Catch Plate is not a work holding accessory of a lathe
Relief Angle controls the chip geometry while turning.
A fit designated as 30H7/g6 is a Clearance fit.
In Clearance fit, the largest permitted shaft diameter is smaller than the diameter of the smallest hole, so that the shaft can rotate or slide through with different degrees of freedom according to the purpose of mating members.
The width of cut produced in gas cutting operation is known as Kerf.