To plumb the jack hole correctly, where should the spider bob be attached from?

Plumbing the jack hole accurately involves ensuring that the hole is perfectly vertical, which is critical for the proper installation and operation of the elevator system. When attaching the spider bob from the top of the hoistway, it allows for straight-line measurement down through the entire depth of the hoistway.

By starting the measurement from the top of the hoistway, you can effectively account for any potential deviations or obstructions as you measure downward. This vertical measurement ensures that the jack hole will be aligned correctly in relation to the rest of the elevator structure. It helps to establish a reliable reference point, which is essential for both safety and function in the installation process.

In contrast, measuring from alternative points such as the center of the machine room or a fixed point on the drill rig may introduce errors, as those locations may not provide a direct vertical line through the entire length of the jack hole, leading to potential misalignment. Similarly, measuring from the bottom of the pit could result in an inaccurate assessment of the verticality of the hole, as it would not account for any above-ground variations.