The main disadvantage of the traditional Rockwell instruments is that the accuracy of testing depends to a large degree on the perfect contact between the work piece and its support, usually called the anvil.
When removing the additional load and returning to the preload, the unique deformation registered by the dial indicator should be the indentation self (see part 3 of the Rockwell method description).
This can happen only if the work piece is in perfect contact with the anvil; if there is a slight oil coat, some grease or other, a little movement occurs during load application, when added to the indentation depth, gives a wrong result, decreasing the hardness value.
Because it is not always possible to work in perfect conditions, for example in heat treatment or workshop environments, this is an important consideration.
To overcome this problem, almost all our instruments work according to a modification to the Rockwell principle, shown in the below figure.
A support on the test surface gives the penetration depth. Therefore, any movement of the part, elevating screw or stand, does not influence the test result.
Thanks to this method, the same advantages of the Brinell and Vickers principle are reached.
A. The assembly (a+b) moves down on to the test piece and the penetrator moves back producing a resistance equivalent to the preload value. Zero setting is automatic.
B. The test load is applied.
C. The load is removed and the dial indicator shows the penetrator’s displacement (1-3).
In case of movement of the test piece, the relation between (a) and (b) stays constant, avoiding the typical error of the original Rockwell principle.
The same method is used also in Superficial Rockwell measurement.
Our instruments have a third component that should not be confused with the penetrator shroud (b).
In bench testers, this component is called clamping shield.
The clamping shield is used for locking of the test piece, avoiding the use of any special support; the clamping shield can be easily removed if needed.
In portable testers, this component is called base, it is interchangeable and helps creating a perfect support on the test piece.