What Is a Sheet Metal Screw?
A sheet metal screw is a type of hardware used for fastening metal objects. It consists of a threaded shank with a flat or rounded head. Unlike wood screws, which are only partially threaded, a sheet metal screw has a shank that is threaded down its entire length. The threads and tip of the screw are sharp, which allows them to easily cut through metal, wood and other objects. These screws are used to secure one metal object to another, or to fasten metal items to wood, plastic or various other materials.
Sheet metal screw size is commonly given using a series of three numbers, which represent the diameter, thread count and length of each screw. For example, a screw listed as 8-32 x 1" has a diameter of size 8, contains 32 threads per inch, and is one inch (2.5 cm) long. The diameter of a sheet metal screw is determined using a chart created by the American National Standards Institute (ANSI), where a size of 8 may equate to a number of different sizes depending on the type of screw.
There are two basic types of sheet metal screws on the market. Standard units have a pointed tip that is designed to cut through metal. The metal must be pre-drilled before these screws are used. Self-tapping or thread cutting screws have a notched tip that can easily cut through metal without the need for pre-drilling.
The head of a sheet metal screw may have one of several different shapes, depending on the intended function. Pan or round-headed screws will extend above the surface of an object once they are installed. Flat or oval countersunk screws will sit flush with the top of an object once they are installed. This reduces wind resistance and may also improve the appearance of the installation. These screws may have Phillips, flat, or square slots depending on the type of tool used for installing them.
Sheet metal screws can be made from a variety of different materials, each with different properties and benefits. Standard carbon steel is the most common, and typically the most economical. These screws are subject to rust or corrosion when exposed to moisture or chemicals, and should not typically be used outdoors. Galvanized or stainless steel screws are designed to resist rust and corrosion, though they generally cost more than standard steel units. Brass or bronze screws are also available, and may be coated with zinc or nickel to modify their appearance.
When you use stainless steel machine screws, or any screws that involve drilling into metal, always wear the appropriate protective eyewear! This is particularly important to remember with metal, because drilling into it causes little particles of metal to be released into the air. If those bits get into your eyes, even one of them can cause cuts and damage...not fun.
If you drill more slowly, or on a lower setting, it can prevent shavings from flying up into the air as much. Another good practice is to pause in drilling every few seconds and wipe away the shavings that are building up so that they do get flung around too much.
Even if you're wearing protective eye wear, metal shavings can still be harmful if you inhale them or get them embedded in your skin. If you are going to be drilling more than a couple of screws, a breathing face mask is a good precaution, and work gloves will protect the skin most likely to be exposed: your hands.
If you're careful, you can safely drill sheet metal for screws while still being quick and efficient.
@hanley79 - There are actually more than two kinds of head for these screws. If you look at a sheet metal screw chart poster, you'll find that there are three types of sheet metal screw head.
The first is called a Pan head. Pan head screws are raised above the surface of the metal they are screwed into, and they have very square corners to the heads. They look a bit mechanical, and can add a certain feel -- I can imagine them on a steamer trunk very easily.
Next up we have Oval head screws. These are similar to Pan head screws, but they are a bit flatter, and they have a nice rounded head to appear more finished. I would recommend these for your steamer trunk -- they look the most like old-fashioned bit round rivets.
Finally, there are Flat head screws. Those are the ones you described that end up being flush with the surface of the metal they are screwed into. You're correct, they dig down into the metal on the bottom of the head; this is called countersinking, and it has a rivet-like effect, holding the sheet metal a bit more firmly to the material behind it.
Any of these types of screws will work just as well as the next for holding sheet metal covering onto the inside frame of a steamer trunk. Be aware, however, that the Flat head screws' countersinking might case the sheet metal to dent downward around the screws slightly.
I'm trying to pick a stainless steel sheet metal screw that would be good for holding together the trim and edges of an old-fashioned metal steam trunk. Any advice on what kind of screw head would work best?
The article says there are ones with rounded heads and others with ones that end up flush flat with the surrounding surface. I think I'd prefer the old-fashioned "studs" look of having the screw heads rounded up from the surface, but before I go ahead on my project I was wondering if the shape of the screw head affects how tightly it can clamp down any.
The rounded head ones have flat bottoms, while the ones that sink down flush have pointed bottoms to the heads, don't they? Doesn't that mean the flush head style punches into the sheet metal more than the rounded head kind does?
@Hawthorne - Yep, they look like drill bits. Sheet metal screw dimensions can look kind of counterproductive to the untrained eye, because at a certain angle from the side the tips looks square, which looks blunt and completely unable to pierce and drill through sheet metal, but trust me on this.
Sheet metal thread cutting screws work extremely well -- even better if you have a good high-powered electric drill to put them in with.
Since your project was an outdoor shed, if you do another similar building project I would also recommend using galvanized or stainless steel screws. As the article says, they are rust resistant, so they won't leave rust streaks across the sheet metal roof where rain drips down.
I don't know about you, but that's one of my biggest pet peeves with metal roofs -- rust streaks make them look dilapidated even when they're otherwise brand new. Hope this helps answer some of your questions about those thread cutting screws.
I've used stainless steel sheet metal screws before that had the pointy tips. I thought the pointy tips would make them easy to cut through the sheet I was attaching to the beams (this was a sheet metal roof for a little garden shed), but I ended up having to stop and pre-drill all of the holes in with a power drill first.
It sounds like I needed some thread cutting screws -- too bad I didn't know about them back then! I'll have to keep an eye out for them next time I'm at the hardware store. Judging by the description in the article, thread cutting screws have tips that look kind of like their own little drill bits, am I right?
Post your comments