TechArchive

This article was originally written in the period 1995-2000

Aluminium framworks for machine construction systems

The quality differences in the aluminium extrusions used for constructing kit-form aluminium frameworks are subtle and become more noticeable as you move down the field. So says Machine Building Systems Managing Director Stuart Mitchell, whose position in this respect is relatively comfortable as sole distributor of the top-flight German Item range. Item in fact aims for half-DIN tolerances on parameters such as straightness and works closely with extruders to achieve it. Running the extrusion presses faster makes the extrusions considerably cheaper, but the tolerances will slip. The significance of this will vary, says Mitchell, depending on whether the objective is to build a simple guard frame or a critical struct in a precision linear slide!

Anders Pihl of Flexlink Systems highlights three factors which determine quality: the aluminium alloy composition, the surface treatment, normally anodising (see later), and the production and cutting tolerances of the extrusion.

Michael Etherington of Alusett, which supplies the Kanya modular assembly system, says that there are differences in the aluminium alloy. It is important to consider the tensile strength for the purpose required. The best alloy, he says, is one known as F25, which is an aluminium-magnesium-silicon alloy.

Nigel Clarke of Boldman Paletti has researched German suppliers and found that most use the same alloy, but most suppliers have their own post-extrusion processes, such as ageing and straightening techniques which are specific to them. Paletti is no exception and has also set up rigorous inspection and quality control procedures. The alloy used is also F25 and the profile is produced using the Strangpress procedure (DIN 17615). Tolerances are held within 0.01mm. Nigel Clarke also highlights the range of standard profiles and the accessories that bind the system together as a major differentiator between suppliers.

Flexlink uses alloy AA 6063-T6 and precision extrusion production is also in accordance with DIN 17615. Beams cut to length will be within +/-0.30 to +/-1.0mm, depending on the length. It is important to remember that the production tolerances must be valid throughout the total economical production time of the extrusion tool – a point to confirm with suppliers.

Digiplan’s Colin Ford doesn’t see major differences in the quality of extrusions of different manufacturers. He sees the main differences in the shape and dimensions of profile sections. Some suppliers tock a wider range of profiles, but of course this implies higher total stocking levels for equivalent availability. Jeff Blackman of Bosch agrees, though he points out that in a few cases the extrusions may be without anodising or of a different alloy.

Interchangeability

Most suppliers agree that there is only limited interchangeability between components from different manufacturers, but do seem divided as to exactly how much compatibility can be achieved. In a sense of course, it is easy to fasten anything to a system, so it is also easy to fasten systems to each other – within reason. On balance, while bemoaning the lack of “open systems standards” in this area, it is probably easier to stay within a single system and to work with components which are designed to fit together. This means that it is very important to choose a supplier most appropriate to both the current application and with future needs in mind.

However, Nigel Clarke of Boldman Paletti explains that the Paletti profile system is designed to be totally interchangeable with the Item range, seen by Paletti as the undisputed market leader. The Paletti Silverline ranges are contained within frameworks of 40 x 40, 30 x 30 and 20 x 20, which guarantee uniformity of all profiles and fasteners with each other. Common to this configuration is the T-Nut fastening system, usually based around an M8 type fastener. The critical area is the shape of the T-slot groove and T-slot nut being able to fit correctly in each others’ grooves.

Many suppliers also manufacture a wide range of additional equipment: Flexlink, for example, makes structural components, enclosures, linear drive units and conveyors, while Bosch has product lines which manufacture pneumatics, hydraulics and PLCs. All the Bosch products are compatible with pneumatics systems and include recent technologies, such as ASI, Profibus and Interbus bus systems. Accessory mounting kits are available to mount value blocks onto frameworks.

Anders Pihl from Flexlink agrees with Nigel Clarke that the ability to exchange components between systems depends on two factors – the standard extrusion modules used and the T-slot/T- slot nut design. Unfortunately, he says, almost every supplier has its own sizes and/or designs of T-slot. FlexLink uses 22, 44, 64, 88, 132 and 176mm as standard sizes for the extrusion range. They are all adapted to be interchangeable with each other, as well as with the company’s ranges of enclosures, linear drive units and conveyors.

Anders Pihl thinks that it is possible to integrate most competitors sizes into the FlexLink framework, because most of the beam sizes are either slightly larger or almost identical to those from other manufacturers.

FlexLink has two standard T-slot designs for structural, enclosure and linear units and one for all conveyor T-slots. FlexLink T-slot nuts with M8, M6 and M5 thread sizes are available. The special range of T-bolts for the conveyor T-slot can also be used as a standard M8 T-bolt fastener for all the T- slots and for those of some competitors (though he doesn’t say which!)

Jeff Blackman of Bosch sees the Bosch slot as being a de facto standard and says that brackets and cover strips, for example, which are compatible to the Bosch 8 and 10mm slot, are also compatible with other Bosch equipment – not surprisingly.

Design effects

Aluminium frameworks are not as rigid as similar welded structures. But except in the highest applications, the frame can be made as rigid as required. In a typical automated production environment making white goods or automotive engines, the forces are not terribly high, so the advantages of using a versatile assembly technology may well outweigh this disadvantage.

Some designers will integrate the welded C-frame of an hydraulic press into an aluminium structure, combining the flexibility and economy of aluminium frameworks with the strength of welded steel where it is needed.

Most systems have a way of strengthening the frame – with careful design and the use of suitable bracing with adequate sections, they can satisfy a large proportion of applications. This means that the basic fastening method must be very strong.

A downside of the welded structure pointed out by Digiplan’s Colin Ford, is that it can’t easily be corrected or modified and the components can’t be reused.

Nigel Clarke of Boldman Paletti agrees that steel frameworks are more rigid and stronger, but says that they are also heavier and are usually over-designed for their purpose. He adds to the limitations of welding, pointing out the grit blasting, painting and machining processes which are often needed.

He says that the benefits of aluminium are that:

• Little or no drawing is required on how the frame is made (or the design can be inferred from the general arrangement drawing)
• Large, free-standing frames can be designed to carry working mechanisms mounted to steel base plates with supported loads of over 1500kg.
• Such a strong and rigid bolted structure allows the builder the freedom to place windows, doors and attach accessories without machining or marking out hole positions to take brackets – all that is required is straight and accurate sawing to length. Mistakes can be rectified without drastic recutting or rewelding.
• Clever design of profile grooves and fasteners creates a highly rigid and permanent joint. It is claimed that the shape of the Paletti T-Nut and anchor bolts with their antitwisting actions and more central fasteneing position within the profile sections make the Paletti system up to 20% more rigid than competitor systems.

The main considerations for the stability of an aluminium framework come down to design, agrees FlexLink’s Anders Pihl in another highly detailed response, which mentions the T-slot/T- slot nut or screw combination, the inertia of the extrusions and the way the framework is put together.

The FlexLink T-slot nut, according to Anders Pihl, provides one of the largest contact surfaces between the steel and aluminium in order to guarantee that the surface tension is always below the yield point. The FlexLink T-slot nut is also claimed to be the only nut available on the market that automatically centers itself into the T-slot due to the metal spring on the back.

The FlexLink range of connectors is designed to be stronger than the beams in order to maximise the passive safety and rigidity of our system.

The moment of inertia (often just called inertia) of the extrusions indicates the stiffness of the cross section and is the most important factor determining the correct extrusion size. A lot of people use the weight per metre as an indication of the strength of the profile, but even though some structures use slightly less aluminium, the slightly larger cross sections can more than compensate for the difference when you compare the inertia! For example, the FlexLink lightweight 44 x 44mm beam weight of 1.59 kg/m has an inertia of 12.7 cm compared to a competitive, lighter, 40 x 40mm profile with a weight of 1.74 kg/m and an inertia of 9.0 cm. The comparison shows that 9% more aluminium in this case resulted in an extrusion with 40% less inertia.

The most important factors FlexLink sees in framework design are to keep the vertical outer corner profile intact and connect the pieces horizontally at both the top and the bottom. By doing this, lightweight 44, 64 and 88 x 88mm extrusions with angle brackets provide a very rigid and cost-effective frame. If the framework becomes greater than 1.5m, bracing with 44 x 44mm extrusions gives a very good result, says Anders Pihl.

The stiffness of aluminium beams can be exemplified by the fact that a 10kN load on a lightweight 44 x 44mm cross section imposes a tensile stress on the beam of only 11% of the yield point. Even so, Flexlink would not recommend anyone using aluminium frameworks when moving high loads (more than 40kg) very fast (more than 1m/s), if trying to keep a high repeatability and positional accuracy (less than 0.1 mm) in a continuous cycle.

Michal Etherington highlights the Kanya PVS connector as the means of solving any rigidity question. A joint can be made in any position, sufficient for the static stresses at least of any application.

Design is the key, agrees Jeff Blackman highlighting the CAD systems available from Bosch which have calculation modules for stiffness, as well as the technical data contained in the catalogues, which illustrate frameworks, machines and conveyors.

Components for Pneumatics

Paletti within the Silverline system offers in total over 90 profiles, many of which have core holes and cavities designed to take different media: compressed air, return air, water, oil, vacuum. Profile core diameters have been designed to suit the standard sizes associated with pneumatic pistons. Paletti designers pay specific attention to the surfaces and density of the extrusion around the core area, which should appeak to users of pneumatics systems.

Paletti has also designed profiles specifically to take high and low voltage cables inside the profile. Electrical wiring, pneumatic and hydraulic pipes are separated via an internal bridge. These profiles have two wide slots on opposing surfaces and allow for the wires or pipes to be pushed in directly, rather than fed in from one end. Once the wires and pipes are in position, there is an aluminium profile cover strip that simply clips into the profile. Power can then be brought up from the floor or down from above.

The complete range of T-Nuts in steel or plastic with threads from M8 to M3 means that the designer controls how accessories such as sensors, valves, switches and cams are fastened.

Digiplan offers components for the attachment of pressurised air to 150psi, so that the section acts as a manifold.

FlexLink provides conduits in aluminium in sizes of 22×22, 44×44, 44×88 and 88×88 mm and has recently launched a range of pneumatic range of end and connector plates that fit the 44 x 44 (G1/4″), 44 x 88 (G3/8″), and 88 x 88 (G1/2″) mm beams. In this range, there also exists a pneumatic connector that can be inserted anywhere along the T-slot (1/8″). A range of brackets for the company’s 18 mm diameter guide rails fits most sensors as well.

Machine Building Systems, from the Item range, has a large range of products, including clip-together trunking, proximity switch mounting brackets in the popular sizes and pneumatic connectors in M5, G1/8″ and G1/4″. Some suppliers, including Festo, incorporate Item grooves in some of their products, other such as MTA make pick-and-place equipment with 20mm or 40mm mounting hole centres, so that their components sit easily on Item extrusions.

Coatings

Anodised depths do vary from supplier to supplier (Paletti’s is the greatest we have found), but all seem to have similar hardness characteristics. Stuart Mitchell from machine Building System says that the hard anodising process is another area where false economies are made, which not only affect corrosion resistancem but also resistance to mechanical surface damage. The hard anodised surface is also very important in enabling the user to cut and machine the extrusion cleanly, leaving no fraze to be removed. This anodising process is not cheap and on lighter profiles can actually cost more than the aluminium content, so it is possible for a manufacturer to save money by reducing coating thickness, again with little or no visual effect. Item has recently introduced some round and angled cover profiles, enabling designers to hide the exposed grooves and alter the shape of the profile totally.

Digiplan’s extrusions all have a 204R1 clear, anodised surface to a depth of 10 microns, the same depth as is specified by FlexLink. For decorative or display purposes, they can be fitted with either black or yellow insert stripes.

The Alusett Kanya range comes with natural matt anodising to a depth of approximately 12um. Powder coating is an option.

Bosch also offers coloured covers and cover strips and FlexLink will discuss specific surface treatments with customers.

Paletti produces natural or black anodised extrusions to E6/EV1, to a depth of 15um and a layer hardness of 280-350HV. Nigel Clarke points out that it is necessary to avoid acids with a low pH value (less than 4) and bases with pH higher than 8, as well as prolonged exposure to chlorinated hydrocarbons, such as trichloroethylene. Again, powder coating in RAL colours is an option, intended for companies specifying workbenches and laboratory furniture. A forming service is available, whereby small sections can be rolled into a variety of curves and shapes.

with thanks to:-

• Machine Building Systems
• Flexlink Systems
• Alusett
• Boldman Paletti
• Parker Digiplan
• Robert Bosch