How is the diameter for wood screws measured?

Written by jane smith
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You stand in the fastener aisle of the hardware store, bewildered by all the choices. You see bins, boxes, and clear plastic packages filled with a myriad of sizes of screws. You pour out the contents of that coffee can on your workbench, where you have been throwing leftover screws from your latest project. And size does matter, if you don't want your carefully crafted bookcase storage bench to fall apart the first time someone sits on it.

There are many different ways to measure wood screw diameter, including visual measurements, comparison to other screws, using outside calipers, laying the screw on a measuring tape, by thread count and pitch, and by matching screws to pre-drilled holes with known diameters.

For the visual method, have your shop apprentice pour the contents of the coffee can onto a shallow tray. Sort the screws into piles according to length and apparent width. Lay one screw from each sorted pile across a yardstick or measuring tape with measurement marks to 1/32. Place all the screws that are both the same diameter and the same length in a bag, with the diameter listed on the bag first, and the length second.

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Use a Precision Instrument

You can use a set of outside calipers to determine the diameter. Place the points of the calipers on the unthreaded portion of the screw, which is also called the shank. Write the diameter measurement on a sticker and label one bin. Write the screw length underneath the diameter.

Wood screws can also be measured at the root, which is the diameter in between threads. They are measured by thread count, or the number of threads per inch, in the United States, and by thread pitch, or the distance between threads, in the United Kingdom.

Compare Known Diameters

Use a drill press to drill one hole for each bit you own, in order from the largest diameter to the smallest, into a 2-inch by 4-inch by 12-inch piece of stock lumber. Label the diameter of each hole as you drill. Have your apprentice check any remaining screws against the drilled measuring template you made in step 4. Sort the screws into the correct bin or bag.

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