The plumb bob is such a simple yet powerful tool. It is pretty much a pointed weight that you attach to a lightweight string. It is a very helpful tool to have around the shop, and you can buy one for around $5. In this blog I will explain how to set up a plumb bob with kite string and explain some common and uncommon uses for it.
The plumb bob is fairly heavy, and the kite string has virtually no mass. By hanging the plumb bob from a kite string, you end up with a true vertical line from the earth’s core since gravity is essentially the only force being applied to the device. The bottom of the plumb bob is considered “plumb” to the kite string, which is where the tool gets its name.
Unscrew the top of the plumb bob and thread your kite string through the hole. Double knot the kite string, and screw the top back on.
Finding a “Plumb” Point
The traditional use for the plumb bob is to find a point that is directly below another point, also known as “plumb”. This is useful in construction, and it is believed that the Egyptians used plumb bobs to construct the pyramids (according to Wikipedia). To get a precise reading, the top of the kite string must be fixed and not held. The plumb bob will initially spin after being released, and when it fully stops it will be “plumb” to your fixed point.
Digging a post hole
When digging a post hole it is important that your hole stay straight and reach the appropriate depth; using the plumb-bob is an easy way to check this. When building our golf net we had to dig a total of 9 post holes that were 12”x10” and 3’ deep. To check each hole quickly, we took two paint sticks matching the two widths of our hole and crossed them at their centers. We then attached the plumb bob to the center of our crossed sticks and adjusted the positioning so the bottom of the plumb bob to the paint sticks was 3’. To check our holes, we hung the plumb bob in the hole until it barely touched bottom. If the plumb bob hit bottom before the paint sticks were level with the ground, the hole was not deep enough yet. This technique also made it very easy to site if our hole was starting to favor one side or the other as it was being dug. Since the sticks were the dimensions of our hole, we could also verify that our hole was the proper width all the way down.
Measuring Distance Down
When you need to know the distance from a particular point straight down, the plumb bob is the way to go. This was necessary so we could precut the golf net posts before placing them in the holes (see The Level for further details on this technique). Basically we had a level point established, and we needed to measure the distance from that level point to the bottom of each hole (not the same as hole depth since the ground is no longer the reference). To do this, take your plumb bob and hang it into your hole until it just barely touches the bottom. Mark the point where your plumb bob kite string crosses your level point with your finger, remove the plumb bob from the hole, and measure the distance from the bottom to where your finger is. Note that there is tension on the string from the plum bob hanging, so keep that tension while measuring to get the most accurate results.
I hope this blog has inspired you to buy your very first plumb bob! Feel free to leave your comments or questions.