When bending a flat bar in a workshop, it is important to maintain its surface finish while ensuring the bend is accurate. The most suitable method for this is:

1. Using a bending block and applying uniform pressure:
   • This method ensures that the force applied to the flat bar is spread evenly across its surface, preventing any localized stress or distortion.
   • The use of a bending block helps in achieving a consistent bend angle while preserving the bar's surface finish.
   • Applying uniform pressure minimizes the risk of surface marks or indentations, which can occur if the force is applied unevenly.

Now, let's examine why the other options are less suitable:

1. Striking the bar with a hammer repeatedly:
   • This method can easily damage the surface finish of the bar due to the impact force from the hammer.
   • Repeated hammering can lead to uneven bends and potential cracking.
2. Clamping in a vice and twisting the bar:
   • This technique is more suited for torsional (twisting) deformation rather than bending.
   • It risks creating stress fractures and damaging the surface due to the uneven distribution of force.
3. Heating the bar and then quenching in water:
   • This process involves thermal stresses and can alter the material properties of the bar.
   • Quenching could lead to surface cracks or a change in hardness, affecting the finish.
   • This approach is typically used for hardening rather than bending.

In summary, the correct and most effective way to bend a flat bar without damaging its surface finish is to use a bending block with uniform pressure. This ensures an even bend and maintains the quality of the bar's surface.