Polishing hardened tool steel to a low Ra value often requires more than one step and long cycle times. Ra is the roughness average of a surface. It is basically a measure of the difference between the microscopic peaks and valleys of surface. The lower the Ra – the less difference between the average peak and valley – the more polished the surface
Generally, any imperfections in the surface must be removed prior to polishing. Machining marks, scratches, etc. will get amplified during any polishing process. This type of abrasive process will generally result in a uniform, matte (high Ra) finish. For tool steel, this step is typically done with Ceramic Media in a tumbler or by blasting with Glass Beads or Steel Shot.
There are several ways to polish the surface. Abrasive blasting is limited as to how fine a finish can be achieved. Typically, 32 Ra is about as good as you can get. Mass finishing using vibratory or barrel tumbling is the best way to get to 16 Ra or better. Tumbling the parts with Steel Tumbling Media, high alumina (‘porcelain’) Ceramic Media, or the proper Premium Treated Tumbling Media are good options. The best choice will depend on the geometry of the part and the final finish required.
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