When long thin parts are ground between centres there is a tendency that they are pushed out of the centre line due to the grinding forces and a 2-jaw steady rest can be used as a support. Case-hardened parts can be deformed and must be pre-ground first. This is normally done without a steady or by using a spring loaded steady. After pre-grinding, the parts are then finish ground to size using a pre-loaded fixed steady. More than one steady can be used.
RULE OF THUMB: a steady should be used when the length/diameter ratio is larger than 1:10
In most applications a fixed steady is used. An 2-jaw spring loaded steady is only then used when a stabilisation (vibration dampening) is needed.
How many steadies can be used?
Fixed steadies should only be used in odd numbers ( 1, 3 or 5) depending on the space available and length of the workpiece.
On which position on the workpiece is the steady used?
The steady is used next to the weakest point on the workpiece.
With long thin parts, the steady should be in the middle of the part.
Important: The steady is positioned on the diameter that is ground first so that there is no size variation in the other diameters due to the steady rest pre-load.
When is pre-grinding necessary?
For long and thin workpieces, it is an advantage if the diameters are pre-ground first. The first diameter that is ground is normally unstable and this could have a negative effect on the roundness or run-out of the other diameters. An exception are Diameters next to the centres or far away from the steady. Case hardened workpieces should always be pre-ground first, in some cases more than once, to be able to reach the required run-out. Eventually, thin hardened workpieces must be stress relieved first.
Rotational speed of the workpiece
The rotational speed of the workpiece must be selected so that no natural vibrations during grinding can occur. Normally, the rpm is below 180 min-1, regardless of the diameter of the workpiece or the grinding wheel.