DAM Ústí nad Labem

Design of machine tool frames

The frame structure of a machine tool is a complex structure comprising parts, components and mechanisms, connected together with flexible or rigid joints, that transfer forces during the machining process and contribute to the static and dynamic rigidity of the system made up of:

the MACHINE – the TOOL – the WORKPIECE AT THE POINT OF OPERATION

The requirements for a machine tool frame structure and the parameters influencing the machine's precision and productivity include:

• static and dynamic rigidity,
• modal-dynamic properties of the frame – natural frequencies and mode shapes, damping for individual mode shapes,
• mass of movement axes,
• low thermal deformations and time-stable dimansions,
• long operating life and high reliability,
• production cost-effectiveness.

The static rigidity of standard steel frames is generally sufficient, often achieved by using large amounts of material for the frame. This results in the system being very heavy, which in turn limits its dynamic rigidity, i.e. its modal properties.

In designing the main parts of a machine frame structure the selection of the right materials plays an important part. The following factors impact on the selection:

• properties of the structural material (strength, density, modulus of elasticity in tension and in shear, internal damping, internal stresses, thermal expansion, specific heat, thermal conductivity),
• production factors (machinability of the material, connection between the parts, production technology – casting, welding),
• economic factors (number of units produced, price).

Types and properties of machine tool structural materials

Iron castings
Advantages	Disadvantages
higher vibration damping	low modulus of elasticity (approximately ½ E of steel)
better machinability compared to steel welded structures	technological limitations for edges and overall shaping
cost-effectiveness for large series	high costs of small-scale production
	heavier weight compared to welded structures of comparable rigidity
	stress-relief annealing

Steel welded structures
Advantages	Disadvantages
high modulus of elasticity,	lower vibration damping
the possibility to create very complex structures	internal stresses
lower costs of small-scale production	worse machinability compared to cast iron

Sandwich composites
Advantages	Disadvantages
significant weight savings	lower operation temperature limits for the parts
good damping properties	more complicated connection with other structural parts
higher flexural rigidity
lower costs of small-scale production

Therefore, the selected criteria for evaluating the design options are:

• availability of the materials used,
• technological demands on processing,
• cost-effectiveness,
• potential weight savings,
• suitability for mass production (in the machine tool sector this means e.g. 10 units/year).