Computer case screws
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Computer case screws are the hardware used to secure parts of a PC to the case. Although there are numerous manufacturers of computer cases, they have generally used three thread sizes. The Unified Thread Standard (UTS) originates from the United States while the ISO metric screw thread is standardized worldwide. In turn, these thread standards define preferred size combinations that are based on generic units—some on the inch and others on the millimeter.
The 6-32 screws are often found on hard disk drives and the case's body to secure the covers. The M3 threaded holes are often found on optical disc drives and floppy drives. On older Serial, Parallel and VGA and DVI cables, 4-40 thumb screws are often found on the ends.
More modern cases from certain manufacturers (Dell, Gateway) and enthusiast cases will lack screws altogether, instead utilizing a tool-less design.
The 6-32 is a UTS screw with a major thread diameter of 0.1380 inches (3.5052 millimetres) and a tpi (threads per inch, imperial threads) of 32 tpi, so the threads of the screws are 0.79375 millimetres apart. It is by far the most common screw found inside computer cases and commonly appears in lengths of 0.15 and 0.25 in. Nearly every brand new computer case comes with a bag of these. They are used:
- To secure a power supply to the case
- To secure a 3.5 in hard disk drive to the case
- To hold an expansion card in place by its metal slot cover
- To fasten case components to one another
- Usually, a 6-32 screw holds the main cover on the case.
They are almost always provided with a Phillips drive, accepting a #2 size tip (the larger of the two most common Phillips screwdriver tips). Sometimes a Torx drive is used instead. Both Phillips and Torx patterns may also be combined with a slot for a plain flat-blade screwdriver. Usually they are provided with a hex head which accepts a 6mm AF spanner. Less frequently, they have a pan head - a low disk with a chamfered outer edge. Because they are used in places where high torque is not required and easy removal and replacement may be desirable (such as on the side panels of the PC case), they are frequently available as thumbscrews with larger, knurled heads that can be removed with one's fingers or tools.
This screw is the second most common screw found in PCs. Of all the screws commonly provided with PCs, M3 screws can be visually identified as having the relatively finest thread pitch.
M3 is a metric screw, and just about every case manufacturer provides a package of M3 screws as well as 6-32 screws. Specifically, the screw type is M3-0.50, and the "0.50" indicates that the screw threads are 0.50 millimetres apart, which is much finer than the 1/32 in (0.79375 mm) pitch of 6-32 screws, and more suitable for fastening into threads made in thinner materials. Under the metric thread standard, the M3-0.50 is actually the coarser of the two M3 thread pitches available (the other pitch is the M3-0.35); however, the "fine" M3 is not used in standard computer case applications.
Like the 6-32 screws, the M3-0.50 screws also typically accept a #2-size Phillips screwdriver tip. M3 screws are used for securing optical disc drives, 2.5-inch hard disk drives (HDDs), solid-state drives (SSDs), and floppy drives. The Raspberry Pi Model B+ and Model A+ single-board computers also have mounting holes suitable for M3 screws. Prior versions of the Raspberry Pi used non-standard mounting holes.
Most cases use threaded brass standoffs (Jack Screw Standoffs) for attaching the motherboard to the case chassis. Sometimes threaded or snap-lock plastic standoffs are used, which are less secure, but equally useful in a stationary computer. The standoff provides a margin of space between the motherboard and the case to keep the multiple solder points below from grounding and short-circuiting.
Usually the standoff has a 6-32 male thread on one end which screws into a threaded hole in the case or motherboard backplate, and a 6-32 female thread in the other end which accepts a screw to retain the motherboard. Less often, the standoff has a female thread in both ends and a second screw is used to attach it to the case. Some standoffs use the M3 female thread (which faces the motherboard) instead of 6-32, and on a rare occasion a mixture of types can be used in the same case.
Version 2.1 of the ATX specification states that the length of standoffs needs to be at least 0.25 inches (6.4 mm), with their cross sections fitting within 0.40 by 0.40 inches (10 mm × 10 mm) square areas centered around each mounting hole on ATX motherboards.
Pairs of 4-40 thumbscrews are used to fasten certain connectors to hardware ports. The screws are typically located on either side of D-subminiature connectors such as on VGA, serial, parallel, and legacy game controller ports. They are also more recently used on DVI connectors. The typical length for a 4-40 screw used in PCs is 0.18 in.
Thumbscrews from an ATX personal computer case
- Rutter, Daniel Dan's Data - Letters 53, "Screwed", 2006-02-26
- "M3 metric fine thread specs". ehow.co.uk. Retrieved December 20, 2014.
- "2.5-inch Hard Disk Drives Installation Guide" (PDF). HGST. January 30, 2007. Retrieved November 16, 2014.
- "ATX Specification, Version 2.1: Section 3.4.2 Secondary (Bottom/Solder) Side Height Constraints" (PDF). formfactors.org. 2012-08-18. p. 18. Retrieved 2014-12-22.
- McDonough, Andy The 11 Tools Every System Builder Should Own retrieved Nov 2, 2014 from crn.com