QMT Features: January 2011
Adding quality
Multisensor vision metrology speeds precision processes at Selective Laser Melting (SLM)  machine manufacturer

MTT Technologies Group, a designer and manufacturer of SLM (selective laser melting) machines for additive manufacturing applications, is a major player in SLM technology. MTT has a turnover in the region of €15-20 million generated by 80 employees at locations across the globe.

SLM is a pioneering, additive manufacturing process capable of producing fully dense metal parts direct from 3D CAD models using a high power fibre laser. Parts are built from a range of fine metal powders (including cobalt chrome, stainless steel, tool steel, titanium, Inconel and aluminium) which are fully melted in a controlled atmosphere, in layers as thin as 0.02mm.

The laser is at the centre of the SLM machine’s operation. From source, it is directed through a series of optical components before entering the X-Y scanning head, which is used to direct the laser. The X-Y head features two highly responsive scanning mirrors capable of translation/positioning speeds of 7000mm/sec, so even the smallest of errors can have a negative effect.

The laser has a focal distance of around 400mm to the object being manufactured, but with machine build tolerances in the region of ±1mm, a reliable method of compensation is required to guarantee accuracy for SLM machine users.
Until recently, the company used manual methods or opted to subcontract the task of calibrating the range of SLM machines it builds at its headquarters in Stone, Staffordshire. Now, MMT use a  SmartScope ZIP Lite co-ordinate measuring machine (CMM) supplied by OGP UK.

“An important function of our ZIP Lite is to scan a standard grid produced by the SLM and to measure various points to check for variations in X-Y,” explains MTT’s group marketing manager, Robin Weston. “From the results we can see precisely how much compensation we need to apply to each individual machine. This is a process we can iterate many times if required, ultimately achieving accuracy down to 0.03mm and beyond. Put very simply, without the ZIP Lite, we couldn’t make machines this accurate.”

Fair and square
The OGP machine at MTT also allows the company to ensure its laser is square and centred perfectly on the build plate. A 250 x 250mm build plate is used on MTT’s standard SLM250 model, and each newly constructed machine is asked to create a square at the extremes of the build area. The ZIP Lite measures the difference at each outside edge and the straightness of each axis, allowing MTT to introduce compensation that guarantees a centrally located build at exactly the correct size. Compensation is then applied in a system calibration file.

With 90% of the company’s SLM machines destined for export markets such as Germany, France, Italy and the USA, MTT says the past couple of months have seen a marked rise in demand as the global economy strengthens. Sectors such as dental, medical/orthopaedic and aerospace are all exploiting the many benefits afforded by SLM, such as the creation of very complex geometries that would prove almost impossible using conventional subtractive machining techniques. Using scanning speeds of up to 2000mm per second and build rates of up to 20cm³ per hour, parts are produced in only a few hours. Furthermore, multiple parts can be constructed simultaneously on the same build plate.

A matter of fact
Another use for the ZIP Lite at MTT is the production of artefacts. While the SLM process is ideal for complex, lightweight parts, the inherent nature of SLM means parts exhibiting substantial cross-sections can sometimes demonstrate minor levels of distortion or shrinkage.  However, using the ZIP Lite to measure the first-off means MTT can identify any problem areas, feed this information back to the CAD model and subsequently produce a perfect artefact.
Installed in the summer of 2010, the precision magnification of the ZIP Lite’s lens is also used by MTT to examine the surface roughness of SLM-produced components, as well as material density.

“The magnification is terrific and we can easily identify any inclusions or flaws,” says Mr Weston. “It helps analyse the process and see how the weld pool is behaving. We’ve also started to use the machine for a limited amount of critical goods-in checks.”

A total of four MTT employees are trained to use the company’s ZIP Lite following two days’ training on site at the 22,500 sq ft Stone facility – each uses the machine for a different function. With MeasureMind 3D software, fully automatic measurement routines can be programmed in minutes, while data outputs can be customised to meet personal reporting formats or used by analysis packages to compute 3D surface profiles, part-to-CAD comparisons, SPC or for reverse engineering.

“The training provided was very comprehensive, which in combination with excellent technical output on the OGP website has provided the foundation for a successful and rewarding experience,” says Robin Weston. “Ultimately the machine has given us a sense of awareness, that if we are going to improve, we have to measure what we’ve done. It’s proven to be a very objective way of boosting business performance.”l
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