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How to attach and remove protractor
accessories

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The locking nut is used to fasten accessories to the rule; combination square set, square head, centre head, protractor head, Starrett The various parts of the combination square set can be removed from and put back on the rule through use of a screw or nut, which secures them to the rule.
A black Starrett protractor head with a locking nut; combination square set, square head, centre head, protractor head This is useful for when you need to use one specific accessory.
The groove on a Starrett rule; combination square set, rule, ruler, blade This can be done because the rule is grooved on one side, allowing the accessories to slide on and off.
The combination square set square head before being reversed; rule, ruler, blade Sometimes the user may find that the rule of the combination square set may have marks on both sides.
The combination square set square head after being reversed; rule, ruler, blade As a result, the accompanying heads may come with locking nuts which can be rotated 180 degrees, to allow the rule to be turned around.
The locking nut mechanism once removed from the square head; combination square set Inside the various heads is the little device which secures the head to the rule.
The locking nut device inside the square head (before being reversed) When the locking nut is turned around…
The locking nut device after it has been turned around); locking nut, square head, combination square set …the device is also turned around.
The locking nut being turned around; square head, combination square set This means that the rule can be used with the groove on “either side”.