Machine screws are essential for many mechanical projects, providing both strength and flexibility in fastening parts together. When choosing the right size, you must balance factors like diameter, length, and thread type. Selecting the wrong screw can affect performance, making it important to understand the size chart before deciding.
Machine Screws Size Chart
| Nominal Size &Threads/lnch & Series Designation | Allowance | Major Diameter | Pitch Diameter | ||||
| Max | Min | Max | Min | Tolerance | |||
| 0-80 1-64 1-72 2-56 2-64 | UNF UNC UNF UNG UNF | 0.0005 0.0006 0.0006 0.0006 0.0006 | 0.0595 0.0724 0.0724 0.0854 0.0854 | 0.0563 0.0686 0.0689 0.0813 0.0816 | 0.0514 0.0623 0.0634 0.0738 0.0753 | 0.0496 0.0603 0.0615 0.0717 0.0733 | 0.0018 0.0020 0.0019 0.0021 0.0020 |
| 3-48 3-56 4-40 4-48 5-40 | UNG UNF UNG UNF UNC | 0.0007 0.0007 0.0008 0.0007 0.0008 | 0.0938 0.0983 0.1112 0.1113 0.1242 | 0.0938 0.0942 0.1061 0.1068 0.1191 | 0.0848 0.0867 0.0950 0.0978 0.1080 | 0.0825 0.0845 0.0925 0.0954 0.1054 | 0.0023 0.0022 0.0025 0.0024 0.0026 |
| 5-44 6-32 6-40 8-32 8-36 | UNF UNG UNF UNG UNF | 0.0007 0.0008 0.0008 0.0009 0.0008 | 0.1243 0.1372 0.1372 0.1631 0.1632 | 0.1195 0.1312 0.1321 0.1571 0.1577 | 0.1095 0.1169 0.1210 0.1428 0.1452 | 0.1070 0.1141 0.1184 0.1399 0.1424 | 0.0025 0.0028 0.0026 0.0029 0.0028 |
| 10-24 10-32 12-24 12-28 1/4-20 | UNG UNF UNG UNF UNG | 0.0010 0.0009 0.0010 0.0010 0.0011 | 0.1890 0.1891 0.2150 0.2150 0.2489 | 0.1818 0.1831 0.2078 0.2085 0.2408 | 0.1619 0.1688 0.1879 0.1918 0.2164 | 0.1586 0.1658 0.1845 0.1886 0.2127 | 0.0033 0.0030 0.0034 0.0032 0.0037 |
| 1/4-28 5/16-18 5/16-24 3/8-16 3/8-24 | UNF UNG UNF UNG UNF | 0.0010 0.0012 0.0011 0.0013 0.0011 | 0.2490 0.3113 0.3114 0.3737 0.3739 | 0.2425 0.3026 0.3042 0.3643 0.3667 | 0.2258 0.2752 0.2843 0.3331 0.3468 | 0.2225 0.2712 0.2806 0.3287 0.3430 | 0.0033 0.0040 0.0037 0.0044 0.0038 |
| 7/16-14 7/16-20 1/2-13 1/2-20 9/16-12 | UNG UNF UNG UNF UNG | 0.0014 0.0013 0.0015 0.0013 0.0016 | 0.4361 0.4362 0.4985 0.4987 0.5609 | 0.4258 0.4281 0.4876 0.4906 0.5495 | 0.3897 0.4037 0.4485 0.4662 0.5068 | 0.3850 0.3995 0.4435 0.4619 0.5016 | 0.0047 0.0042 0.0050 0.0043 0.0052 |
| 9/16-18 5/8-11 5/8-18 3/4-10 3/4-16 | UNF UNG UNF UNG UNF | 0.0014 0.0016 0.0014 0.0018 0.0015 | 0.5611 0.6234 0.6236 0.7482 0.7485 | 0.5524 0.6113 0.6149 0.7353 0.7391 | 0.5250 0.5644 0.5875 0.6832 0.7079 | 0.5205 0.5589 0.5828 0.6773 0.7029 | 0.0045 0.0055 0.0047 0.0059 0.0050 |
| 7/8-9 7/8-14 1-8 1-12 1-14 | UNG UNF UNG UNF UNS | 0.0019 0.0016 0.0020 0.0018 0.0017 | 0.8731 0.0834 0.9980 0.9982 0.9983 | 0.8592 0.8631 0.9830 0.9868 0.9880 | 0.8009 0.8270 0.9168 0.9441 0.9519 | 0.7946 0.8216 0.9100 0.9382 0.9463 | 0.0063 0.0054 0.0068 0.0059 0.0056 |
| 11/8-7 11/8-8 11/8-12 11/4-7 1114-8 | UNG UN UNF UNG UN | 0.0022 0.0021 0.0018 0.022 0.0021 | 1.1228 1.1229 1.1232 1.2478 1.2479 | 1.1064 1.1079 1.1118 1.2314 1.2329 | 1.0300 1.0417 1.0691 1.1550 1.1667 | 1.0228 1.0348 1.0631 1.1476 1.1597 | 0.0072 0.0069 0.0060 0.0074 0.0070 |
| 11/4-12 1 3/8-6 1 3/8-8 1 3/8-12 11/2-6 | UNF UNC UN UNF UNC | 0.0018 0.0024 0.0022 0.0019 0.0024 | 1.2482 1.3726 1.3728 1.3731 1.4976 | 1.2368 1.3544 1.3578 1.3617 1.4794 | 1.1941 1.2643 1.2916 1.3190 1.3893 | 1.1879 1.2563 1.2844 1.3127 1.3812 | 0.0062 0.0080 0.0072 0.0063 0.0081 |
| 1 1/2-8 1 1/2-12 1 5/8-8 1 3/4-5 1 3/4-8 | UN UNC UN UNC UN | 0.0022 0.0019 0.0022 0.0027 0.0023 | 1.4978 1.4981 1.6228 1.7473 1.7477 | 1.4828 1.4867 1.6087 1.7268 1.7327 | 1.4166 1.4440 1.5416 1.6174 1.6665 | 1.4093 1.4376 1.5342 1.6085 1.6590 | 0.0073 0.0064 0.0074 0.0089 0.0075 |
| 1 7/8-8 2-4 1/2 2-8 2 1/4-4.5 2 1/4-8 | UN UNC UN UNC UN | 0.0023 0.0029 0.0023 0.0029 0.0024 | 1.8727 1.9971 1.9977 2.2471 2.2476 | 1.8577 1.9751 1.9827 2.2251 2.2326 | 1.7915 1.8528 1.9165 2.1028 2.1664 | 1.7838 1.8433 1.9087 2.0931 2.1584 | 0.0077 0.0095 0.0078 0.0097 0.0080 |
| 2 1/2-4 2 1/2-8 2 3/4-4 2 3/4-8 3-4 | UNC UN UNC UN UNC | 0.0031 0.0024 0.0032 0.0025 0.0032 | 2.4969 2.4976 2.7468 2.7475 2.9968 | 2.4731 2.4826 2.7230 2.7325 2.9730 | 2.3345 2.4164 2.5844 2.6663 2.8344 | 2.3241 2.4082 2.5739 2.6580 2.8237 | 0.0104 0.0082 0.0105 0.0083 0.0107 |
| 3-8 3 1/4-4 3 1/4-8 3 1/2-4 3 1/2-8 | UN UNC UN UNC UN | 0.0026 0.0033 0.0026 0.0033 0.0026 | 2.9974 3.2467 3.2474 3.4967 3.4974 | 2.9824 3.2229 3.2324 3.4729 3.4824 | 2.9162 3.0843 3.1662 3.3343 3.4162 | 2.9077 3.0734 3.1575 3.3233 3.4074 | 0.0085 0.0109 0.0087 0.0110 0.0088 |
| 3 3/4-4 3 3/4-8 4-4 4-8 | UNC UN UNC UN | 0.0034 0.0027 0.0034 0.0027 | 3.7466 3.7473 3.9966 3.9973 | 3.7228 3.7323 3.9728 3.9823 | 3.5842 3.6661 3.8342 3.9161 | 3.5730 3.6571 3.8229 3.9070 | 0.0112 0.0090 0.0113 0.0091 |
Machine Screw
| Nominal Screw Size | O thru l2 | 1/4 thru 3/4 |
| Nominal Screw Length | Tolerance on Length | |
| Up to 1/2 in., Incl. Over 1/2 to 1 in., incl. Over 1 to 2 in., incl. Over 2 in. | Up to 1/2 in., Incl. Over 1/2 to 1 in., incl. Over 1 to 2 in., Incl. Over 2 in. | -0.03 -0.03 -0.06 -0.09 |
Understanding Machine Screw Sizes
Understanding machine screw sizes is key to ensuring a secure fit in any project. These screws are classified by diameter, length, and thread pitch. While it’s tempting to focus on just one factor, balancing all three is crucial.
Choosing a screw with the wrong diameter can weaken the connection, while an incorrect length might cause the screw to protrude or fail to hold parts together.
Similarly, thread pitch affects how tightly components are fastened. Each of these factors plays a vital role, and overlooking one could lead to complications in your project’s performance.
Machine Screw Diameter and Length Explained
Machine screw diameter refers to the width of the screw’s shaft, while the length measures from the head to the tip. Choosing the right diameter is crucial; a screw that’s too wide can damage the material, while one too narrow may not hold properly.
Length is equally important—if the screw is too long, it may stick out or loosen over time, while a shorter screw might not secure the parts firmly. Balancing these factors is essential, as using the wrong size can compromise the strength and durability of your project. Understanding these dimensions helps ensure a perfect fit.
Machine Screw Head Styles and Drive Types
Machine screws feature various head styles, such as flat, pan, and hex, each designed for specific applications and tool compatibility. Choosing the right head style depends on both the look and function you need. A flat head sits flush with the surface, offering a smooth finish, but it requires precise countersinking.
On the other hand, a pan head is easier to install but may protrude, which could interfere with other components. Hex heads allow for greater torque but require specific tools. Balancing appearance, ease of installation, and functionality is key to selecting the right head style for your project.
How to Select the Right Machine Screw for Your Project
To select the right machine screw for your project, you need to evaluate several factors, including material, load requirements, and environmental conditions. For example, steel screws provide strength, but in humid or corrosive environments, stainless steel or coated screws may be more durable.
While stronger screws can handle higher loads, they might also cost more or require special tools for installation. Additionally, the surrounding conditions—like exposure to moisture or extreme temperatures—will affect the screw’s longevity. Balancing these factors ensures that your machine screws will meet both the functional and environmental demands of your project.
FAQs
1. What is the difference between coarse and fine thread machine screws?
Coarse threads have fewer threads per inch, making them quicker to install and more resistant to wear in softer materials. Fine threads have more threads per inch, offering greater strength and precision but are harder to align and more prone to stripping in softer materials.
2. How do I know which material to choose for my machine screw?
The material depends on your project’s environment and load requirements. Steel screws are strong and affordable but may rust in wet conditions. Stainless steel is corrosion-resistant, making it ideal for outdoor or humid environments, while brass is softer and better suited for light-duty applications.
3. Can I use a longer screw if I can’t find the exact size?
While you can use a slightly longer screw, it’s not always recommended. Using a screw that is too long may result in it protruding from the material, potentially interfering with other parts or weakening the overall structure. Always try to match the size to your project’s specifications for optimal performance.