3D off-line programming speeds complex machining

Complex and accurate CNC machining of a variety of features requires a fast off-line programming system, which is why Strachan and Henshaw chose LICOM's AlphaCAM.

Complex and accurate CNC machining of a variety of features in tough, often difficult-to-machine alloyed stainless steels is not easy work.

Programming is complex, and a system which is easy to use and provides ready access to a wide-ranging, updatable methods and materials database, is essential.

These are the reasons why Strachan and Henshaw (S and H) chose LICOM's AlphaCAM for the off-line programming of its CNC machine tools.

A member of the Weir Group of companies, S and H is currently engaged in a mix of civil and military contracts.

On the civil side, the company manufactures specialised, all-stainless steel, computer-controlled cranes, and nuclear waste handling equipment for British Nuclear Fuels Ltd.

(BNFL).

On the military side, weapons handling equipment is being produced, typically for the new Astute class of nuclear submarines.

All this work involves CNC 3D-milling, drilling, tapping and other operations in often very tough castings and forgings.

Chief programmer at S and H, Roger Duffner, says that before AlphaCAM was installed, a 2D system had replaced the very lengthy hand-written programming procedures.

Following a number of years of reliance on the 2D programming system, the decision was taken to acquire a 3D programming replacement.

"We narrowed the choice down to six suppliers," says Roger Duffner.

"We invited their sales engineers to come into the factory and gave each one the same job to program.

LICOM's engineer went straight into the work and produced the program very quickly.

He did this without any fuss or bother.

We were very impressed.

Time saved.

The programming team at S and H key in all data directly from the engineering drawings.

When using the 2D system, constant changes had to be made from one view to another.

"With LICOM's AlphaCAM, there is no longer any time wasted switching and waiting between views, for zero-ing and so on.

We can carry out full 3D simulation and check for collisions.

For example, part of a weapons handling system is a forged nickel-aluminium-bronze valve plate approximately 500mm square and 100mm thick.

Features, which have to be milled and drilled, include an oval through aperture with a raised sealing profile and location and bolt holes.

Some of the holes are countersunk and counterbored, chamfered and have raised sealing lips.

Other milled features include shoulders and bosses.

The valve plate is machined in two set-ups on CNC millers, one having an angle head.

Altogether, some 40% of stock material is removed.

The problem with the 2D system was that after a cutter change, it was too easy for the programmer to accidentally remove small bosses or 'nick' features in the valve plate, especially if the new cutter was of a larger diameter.

"Whereas on the LICOM system, the programmer can 'zoom-in' on details and check that no collision has occurred," said Roger Duffner.

Total programming time on the valve plate is a good four days.

The time would have been doubled using the earlier 2D system.

It took only two days' training with LICOM for S and H's programmers to operate the system.

"We found the LICOM system very easy to use, though naturally, our previous programming experience did help," said Roger Duffner.

The programming office is operating LICOM on two seats based on Hewlett Packard workstations linked into the CAD seats on an open net.

The machine tools are not DNC connected.

Instead, programs are downloaded onto 3.1/2in.

floppy discs.

The discs are loaded to a FACET disc reader on the shopfloor.

The FACET is linked into the CNC machine tools via RS232 interfacing.

The operator loads the disc and copies it directly into a machine tool CNC.

Among the machines at S and H are a large Asquith gantry borer with an 8m X 4m bed and is equipped with Heidenhain 426 CNC, as is a Correa horizontal machining centre.

There are also two Pollard Mori Seiki horizontal machining centres and an AB Marwin (Asquith and Butler) 3000mm long bed machining centre.

Most of the turning is performed in batches on 'teach-in' NC lathes.

Further examples of work performed at S and H include handling equipment for torpedo tubes.

Typically, the outboard length, a massive three-piece, welded forging assembly in QIN steel, has wear features MIG weld-cladded with Inconel 625.

There are a variety of work planes and some 3D geometrical features to be machined in a top face.

Again, the ability to switch easily between views saves time on a complex workpiece like this, said Roger Duffner.

Helicopter landing grids.

On the AB gantry borer, S and H produces helicopter landing grids.

A typical grid has a top and bottom plate divided by turned spacers.

The material specification depends on the environmental location.

For example, under Arctic conditions, Aubert and Duval Marval X12 precipitation-hardening stainless steel is specified.

For tropical climates, APX 4W through-hardened martensitic stainless steel provides a tough machining proposition, owing to the presence of 'hard spots.

Each grid plate is 2750mm diameter X 30mm thick.

Each plate has to be skimmed both sides and machined on the circumference.

The difficult part is the machining of some 3000 holes, some of which have to be drilled and tapped and counter-bored with a developed radii to produce a specified location profile.

This feature allows the helicopter's locating arm to locate and clamp the helicopter in position.

So in a top plate, the requirement can be, typically, 339 51mm diameter, M12 tapped holes and 1303 51mm diameter holes with the developed counterbore profile.

Step drills are used for combined drilling and countersinking.

Interpolation cutting produces the counterbore profile.

Rough machining and drilling time is around 8h.

Total machining time per grid plate is about 16h.

When finished, some 50% or more of the plate stock will have been machined away.

"The LICOM system came into its own on these jobs," said Roger Duffner.

"It took about a week to program each grid and many engineering modifications were requested during programming and editing.

It was not just the ease of programming that attracted S and H's engineers to LICOM AlphaCAM.

"The maintenance contract was also very competitive.

For about GBP1000 the contract includes two software updates/year as well as 24h support fort rouble-shooting," said Roger Duffner.

"We get immediate attention for queries such as 'how do we program this'.

In all, it is a cheap system to run, is reliable, and we are very impressed.

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