Custom motherboard standoffs

[robbee]

Hi there,

For a project I'm doing, I need motherboard standoffs (spacer bolts i think is the right word) with a height of 20mm. I've found a lot of those already:

Problem is, the bolt part that's 6mm long in the picture, needs to be shorter (about 4mm max). Does anyone know where to get it, or if they'll still be stable if i cut them shorter?

Also not super sure if i need M3 or M2.5 for the standoffs...

 

[iFreilicht]

Also not super sure if i need M3 or M2.5 for the standoffs...

M3.5 would be ideal, but M3 should work fine as well.

Problem is, the bolt part that's 6mm long in the picture, needs to be shorter (about 4mm max). Does anyone know where to get it, or if they'll still be stable if i cut them shorter?

Yes, you can do that. M3 needs about 1mm of thread engagement for a proper connection, so everything above 1.5mm should be fine.

EDIT: As pointed out by @imparanoid, rule of thumb in engineering is to have an engagement distance of 1.5 times the major diameter of the thread, so 4.5mm for M3. This is a good rule, because it will make sure that your connection is absolutely rock-solid, so if you're building something heavy, use that!
But, in the case of a PC mainboard with a moderately sized cooler, 1mm should be enough, though it is of course the minimum I'd go for. If you have room, get the thread length as close to the 4.5mm as possible.

 

[robbee]

Very useful information, thanks!

 

[EdZ]

Top tip if cutting them down: thread a nut on first, then cut it, then remove the nut. The action of removing the nut 'cleans up' the threads in the cut area (that would be mangled by the cutting process) as it passes over them. Otherwise, it will be difficult to thread anything else onto them after cutting.

 

[imparanoid]

I disagree with IFreilicht. Rule of thumb I've been taught is 1.5 times major diameter of the thread (M3=3mm in this case). For this reason you'll find that Mcmaster, Fastenal, and Misumi has smallest threaded length available as 4.7mm.

Go the way of EdZ and cut it down if needed, but remember the shorter you go the less stability is offered.

 

[ImperialAlex]

Problem is, the bolt part that's 6mm long in the picture, needs to be shorter (about 4mm max). Does anyone know where to get it, or if they'll still be stable if i cut them shorter?

Remember that, at least according to the ATX standard (and, in a trickle-down fashion, also mATX and ITX), the motherboard standoff distance has to be at least 6.5mm. That means that if you have a chassis wall or similar element closer than 6.5mm from the bottom of the board then you might find that some components (probably most relevant if there are M.2 slots on the rear side) do not have sufficient clearance.

More on point for your actual question: Why don't you just grind down a few 6mm standoffs to your desired height? Standoffs are pretty abundant spare parts because every large case ships with the quadrillion standoffs required for the largest mainboard so you'll most likely either have spares already or your local electronics/computer/etc store will have a bin of them.

 

[Kmpkt]

20mm > 6.35mm

He's only cutting the threaded portion. The actual standoff height will remain unchanged at 20mm.

 

[ImperialAlex]

20mm > 6.35mm

He's only cutting the threaded portion. The actual standoff height will remain unchanged at 20mm.

Just realized my mistake, should have read more carefully

Ironically I've actually done exactly what he's proposing with standoffs for a prototype I was throwing together (i.e. just sanded down the threads). However it was only a 6.5mm standoff (shorter lever might mean less force compared to 20mm standoffs?) and I only had to take off 0.75mm so I wasn't worried about stability.

The easiest way to test this, assuming you have a set of metric taps, would be to grab a throwaway bit of metal, drill and tap it M3 and then test the strength of a normal and shortened standoff. Since this is the SFF forum I'm going to assume you won't mount 5kg of copper heatsink on the mainboard anyway so some unscientific "hit it with a hammer" should suffice.

 

[iFreilicht]

I disagree with IFreilicht. Rule of thumb I've been taught is 1.5 times major diameter of the thread (M3=3mm in this case). For this reason you'll find that Mcmaster, Fastenal, and Misumi has smallest threaded length available as 4.7mm.

Go the way of EdZ and cut it down if needed, but remember the shorter you go the less stability is offered.

Well rules of thumb are just that. It's a good rule because it will prevent bad things from happening if you don't want to think at length about a possible design, but the engagement that you actually need depends entirely on the load, and we're not talking about heavy structural fastening here. Though it did just occur to me that nowhere in the thread it says what sort of project this will be used for, so you're probably right that it's better to give more conservative advice.

If the standoffs are holding a motherboard in place that uses even a moderately-sized CPU cooler and they are screwed into steel, I'd bet you 1mm engagement would be enough to hold the board in upside down, even when shaking the whole rig.

Lian-Li also frequently uses M3 screw-in standoffs in 1mm thick aluminium sheet.

I'll edit my answer so nobody gets wrong ideas.

 

[aquelito]

According to the guys from one of the workshop I'm working with : a screw fastening in aluminium is fully secure with one full turn and a half (for heavy duty loads on store furnitures, etc.).

 

[robbee]

Though it did just occur to me that nowhere in the thread it says what sort of project this will be used for, so you're probably right that it's better to give more conservative advice.

It will be fastened into MDF, so a little extra would be welcome. The plate is 5mm thick though, so i've got plenty of room there.

 

[EdZ]

It will be fastened into MDF

Are you using press-insert nuts, or a nut on the backside? Threading straight into MDF will not be very secure with any machine screw (wrong kind of thread).

 

[robbee]

Are you using press-insert nuts, or a nut on the backside? Threading straight into MDF will not be very secure with any machine screw (wrong kind of thread).

I was looking at nuts but they seem to have the same issue (6mm length at least), so now I'm looking at these:

 

[aquelito]

Are you using press-insert nuts, or a nut on the backside? Threading straight into MDF will not be very secure with any machine screw (wrong kind of thread).

Indeed, you would need to screw or press that kind of threaded insert into the MDF first (don't know the english name for it) :

 

[robbee]

Indeed, you would need to screw or press that kind of hardware into the MDF first (don't know the english name for it) :

Isn't that second one supposed to be used on the opposite side of the screw? Either way, I'm not really finding any suitable nuts that won't stick all the way through the panel (want to keep the opposite side clean...)

 

[Ceros_X]

Yeah, finding one that is 4mm or less is proving difficult. You'd want to use something like the second one (coupled with some epoxy) for best results. Might check into some speaker forums, they use MDF a lot on custom enclosures. Seem to see a lot of recommendtions for Tee nuts or hurricane nuts.

Isn't that second one supposed to be used on the opposite side of the screw? Either way, I'm not really finding any suitable nuts that won't stick all the way through the panel (want to keep the opposite side clean...)

No, from the way these work you drill a small hole in the panel the size of the shaft, then hammer it in. The fingers dig into the wood/mdf and then keep the nut from moving. The bolt should come from the top and screw towards the bottom and not break through the panel.

 

[imparanoid]

If you want the back side to look neat this might seem silly but what about using a countersunk/bore on the backside to at least make a nut flush. Or a countersunk/bore bolt on the backside with a female-female standoff. Bolt head would be flush with the backside of the panel, and you can easily find a 20mm female-female hex standoff? Just a thought. For reference, an M3 flat head cap screw has a head height of ~2mm leaving ~3mm of panel for strength.

 

[iFreilicht]

Countersunk screw with female-female standoff seems like the way to go. Inexpensive and solid solution.