To build a high bed to specific dimensions, both the construction material and the manufacturing process must be taken into account. The bed is one of the most important parts of a machine tool, as the entire machine is built and mounted on it. Beds are characterised by their rigidity and their ability to support large workpieces.
What is the bed?
The bed is the fixed structure of any machine tool. It is the foundation on which the machine is built, which is why it is also called the base or bench. The main characteristic of any bed, whatever its type, is its rigidity. This refers to the materials it is made of (they must withstand high temperatures), but also to its ability to cope with machining speeds.
It is important to bear in mind that if a bed vibrates or moves during machining, the desired results may not be achieved. Beds are needed in machine tools because deformation of the machine structure would otherwise end up compromising machining accuracy.
The other components of the machine are mounted on this part of the machine tool, but supporting these elements is not the bed's only job. It must also absorb cutting forces and allow the machine's moving parts (such as the carriages) to travel correctly. Besides being the base and structure of a machine, the bed is what the moving parts of the machine travel along. These may be the carriages or the table. They move forwards and backwards along guideways mounted on the bed, which are special bearing surfaces.
Types of beds
Despite their rigidity, beds do not always have to be positioned horizontally relative to the workpiece headstock; they can be built to suit the requirements of the machine. This is why there are different types of machine beds, such as flat beds, slant beds and the high bed, which is taller than the other two.
Beds vary depending on the machine tool in question. On lathes, for example, the flat bed stands out; machines with this bed are known as toolroom-type lathes and can machine large workpieces. Slant-bed lathes are used to machine small parts such as fittings.
On more compact machine tools, such as milling machines, bench drills and grinding machines, among others, a high bed is common: shorter in length than a horizontal bed, but taller.
High beds: dimensions and technical requirements
As already mentioned, the high bed is usually found on smaller machine tools that work vertically or on a bench. What requirements must a bed meet to be effective? To begin with, this part of the machine must provide the correct arrangement for the machine's other components; this ensures machining accuracy.
Beds must also offer static and dynamic rigidity, include bridges and columns and have high resistance to wear caused not only by the guideways that travel over them, but also by the vibrations generated during machining.
Construction material
Choosing the right material for a bed depends on the working conditions in the workshop, the type of machining carried out and, in general, how the machine tool is used. Today, despite differences in their dimensions, almost all beds are built from the same materials:
- Grey cast iron. This is the most common material for machine beds. When the guideways are part of the bed itself rather than components mounted on it, the iron must be highly wear-resistant.
- For smaller machines, such as DIY machines that do not carry out heavy machining, the high bed can be made of CY 18-36 cast iron, which has a hardness of HB-60-230.
- When the high bed has a simple walled configuration (between 10 and 30 mm thick), CY 21-40 cast iron is used; if the thickness increases to between 20 and 60, the iron will be type 28-40. As wall thickness increases, so must the grade of cast iron used.
Manufacturing a high bed and its dimensions
There are two methods for manufacturing beds, whatever their type:
- Casting: this applies to beds made of cast iron.
- Welding: for beds made of carbon steel.
The choice of bed manufacturing method depends on the economic and technical factors of each construction method. Cast-iron high beds, for example, are less complex to make than steel beds, which makes them more economical; this is the most widely used technique for series production of machines.
Welded beds, on the other hand, are built when machines need to be produced in a short time and when making iron casting moulds is not advisable.