Screw decisions for assembly

[levi]

FWIW, both the screws in my lid and the screw I used to replace the on I broke in my base (which was designed to be identical to the one I replaced) are both non-self-tappers. I assume they all are, though I can't get any of the base screws out of my Pandora without turning it off.

 

[God Ginrai]

I don't get it... why do they need a clip?

-God Ginrai

 Screw enters from the bottom of the device.  Current design is to have it thread into a pillar of plastic on the upper case.

Replacing the pillar of plastic with something metal is a good idea - you get that, right?

In order to have that metal interface it has to anchor to the UNDER side of the upper case plastics and stay put during assembly.  Clip on nuts would clip to the plastic (replacing the pillar) and hold-put during assembly and not fall out on the floor.

If standard hex nuts were used - how wold they interface with and hold onto the upper plastics?

More realistically, if this is to be done 'right', plastic (carbon fiber?) pillars should be OK.  However, to make a set of reusable threads in soft material, it would be better to machine out a set of coarse and deep threads (thread depth/cut not screw length).  Then use a matching screw.  Think of a wood/deck screw with the point removed going into a pre-tapped socket.

The Pandora's screws were relatively fine threaded machine screws - not much depth to the thread AND they were expected to self-thread.

Conversely - machine threading the screw holes on every case would probably add $5 to each case.  The outcome would be robust, but a tap drilling robot would have to be set up to tap out each screw pillar in the top plastics.

Ah, I was afraid this was what you were suggesting... I didn't want to replace plastic threads with metal ones. I wanted a screw and washer nut to push the two parts of the case together. Hence, no need for a clip.

-God Ginrai

 

[levi]

Are you suggesting loose washers inside the case, possibly inside a recess to stop them spinning or embedded into the case? If the former that sounds like it would make the assembly very fiddly, and make the case weaker as rather than having a thick wall the screw screws into, you'd have to have the case top fold back on itself making it more liable to fracture (a problem which would affect clip on nuts, for that matter). If embedded in the plastic, the solution I usually see is more star shaped than hex, since there is no requirement to slide a spanner on to it, and the pointier, uh, points resist turning better.


A few thousand nuts might be cheaper though. Dunno.

 

[God Ginrai]

I'm suggesting neither.

Screw

Case

Washer Nut

^ That's what I'm suggesting.

-God Ginrai

 

[levi]

Hang on, why am I talking about washers?  Where I come from, washers aren't threaded, so don't stay on threads by themselves.  What you posted a picture of appear to be nuts.

To clarify, I've made this picture:

The first picture is my representation of the Pandora design, screwing into plastic. The other two pictures are in order, my interpretation of your words, firstly with a loose nut in a recess, and secondly as a nut embedded into the top case.  It sounds like you mean the middle one, which I have criticised as fiddly and fragile.

 

[God Ginrai]

Yea, I accidentally started saying Washer, sorry. <.< The closest to what I have said is the one in the middle. However, I'm talking about both the Nut and screw head existing on the outside of the case. This way, you can hold the screw head in place as you remove the nut with a socket wrench of some sort.

-God Ginrai

 

[rygD]

I agree with the title, screw these decisions.

 

[levi]

Oh, you mean a bolt that goes right through the case and out the other side?  Hmm, you generally need the bolt thread to go through the nut and out the other side, as the low-friction metal-to-metal interface will work loose over time if both ends are free to rotate.  To minimise that you normally add two nuts and twist them against each other to jam the thread and lock them in place.  How would you do that and avoid the bolt sticking out and either scratching the table you put it on, or stopping the lid from closing?

Thinking about it, the screw working loose might be an issue with an embedded nut type arrangement as well.  Not sure why my multimeter battery cover screw doesn't work loose - possibly because the rest of the case is so rigid that there's no flex causing it to move, dunno.

 

[God Ginrai]

I'd cover it with one of those rubber covers that screws get covered with in a lot of consumer products. The screw shouldn't work loose or scratch the table with that in the way. As for stopping the lid from closing, the screw well would be recessed, so there would be no problem.

-God Ginrai

 

[Yoyobuae]

What about the nut then? Those can't be recessed (couldn't reach them then).

 

[God Ginrai]

Why can't it be recessed? If you make the well large enough that a socket can get in, then you can reach it.

-God Ginrai

 

[comradekingu]

Exposed and flush torx-screws and flange nuts 

on the back of the screen and on the bottom of the device along with some threadlocker for good measure would be very nice and easy enough to do.

What makes this easier is the nut doesnt have to be glued or forged into the plastic.

However there isnt enough space on the bottom of the device (pandora) to do that, two of them are just on the outside of the battery cover, and one is underneath the hinge.

Edit: i looked at the pyra, and there is only 4 screws underneath for that, which makes sense. I think maybe moving the upper two screws put them in the L/R buttons.

I doubt that makes the middle of the PCB flex any more, dont know how much that matters, but it was a non-issue on the pandora.

 

[God Ginrai]

Well yes, but if you keep such a solution in mind as you are designing the product, you can look for places where you can hold the device together in such a way, can you not?

-God Ginrai

 

[Yoyobuae]

Why can't it be recessed? If you make the well large enough that a socket can get in, then you can reach it.

-God Ginrai

If you make the well large enough for that then your 6-button layout wont fit... :rolleyes:

But seriously, larger screw wells do take away some PCB/keymat/etc space.

 

[Bosbeetle]

Ahem - those nuts are not appropriate for the application. Try these instead:

http://www.mcmaster.com/#clip-on-nuts/=rw9hd7

This is actually a nice idea...

however I'm fine with torx or even philips.

 

[fusion_power]

Maybe the screws could be part of the design if done right, like this nice iPhone protection hull here:

https://www.kickstarter.com/projects/1104350651/taktik-premium-protection-system-for-the-iphone

Looks really durable. ^^
 

 

[comradekingu]

Thermoset threaded inserts could be hidden in the gap on either side of the battery. With flush torx screws on the upside that would be really nice, but might not be the best technically.

Hexnuts atop with torx screw would mean anyone with a spanner could open them.

Or you could have ones that are just inserted into the holes underneath.

Both of these return to the question of moving the two screwholes underneath the battery over to the L/R side chamber. (like it is on the other side)

Usual size M2.5, which is a standard of sorts, but can be had smaller. M1.4 and 1.6 ive seen mentioned, but check out microbarb M2

http://www.kvt-fastening.de/media/20f2a6d9-4dd5-c410-80f6-91ad49e8aa1f/KVT_TAPPEX_EN_04-03-2013_web.pdf

Regular press fitting

In a perfect world the L/R screws would also double as how to screw the L/R switches, some switches, especially the dual (L1 L2) require the hinge part to be screwed, but i dont see

that breaking with just a plastic insert, as it isnt where

Also thats directly below the speaker, so thats out. Otherwise the L and R would be hinged the other way.

The idea with a screw is to depress only the switch part, after max actuation is reached, excessive force is taken by the screw assembly into the case.

Its not so much for user failure, but moreso if you drop the unit and it lands on its tip edge.

Edit: I really like the new way of being able to take the screen out without further assembly. Not being able to unscrew the case before removing the battery has its upsides too.

If the L/R button is a standalone unit then it would be easy enough to ship even bigger batteries with new bottom covers.

 

[Binky]

Has anyone mentioned Allen bolts yet?

or captive nuts held in the plastic?

 

[TrashyMG]

Well they have special threaded inserts for plastic molding..

http://www.tristar-inserts.com/threaded-inserts.htm

 

[comradekingu]

Hehe, would be nice with little thumbscrews on the bottom to double as legs. I suppose that ruins pocketability a bit for some, but certainly a nice casemod.

Edit: Allen key nuts are called hexnuts elsewhere.