Variable speed on a grinder is a feature that allows us to do jobs and use abrasive discs that we might not otherwise be able to use. The angular velocity determines, according to the diameter of the disc, the linear velocity on the surface or edge.
All abrasive discs specify the maximum speed at which they can operate within the safety margins; this speed can be expressed in meters per second on older discs, or in revolutions per minute on newer ones. Variable speed in a grinder allows you to work with different abrasive or polishing discs safely and without changing machines or using expensive choppers or external drives.
Variable speed on a grinder – what for?
The cheapest grinders usually have one or two fixed speeds; thus, they can only be used with certain abrasive discs – those whose maximum linear speed is higher than that offered by the grinder – so, in that sense, they are limited machines.
The variable speed in a grinder allows the use of different discs, with their corresponding uses, which makes this machine a much more versatile tool. Speeds generally range from 900 RPM to 12,000 RPM; sufficient range to cover the vast majority of abrasive and polishing discs currently on the market.
Angular velocity and linear or tangential velocity
Angular velocity — expressed in revolutions per minute or rpm — as it varies, also determines a change in linear velocity at the edge of the disk. Naturally, at different diameters, for the same angular velocity, we will have different linear velocities at the edges. The formula:
allows to calculate, from the speed in rpm, the speed of the disk in radians per second. Here 𝛚 indicates the angular velocity, measured in radians per second. T is the time, in seconds, it takes for the disk to rotate 360º, and F is the speed in revolutions per second.
Tangential velocity t (or linear) will be given by the product 𝛚 · r. If, for example, we have a grinder with an attached disc that rotates at 1000 rpm, and its diameter is 28 mm, we will have a tangential speed:
t = two · π · F · r
t = two · π · (1000/60) · 14
t ≅ 1465.5 mm / s
This gives us a value approximately equal to 1.5 m per second (standard unit used in the abrasives industry), which is equivalent to 90 meters per minute.
Why is it necessary to vary the speed of the discs?
Not all surfaces support the same abrasion rate. There are also discs that are not abrasive: brushes, tow and others are used for polishing and polishing; their maximum speed must be reduced so as not to burn or break them.
The discs express their maximum speed in revolutions per minute (rpm). In general, this maximum speed is in relation to the original diameter of the disc; as it wears, the tangential speed, at the same rpm, decreases, so there will never be any risk of operation.
Some top speeds
The maximum working speeds are coded according to a color code system, originating from the German standards DIN EN 12413, DIN EN 13743 and DIN EN 13236, and are expressed in meters per second – tangential speed at the edge of the disc.
|Color||Maximum speed (in meters per second)|
This system is falling into disuse, in favor of printing the maximum speed in revolutions per minute, something much more intuitive and easy for the operator to calculate.
There are different materials that have a limiting working speed – expressed, again, in meters per second – that can be tabulated as follows:
|Material||Maximum speed in meters per second||Speed in RPM for 127 mm diameter discs|
|Pressed hard felt discs||37||5565|
|Brush thousand sheets of small foils||40||6016|
|Abrasive flap discs||40||6016|
|Kronenflex cup wheels||fifty||7520|
|Cleaning disc NCD 200||63||9475|
|Kronenflex Parting and Grinding Discs||80||12032|
|Diamond cutting discs||80||12032|
|Diamond sanding plates||80||12032|
On most grinders the speed control is a dial located on the back of the tool; By turning it, it is possible to select different positions, which correspond to pre-established speeds. These can be five, six or four; all of them include a table in the user manual where it is possible to consult these operating speeds.
The speed can be varied while the machine is rotating; This is useful when the wheel has an application that allows it to be used at low or medium speed, such as some polishing brushes. In other cases, it will be advisable to select the speed before starting the engine, to avoid wear due to insufficient speed, or accidents due to excessive speed.