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What is the centre head used for?

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Starrett centre head with the angle between the arms labelled "90 degrees"; combination square set, centre finder, The centre head is the part of the combination square set which has two “arms”. These “arms” meet along one edge of the ruler, at a 90 degree angle.
The centre head on the rule, labelled with a 45 degree angle from the edge of the rule to the arm; combination square set, ruler, blade This means that the angle from one of the “arms” to this edge is 45 degrees.
Finding the centre point of the end of a circular rod l with a centre head; combination square set, rule, ruler, blade, centre finder The centre head can be used to determine the centre of circular objects, such as the end of rods.
The centre head placed on a square corner, 90 degrees, angles Because the “arms” rest at 90 degrees from each other, they can also be used to determine whether the corner of an object is perfectly square.
A centre head measuring a 45 degree angle from the corner of a piece of wood; combination square set, rule, ruler, blade When placed on the corner of an object with the rule attached, the user can draw a 45 degree angle from that corner.