Tall tale of record-breaking Chinese CMM

An ITP product story

Edited by the Engineeringtalk editorial team Jun 20, 2006

A 7.3m-high co-ordinate measuring machine built for Beijing Space Authority to measure satellites may be the tallest CMM in the world.

Rugby-based ITP Group believes that the 7.3m-high co-ordinate measuring machine (CMM) it has built for Beijing Space Authority to measure satellites may be the tallest CMM in the world.

The company makes large scale CMMs for the motor industry, with truck makers among its regular customers.

But, says founder and Managing Director Dick Turpin, it has also built up a business in making bespoke machines for sectors such as nuclear power and Formula One.

"Three years ago, we opened a small office in China, with the main purpose being to buy granite for our machines, but also to sell into China", said Turpin.

ITP was approached by the space group to build a single CMM to measure small commercial satellites.

Having delivered one machine in 2003, it was contracted last year to build a bigger one, with the possibility of two more being ordered later.

"The satellites are 6m vertical and 3m square", said Turpin.

The machines can measure all faces of the satellites with an accuracy of 0.15mm volumetric.

The machine design has a measuring/machining capacity of 5m in the x-axis, 6m in the z-axis and 1.6m in the y-axis.

The machine is constructed from high-quality stress-relieved fabrications and uses the highest accuracy Schneeberger linear slides.

The machine is controlled through a machine-tool controller, specially adapted to also allow measurement and light machining operations such as fly cutting of aluminium surfaces and drilling of materials such as aluminium and carbon fibre.

The machine is equipped with a two axis manually controlled indexable head for machining and a two axis CNC controlled indexable head for measurement.

The software for measurement is ITP Group's standard 32bit three-axis measurement software, with modules for geometric and surface measurement.

The machining software uses standard ISO GMT codes.

The machine can either be used manually via a joystick, or under full CNC control for both measurement and machining operations.

Entire satellite constructions can be mounted on the 2m diameter CNC controlled rotary table, which is used for indexing to the correct radial position prior to measurement and machining.

Machine drives are spiral rack and pinion for the x-axis and friction drives for the y- and z-axes.

Counterbalance of the z-axis is achieved mechanically and the y-axis includes a unique counterbalance system to compensate for any deflection of the arm.

The linear three-axis position is given from Renishaw RG2 linear scales, having a resolution of 0.001mm.

The machine accuracy falls within 0.15mm volumetric.

To accommodate the new machine inside its factory, ITP has had to dig a pit in its factory floor.

It has also changed the fundamental design of the machine: where standard large-scale CMMs have a bridge structure, ITP's machines mostly have a horizontal arm extending from a column.

But for this huge machine, Turpin has designed one in which the "column" is a structure composed of angle plates with the arm coming through the middle.

This, said Turpin, gives it "extra rigidity" and enables the tolerances to be achieved at full extension.

A tapering design on the cross-arm also adds to accuracy.

The CMM has a Renishaw touch probe as standard and is also fitted with an indexable milling insert for light machining.

Turpin says the combination of measuring and milling has been successful in recent years, especially in milling clay car-body models in prototyping for new vehicles.

And it offers milling to many customers, with NC controls built into the CMM operating system.

Building CMMs for Chinese satellite makers might seem like a bit of a niche, but these large scale multifunction CMMs are suitable for any automotive, aerospace, or nautical application where traditional machine tools are too cumbersome or inaccurate.