Completed STX160.0 - The most powerful ATX unit, in the world!
- Thread starter jeshikat
- Start date
As I alluded to at the end of Part 5, my struggle getting the standoffs sorted out wasn't done quite yet!
Designing the case part 5b: The standoffs strike back
So I left off last time having made the decision to invert the motherboard, both because of standoff height issues and to help with CPU cooling by giving it dedicated intake vents. But I now had to figure out a way to secure the motherboard up in the air in the middle of the case. My solution? Standoffs on top of standoffs on top of standoffs! It's standoffception
A bit kludgy, but it's an easy to implement solution. There was just one issue though...
The root of the problem was because of a secondary design goal I have in mind. Even though it would result in poor CPU cooling with the video card installed, part of the reason I originally had the motherboard facing up was to experiment with using a taller CPU heatsink (when not using a graphics card) than would fit in the ASRock DeskMini 110 chassis:
Useful tip: from the top of an Intel CPU's Integrated Heat Spreader (IHS) to the top of the motherboard is typically 8mm. 75mm - 8mm = 67mm. so that leaves just enough room for my favorite low-profile heatsink, the Noctua NH-L9x65. Obviously with the motherboard flipped upside down the IO cutouts would not line up, but I still wanted the ability to test this configuration in consideration of designing a variant of the case for it.
To do that meant using 8mm PEM standoffs to leave room for the CPU heatsink in that layout and then stacking 13mm and 14mm male-female standoffs on top of them to hold the motherboard in the inverted, GPU layout:
This is as SFF as it gets, the top of the SODIMM memory sticks will be almost touching the case and even then I'll probably have to put a sheet of plastic between the mobo and the video card so nothing shorts
You may be wondering, why is there a bit of a gap between the video card and the top of the frame? Couldn't I push the video card right up against the top to increase the clearance between the card and the board? Not really, but more on why in a later post
So where things went wrong is that most of the readily available 13mm and 14mm M3 male-female standoffs I found have 6mm length for the male threaded stud. Luckily McMaster-Carr has some with just 4.7mm lengths for the male threads and as a bonus the hex size is 6mm (measured from flat to flat) to give more surface area for the board to rest on, sweet.
BTW, McMaster-Carr is a wonderful, wonderful resource for American tinkerers, hardware hackers, case designers, etc. because they have a humongous catalog of nuts, bolts, screws, fittings, gauges, and so on in a clean, easy to navigate website with CAD models available for most things! They're willing to sell and ship even very small quantities too, which is great for a prototype when you only need 4 of a particular type of standoff (like me). Their pricing isn't very good for bulk volumes though so look elsewhere for larger production runs where you need dozens of screws per case for example.
Anyway, while reviewing the PEMNET standoff catalog, I noticed a problem:
The F dimension refers to the minimum depth of the threads of the standoff. As I said above, I needed the 8mm long standoffs to fit the secondary motherboard layout, but that meant a thread depth of only 4mm, which is less than even the 4.7mm male threads of the McMaster-Carr standoffs! The standoff could bottom out and the motherboard then wouldn't sit at the right height for everything to fit
But I can fix this! These are the Blind Threaded standoffs, the "blind" part referring to the fact that the hole does not go all the way through the part, you can't see through the hole. The alternative would be Through-Hole standoffs, where the hole does goes all the way through the standoff. As you might have noticed, engineers suck at naming things
Problem solved!
The downside is that now the threaded holes for the standoffs will be visible on the outside of the case but it's a small price to pay to make sure everything fits right?
And I guess I could take those male-female standoffs, screw them into those now exposed threads on the outside of the case and have the world's smallest open-air test bench
*insert heavy dose of sarcasm here
Table of Contents
Next Update
Designing the case part 5b: The standoffs strike back
So I left off last time having made the decision to invert the motherboard, both because of standoff height issues and to help with CPU cooling by giving it dedicated intake vents. But I now had to figure out a way to secure the motherboard up in the air in the middle of the case. My solution? Standoffs on top of standoffs on top of standoffs! It's standoffception
A bit kludgy, but it's an easy to implement solution. There was just one issue though...
---------------------------
A tale of two layouts
The root of the problem was because of a secondary design goal I have in mind. Even though it would result in poor CPU cooling with the video card installed, part of the reason I originally had the motherboard facing up was to experiment with using a taller CPU heatsink (when not using a graphics card) than would fit in the ASRock DeskMini 110 chassis:
So much room for activities!
Useful tip: from the top of an Intel CPU's Integrated Heat Spreader (IHS) to the top of the motherboard is typically 8mm. 75mm - 8mm = 67mm. so that leaves just enough room for my favorite low-profile heatsink, the Noctua NH-L9x65. Obviously with the motherboard flipped upside down the IO cutouts would not line up, but I still wanted the ability to test this configuration in consideration of designing a variant of the case for it.
To do that meant using 8mm PEM standoffs to leave room for the CPU heatsink in that layout and then stacking 13mm and 14mm male-female standoffs on top of them to hold the motherboard in the inverted, GPU layout:
This is as SFF as it gets, the top of the SODIMM memory sticks will be almost touching the case and even then I'll probably have to put a sheet of plastic between the mobo and the video card so nothing shorts
You may be wondering, why is there a bit of a gap between the video card and the top of the frame? Couldn't I push the video card right up against the top to increase the clearance between the card and the board? Not really, but more on why in a later post
---------------------------
Discovering the problem and the solution
So where things went wrong is that most of the readily available 13mm and 14mm M3 male-female standoffs I found have 6mm length for the male threaded stud. Luckily McMaster-Carr has some with just 4.7mm lengths for the male threads and as a bonus the hex size is 6mm (measured from flat to flat) to give more surface area for the board to rest on, sweet.
---------------------------
BTW, McMaster-Carr is a wonderful, wonderful resource for American tinkerers, hardware hackers, case designers, etc. because they have a humongous catalog of nuts, bolts, screws, fittings, gauges, and so on in a clean, easy to navigate website with CAD models available for most things! They're willing to sell and ship even very small quantities too, which is great for a prototype when you only need 4 of a particular type of standoff (like me). Their pricing isn't very good for bulk volumes though so look elsewhere for larger production runs where you need dozens of screws per case for example.
---------------------------
Anyway, while reviewing the PEMNET standoff catalog, I noticed a problem:
The F dimension refers to the minimum depth of the threads of the standoff. As I said above, I needed the 8mm long standoffs to fit the secondary motherboard layout, but that meant a thread depth of only 4mm, which is less than even the 4.7mm male threads of the McMaster-Carr standoffs! The standoff could bottom out and the motherboard then wouldn't sit at the right height for everything to fit
But I can fix this! These are the Blind Threaded standoffs, the "blind" part referring to the fact that the hole does not go all the way through the part, you can't see through the hole. The alternative would be Through-Hole standoffs, where the hole does goes all the way through the standoff. As you might have noticed, engineers suck at naming things
Problem solved!
And I guess I could take those male-female standoffs, screw them into those now exposed threads on the outside of the case and have the world's smallest open-air test bench
---------------------------
Next update I'll go over screw thread types, how exciting!*
*insert heavy dose of sarcasm here
Table of Contents
Next Update
Last edited:
Excellent movie reference !
So that leaves you with 27mm height for the CPU heatsink in that orientation ? That's awfully tight !
Less actually, 35mm includes the sheet thickness. So subtracting that leaves 25.7mm for the heatsink. I'm planning to use the SilverStone AR04 but if anyone has suggestions I'd love to hear it.
And I just realized if I used spacers so the male threads on the outside standoffs don't interfere with the inside standoffs I could mount both Mini-STX boards at once. So while some people have a system with room for two ATX power supplies, I'll have an ATX unit with room for two systems
Is there a reason you're not sourcing self-clinching standoffs that are 35mm instead of stacking standoffs? Since that seems to be your minimum clerance on the CPU side.
Speaking of standoffs, there's a parameter named "Min. Dist. Hole Centerline to Edge". Is that from the centerline of the hole to the edge where the bend radius starts, or centerline to the edge where the bend's inner wall ends up?
Speaking of standoffs, there's a parameter named "Min. Dist. Hole Centerline to Edge". Is that from the centerline of the hole to the edge where the bend radius starts, or centerline to the edge where the bend's inner wall ends up?
Partly because I want to experiment with having the motherboard on top the 8mm standoffs without the GPU installed so I can fit a larger heatsink. And partly because there aren't 35mm PEM standoffs. There may be some on Alibaba but I don't want to go to the trouble for just 4 standoffs.
I'm going to go over that in a later update but Penn Engineering actually defines it from the centerline to the outside of the bend radius. Page 12: http://www.pemnet.com/fastening_products/pdf/Handbook.pdf
I've noticed that with thicker sheets and bigger bend radiuses the "Minimum Distance Hole Centerline to Edge" number can be far too low though.
Ok, I know you're probably bored to death of talk about threads after Part 5 and Part 5b but being able to fasten the parts of a case together is important so bear with me.
A brief primer on screws
I'll briefly go over a few aspects of screws we need to know for designing SFF cases. For a more in-depth practical overview this is a great guide: http://store.curiousinventor.com/guides/Metal_Working/Screws/
Screw types
To the layman, once you've seen one screw you've seen them all, but there are actually many different types of screws for different purposes: decking, drywall, concrete, wood, self-tapping, etc.
For sheet metal cases there are two types we're interested in.
Machine screws
The "normal" type of screws used with computers. There isn't a real solid definition of what constitutes a machine screw, but generally they are non-tapered screws of smaller diameters designed to be used with existing threads.
Self-tapping
As the name implies, these types of screws create their own thread (tap) when driven into the material. Most commonly used for fan screws, where they cut their own thread into the plastic frame of the fan.
They could also be used for a plastic case, and there are self-tapping sheet metal screws, but in general I would not recommend using them for any application that would require more than a few installations/removals over the life of the case. The threads they create are not reliable and repeated use will cause the thread to wear out, causing the screws to loosen up. For a fan this may not be the end of the world but if they're holding the case together that would be a problem
Screw heads
There are about as many screw head types as there are screw types. Main thing I want to explain is "flat head" screws, which are probably not what you're thinking of.
(a) pan, (b) dome (button), (c) round, (d) truss (mushroom), (e) flat (countersunk), (f) oval (raised head)
Image source
Pan head
A general-purpose screw head, these are readily available and are good for general case screws that don't require the surface to be flush.
Of note: Pan head screws (and the other non-countersunk screws) have their length measure from the bottom of the head to the end of the screw.
Flat head (countersunk)
What is colloquially called "flat head" screws actually refers to slotted drive screws (more on that in a bit). The term flat head screw properly refers to a screw head that has a flat outer side and a tapered inner side. When used in combination with a countersunk hole a flat head screw will be flush with the surface.
This is particularly useful for instances where, due to aesthetics or functionality, the surface of the sheet should be flat with no protrusions. Common uses are exterior panels and for the inner frame when other components need to be mounted up against where the screw is located.
Of note: Flat head screws have their length measured from the top of the head to the end of the screw.
Screw threads
Normally I would do a detailed explanation but even as much of a dork as I am about manufacturing, I find this topic extremely dull. If you're suffering from insomnia though it may be helpful reading material: https://en.wikipedia.org/wiki/Screw_thread
Luckily, there are only a few aspects of screw threads that are useful to know for designing a case.
Major Diameter and Pitch
Don't worry, it's not necessary to understand everything going on in this diagram for the purpose of designing SFF cases.
The Major Diameter (D maj) is the largest diameter of the thread. For a male thread like a screw this would be the Outside Diameter (OD).
The Pitch (P) is the distance between the tip of one thread to the next.
Thread standards
There are two thread standards used for computers.
UTS (Unified Thread Standard)
A thread standard common in the USA and Canada. Naming scheme is Major Diameter, then a dash, then Pitch in Threads Per Inch (TPI). For each size there is also optional letters for the different Pitch groups: UNC (Coarse), UNF (Fine), and UNEF (Extra Fine).
ISO metric
A thread standard set by the International Organization for Standardization (ISO). And no, I don't know why it's abbreviated ISO and not IOS Naming scheme is Major Diameter, then "x", then Pitch, both measured in millimeters.
This is a contentious topic but as you may have noticed, I generally prefer metric wherever possible. The SFF community is very much an international one and I think it makes sense for small form factor case designers to standardize on metric for measurements. Especially since everyone seems to have settled on measuring SFF cases in millimeters, which conveniently allows for easy volume calculations (multiple the dimensions in mm, then move the decimal in the result 6 places to the left to get the volume in liters).
That said, it's very difficult to ditch Standard units entirely since things like 3.5" drives and power supplies use #6-32 screws and those standards are very unlikely to change anytime soon. But for most everything else it should be practical to use M3 screws (countersunk M3 in 1mm thick or less sheet could be problematic though).
Also, if you're getting cases manufactured in the USA (or Canada too depending on the company) then almost certainly the company will be working in Standard units. This can cause issues where measurements get rounded when converting between measurement systems, so pay attention if you set your CAD program defaults to metric but the manufacturer uses Standard (or vice versa).
Fun fact: The USA uses the United States Customary System, commonly referred to as "Standard units". This is not the same as Imperial units, there are slight differences for certain measurements between the two systems, especially volume. Also, the USCS is mostly defined in terms of metric units and has been since 1893.
Clearance holes
If you need to make a hole in a part for a screw to pass through to attach to something on the other side, what diameter should you make the hole? Whatever you do, don't just make it exactly the same as the nominal Major Diameter, like a ⌀ 3.0mm hole for a M3 screw.
Like I went over in Part 3b, you need to account for the thickness of the finish on the parts, the tolerances of the manufacturing processes, and clearance fits.
Rather than making up arbitrary diameters though, there are standard tables of clearance hole diameters, usually in a range of fit classes (close/free or close/normal/coarse). Here are some examples:
UTS: http://www.phy.mtu.edu/~suits/misc/tapsizes.html
ISO metric: http://www.metrication.com/engineering/threads.html
Which fit class to use depends on the application. Generally, if there is a good chance that the parts will have some misalignment then go with one of the looser classes.
Screw drives
There is a dizzying number of different types of screw drives (and Apple adds to the list pretty regularly in a poor attempt to keep people from working on their products) but there are only a few of interest to us.
Phillips
By far the most common screw drive type for computer screws.
The bits come in different sizes, but Phillips PH #2 is the most typical. The smaller PH #1 is sometimes used for smaller countersunk screws.
Slot
A simple slot used with a "flat-head" screw driver like pictured above. Not to be confused with the flat head screw (as in countersunk) mentioned earlier.
These are not commonly used by themselves in computer screws.
Note: if you're looking for a really odd size screw you may come across some with "cross-slot" or "double-slot" drives. They look similar to Phillips at a glance but they're actually slot drives, just with two slots in stead of the usual one, and they're oriented perpendicular to one another.
Thumbscrews
Can be fastened and unfastened by hand, hence the name. Often though they have Phillip, slot, or combo Phillips/slot drives on them to aid in removal for when idiots tighten them too much during installation.
Torx
Actually pretty commonly used in computers, but you probably never noticed because it's almost exclusively for the internals of hard drives
While they are designed to be much more resistant to cam-out than the common Phillips screw, computer case screws don't generally need to be torqued down hard enough for that to be a problem.
I generally wouldn't recommend using these because most users won't have Torx bits on hand. However they may be justifiable for particularly high-end boutique cases where the "exoticness" of Torx fits the theme.
Allen
Also called Allen key, hex key, hex socket, and a few other names.
Almost never used in computers. They may fit in as exterior screws for a rugged, industrial-styled case though.
There is much more that could be said about screws, finishes (zinc, black oxide, chrome, phosphate, etc.) is a big one I haven't touched on but I'll leave that as an exercise for the reader.
So that's it right? We're done talking about threads, screws, standoffs, and the like? Nope!
There's one more post I need to do, and that is actually implementing PEMSERTs in your case. Then I'll move on to a different topic, I promise
Bonus content: Standoffs
I didn't want to make a post just for these so I'll add this extra bit about standoffs here.
Standoffs are typically used in computers to space the motherboard away from the chassis. This is done to prevent the leads on the backside of the motherboard from shorting with the chassis, and to provide room for things like the heatsink backplate and M.2 connectors.
I talked about pressed-in standoffs in Part 5 so this is just about separate male-female screw-in standoffs like in the front row of the above picture. There are a few attributes of these types of standoffs to be aware of:
So should you use pressed-in standoffs or these? The answer depends on the a few factors.
If the standoffs need to be movable, then obviously pressed-in standoffs won't work. Full adherence to the microATX spec (page 10, standoff locations R vs S) for instance requires this. Luckily for SFF case design, all 4 standoff locations of the Mini-ITX form factor are all required.
If being able to move/remove the standoffs is not required, then I would recommend using pressed-in standoffs. Screw-in standoffs require clearance space on the backside of the sheet for the male threads, while pressed-in standoffs are flush with the sheet.
Table of Contents
Next Update
A brief primer on screws
I'll briefly go over a few aspects of screws we need to know for designing SFF cases. For a more in-depth practical overview this is a great guide: http://store.curiousinventor.com/guides/Metal_Working/Screws/
---------------------------
Screw types
To the layman, once you've seen one screw you've seen them all, but there are actually many different types of screws for different purposes: decking, drywall, concrete, wood, self-tapping, etc.
For sheet metal cases there are two types we're interested in.
Machine screws
The "normal" type of screws used with computers. There isn't a real solid definition of what constitutes a machine screw, but generally they are non-tapered screws of smaller diameters designed to be used with existing threads.
Self-tapping
As the name implies, these types of screws create their own thread (tap) when driven into the material. Most commonly used for fan screws, where they cut their own thread into the plastic frame of the fan.
They could also be used for a plastic case, and there are self-tapping sheet metal screws, but in general I would not recommend using them for any application that would require more than a few installations/removals over the life of the case. The threads they create are not reliable and repeated use will cause the thread to wear out, causing the screws to loosen up. For a fan this may not be the end of the world but if they're holding the case together that would be a problem
---------------------------
Screw heads
There are about as many screw head types as there are screw types. Main thing I want to explain is "flat head" screws, which are probably not what you're thinking of.
(a) pan, (b) dome (button), (c) round, (d) truss (mushroom), (e) flat (countersunk), (f) oval (raised head)
Image source
Pan head
A general-purpose screw head, these are readily available and are good for general case screws that don't require the surface to be flush.
Of note: Pan head screws (and the other non-countersunk screws) have their length measure from the bottom of the head to the end of the screw.
Flat head (countersunk)
What is colloquially called "flat head" screws actually refers to slotted drive screws (more on that in a bit). The term flat head screw properly refers to a screw head that has a flat outer side and a tapered inner side. When used in combination with a countersunk hole a flat head screw will be flush with the surface.
This is particularly useful for instances where, due to aesthetics or functionality, the surface of the sheet should be flat with no protrusions. Common uses are exterior panels and for the inner frame when other components need to be mounted up against where the screw is located.
Of note: Flat head screws have their length measured from the top of the head to the end of the screw.
Countersinks
Countersinks are cone-shaped holes made at the surface of the part for the flat head screws (or other items like rivets) to fit in so they sit flush with the surface. An important aspect of countersinks to be aware of is the angle of the countersink.
As I hope is clear from the above diagram, the larger the angle the shallower the depth of the countersink relative to the surface. This is important to note because depending on the thickness of the material, you may be limited in which screws you can use since with some combinations of countersink angle and screw size, the depth of the countersink can exceed the thickness of the sheet. The problem with this is the lower part of the flat head screw's taper would stick out from the back side of the sheet, which can prevent if from sitting flat against the frame or whatever it's attached to.
I'll go into screw threads next but generally ISO metric countersunk screws are only available in 90°, while UTS are typically 82°. There are shallower 100° UTS screws available but they're not as common.
Shallow countersunk metric M3 exist, most notably Lian Li uses them, but they seem to be custom made because I can not find them available anywhere.
Countersinks are cone-shaped holes made at the surface of the part for the flat head screws (or other items like rivets) to fit in so they sit flush with the surface. An important aspect of countersinks to be aware of is the angle of the countersink.
As I hope is clear from the above diagram, the larger the angle the shallower the depth of the countersink relative to the surface. This is important to note because depending on the thickness of the material, you may be limited in which screws you can use since with some combinations of countersink angle and screw size, the depth of the countersink can exceed the thickness of the sheet. The problem with this is the lower part of the flat head screw's taper would stick out from the back side of the sheet, which can prevent if from sitting flat against the frame or whatever it's attached to.
I'll go into screw threads next but generally ISO metric countersunk screws are only available in 90°, while UTS are typically 82°. There are shallower 100° UTS screws available but they're not as common.
Shallow countersunk metric M3 exist, most notably Lian Li uses them, but they seem to be custom made because I can not find them available anywhere.
---------------------------
Screw threads
Normally I would do a detailed explanation but even as much of a dork as I am about manufacturing, I find this topic extremely dull. If you're suffering from insomnia though it may be helpful reading material: https://en.wikipedia.org/wiki/Screw_thread
Luckily, there are only a few aspects of screw threads that are useful to know for designing a case.
---------------------------
Major Diameter and Pitch
Don't worry, it's not necessary to understand everything going on in this diagram for the purpose of designing SFF cases.
The Major Diameter (D maj) is the largest diameter of the thread. For a male thread like a screw this would be the Outside Diameter (OD).
The Pitch (P) is the distance between the tip of one thread to the next.
---------------------------
Thread standards
There are two thread standards used for computers.
UTS (Unified Thread Standard)
A thread standard common in the USA and Canada. Naming scheme is Major Diameter, then a dash, then Pitch in Threads Per Inch (TPI). For each size there is also optional letters for the different Pitch groups: UNC (Coarse), UNF (Fine), and UNEF (Extra Fine).
#6-32 UNC is a UTS thread used in PCs. Sometimes called the "coarse" computer thread, this is used for 3.5" hard drives, power supplies, and commonly used for case screws on mass-produced cases.
#6 means a Major Diameter of 0.1380in \ 3.5052mm. 32 TPI is considered the coarse pitch for #6, hence the UNC at the end.
#6 means a Major Diameter of 0.1380in \ 3.5052mm. 32 TPI is considered the coarse pitch for #6, hence the UNC at the end.
ISO metric
A thread standard set by the International Organization for Standardization (ISO). And no, I don't know why it's abbreviated ISO and not IOS
Naming scheme is Major Diameter, then "x", then Pitch, both measured in millimeters.M3x0.5 is an ISO metric thread used in PCs. Sometimes called the "fine" computer thread, this is used for 2.5" drives, 5.25" drives, and for case screws by some case manufacturers like Lian Li.
M3 means a Major Diameter of 3mm. 0.5 means a Pitch of 0.5mm, simple enough.
M3 means a Major Diameter of 3mm. 0.5 means a Pitch of 0.5mm, simple enough.
- Of note, 0.5mm is considered the Coarse pitch for M3, so while M3x0.5 is fine in comparison to #6-32, there is a fine pitch M3, which would be M3x0.35.
- Another note, M3x0.5 is by far the most common version of M3 screws, M3x0.35 is rarely ever used from what I've seen. So it's not uncommon for manufacturers and suppliers to omit the x0.5 part. This is important to know because length is often of the "x #mm" format. So M3 x 5mm usually means a M3x0.5mm thread screw that is 5mm long. So be sure to double-check to make sure you don't accidentally order the wrong thing.
- Related to the above, the next size up is M3.5, usually in the coarse M3.5x0.6 thread. The x0.6 is also commonly left off so be sure not to confuse M3x0.5 with M3.5.
---------------------------
Metric or Standard?
This is a contentious topic but as you may have noticed, I generally prefer metric wherever possible. The SFF community is very much an international one and I think it makes sense for small form factor case designers to standardize on metric for measurements. Especially since everyone seems to have settled on measuring SFF cases in millimeters, which conveniently allows for easy volume calculations (multiple the dimensions in mm, then move the decimal in the result 6 places to the left to get the volume in liters).
That said, it's very difficult to ditch Standard units entirely since things like 3.5" drives and power supplies use #6-32 screws and those standards are very unlikely to change anytime soon. But for most everything else it should be practical to use M3 screws (countersunk M3 in 1mm thick or less sheet could be problematic though).
Also, if you're getting cases manufactured in the USA (or Canada too depending on the company) then almost certainly the company will be working in Standard units. This can cause issues where measurements get rounded when converting between measurement systems, so pay attention if you set your CAD program defaults to metric but the manufacturer uses Standard (or vice versa).
Fun fact: The USA uses the United States Customary System, commonly referred to as "Standard units". This is not the same as Imperial units, there are slight differences for certain measurements between the two systems, especially volume. Also, the USCS is mostly defined in terms of metric units and has been since 1893.
---------------------------
Clearance holes
If you need to make a hole in a part for a screw to pass through to attach to something on the other side, what diameter should you make the hole? Whatever you do, don't just make it exactly the same as the nominal Major Diameter, like a ⌀ 3.0mm hole for a M3 screw.
Like I went over in Part 3b, you need to account for the thickness of the finish on the parts, the tolerances of the manufacturing processes, and clearance fits.
Rather than making up arbitrary diameters though, there are standard tables of clearance hole diameters, usually in a range of fit classes (close/free or close/normal/coarse). Here are some examples:
UTS: http://www.phy.mtu.edu/~suits/misc/tapsizes.html
ISO metric: http://www.metrication.com/engineering/threads.html
Which fit class to use depends on the application. Generally, if there is a good chance that the parts will have some misalignment then go with one of the looser classes.
---------------------------
Screw drives
There is a dizzying number of different types of screw drives (and Apple adds to the list pretty regularly in a poor attempt to keep people from working on their products) but there are only a few of interest to us.
Phillips
By far the most common screw drive type for computer screws.
The bits come in different sizes, but Phillips PH #2 is the most typical. The smaller PH #1 is sometimes used for smaller countersunk screws.
Slot
A simple slot used with a "flat-head" screw driver like pictured above. Not to be confused with the flat head screw (as in countersunk) mentioned earlier.
These are not commonly used by themselves in computer screws.
Note: if you're looking for a really odd size screw you may come across some with "cross-slot" or "double-slot" drives. They look similar to Phillips at a glance but they're actually slot drives, just with two slots in stead of the usual one, and they're oriented perpendicular to one another.
Thumbscrews
Can be fastened and unfastened by hand, hence the name. Often though they have Phillip, slot, or combo Phillips/slot drives on them to aid in removal for when idiots tighten them too much during installation.
Torx
Actually pretty commonly used in computers, but you probably never noticed because it's almost exclusively for the internals of hard drives
While they are designed to be much more resistant to cam-out than the common Phillips screw, computer case screws don't generally need to be torqued down hard enough for that to be a problem.
I generally wouldn't recommend using these because most users won't have Torx bits on hand. However they may be justifiable for particularly high-end boutique cases where the "exoticness" of Torx fits the theme.
Allen
Also called Allen key, hex key, hex socket, and a few other names.
Almost never used in computers. They may fit in as exterior screws for a rugged, industrial-styled case though.
---------------------------
There is much more that could be said about screws, finishes (zinc, black oxide, chrome, phosphate, etc.) is a big one I haven't touched on but I'll leave that as an exercise for the reader.
So that's it right? We're done talking about threads, screws, standoffs, and the like? Nope!
There's one more post I need to do, and that is actually implementing PEMSERTs in your case. Then I'll move on to a different topic, I promise
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Bonus content: Standoffs
I didn't want to make a post just for these so I'll add this extra bit about standoffs here.
Standoffs are typically used in computers to space the motherboard away from the chassis. This is done to prevent the leads on the backside of the motherboard from shorting with the chassis, and to provide room for things like the heatsink backplate and M.2 connectors.
I talked about pressed-in standoffs in Part 5 so this is just about separate male-female screw-in standoffs like in the front row of the above picture. There are a few attributes of these types of standoffs to be aware of:
- Typically the length listed is for the body only, the male threads are measured separately.
- The thread type of the male and female threads won't necessarily match. It's possible to have a standoff with #6-32 male threads but M3 female threads or some other combination.
- The body is usually hexagonal for use with nut drivers, the diameter is measured from flat to flat.
- The common motherboard form factors (ATX, mATX, Mini-ITX, etc.) have a minimum 6.35mm (0.25in) clearance between the board and the chassis so that's the typical length of computer standoffs.
- Steel and aluminum standoffs are used, but brass is a very common material (I think due to ease of machining).
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So should you use pressed-in standoffs or these? The answer depends on the a few factors.
If the standoffs need to be movable, then obviously pressed-in standoffs won't work. Full adherence to the microATX spec (page 10, standoff locations R vs S) for instance requires this. Luckily for SFF case design, all 4 standoff locations of the Mini-ITX form factor are all required.
If being able to move/remove the standoffs is not required, then I would recommend using pressed-in standoffs. Screw-in standoffs require clearance space on the backside of the sheet for the male threads, while pressed-in standoffs are flush with the sheet.
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This entire log should be in the references section lol, would answer so many of the "I want to build a scratch case, where do I start?" styled questions that eventually pop onto forums like this.
Oops, forgot the section on screw drive types. Fixed.
I'm planning to go through and reorganize and clean them up to post as a series on the news site. Eventually...
I'm planning to go through and reorganize and clean them up to post as a series on the news site. Eventually...
I was thinking the same thing: "this should all be in the Wiki". I tip my hat to you, sir !
My beef is with the outdated measurement system we use
Also:
Excuse the blurry pic
Also:
Excuse the blurry pic
Im honestly surprised that I didnt see the old plug style stand offs in the bonus content. They are hard to find now since they were primarily used in HP prebuilts when the core 2 duo launched lol. I still have a drawer of them though, They are super convenient for scratch builds since you cut a slit and just slide them in from different angles and use slight pressure to keep them in place.
He's installing one of the PEMSERT nuts.
That's mainly why. They're not common and this writeup is mostly aimed for those designing a case for production rather than a comprehensive documentation.
It works! At least at idle
So this is the ASRock H110M-STX + GTX 1060 running off the HDPLEX 160W AC-DC + 160W DC-ATX.
So this is the ASRock H110M-STX + GTX 1060 running off the HDPLEX 160W AC-DC + 160W DC-ATX.
I bought the DeskMini 110W barebones to get it: http://www.newegg.com/Product/Product.aspx?Item=N82E16856158048
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