[Arm-netbook] 15in laptop casework progress

[Luke Kenneth Casson Leighton]

On Wed, Jul 1, 2015 at 11:05 AM, Ken Phillis Jr <kphillisjr at gmail.com> wrote:
> Looks interesting, and does this include options to use a laptop Hard drive
> or Laptop CD/DVD Drive?

 no - there's no space, and it's not safe to do so (mechanically):
i'll explain why.

 ordinarily, laptop casework would be made as a "single unit", right?
the base for example is a single huge piece, requiring something like
60,000 kg of force to push the plastic through the injection mold,
spreading out across an area 35cm x 20cm at a depth of only around 1mm
in depth.

 throughout that base there are various "struts" which help support
the base: strengthening around holes (such as connectors, cd/dvd
drives, and so on), and internal dividers for PCBs, as well as random
bits that stop the case from warping when it cools.  the warping
occurs because the extreme pressure results in different stresses
across different parts of the plastic, so you get some parts "pulling"
and others "pushing" - end result: warping.  bunnie huang has a very
good article about the process if you're interested:

 http://www.bunniestudios.com/blog/?p=4146

 so for a huge hole like a CD/DVD, you can put a huge amount of
internal "support", you can create a structural "bay" that is anchored
to the rest of the casework, compensating for the fact that, on the
outside, you've a huge weakness on the outside edge.

 however what i am doing is completely different: i don't have a 3D
printer that is capable of printing a single block on a 400 x 300mm
bed, and even if it was possible to do i still would not do it because
that would require about 50 continuous hours of sitting there
monitoring the print to make absolutely absolutely 100% absolute
without fail absolute certainty that it printed without a fault.  any
mistake even at time 49 hours 59 minutes, the end result must be
thrown away.  i'm not tolerating that:

 http://motherboard.vice.com/read/how-to-3d-print-a-laptop

 so instead i am designing this in a series of parts, each of which is
a maximum of 2 metres of 3mm plastic (about 18 grams), and there are
around 15 separate parts.  each may be printed separately: one error
clearly does not result in the entire lot being thrown away.

 also, the middle areas, in order to save plastic, i aim to use bamboo
or other wooden laminate.  these are *not* joined to the rest of the
parts (plastic): they are *not* structural: they are merely
protective, and the interior parts will be taped down with
double-sided sticky tape (in a "sandwich" effect) with the bamboo
being top and bottom, and jammed into and surrounded on all 4 edges by
plastic.

 so - and this is where you get your answer, ken - each part needs to
be strong *on its own*.  i have interior cross-hatching inside the
middle strut (running from front to back) which ensures that the strut
does not significantly bend or twist.  the 4 front and back parts
(left and right) all require interior bracing.  i have not arranged
any holes in these 4 front and back parts, so they are quite strong.

 however, the left and right edges are where the connectors are poking
through, and it is here where there is vulnerability to stress.  in
particular, the EOMA68 connector is quite long - 55mm - and 5.5mm
deep. that leaves only 3mm *above* the CPU card and only 2mm *below*
the CPU card, for a total depth of 10.5mm in the casework.  that left
edge is only 12mm wide...

... can you see how that might not be very strong? :)

i can't add any extra support anywhere else, so i am going to have to
rely on the PCB itself (1.2mm thick) to provide some structural
support, here.  down the entire length of the left (and right) edges i
have added a 3mm high strut, and underneath the CPU Card i have filled
that in to the maximum possible height (which is only 2mm), but i
consider this to be marginal, even though it is the best that can be
done.

regarding a hard drive bay: yeahhh there just isn't enough space.  i
had to move the touchpanel / touch-LCD over slightly, due to its
internal connectors coming out in ways that were slightly awkward.
that reduced the depth of the (only) available interior space - front
left corner.

so the only available space for a hard drive is just a tiny bit too
small, but it *is* enough for a USB memory stick (or USB WIFI, or USB
3G).  i have made space for 2 such USB dongles.

the entire right-hand side is taken up with a battery (10Ah).  it's
actually a bicycle battery, and as such is far lower cost than
anything that's actually "designed" for a laptop.  the reason why the
battery is not at the back (and removable) is, again, as you can see
from the above: strength.  mechanically it is too difficult to arrange
a strong compartment at the back of the laptop.

hope that explains why it is not really possible in this design to fit
a hard drive or a CD/DVD.

the other reason why it is not possible is because there is simply
insufficient power.  i have had a heck of a job finding a suitable
low-cost PMIC that can provide the amount of current required to drive
the LCD and the CPU Card *and* 4 USB devices (the LTC 4155) - it can
provide up to 4A @ 5V and it isn't hugely expensive.

if there was an internal DVD and an internal HDD, that power
requirement would go up to 6A @ 5V (30 watts).  now that's starting to
get expensive, as well as almost certainly requiring fans (to cool
down the PMIC board).

also it would be necessary to start investigating 2-cell or 3-cell
PMICs for battery charging, because 6A @ 5V is a hell of a lot (4A is
already into 2oz copper) - and that not only starts to get complicated
but also gets costly, *and* i would have to abandon the simple
single-cell approach *and* start the process of contacting
suppliers... *again*.... asking around for different-sized batteries.

so in summary, can you see, ken, how such a simple-sounding question
has absolutely massive implications which, in effect, require
completely abandoning absolutely everything that i've done in the past
6 months, in effect starting *entirely* from scratch, abandoning all
the component sourcing, PCB design work *and* the casework designed so
far *and* requiring to research an entirely new mechanical design
approach (from scratch) in order to accommodate the
innocently-asked-for parts? :)

this is a simple laptop, with a very straightforward design, where the
target sale price (even in 1k volumes) i would like to be around the
$USD 350 mark.  there's plenty of storage space: two Micro-SD cards
(one internal in the laptop, one in the CPU Card), on-board NAND flash
for OS, and there are 2 internal full-sized USB connectors behind a
removable compartment.

l.