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Wood Screw Sizes Explained – A Beginners Guide

– In today's video, we're simplifying the complex world of wood screw sizes, types and options. Plus you'll have a chance to download three different charts to help you make sense of it all. Just so everybody's starting from the same place, we need to first briefly talk about the anatomy of a traditional wood screw. Starting at the top, we have the screw head, the shank, the threads and the tip. Now each one of these can vary as we'll see later. There are four main measurements that can be taken from a wood screw, the head diameter, the shank diameter, the thread diameter which is also called the major diameter, and then the root diameter also called the minor diameter. The head size for wood screws is sometimes labeled as a max measurement on screw charts. For example, the number six or 3.5 millimeter screw has a max head diameter of .279 inches or seven millimeters. It's most likely labeled as a max size because a lot of wood screws actually have smaller heads than the max size diameter. Next is the shank size. This is the diameter of the smooth part of the screw. You will also notice that the first two sizes of traditional wood screws generally don't have smooth shanks. That's simply because they're just too small for one. Next is the thread diameter, this is also known as the major diameter. This is the measurement from the outside thread on one side, to the outside thread on the other. The last measurement is called the root or minor diameter. This measurement is taken from the point in between each thread. Moving on, the next important thing to know is TPI or threads per inch. This is similar to teeth per inch on a jigsaw blade, but threads per inch are basically the number of threads per inch of screw length. Lastly, to finish off this section, we need to talk about how the length of the screw is actually measured. But in order to know that, you first must know a few things about the different types of screw heads available. Let's look at three of them. The first one here is a standard flathead and it's characterized by, well, you guessed it a flat top. And the sides are actually angled at 82 degrees which allows for the screw to be countersunk. The second one is the pan head. These are the most common of all the rounded top screws and it's characterized by a broad rounded top. The last group combines the previous head styles together, and this one's called an oval head. So why did I show you these three? Well, because each one of these is actually measured slightly different than the other one. The rule to determine the screws length, is the amount of screw that sits beneath the surface of the wood. For example, this two and a half or 63 millimeter long flat head is measured from the top of the screw to the tip, why? Because this screw's designed to be installed flat or flush with the surface. However, the other two screws are measured from a different starting point. If we look at them now, both of the screws I have, are an inch and a half or 38 millimeters long. But as you can see this one here is actually taller. Why is that? Well it's because for the pan head, the inch and measurement is taken from the underside of the head. Again, once this screws installed, the head will remain above the surface of the wood. Now, as far as the oval head, this one will be countersunk up to about the halfway point, leaving the rest of the head above the surface. Why are all these things important to know? Well, it's the foundation to understanding the numbers on the side of a screw box, or a bolt or machine screws for that matter. And speaking of a machine screw, let's start there for our first example. Marked out on the front the package, is three numbers. An eight, a 32 and an inch and a half. The first number is this screw's diameter, which in this case is a US gauge number eight which has a shank diameter of five 30 seconds. The second number is the TPI or threads per inch. Here in the US, for every inch of thread length, there is a certain number of threads. And for this screw, that is 32 threads. The last number is the length of the screw. And as we already covered, that measurement is taken from where the head sits flat to the surface, to the tip. Because metric has used so much as well, let's quickly look at a metric machine screw. This time instead of having the gauge number, like a number eight, we have a diameter size noted as an M6, which is a six millimeter diameter screw. The second number is not threads per inch, but rather a measurement between the threads or millimeters per thread. And the last number is the length, and in this case, that's 25 millimeters. So now that we looked at the numbers on a machine screw, let's go ahead and look at the numbers on a wood screw. Here on the front of the package, you can see that there are only two now numbers this time, the gauge and the length. In this case, if the TPI is not listed, you can assume that the screw has the standard number of threads. And for a number 10, that is 13 threads per inch. Up to this point, we've covered almost everything except for the different types of drive options. Here in the US, there are three types of drives most commonly used. And the first one is a Phillips drive, the next is a square drive and the last is a Torx or better known as a star drive. Phillips is still widely used, and for the most part, they work great just as long as you use the right size bit for the screw. Now, when you're looking for a more positive connection, the star drive does provide that with very little chance of stripping the screw under high force. And of course, last but not least, is the square drive is this two provides a great connection between the driver and the screw. Bottom line, if you're gonna be installing a lot of screws like say for a deck or a bigger project, choosing a bit or a driver that has a really secure positive connection, is gonna be the way to go. And I think in that case, it's gotta be the star drive. Moving on, let's quickly talk about pre drilling and correct screw depth. First, screws holds securely because of the threads not because of the shank. Therefore what pre-drilling does, is removes the thickness of the shank material, so not the threads can grab the wood for a very strong connection. Now, I won't get into the specifics about how to pre drill because I've already done a video on that and I'll leave a link up here and down in the note section. Second, as a general rule of thumb, whenever you're screwing two pieces together, the screw should enter at least halfway through the thickness of the material you're screwing into. For example, if you're screwing two pieces of three quarter inch, or 19 millimeter flat stacked together, you want to choose a screw length that will drive at least halfway through the bottom. So in this case, the closest screw length would be an inch and a quarter or 31 millimeters. Before we end for the day, let's look at the available sizes for the si

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