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Measure and cut the top deck members and pivot support as per the details below.
The pivot support tube hole for mounting the pivot bush, requires a relative amount of accuracy to ensure a snug fit for the bush as well as being square to the upper surface of the pivot support.
Use a 60mm (2 3/8") bi-metal holesaw suitable for cutting steel. Pre drill a small centre hole on both sides of the pivot support and clamp the RHS to the drill press bed.
Use a slow cutting speed and plenty of cutting fluid to keep the holesaw cool and lubricated.