
Determining the correct screw size for a hole is a common problem, especially when dealing with second-hand items or products that come without assembly screws. To find the right screw, you need to measure the hole's diameter and thread type. You can use a ruler, measuring tape, or caliper to measure the diameter of the hole and the inner diameter of the screw. A thread gauge can help determine the number of threads per inch or the thread pitch. You can then use a machinist's handbook to find the exact screw you need. For wooden holes, a nut thread gauge can help determine the size of the threads and the drill bit used. Metal holes may require grinding to remove shavings and rough edges before accurate measurements can be taken.
Characteristics | Values |
---|---|
Screw length | Measured from the tip of the screw to the head. For countersunk screws, measure to the flat part of the head. |
Screw diameter | Measured across the width of the thread. For the imperial system, this is represented by a gauge number or a fraction of an inch. For the metric system, it is measured in millimetres. |
Thread spacing/pitch | In the imperial system, this is the number of threads per inch. In the metric system, it is the distance between threads, measured in millimetres. |
Thread gauge | Used to find the size, expanse, and type of thread. |
What You'll Learn
Measure the hole with a tape measure
To determine the correct screw size for a hole, you can measure the hole with a tape measure. Here's a step-by-step guide to help you through the process:
Firstly, it is important to understand the different parts of a tape measure. The case is the square-shaped housing for the tape, typically made of plastic or metal. The hook is a bent piece of metal that allows you to extend the tape and hold it in place for measurement. The hook slot is the hole at the end of the tape, which lets you latch onto an anchor point, such as a nail or screw head. The tape or blade is usually yellow or white, made of metal coated in plastic, and can be extended or retracted. The thumb lock is a button that holds the extended tape in place, and the belt clip lets you hook the tape measure onto your belt or pocket.
Now, let's get into the measurement process:
- Extend the tape measure and hook it over the edge of the board or surface with the hole.
- Measure the hole at its widest point. For accuracy, measure to the closest millimetre.
- For the length, you can insert a slim object, such as a bamboo skewer, into the hole. Measure the skewer to match the width of the material you're trying to screw together.
- To determine the screw length, consider how much of the screw needs to go through the material and how much thread you need for a secure hold.
- Washer size is less specific, but a wider washer means more surface area to hold weight, so it's easier on the wood. Just ensure the washer isn't wider than the wood itself to avoid any scraping issues.
- If you're dealing with a nut, you'll also need to consider thread size. There are typically two common thread widths used for furniture, so examine the nut to see if the threads are close together or wide enough to run your fingernail along.
- If you need to match an existing screw, take it with you to the hardware store for a direct comparison.
- For added accuracy, "burn an inch" by starting your measurement at the 1-inch mark on the tape measure, and then subtract that inch from your final measurement.
By following these steps and taking precise measurements with your tape measure, you'll be able to determine the correct screw size for your project.
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Use a nut thread gauge to find the size, expanse, and type of thread
Using a nut thread gauge is an effective way to find the size, expanse, and type of thread for a screw. This method is especially useful when working with internal threads, which are harder to measure. A nut thread gauge, also known as a nut gauge or nut/bolt gauge, comes in various types, including flat card-style, wall hangings with threaded posts, and a series of headless fasteners.
To use a nut thread gauge, start by identifying the type of thread you are dealing with. External threads, known as male threads, are visible on the outside of a bolt or screw, while internal threads, known as female threads, are found inside a part such as a nut or tapped hole. If you are working with male threads, simply choose the appropriate leaf of the gauge and hold the teeth against the fastener. If the teeth fall perfectly into the threads without any visible gaps, you have found your match.
For female threads, the process is a bit more involved. You can measure the mating fastener for the female threads if one is available. Alternatively, use a nut gauge by carefully threading the two pieces together to avoid damage. The final option is to guess and check with a series of fasteners, preferably of known sizes. Measuring the internal diameter of the nut or tapped hole beforehand can help narrow down the search.
In addition to size, a nut thread gauge can also help you determine the thread pitch, which refers to the distance between adjacent threads. This information is crucial for ensuring proper mating and compatibility with corresponding nuts or bolts. By using a nut thread gauge, you can accurately identify thread types, measure thread dimensions, and ensure the correct thread pitch, leading to secure and long-lasting mechanical connections.
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Measure the screw length in either inches or mm
Measuring the length of a screw is straightforward but can be complicated by different types of screws. The length of a screw is the distance between one end of the screw and the other. The nominal length of a screw is the length used to define the size of the fastener and is usually found in product names or catalogue listings. Nominal length is a simplified term used to refer to a screw when literal sizes or micron-accurate measurements would be inconvenient. Nominal length is usually measured from below the head to the tip of the screw. However, this can vary depending on the type of screw.
For most machine screws, the nominal length is measured from below the head to the tip of the screw. For flat head, countersunk screws, the length is measured from the top of the head to the tip, as the head sits below the surface. Hex and hex washer head screws are measured from the bottom of the head, even though they are flat on both the top and bottom. Oval head screws are measured from the widest point of the head to the tip. Self-drilling screws are measured to the end of the drill point tip.
For screws with no head, such as a threaded bar or a grub screw, the screw is measured from end to end. For shoulder screws, the nominal length is usually defined by the size of the shoulder, as there is little variety in thread length. For a raised countersunk screw, the raised section sits above the required hole, so it is not counted as part of the nominal length. For screws with specialised tips, such as nylon tip grub screws, it is important to check with the manufacturer whether the tip length is counted towards the total length.
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Measure the width of a thread to get the diameter
To determine the correct screw size for a hole, you need to measure the width of the threaded part of the screw. This will give you the diameter, which corresponds to the first number of the thread size.
If you are working in the imperial system, you can measure the diameter with a ruler, but a caliper will give you a more accurate measurement. Measure the width of the threaded area in inches, from one side of the thread crest to the other. If the diameter is less than 1/4", it will be indicated by a number (e.g. 8 means 0.164").
If you are working in the metric system, use a ruler or caliper to measure the width of the threaded area in millimetres. The diameter will be the number of millimetres preceded by the letter M (e.g. M6 means 6mm).
Once you have the diameter, you can use a thread gauge to determine the number of threads per inch (TPI) or the pitch (distance between threads). For imperial measurements, count the number of threads in one inch. For metric, measure the distance between threads in millimetres.
Now you have the diameter and the TPI or pitch, you can look up the correct screw size in a machinist's handbook or thread chart.
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Measure the thread count/pitch
Thread count, also known as thread pitch or threads per inch (TPI), is the distance between the same point on two adjacent threads of a screw. It is important to determine the thread count to ensure that a screw will thread into a bolt properly.
To measure the thread count, you will need a ruler or a caliper. First, lay the screw down on a flat surface. Let the head of the screw hang off the edge of the surface so that the screw threads are lying flat. Then, place a ruler or caliper down the axis of the screw. It is important to note that the first thread on the screw is counted as zero, not one.
To calculate the thread count, you will need to measure the distance between two adjacent thread crests or roots. This can be done in millimetres or inches. If the threads sit on the millimetre scale, you have a metric thread. If a certain number of threads fit within an inch, you have an imperial thread.
For metric threads, the pitch is the distance between two threads, usually expressed in millimetres. For imperial threads, the pitch is the distance between a certain number of threads per inch. To calculate the TPI for an imperial thread, divide the number of threads by the length in inches.
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Frequently asked questions
You need to measure the diameter of the hole. Then, you can use a standard screw size that corresponds to that diameter. For example, if the hole diameter is 2mm, you can use an M2 screw. However, for optimal results, it is recommended to choose a screw with a major diameter that is slightly larger than the hole diameter to ensure a secure fit.
You can use a caliper, ruler, or tape measure to determine the size of a screw hole. If the hole is in a soft material like wood, you may need to clean it with a metal wire brush to remove any debris before taking measurements.
You can measure the length and diameter of the screw. The length is measured from the tip of the screw to the underside of the screw head. The diameter is measured across the width of the threads. You can use these measurements to find a screw that matches the hole size.