Z (towards and away from the front of the machine), with the spindle mounted on it moving in Y (vertically), while the pallet moves side to side, in X. These machines offer higher levels of rigidity than their linear guideway counterparts but they do have their limitations i.e. slower axis movements, slower pallet and tool-change times, less – effective chip control and removal (swarf had to come out the side of the machine) and, additionally, relatively poor operator visibility, as the column moves past the operator, obscuring his view.

With the single-piece, base casting linear guideway design, the column moves in X, the spindle mounted on it moves in Y, and the pallet now moves in Z (the pallet rotates in B, as before). The benefits linear guideway machines have over the box guideway versions include: faster axis travels; faster tool and pallet change; improved chip control, (due to splashguard designs); improved swarf handling, as it can now be taken out the rear of the machine; plus improved operator visibility.

“Linear guideway machines have a number of positive attributes but, on the downside they do not provide the same rigidity of box guideway machines. As a result linear guideway machines tend to be (and are perceived as being) light duty, high speed machines,” says Andrew Jack, Mills CNC’s Technical Director.

For many years, through the 90s and into the new millennium, the two designs existed side by side, with the box guideway machines being typically equipped with a 50 taper spindle and the linear guideway machines in many cases utilising a 40 taper spindle.

However with the advent of the roller type linear guideway, the demand for box guideway designs fell.

This meant that many machine tool builders ceased or scaled back production of box guideway machines below 800 mm pallet size, Andrew Jack says, but adds that Doosan continued with the manufacture of box guideway designs from the 500 mm pallet size and upwards, and became a niche market builder.

“Doosan is probably the global number one in the production of box guideway horizontal machining centres,” he suggests. “And last year, more than half the 70 horizontal machining centres/month produced were of box guideway design”, he continues.

Challenging conventional thinking

Doosan has now developed a series of horizontal machining centres that sees an improved box guideway design - which has not seen fundamental design change for 30 years - alongside a linear guideway series, with a drive for common parts applied to help reduce costs as well as benefitting production efficiency and delivery times.

The results are the NHM5000/6300/8000 box guideway machines and the NHP5500/6300/8000 linear guideway models (500, 630 and 800 mm pallets in each case), both with 50 taper spindles, although the NHM series have gearboxes, while the NHPs feature direct-drive, built-in spindles.

Single piece difference

The key design point here is that the NHM (box guideway) machines feature a T-shaped single-piece base casting, with the same axis movements of the linear guideway machine.

This feat has been achieved by employing an innovative saddle casting that sits on the single-piece base casting to support the side-to-side column traverse. The box guideways are an integral part of the casting in both X (saddle) and Z (base casting) axes, being induction hardened and then ground separately.

The linear guideway machines have no need of the saddle device, since there's no guideway machining, so their one-piece casting, a different one, has a stepped vertical height guideway system for the column. This stepped approach offers the possibility of a smaller, lighter column, and gives a larger width between the guideways (albeit in an inclined plane) but still delivers the benefit). “And that is the only difference between the two machine lines, everything else is the same and that is a big advantage,” Andrew Jack says.

And that simple statement that everything else is the same, means that for the box guideway machines, the performance in tool changing, pallet changing, chip control and swarf handling, plus operator visibility is improved, coming up to the linear guideway machine standard.

But in addition to those improvements, there is improved static and dynamic rigidity for the NHM versus previous models (helped by latest finite element analysis software). In addition rapid traverse rates are 32 m/min (36 m/min option), up from 24 m/min; there's a semi-floating column supported by an air cushion; spindle bearings, gearbox, motor flange and ballscrew nuts are all oil-cooled; and the tool and pallet changers are servo-controlled too.

“In creating both NHM and NHP from the same baseline and designing all models at the same time, the optimum technical specification was established, with the various parameters set by benchmarking key competitors. The result is a 'best in class specification', in terms of workpiece envelope, tool size capacity, spindle performance and operator convenience,” underlines Mills CNC's Technical Director.

The first UK NHM 6300 was ordered at the last MACH and has been purchased by a Mills customer operating in the oil/gas sector. The NHM range is in full production as are the NHP 6300 machines. The other two NHP models will be available in 2013.

Common parts

Parts that are common to both NHM and NHP series include: the automatic tool-change mechanism and the magazine; spindles (all NHM are common and all NHP are common but this commonality does not exist across both series); the pallet table, with 1° or full B-axis capability); splashguard; coolant tank; CNC functions.

Tool capacity calculation

The NHM/NHP machines can change a tool with up to a 320 mm diameter – and that’s good news for manufacturers machining large components.

Says Andrew Jack:

“If you machine an engine block, you have to machine faces in one pass; you can't have a blend line. So, by having a 320 mm tool diameter capacity, it means that a nominal 300 mm diameter face mill, fitted with 45° inserts, can be used. On competitors’ machines which, say have a 300 mm diameter tool capacity it simply wouldn't be possible. It’s that attention to detail that the designers at Doosan bring to machine tool development that enables them to create machines with the optimum, market-leading specification.”

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