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November 25th, 2003 admin

Computer Case Screws by hi joiney

M3 screw

The M3 screw (center) has a pitch of 0.5 mm, which is finer than the pitch of a 6-32 UTS screw (top).

This screw is the second most common screw found in PCs. This is a metric screw, and just about every case manufacturer provides a package of M3 screws as well as 6-32's. Specifically, the screw type is M3-0.50, meaning the threads of the screws are 0.50 mm apart. These screws, like the 6-32 screws, also typically accept a #2 size Phillips screwdriver tip.

M3 screws are used for fastening CD-ROM drives and floppy drives. They will fit in 6-32-threaded holes, but their narrower diameter and finer thread pitch causes them to seat only loosely in the hole. Nevertheless they are often placed there by amateur PC builders who are unaware of the distinction.

M3 screws are also the most common screw used to secure a motherboard to motherboard standoffs.

Of all the screws provided with PCs, M3s are usually identified by them having the finest thread pitch of all the screws provided.[citation needed]

6-32 screws will strip the threads of M3-threaded holes. When the M3 hole is drilled into sheet metal (as is the case with floppy and CD-ROM drives), a 6-32 screw that damages the threading will typically stay in the hole, and can still be removed fairly easily. When the M3 threading is into solid metal (such as a motherboard standoff), the screw will get stuck before it can be screwed in all the way.

Motherboard standoff

Various types of motherboard standoffs

Most cases use threaded brass male-female standoffs for attaching the motherboard to the case chassis. Sometimes threaded or snap-lock plastic standoffs are used, which are less secure mechanically. Metal standoffs provide an electrical ground connection between the motherboard and case, while plastic standoffs do not. The standoff provides a margin of space between the motherboard and the case. Without the space, the motherboard would be in physical contact with the case, which would cause it to not function because its various solder points would be in contact with the case's metal, which forms a short circuit with the computer's electrical ground.

There is no rigid standard for what measurements a case's standoffs use. However, use of M3 and 6-32 threads is nearly universal.[citation needed] There is also no single standard for the height of the standoff; one quarter inch is common but not universal.

Each standoff has a "male" portion that screws into the case, and a "female" portion that receives a screw that has passed through a specially grounded pre-drilled hole in the motherboard. Standoffs that use strictly M3, strictly 6-32, or a combination (male portion 6-32, female portion M3) are all fairly common.

It is good practice to tighten the standoff to the chassis with a nut driver or pliers, or use a thread locking compound between the male portion of the standoff and the chassis sheet metal. This ensures that if when detaching the motherboard, the screw that holds the motherboard onto the standoff will unfasten instead of the standoff becoming unfastened from the chassis. If tightening the standoff with a nut driver or pliers, be careful not to over-tighten, as the soft metal threads of the standoff and chassis are easily stripped or broken. Using a thread locking compound between the motherboard fastening screw and the female portion of the motherboard standoff is neither needed nor desired. Thread locking compound will not adversely affect the electrical ground path between the motherboard and chassis.

It is a frequent mistake when building PCs to accidentally use 6-32 threaded screws into motherboard standoffs expecting an M3 screw.[citation needed] The result is that the screw will not go all the way, and the threads become stripped and "lock". Once locked, attempts to remove the top screw typically result in the entire standoff unscrewing from the case and potentially flexing the motherboard. In that situation, the screw and the standoff cannot be separated without removing the entire motherboard and subjecting it to potential damage while it is flexed in place.

4-40 jackscrews

Pairs of 4-40 UTS jackscrews 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 female 4-40 is usually just a removable standoff that has been screwed into the port with a relatively tight torque. This standoff also often serves as a mechanical mounting either attaching a connector on a flylead to the case or attaching the backplate to an exansion card (connectors on the ATX IO plate are not normally attached to the plate). The screw has 40 threads per inch, and despite its threads visually appearing similar to M3, they are not interchangeable. In SI units, the 4-40 has a pitch of 0.635 mm while the M3 has a finer pitch of 0.5 mm.

A typical length for a 4-40 screw used in PCs is 0.18 inch.

Gallery

6-32 screw with a pan head, commonly provided in PC cases.

Thumbscrews from an ATX personal computer case

Close-up of a 6-32 screw with a hex cap/Phillips head

References

McDonough, Andy The 11 Tools Every System Builder Should Own retrieved May 18, 2007 from crn.com

^ Rutter, Daniel Dan's Data - Letters 53, "Screwed", 2006-02-26

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