Cutting speed is one of the most important parameters of work of machining devices like lathes, mills, and others, designed for chip processing. Such parameters describe the speed of contact between material removal elements (like bits, blades, or cutters) and the material surface. Right cutting speed has a direct impact on tool longevity and the final effect of the machining surface.
What is a cutting speed (definition)
The definition of cutting speed (Vc) is pretty simple: it is the movement speed (linear) of a machined surface with respect to a machined element.
In other words – it is a measure of travel in time, described in meters per minute (or feet per minute).
Machining speed is one of the key parameters of machining – the chip processing; on the other hand, remains important for the grinding process.
In measurements and technical documentation that parameter is described as Vc (V = velocity; c = cutting). Such a parameter is often called 'cutting velocity', but this is not fully accurate (due to differences between these two).
Cutting speed is often given by a cutting tool manufacturer (cutter, turning bit, etc.) in regard to typical materials with known machinability properties. Selection of the best possible cutting speed depends on the operator's awareness of aspects like:
- type of machined surface (ng. different in the initial machining process than in the finishing process),
- a particular type of machined material and its mechanical and processing properties (eg. type of steel alloy or polymer),
- stiffness of the machining device,
- required efficiency of the process.
Cutting speed for mills requires the movement velocity of the outer surface of the spindle in relation to the shaped object. A similar description fits also grinders (speed is calculated based on the used tool). That parameter in lathes machining describes the distance in time of the machined surface in relation to the cutting bit.
As you can see – outer surfaces of the objects (machined and tools) are very important.Their speed is a base to calculate the actual cutting speed. It is worth mentioning the parameter is strongly connected with the revolution speed–cutter on the spindle of the lathe head. Precise selection of that parameter has also an impact on the degradation of machining blades – too low or too high can cause faster tool degradation. It is similar to the usage of the wrong tools to work with particular types of material.
How to calculate cutting speed – pattern
In a nutshell – cutting speed (Vc) can be calculated with the following pattern:
Vc = (Π x d x n)/1000
- Vc is a cutting speed,
- Π (≈ 3,141592),
- d is the diameter of the object in time, often described as a fi,
- n is an amount of revolution in a given time, most often described as a RPM – revolutions per minute.
Such a formula for cutting speed allows quick calculation of the cutting speed. On the other hand – it is a parameter that is often given by a tool manufacturer – most often in the form of range for particular types of material.
Selection of the right cutting speed is still the responsibility of a machining operator. Based on their knowledge and experience, such specialists can calculate the right parameters for machined material, tools, and even particular machines and their possible tolerances.
It is worth knowing the mentioned pattern, even if you can find it easily on the internet cutting speed calculators.
How to match cutting speed to a particular material?
Before launching production based on the machining it is very important to specify working parameters such as:
- cutting tempo (in other words: how fast amount of material will be removed),
- feed rate (for the blade),
- revolution speed of the lathe head or milling machine spindle,
- and finally – cutting speed for a particular type of material.
Each of the machining parameters is dependent on the selected material and its machining susceptibility. Among others – this is the reason why the cutting speed for steel machining will be different than the parameters recommended in the case of aluminum machining.
On top of the recommendation related to the bigger groups of material, there are differences between series or types of alloys. For example – milling aluminum of series 2xxx requires different parameters in the case of series 7xxx.
These differences are easy to identify by experienced operators of machining devices.On the other hand – while using new tools, operators should closely observe the effectiveness of recommended parameters to choose the perfect one.
Even when a tool manufacturer adds information about recommended Vc for a particular material, it is still good to know how to calculate that parameter – based on observation and knowledge. It is worth mentioning that there is a correlation between the hardness of the machined surface and the cutting speed value.
The higher the hardness of the material, the lower the Vc parameter should be.
Most of the operators agree on the following 'basepoint' Vc:
- for steel machining – between 80 and 120 m/min (262-393 ft/min),
- for aluminum machining – between 600 and 800 m/min (1900-2600 ft/min),
- for plastics/polymer milling – between 50 and 100 m/min (160-320 ft/min).
Cutting speed and degradation of the blades
Each tool, even used within specification, can degrade in time – this is a completely natural process. But there are some things that can impact the tool's life – among many: cutting speed. Wrongly selected can cause numerous problems, among many – tool degradation and a decrease in the surface quality of the machined part.
There are four main reasons for tool degradation:
- adhesion – in other words a buildup on the surface of the cutting tool, made of chips of machined materials. Such a process has a strong impact on the quality of the surface (eg. chipping) and the efficiency of machining. The reason is simple – machining blades impacted by adhesion do not cut with their blades but with unwanted layers of build-up. Such a problem can be caused by too low a cutting speed. The solution is simple – increasing the Vc on machined material (to the right level) and cleaning up the cutting tool,
- diffusion and oxidation – are the effects of the machined material particles deposition on the blade, caused by too high temperature. The reason for that process is often very simple – lack of heat dispersion equipment or too high a cutting speed. Solutions for these problems are most often twofold: decrease the cutting speed (to recommended values) and/or provide more efficient heat dispersion systems,
- mechanical abrasion – It is a completely natural process of tool degradation due to its usage. Some tool manufacturers even deliver estimations of the tool life based on previous observations of work.
The selection of the right cutting speed is important while working with automatic machining devices, but is even more significant when used in numerically controlled mills or lathes.
CNC operators early observe the whole machining process – that is why the selection of the right parameters is so important. On the other hand – the selection of said parameters is the responsibility of a professional CNC machining operator.
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