A topic I haven't talked about in too much detail yet.

Its something I'm constantly thinking about whenever I change something in the design. The tricky part is that I want to reach two main target audiences which require somewhat conflicting mechanical design.

The first one being the general open source crowd, no matter if they are interested in hardware or not, there is quite a lot of interest from people who are interested in Linux and open development. They mostly want to see a tablet that comes close to a polished commercial tablet, from the feedback I've got it seems they are fine with something thicker but it didn't seem there was a ton of interest in fiddling with hardware or getting something that works but looks ugly. Small to mid sized industrial companies are also pretty interested in something like this, they would require a more rugged case and are also fine with thickness but also require a finished and polished product that is well integrated.

Catering to this crowd is actually not hard, you require a few injection moulded pieces of plastic (I try to keep it as simple as possible, so far I see about 2-3 large pieces of plastic) and make a design that is constrained and tries to squeeze the hardware into any space there is to yield a tablet that looks nice and tidy, just as a tablet would look like that you bought from the usual Manufacturers. Sure there will be a few iterations until it comes out perfect but injection moulding is a well understand process and can be done in any quantity now a days. There is polyurethane casting for prototyping and small runs of 10-100pcs.

Next step would be injection moulding with an aluminium tool, they are a lot cheaper to manufacture than the high-end steel ones and yield y few hundred to maybe 1k pieces and if you are going really big you can scale up to steel tools and produce several 10k-100k per tool ( I pulled those numbers out of my nose but this is the range I remember).

The issue is that I can't design the mechanical parts so that they are only feasibly manufacturable with injection moulding.

This is because I have a second target audience that I personally find most exiting and is what drives me personally. It's the hardware hacker crowd, the educational crowd, the mid-size company that wants 10 super specialized tablets that integrates their own weird oddball hardware.
Those people want to modify the hardware, they want to experiment, they want to make their own spin-off.

For this audience I need to think about hack-ability. It doesn't mean they don't want a somewhat good looking tablet, but extendability is key here.

This is one reason why I design the electronics in a very modular way, no peripheral is fixed, everything is changeable. Don't need HDMI output? Then don't get the HDMI PCB. Need it 2 years later? Just buy it for a small amount and add it to your tablet, no need to buy a whole new thing. This not only allows upgrading to some degree, it also allows for great repairability which is something pretty much everyone likes.

Now there is a catch here, how do I add an HDMI port to a case that doesn't have an opening for it?
This why I decided to design the case in the following way. There will be a display assembly that consists of LCD, Touchscreen (the two will not be bonded for repairability) and a plastic 'spacer' that creates a frame around the LCD. This assembly is then acting as a flat surface where the aluminium plate will be attached to.

This plate will have a sort of pegboard pattern of 2.5mm threaded holes where the electronics can be attached to, this allows for free placement of all peripherals and the motherboard. This plate can be easily manufactured in lots of different processes, its thin enough to be done in a sheet metal process, using water-jets or lasers, or more expensively, on a milling machine.

The tapping will be a bit more expensive than just having threads in a limited amount of places for a fixed set of peripherals but this can be done with CNC (not actual milling) and is a standard part of a sheet metal workflow.

So far so good, none of these parts need to be exchanged if you were to change a peripheral or anything else in the electronics design.

Only the last part gets tricky, its the back. This is a plastic piece that will serve as nothing but the cover for the electronics, nothing will attach to it, it just covers it up. It's also the driving factor of how the the Tablet will look.
The way I have the design in mind so far is that this back can be produced either in a high-end manufacturing way (e.g. moulding) or in a home shop through 3D printing or milling, heck even laser cutting can work, you just need to laser spacers to put between the peripheral connector and have one large plate cover everything up.

The possibility for easy prototype manufacturing will be important for testing out your own variants of the tablet, this is important for business users and of course hackers.
There is a not so small market that is in need of very specialized portable electronics where a tablet is 90% of what you need but the remaining 10% is their own specialized hardware that they just need to integrate into the tablet, DLT will serve as a perfect basis for these kind of projects.

Now back to our average Linux lover who wants to add an HDMI port because his needs changed.
I want to offer him several ways of doing that, one I just described right now, visit your hackerspace and make something yourself. That's something very few will do but they have the option.
The next option will be getting a new back, maybe you opted for the integrated back cover before, unfortunately you will have to ditch this, right now I don't see any feasible way around this but you can keep the most expensive and resource hungry parts of the tablet.

You will then have to get the second variant that I plan to offer, it will be similar to the first but the sides of this variant will be open except for the corners, the corners are where the large screws go in, to hold everything together (look at the early mechanical design concept for a visualization).
Now you can get a set of pre-made 'jackets' for each peripheral so they are encased nicely and a set of spacers that fill the holes between the peripheral connectors, this allows for a pretty free placement so everything is located where you want it to.

These spacers and jackets will be screwed down onto the aluminium plate just like the peripheral PCB's. Thanks to counter sunk screws they will end flush and you can put the finishing back plate onto everything just like you did before.

I'm not sure if I will go with these two variants, maybe I will only make the customizable one, this really depends on how much my customers are willing to do some end assembly but from the current feedback I feel there is a large crowd that will be happy with a minimal set of peripherals and a finished product that requires no assembly and for those the cheapest and easiest option will be a 'unibody' plastic back.

Of course there is also the possibility of made to order tablets where the customizable variant is build for you but this will jack up prizes and is not something I see as viable in the near future.

I try to add some picture to this blog post later on to illustrate the concepts better, for now the wall of text has to suffice :)

I focused solely on the mechanical manufacturing part here, getting PCBs assembled is something so standard I don't think I have to go into details here. There is nothing particular unique in terms of parts for DLT.