Surface treatment systems for aerospace

Aircraft Engineering and Aerospace Technology

ISSN: 0002-2667

Article publication date: 1 October 1999




(1999), "Surface treatment systems for aerospace", Aircraft Engineering and Aerospace Technology, Vol. 71 No. 5.



Emerald Group Publishing Limited

Copyright © 1999, MCB UP Limited

Surface treatment systems for aerospace

Surface treatment systems for aerospace

Keywords: Surface treatment, Aerospace industry, Peening, Cleaning

Shot peening, abrasive blasting, surface activation, ice cleaning, names from an alchemist's lexicon? Far from being medieval "Black Arts" these are the technologies of the new millennium. Medieval alchemists may have failed to turn lead into gold; these technologies, however, are the unseen sciences that ensure that you and I fly in safety, protecting aircraft from metal fatigue and corrosion, not to mention removing unsightly tail logos. In order to expand and develop these technologies several surface preparation companies have come together to create the USF Surface Preparation group. Each of these companies has a wealth of global applications expertise within the aerospace industry. The group's combination of experience and innovative capability provides surface preparation solutions for a wide variety of aerospace applications. These include aircraft brake components, engine components, landing gears to the coating removal of complete aircraft fuselages.

The process known as shot peening involves the bombardment of a metal surface with a precisely controlled stream of spherical media, usually steel shot or glass or ceramic beads, plastically deforms and induces a residual compressive stress in the surface of the metal. This prolongs the fatigue life of metal components operating under cyclic stress by more than 100 per cent and inhibits stress corrosion cracking.

Ice cleaning is a blast cleaning system which employs water-ice pellets. Using this system, surface contamination can be removed in a single operation without damaging the surface or affecting existing coatings. The pellets quickly melt after blasting and no ice residue remains. The system is said to be especially suitable for gentle surface cleaning applications such as the removal of dust or rubber deposits from aircraft brake components.

State-of-the-art Vacu-Blast peener for Fiat Avio

The innovative capability of the group was recently demonstrated by one of its members, Vacu-Blast International, which recently designed and manufactured an advanced universal CNC shot peening system for the Fiat Avio aeroengine repair and maintenance facility in Turin, northern Italy. A second special Vacu-Blast machine is also due to be installed later this year at Fiat Avio's Brindisi plant.

Reported to be the state-of-the-art in aerospace shot peening, the new system can process the widest variety of parts from aeroengines of any manufacture, to improve or restore their resistance to fatigue. These include turbine discs, fan discs, compressor spools, blade aerofoils and roots, as well as transmission components such as gears and shafts, made from Inconel 718, steel and titanium. Parts processed include those for the "Eurofighter Typhoon", CF6-80 and GF-90 aeroengines. S110 and S170 steel shot will normally be used.

The machine has a 2.5m x> 2m x 2.5m peening enclosure, is fully programmable and operates automatically to the highest standards of precision - meeting the strict requirements of P11TF8 S5, AMS 2340 and the latest General Electric aerospace specifications. It incorporates two high-accuracy three-axis nozzle manipulators - each with vertical, horizontal and wrist axes -- which can operate independently or in tandem. The machine also has an indexing component turntable which moves automatically in and out of the enclosure. Additionally, there are special nozzle attachments for the high-precision processing of small holes, slots and internal bores down to 6mm in diameter. This provides a total of ten adjustable processing axes.

In-cycle shot-and-shape classification is included, to ensure that the highest peening integrity is maintained throughout each peening operation.

Other important features include: a Vacu-Blast "Waffle-Floor" pneumatic media recovery floor; a precision indexing component turntable mounted on a carriage and running on rails; an overhead hoist for loading/unloading; an external operator station for manual processing; and special acoustic construction to minimise external noise levels.

Vacu-Blast, has also supplied a "whole aircraft" plastic media stripping (PMS) facility to the Naval Aircraft Repair Organisation (NARO), Fleetlands, Portsmouth. This follows the proven operation of similar PMS facilities by the Royal Air Force at RAF St Athan in South Wales and RAF Kinloss in Scotland.

NARO undertakes work for all three UK services - the Royal Navy, the Army and the Royal Air Force - and the facility is used for the controlled stripping of paint and other surface coatings from complete helicopter fuselages, mainly from Sea King Lynx, Chinook and the new Merlin (EH101) aircraft, during regular repair, overhaul and re-paint operations. The facility is also used to strip coatings from critical mechanical parts prior to non-destructive testing.

The PMS process can be controlled closely to remove coatings selectively layer-by-layer, when required, and does not damage delicate substrates such as anodised surface. It also has inherent health and safety and environmental benefits, since no potentially hazardous chemicals or liquid effluent are involved. Housed in a new surface finishing complex at the Fleetlands site, the PMS sure measures 23m long x 7 m wide x 8.5 m high and incorporates "Waffle-Floor" pneumatic blast media recovery system, a Vacu-Blast invention.

The Waffle-floor system utilises a constant down draught of air which continuously directs used blast media, dust and debris through a heavy duty floor grille into a series of under-floor "mini-hoppers". These are connected to a common duct through which the material is conveyed pneumatically to a generator-reclaimer. This automatically recycles reusable blast media to the blast nozzle, thereby minimising media consumption. Degraded material and debris are directed into the dust collector for safe disposal as dry waste.

Because there are no moving parts, the system is believed to be virtually maintenance-free and the floor design removes the need for pre-loading with non-recoverable blast media. The under-floor hoppers and ductwork are automatically purged free of blast media, which allows the media grade to be changed easily. The constant down draught of air has the advantage of enhancing operator visibility.

In the case of the Fleetlands facility, the Waffle-Floor grid sections and connecting ductwork are incorporated into the existing concrete floor of the hangar and can withstand a load of 5,000kg, transmitted through an aircraft jack with an 8in. diameter footprint.

Other features of the NARO facility include high-integrity media/debris separation to ensure that only good quality plastic media is recycled and process consistency maintained, in-process monitoring of media flow, and the use of acoustic construction techniques to minimise noise levels. The enclosure incorporates heating and cooling for the comfort of operators, and all processing parameters are controllable from an external console situated by an observation window.

Although the Vacu-Blast facility uses high-quality thermoplastic acrylic abrasive media, the design of the blast generator and recovery system allows for the possible future use of an alternative starch-based media which has different flow characteristics.

Advanced surface treatment machines

The depth of Vacu-Blast's experience was called on with the installation of state-of-the-art controlled abrasive blasting and precision shot peening machines which have recently been installed by Rolls-Royce plc for surface treatment operation on a variety of critical engine components at two of its UK manufacturing facilities. The machines are reported to have substantially increased surface processing capacity.

The total value of the order, one of Vacu-Blast's largest in the aerospace industry, was in the region of £1 million sterling.

The equipment includes two of Vacu-Blast's latest "Vector" automated cabinet machines. One is a fully programmable unit, controlled by a PLC, which incorporates a 14-nozzle array and is used for the high-throughput surface preparation of turbine blade aerofoils with aluminium oxide, prior to coating. The other Vector automatic machine is employed for the surface preparation of selected parts, including seal areas components with heat-resistant coatings, before plasma spraying. This machine operates under Vacu-Blast's advanced "Vacutrol" Windows-based process control system.

A specially engineered Vacu-Blast programmable steel shot peener for the fatigue life, improvement of turbine blade roots has also been installed, to provide additional capacity over a machine supplied by the company in the 1980s.

All the machines used in turbine blade work include special Vacu-Blast polyurethane tooling to allow blades of widely differing sizes to be processed.

In addition, five Vacu-Blast "Ventus AXT 150" multi-nozzle automatic cabinet machines for glass bead peening and aluminium oxide surface preparation of machined parts have been introduced into a number of manufacturing cells. A special "Ventus 150" manual cabinet machine has also been installed. This features a tilting work table to allow improved access for the surface preparation of selected seal areas. Finally, four smaller "Ventus 92PR" cabinet machines are being used for the general manual processing of small components.

MP 1500 Ti

News from another company in the group, USF Wheelabrator Sisson Lehmann, is that the latest version of its MP range, the MP 1500 Ti, has successfully completed its final qualification tests. Recently developed for shot peening and surface preparation of aerospace components, the MP 1500 Ti is said to be an accurate multi-axis machine including the latest technical improvements in the field of self-regulation and process control.

The basic machine includes an accurate turntable mounted on a revolving arm, allowing the loading of the components outside the blast chamber and a four-axis robot (patented). The table accepts components of maximum diameter 1,500mm, 1,000mm high, and a maximum load of 500kg. The four axis robot fitted on the roof of the machine allows moving and accurate positioning of the blast nozzle with a horizontal stroke of 1,500mm, a vertical stroke of 1,000mm, and a rotating wrist equipped at its bottom end with a tilting axis (þ 135ú).

Various options are available as: second nozzle manipulator (3/4 axis) on top or side wall; table with four or 20 numerical satellites for fan blades and airfoil batch treatment; bores and slots including ID lance spindle; hydraulic vertical sliding door.

The large blast chamber is fitted with doors for easy access and programming with a pitless pneumatical recovery. The standard shot recycling system includes dedicated devices for size and shape calibration of two kinds of shot sizes in accordance with SAE and Mll, standards. Other classification systems for glass beads, ceramic beads, aluminium oxide or mixtures are available. The control of the shot projection is by a G7 automatic refilling pressure pot with shot flow end air pressure regulation.

The machine is operated through a multi-axis interpolated CNC programmable in ISO language and "Teach in" mode. The machine is supplied with a PC, printer and USE Wheelabrator Sisson Lehmann supervision software including the following facilities: maintenance assistance with real time flow-charts equipment and process control procedure; storage/transfer of part programs with graphic identification facilities; dynamics recording and storage of quality reports for each part processed.

The two machines have been delivered to Super Precision, Taiwan and Xian, China. The third machine is destined for MTU, Munich. It is equipped with a second wall side robot, two spindles BR 2/10 and a four-shot classification system.

MP 1250 airblast peener

The company has also delivered an automatic shot peener to General Electric for its aero-engine maintenance facility at Cincinnati. This advanced technology CNC peening system is used to enhance the operation life and resistance to fatigue cracking of turbine discs, fan discs, compressor spools and shaft from General Electric's engines series.

The peener features a twin straight and ID lance nozzle system operated by a seven interpolated axis robot for processing external surfaces, bores, slots and cavities, without any set up. Components up to 250kg are loaded onto a CNC-controlled turntable with horizontal movement, continuous and variable speed rotation or indexing positioning. The very high accuracy of the system guaranties the repeatability of all linear movement within a tolerance of 0.02mm, and similarly, 0.5 minutes of angle for all spinning movement.

The plant's process control system constantly monitors air pressure and shot flow as per the programmed setting, as well as parts and axis movement. The PC controller supervises and registers all the process control records for each component peened.

Shot is supplied from 2 off G7 type continuously cycling dual pressure vessels. It is fed from one or two hoppers, one for each of the two shot size available, depending on the required peening intensity. Change over from one sizes to the other is automatic and safety features are included to prevent cross-contamination.

The quality of the shot is constantly controlled and worn media is automatically removed by means of full in-cycle shot size-shape classification. The peening enclosure has a pneumatic recovery system and full flow reclaim.

Two shot peening machines are also being supplied to Messier Dowty, Bidos for treatment operations on a variety of critical landing gear components for civil and military aircraft. The components having a maximum length of 3m, width of 1.8m, total load 1 ton.

The combined installation consisted of one CNC controlled five high efficiency wheelblast machine for OD peening treatment and one CNC controlled two-axis airblast machine. The latter is equipped with one continuously-cycling dual controlled pressure vessel type 7V7 for ID peening treatment.

Both machines are fed by means of one single monorail conveyor and equipped with the suitable tooling to locate the parts during the process.

The quality of the shot is constantly controlled by a common recovery and recycling system where the worn media is removed by means of full in-cycle shot flow monitoring and size-and-shape classification, one automatic refilling device is available to compensate the shot consumption.

The machines are operated each through a NUM 1060 CNC and the process is controlled with a PC printer and supervision software.

The two plants' process control systems measure and constantly monitor shot flow against the program setting as well as speed of projection, air pressure and parts and axis movement for total quality insurance.

Starch media dry stripping

USF Schlick, another member of the group, is well-known throughout the aircraft industry for its delivery of numerous dry-stripping systems. According to Schlick, this modern technology continues to replace chemical stripping systems, which they believe are, for the most part, more expensive and more harmful to the environment.

A SMDS (starch media dry stripping) unit operating with starch media, which is thought to be environmentally friendly, was recently put into service in Australia.

Designed by Schlick to strip paint from aircraft components, this pressure blasting unit can be used to treat aluminium parts. Although the unit was designed for fully automatic operation, blasting can also be performed manually.

All of the necessary components that make up the system have been fine-tuned to work together and to guarantee compliance with the required specifications.

Fully automatic stripping of workplaces is carried out with a specially developed flat blast nozzle that is guided by a process-controlled six-axis robot. The robot was installed hanging in a head first position from the centre of an overhead linear axis in an acoustically insulated blasting hall (W x L x H: 4 x 8 x 4).

The linear axis gradually approaches the workplaces and blasts their surfaces. The starch abrasive used in the blasting process is collected in a closed-loop processing system. This system employs a time-tested scraper conveyor and a back extraction channel to feed the recovered abrasive to an elaborate cleaning station.

The abrasive passes through a cyclone separator, a magnetic separator, a large-mesh screening unit, and an additional high-performance, B-stage cascaded air separator. Two integrated de-ionization units dissipate static charges.

The recovered starch abrasive is subsequently fed to a storage silo. A specially designed, double-chambered pressure pot makes continuous blasting possible. The silo has been equipped with large outlet openings in order to prevent bridging of the abrasive and to ensure complete emptying of the storage silo.

A belt-type dosing device guarantees a uniform, repeatable flow of blast abrasive.

The SMDS unit is controlled from a menu-driven operator panel. All of the required blasting parameters can be monitored from this panel.

During the engineering and design phases of the SMDS unit, a modular construction concept was developed that takes into account the aircraft industry's need for an easy to expand unit that offers many different design variations.

Blasting systems of this kind are suitable for SMDS stripping cabins and rooms for aircraft components and for fully automatic or manual operation.

Further details of the USF Surface Preparation Group are as follows:

  • Abrasive Developments. Tel: +44 (0) 1564 79 2231; Fax: +44 (0) 1564 79 5479; E-mail:

  • Pangborn Europe. Tel: +39 (0) 2 9639951; Fax: +39 (0) 2 9656741; E-mail:

  • Schlick (Greven). Tel: +49 (0) 25 75 31 0; Fax: +49 (0) 25 75 31 150; E-mail:

  • Schlick (Metelen). Tel: +49 (0) 25 56 88 0; Fax: +49 (0) 25 56 88 150; E-mail:

  • Spencer Halstead. Tel: +44 (0) 124 27 6303; Fax: +44 (0) 124 27 7829; E-mail:

  • Vacu-Blast International. Tel: +44 (0) 1753 526511; Fax: +44 (0) 1753 538093; E-mail:

  • Wheelabrator Berger. Tel: +49 (0) 221 8394 0; Fax: +49 (0) 221 8394444; E-mail:

  • Wheelabrator Sisson Lehmann. Tel: +33 (0) 3 24 336300; Fax: +33 (0) 3 24 37 39 37; E-mail:

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