Poor choice of mounting holes: should be 3 mm, not 2.9 mm !


22 posts
by ahi » Mon Jan 21, 2013 2:13 pm
Poor choice of mounting holes: should be 3 mm, not 2.9 mm !

I have tested this myself: a properly drilled hole using a good quality 3 mm drill bit will accept a 3 mm machine screw with no problems whatsoever.

But a 2.9 mm hole will need 2.5 mm screws; these are hard to find and expensive, especially when the only place that sells them will add a substantial amount of shipping/postage and handling charges, and you do not wish to order anything else from them.

The Raspberry Pi foundation advices against re-drilling those holes with a bigger drill bit, and warns that doing so will damage the raspberry pi circuit board.

Question: Does this warning apply even to re-drilling with a 3 mm drill bit ? After all, the difference is only 0.1 mm, but would allow to use mounting screws that are easily and cheaply available, even in small quantities.

Like I have already written, 2.5 mm screws must be purchased in a plastic bag containing one hundred screws and the same goes for 2.5 mm nuts as well. And, since local stores do not sell this size, that will add postage fees as well, totaling over 11 euros just for two screws and two nuts for them !

Even big electronics stores like mouser.com, they do sell 2.5 mm machine screws in single quantities but NO matching 2.5 mm nuts: so just screws without matching nuts are useless! And, theit price for 2.5 mm screws is much more expensive than a local shop's price for 3 mm screws. The benefit here would be that if you order other products from mouser anyway, there will be no extra shipping costs for the screws; instead, if your order is at least 65 euros, they offer free shipping, and no handling fees anyway.

Just for information, here's the only shop's prices in Finland that sells 2.5 mm screws and nuts:

2.5 mm nuts, a bag of 100 pieces, 1.51 euros.
2.5 mm screws (threading length 12 mm), a bag of 100 pieces, 2.93 euros.
6.90 euros shipping and handling fees at minimum. (express shipping more expensive)

So, this means that just to get 2.5 mm screws and bnuts, a total order will be:

1.51 + 2.93 + 6.90 = 11,34 euros which buys you 100 screws and 100 nuts.

Prices above include the finnish VAT of 24%.
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by jamesh » Mon Jan 21, 2013 2:27 pm
It's worth noting that these are not actually mounting holes - they are holes to help locate the PCB during the manufacturing process (which is why they are the size they are). So although they can be used as mounting holes that is not their primary purpose.
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by MattHawkinsUK » Mon Jan 21, 2013 2:39 pm
Just order some 2.5mm nuts/bolts from eBay in bags of 10. I'm sure the ones I ordered were not that expensive and the postage was minimal given their weight.
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by rurwin » Mon Jan 21, 2013 2:40 pm
What about this example on eBay? 10 to 50 screws nuts and washers. £1.99 for 10, £4.75 for 50. £2.90 P&P to Finland by the 31st January.
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by Gert van Loo » Mon Jan 21, 2013 6:26 pm
Interesting, I tried several 3mm screws and they all fitted through the holes.
So I looked up the M3 standard and indeed an maximum size ISO M3 would not fit through the hole.
But that is theoretic. Practically there does not seem to be a problem to find a fitting screw.

Time for one of may favorite sayings:
"Theoretically, theory and practice should be the same. Practically they are not"
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by joan » Mon Jan 21, 2013 6:35 pm
My 3mm fixings fit as well. A tight fit but they go through.
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by aTao » Mon Jan 21, 2013 6:46 pm
The standard M3 screw has a thread depth of 0.24mm. All you need to do is file off 0.05mm radius and an M3 screw will fit and still hold a nut.
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by techpaul » Mon Jan 21, 2013 7:49 pm
Local hardware shop, or builders merchants or DIY store should have M2.5 screws and nuts some might even have M2.5 spacers (a more rare bread).

I could find them in lots of places locally and ata push Maplins locally here in UK.
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by Joeker » Wed Jan 23, 2013 2:13 pm
The choice of the diameter for the mounting holes is really very poor.
I think the best would be, if the holes have a diameter of 3.2 mm
This is the ISO through hole diameter for M3 srews.
I hope it would be changed!
Cause M3 screws are often much cheaper then M2.5 screws and easier to get in small packages at a local store.
There even alternatives to standard screws like self-adhesive feet PCB spacers,If the holes would have a size of 3.2mm.
I bought some and they fit very tight in the holes after I compressed them with a
calliper.
But I think i have to destroy them, if I want to remove the Raspberry Pi from the PCB spacers
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by Jessie » Wed Jan 23, 2013 11:42 pm
A 1/8th in drill bit is 3.175 mm.
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by clive » Wed Jan 23, 2013 11:50 pm
Joeker wrote:The choice of the diameter for the mounting holes is really very poor.


jamesh wrote:It's worth noting that these are not actually mounting holes - they are holes to help locate the PCB during the manufacturing process (which is why they are the size they are). So although they can be used as mounting holes that is not their primary purpose.
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by AVIESTech » Thu Jan 24, 2013 9:47 pm
clive wrote:
Joeker wrote:The choice of the diameter for the mounting holes is really very poor.


jamesh wrote:It's worth noting that these are not actually mounting holes - they are holes to help locate the PCB during the manufacturing process (which is why they are the size they are). So although they can be used as mounting holes that is not their primary purpose.


Use 4-40 screws and nuts if you are looking for a perfect fit, they are about 2.85mm.
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by techpaul » Thu Jan 24, 2013 9:54 pm
AVIESTech wrote:Use 4-40 screws and nuts if you are looking for a perfect fit, they are about 2.85mm.

Which also happens to be the standard size of screws for D type connector locking screws plenty of 4-40 spacers out there...
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by pygmy_giant » Thu Jan 24, 2013 10:08 pm
I tried square pegs but they also would not fit in this round hole.
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by ceteras » Fri Jan 25, 2013 10:28 am
jamesh wrote:It's worth noting that these are not actually mounting holes - they are holes to help locate the PCB during the manufacturing process (which is why they are the size they are). So although they can be used as mounting holes that is not their primary purpose.


That sounds just terrible. When revision 2 was announced, mounting holes were said to be there for the purpose of mounting.
For manufacturing purposes, the boards are panelled in 6 packs, and these 6 pack panels have the 4 tooling holes for controlling the panel location in the machines and 3-4 fiducial marks for guiding the pick-and-place machine so it knows precisely where to put the parts. Also I see 4 of these fiducial marks on the Pi itself.
I get the feeling from your comment that you treat your readers like complete noobs, or perhaps Sony doesn't use standard PCB manufacturing techniques... Guiding with holes, never heard about.
This sounds like S.Jobs "You're holding it wrong".
I was hoping that these holes will fit the standard mounting screws used in computer cases for installing optical units (or 2.5" drives in laptops, I've got tons of them). And one of my Pies warranty went away after I've enlarged the holes to fit just that kind of screw.
Still works, YMMV.
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by rurwin » Fri Jan 25, 2013 10:39 am
If I remember correctly, they are for automated electrical test, rather than pick-and-place or stencil positioning.

It has always been clear that their primary purpose was not as mounting holes and that care should be exercised in using them for such: http://www.raspberrypi.org/archives/2691

ETA: It's probably the last scene in this video that uses the holes. That jig she puts the cards in must measure signals at various places across the board and the holes are used to align the probes. It certainly isn't aligned by image-capture.
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by RaTTuS » Fri Jan 25, 2013 10:39 am
ceteras wrote:...
That sounds just terrible. When revision 2 was announced, mounting holes were said to be there for the purpose of mounting.
....

no - they never have said they are mounting holes they have mentioned they can be used as mounting hoes but they are not mounting holes as the 6lasyer board can be damaged by too much pressure on them ;)
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by aTao » Fri Jan 25, 2013 10:43 am
A couple of 2 1/2" or 3" nails oughta do it.
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by ceteras » Fri Jan 25, 2013 10:52 am
RaTTuS wrote:no - they never have said they are mounting holes they have mentioned they can be used as mounting hoes but they are not mounting holes as the 6lasyer board can be damaged by too much pressure on them ;)


http://www.raspberrypi.org/archives/1929 wrote:"Mounting Holes!"


Ok, I'm sorry for the rant, the message was clear back then when it was announced.
I guess I am a noob at reading comprehension after all. :oops:
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by AVIESTech » Fri Jan 25, 2013 3:57 pm
techpaul wrote:
AVIESTech wrote:Use 4-40 screws and nuts if you are looking for a perfect fit, they are about 2.85mm.

Which also happens to be the standard size of screws for D type connector locking screws plenty of 4-40 spacers out there...


Indeed :)
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by mahjongg » Sun Jan 27, 2013 1:27 pm
ceteras wrote:
RaTTuS wrote:no - they never have said they are mounting holes they have mentioned they can be used as mounting hoes but they are not mounting holes as the 6lasyer board can be damaged by too much pressure on them ;)


http://www.raspberrypi.org/archives/1929 wrote:"Mounting Holes!"


Ok, I'm sorry for the rant, the message was clear back then when it was announced.
I guess I am a noob at reading comprehension after all. :oops:

As in mounting them in the Automatic Test Equipment, (http://en.wikipedia.org/wiki/Automatic_test_equipment) see this sentence in the very same blog about it.
Two 2.5mm (drilled 2.9mm for M2.5 screw) non plated mounting holes have been provided to assist with ATE test mounting. Positions of these holes relative to the bottom left of the PCB (Power Input Corner) are:.....


additionally in the very first announcement of these holes it was made very clear they were never intended to be used to mount the PI, but were used for testing purposes. From that blog.
Pete Lomas always asks me not to refer to the holes, which are there to aid the robots in the production process, as mounting holes

But I get that people that desperately wanted to mount their PI boards just glanced over that.
In fact most enclosures for the PI have their own mounting fixtures, and the PI is already fixed in the enclosure without the need of "mounting holes".
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by Schorschi » Sat Mar 30, 2013 5:30 am
Those of us that mount Pi to panel boards for big relay or custom PCB boards run into this issue all the time. I use standard 3M female offset mounting screws, I just have to slight enlarge the 'mounting' holes. This can be done using a hand tool, there is just enough board material between the 2.9mm hole and the copper ring to honor a 3M female offset mounting screw. Just about everyone has 3M female offset screws... almost every computer case comes with them. Heck, you can even get brass or plastic bulk units of 50 on Ebay!
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