Guards are safety components for machine tools designed to protect persons from parts flung out of the machine's working zone. The latest technical development towards high speed machining calls for new material concepts providing better energy absorption than conventional guard materials. Sandwich structures made of sheet steels and an energy-absorbing aluminium foam core principally seem to be a suitable solution. The project was aimed to lay the basis for the design of guards used on high-speed machine tools.
The impact resistance of different types of sandwich structures made from steel sheets and an aluminum foam core was examined in impact tests. Testing was done by means of blunt cylindrical steel projectiles described in the applying European standards for turning machines (EN 12415 "Safety of machine tools - Small numerically controlled turning machines and turning centres") and machining centres (EN 12417 "Safety of machine tools - Machining centres"). The aluminium foams were developed gradually together with the Fraunhofer Institute for Manufacturing and Advanced Materials, Shaping and Functional Materials Department (IFAM).
As the results of the shot tests show, at the current state of the art, aluminium foams do not constitute an alternative to solid metal materials, at least with regard to the impact resistance of machine tool guards. For structures which are required to exhibit high impact resistance even at very high anticipated impact energies, austenitic steels are among the suitable alternative materials. The shot tests performed demonstrated that sandwich designs comprising 1mm-thick steel outer sheets and a 20mm-thick aluminium foam core provided at best twice the impact resistance of similar designs without an aluminium foam core. Variation in the foam thickness and heat treatment for optimization of the mechanical properties had only a minor influence upon the impact resistance. A 20mm-thick aluminium foam core makes approximately the same contribution to the impact resistance as two separate 1mm-thick steel sheets. The use of aluminium foams for structures exposed to impact such as machine tool guards thus offers no benefit, not even one of weight saving, over exclusively steel guard designs. Although the density of aluminium foam is only around 10 % that of steel, approximately ten times the quantity of foam is required in order to attain an equivalent impact resistance.
mechanical engineeringType of hazard:
Sicherheitstechnik, Maschinensicherheit, Mechanische GefährdungDescription, key words:
guards, safety components, machine tools, flying parts, injuries, high-speed machining, sandwich structures