About cutting

Separation of the specimen from the workpiece is the essence of materialographic cutting.

The requirements listed below for the extraction of the specimen are linked to how to select the specimen, but also to the effects introduced to the material during the cutting process and to the cutting process itself.

• The specimen must represent the features of the parent piece or component from which it is taken.
• The cut-off wheel must not get blocked when cutting.
• The cut-off wheel must endure only minimal wear.
• The specimen must be cool to touch when taken out of the machine.
• There must be no thermal damage/burning of the specimen surface.
• There must be a smooth and uniform surface with homogeneous scratches.
• There must be a minimum of burrs.

For the sectioning itself, a broad variety of methods can be used, but to meet the demands for a materialographic cut, only very few techniques are feasible, of which wet abrasive cutting will be described below.

How to Cut

Select cut-off machine

The size of the workpiece determines the size of the cut-off machine as the piece should fit into the cutting chamber, and the cut-off wheel diameter is large enough to cut through the piece.

• Moving the workpiece or the cut-off wheel in three directions in the machine (x, y, and z) can be pre-defined in the programming of an automatic cut-off machine or done by an operator in a manual cut-off machine.

• Cut-off machines will offer different possibilities for selecting cutting modes which can expand the size and material range of the workpiece.

• The precision of the cut, the ability to hit a determined cutting line close to the area subject for the examination, for example, is also defined by the equipment. Precision cut-off machines can position the workpiece with an accuracy down to a few microns.

Choice of Cut-off Wheel

The wheel's characteristic is a crucial factor, and the choice of wheel depends primarily on the material to be cut. Abrasive cut-off wheels are defined by:

• Design of the wheel
• Type of abrasives
• Type of bond

Differences in the design include dimensions, like diameter and thickness, and the placement of the abrasives as either a continuous rim in the circumference or distributed in the complete body of the cut-off wheel. Differences in the size and concentration of the abrasives offer possibilities to optimize the performance.

The optimum choice of abrasives for different materials is given below for bakelite-based cut-off wheels:

Diamond abrasives in a metal bond are mostly used for non-metals with moderate hardness such as:

Cutting mode

Direct cut is the most used cutting mode where the cut-off wheel is moved directly into the workpiece.

For the improvement of the direct cut or expansion of the application range for a machine or cut-off wheel, other cutting modes can be beneficial. In general, the improvement is linked to decreasing the contact area throughout the cutting process, which can give several advantages:

• When the force per contact area increases, harder materials can be cut. Standard materials are cut faster, and the wheel wear can be reduced. A harder wheel than that used for a direct cut can be used for a given material.

• Since the contact area constantly changes, cooling is improved. In turn, this means a reduced risk of damages to the workpiece and reduced work in the preparation following the cutting.