Standoff size and side height

Standard PCB templates and project cases

Standoff size and side height

Postby ian » Thu Jan 19, 2012 8:49 am

ian wrote:I like both Tayken's concepts, and we could easily sell the same top-sheet with slots cut in anticipation of sides. The sides do raise the question of - what thicknesses do we support?

My material of choice is 3mm acrylic, but that can change. It is really easy to cut and durable.

For height I'm thinking about using standard standoffs in calculations and those can come with cases too. Something like:
- 5mm standoff + 1.6mm PCB + 10mm standoff = 16.6mm side
- 5mm standoff + 1.6mm PCB + 15mm standoff = 21.6mm side
- 5mm standoff + 1.6mm PCB + 18mm standoff = 24.6mm side

Here is a catalog page I used, standoffs can be metal or nylon but I like 5.0 hex diameter aluminum or steel ones.
User avatar
ian
Crew
Crew
 
Posts: 10803
Joined: Mon Jul 06, 2009 6:14 am

Re: Standoff size and side height

Postby ian » Thu Jan 19, 2012 9:14 am

PCB hole is intended for M3 or 3mm diameter screw threads.
The HEX part looks to be 5MM in diameter on the typical standoff.

At mouser it looks like I can get 3,4,5mm,etc. I will order a range of sizes, including some small ones to find the best size for the bottom of the PCB. How about: 3,4,5,8,10,12,15,18? I don't know how useful 3mm is in terms of depth from either side.
User avatar
ian
Crew
Crew
 
Posts: 10803
Joined: Mon Jul 06, 2009 6:14 am

Re: Standoff size and side height

Postby tayken » Thu Jan 19, 2012 9:32 am

Standard PCB library has 3.2 mm diameter holes. Which is good if we consider tolerances etc.
The one I proposed has M3 screw threads and 5 mm HEX (which is the distance between parallel surfaces).

I was thinking about having a female-female standoff at the bottom, adding a male-female standoff to the top and fixing the acrylic cases with some M3 screws + washers. Bottom one has to cover both screw length + male-female standoff thread length (8 mm according to the catalog). Shortest M3 screw thread length available is 6 mm on that page. So:
8 mm + 6 mm - 1.6 mm (PCB thickness) - 3 mm (acrylic piece thickness) = 9.4 mm which corresponds to L = 10 mm female-female standoff for the bottom and any other male-female standoff for the top.

Another case is to use a really short female-female standoff at the bottom so that we use the top male-female standoff threads to fix the acrylic case with some hex nuts. For that what we need is:
8 mm - 1.6 mm - 3 mm = 3.4 mm is the maximum female-female standoff length which is not possible as they start from 5 mm.
User avatar
tayken
Developer
Developer
 
Posts: 1520
Joined: Sat Dec 12, 2009 10:58 am
Location: Tokyo, Japan

Re: Standoff size and side height

Postby dsm » Thu Jan 19, 2012 2:44 pm

minor comments

Some designs like Bus Pirate have a main IO connector that wants to be accessed from the top of the case.
Assuming that the IO connector is 8.5 mm high and you use a 3.0 mm thick top acrylic window, then the distance from the top of the PCB to the inside of the top acrylic window is 5.5 mm. Assuming that the IO connector is 0.5 mm proud of the top surface of the top acrylic window, then you probably want a 5.0 mm top standoff. I suspect the length of the screws in the male-female standoffs might be a problem in this case. You might also want to consider the height of any lock washer hardware needed to keep the standoff screws from slipping.

Another possibility would be to swage threaded standoffs onto the bottom of the Bus Pirate board and attach it to the bottom acrylic window using machine screws. No top standoff would be needed, but then how would the top acrylic window be held in place?

I tend to think that the 3.0 mm acrylic looks a bit clumsy, especially for the smaller boards.

Thanks for your time.

dsm
dsm
Full Member
Full Member
 
Posts: 216
Joined: Sun Jul 10, 2011 3:04 pm

Re: Standoff size and side height

Postby ian » Fri Jan 20, 2012 4:27 am

I am open to 1.5mm acrylic, but it has been much cheaper to prototype with 3mm.
User avatar
ian
Crew
Crew
 
Posts: 10803
Joined: Mon Jul 06, 2009 6:14 am

Re: Standoff size and side height

Postby dsm » Fri Jan 20, 2012 6:02 am

1.5 mm versus 3.0 mm

Are you planing to build these cases in the Iowa (which would be a really good excuse to buy a laser cutter)?
Are you planing to get Seeed Studio to build these cases?

If the former, 1.5 mm extruded acrylic is roughly 60% of the cost of 3.0 mm extruded acrylic in volume.
If the latter, perhaps it would be worth asking Seeed Studio what thickness they can most readily obtain.

I notice that Proto Labs has some transparent plastics on their list of resins (link) (link).

Thanks for your time.

dsm
dsm
Full Member
Full Member
 
Posts: 216
Joined: Sun Jul 10, 2011 3:04 pm

Re: Standoff size and side height

Postby ian » Fri Jan 20, 2012 6:23 am

It will be done by Seeed for sure. I'm sure they can get 1.5mm. After we finalize this design and have a set of files to test, I will send off the useful case sizes and PCB templates and we can have a once-over test of everything. I'm sure we could get 1.5mm for that test too, so maybe the smaller cases be done at both thicknesses for comparison.
User avatar
ian
Crew
Crew
 
Posts: 10803
Joined: Mon Jul 06, 2009 6:14 am

Re: Standoff size and side height

Postby dsm » Fri Jan 20, 2012 6:33 am

alternate case design suggestion

This may be the wrong forum thread to make this suggestion, but since I was thinking about the standoff size...

What if the standoffs that hold the case together were different from the standoffs that hold the board in position?
The case standoffs could be placed outside the side walls.
The board standoffs could be placed inside the side walls (typically between the bottom acrylic window and the board).
Or perhaps swaged (link) onto the bottom of the board.

The hardware costs would go up somewhat, but not that much. The number of spacers would be the same, although the lengths would be slightly different. The number of screws might go up, but screws are cheap. Only more available, less expensive female-female spacers would be needed, instead of less available, more expensive male-female spacers.

Thanks for your time.

dsm
Last edited by dsm on Sun Jan 22, 2012 1:47 am, edited 1 time in total.
dsm
Full Member
Full Member
 
Posts: 216
Joined: Sun Jul 10, 2011 3:04 pm

Re: Standoff size and side height

Postby tayken » Fri Jan 20, 2012 6:39 am

I will prepare case designs for 1.5 mm and 3 mm for the smallest size. I'm trying to learn how to write a SketchUp script. :)
User avatar
tayken
Developer
Developer
 
Posts: 1520
Joined: Sat Dec 12, 2009 10:58 am
Location: Tokyo, Japan

Re: Standoff size and side height

Postby ian » Fri Jan 20, 2012 10:50 am

Here is the M3 list of parts from Keystone, the distributor Mouser carries. I also got it all into a cart, along with a couple screw lengths:
http://www.mouser.com/ProjectManager/Pr ... 66bf0a60b1

I need to decide exactly what to get, but I think a kit of every type is probably needed for experimentation.
Attachments
m3-hexstand.png
parts.png
m3screws.png
User avatar
ian
Crew
Crew
 
Posts: 10803
Joined: Mon Jul 06, 2009 6:14 am

Re: Standoff size and side height

Postby arakis » Sat Jan 21, 2012 7:01 am

we , might use those screws that have a angled head, so that the top of the head drives into the acrylic and is flush with it. Making holes to accept then is also easy, you just need one drill bit to drill a hole straight through, and another one that is the diameter of the head to drill only to the thickens of the board, not straight through. this way the hole will have one exit the diameter of the screw thread, and the other exit will have the diameter of the head.
here is a pic to illustrate what I mean
Attachments
screw.jpg
best regards FIlip.
arakis
Crew
Crew
 
Posts: 1044
Joined: Wed May 25, 2011 11:15 am
Location: Belgrade, Serbia

Re: Standoff size and side height

Postby dsm » Sun Jan 22, 2012 3:09 am

standoff schemes

The range of lengths for Keystone standoffs and screws (link) is more limited than I had thought...
Assume having nuts on the outside of the case is esthetically undesirable, so consider...

Scheme A ~ male/female threaded standoff above + female/female threaded standoff below

    3.0 mm top acrylic window +
    10.0 mm top male/female threaded standoff +
    1.7 mm board +
    10.0 mm bottom female/female threaded standoff +
    3.0 mm bottom acrylic window
    = 27.7 mm (plus twice the height of the screw heads)
Assume Keystone 29311 M3 x 6.0 mm screws for now. 8x needed.
Assume Keystone 24289 10.0 mm top male/female threaded standoffs for now (shortest available in M3). 4x needed.
The bottom female/female threaded standoff length is driven by thread lengths.
6.3 mm (8.0 minus 1.7) + 3.0 mm (6.0 minus 3.0) == 9.3 mm => 10.0 mm bottom female/female threaded standoff.
Assume Keystone 24473 10.0 mm bottom female/female threaded standoffs for now. 4x needed.
(1.5 mm acrylic windows implies 12.0 mm bottom female/female threaded standoffs.)

Scheme B ~ separate case threaded standoffs and board mounting scheme
    1.5 mm top acrylic window +
    10.0 mm box female/female threaded standoff +
    1.5 mm bottom acrylic window
    = 13.0 mm (plus twice the height of the screw heads)
Assume Keystone 29311 M3 x 6.0 mm screws for now. 8x needed.
Assume Keystone 24473 10.0 mm female/female threaded standoffs for now. 4x needed.
Note that the side wall height is only dependent on the box threaded standoff height (simplified overall mechanical model).
Suitable hardware choices would allow the Bus Pirate IO connector to protrude from the top of the case.
Mounting the board is handled through a separate mechanism such as
    swaged threaded standoffs (4x) and screws (4x)
    female/female threaded standoffs (4x) and screws (8x) ~ may need shorter standoffs and screws
    round clear hole standoffs (4x) and screws/nuts (4x) ~ may need shorter standoffs and screws
    nylon lock-in supports (4x)
Scheme C ~ female/female threaded standoff above + female/female threaded standoff below
    3.0 mm top acrylic window +
    6.0 mm top female/female threaded standoff +
    1.7 mm board +
    5.0 mm bottom female/female threaded standoff +
    3.0 mm bottom acrylic window
    = 18.7 mm (plus twice the height of the screw heads)
Assume Keystone 24311 M3 x 6.0 mm screws on top. 4x needed.
Assume Keystone 24471 6.0 mm female/female threaded standoffs for now. 4x needed.
Assume Keystone 24470 5.0 mm female/female threaded standoffs for now. 4x needed.
Assume Keystone 24314 M3 x 12.0 mm screws on bottom. 4x needed.
(1.5 mm acrylic windows implies M3 x 4.5 mm screws on top and M3 x 10.5 mm screws on bottom.)
Suitable hardware choices would allow the Bus Pirate IO connector to accessed through the top of the case.

Scheme D ~ female/female threaded standoff above + round clear hole standoff below
    3.0 mm top acrylic window +
    6.0 mm top female/female threaded standoff +
    1.7 mm board +
    3.2 mm bottom round clear hole standoff +
    3.0 mm bottom acrylic window
    = 17.9 mm (plus twice the height of the screw heads)
Assume Keystone 24311 M3 x 6.0 mm screws on top. 4x needed.
Assume Keystone 24471 6.0 mm female/female threaded standoffs for now. 4x needed.
Assume Keystone 3403 3.2 mm clear standoffs for now. 4x needed.
Assume unknown M3 x 11.7 mm screws on bottom. 4x needed.
(1.5 mm acrylic windows implies M3 x 4.5 mm screws on top and M3 x 10.2 mm screws on bottom.)
Suitable hardware choices would allow the Bus Pirate IO connector to be accessed through the top of the case.



Keystone 24289 ~ 10.0 mm male/female stainless steel threaded standoff ~ $1.13 @ 1000
Keystone 24337 ~ 10.0 mm male/female aluminum threaded standoff ~ $0.166 @ 1000
Keystone 24473 ~ 10.0 mm female/female stainless steel threaded standoff ~ $0.98 @ 1000
Keystone 24433 ~ 10.0 mm female/female aluminum threaded standoff ~ $0.124 @ 1000
Keystone 24471 ~ 6.0 mm female/female stainless steel threaded standoff ~ $0.878 @ 1000
Keystone 24431 ~ 6.0 mm female/female aluminum threaded standoff ~ $0.114 @ 1000
Keystone 24470 ~ 5.0 mm female/female stainless steel threaded standoff ~ $0.877 @ 1000
Keystone 24430 ~ 5.0 mm female/female aluminum threaded standoff ~ $0.13 @ 1000
Keystone 3403 ~ 3.2 mm round clear hole aluminum standoff ~ $0.083 @ 1000
Keystone 29311 ~ M3 x 6.0 mm screw ~ $0.135 @ 1000
Keystone 29314 ~ M3 x 12.0 mm screw ~ $0.135 @ 1000
Keystone 8843 ~ 3.0 mm nylon lock-in support ~ $0.145 @ 1000

Scheme A seems to lead to thicker cases and more expensive hardware.
Scheme B seems to require slightly more hardware, but allows thinner (although possibly slightly wider) cases.
Scheme C seems to lead to thicker cases and require finer control over the screw length.
Scheme D seems to lead to thicker cases and require finer control over the screw length.
I'm sure you can find cheaper hardware from Chinese manufacturers, but still...

How does the choice of hardware affect the case design?

Thanks for your time.

dsm
Last edited by dsm on Tue Jan 24, 2012 7:06 pm, edited 1 time in total.
dsm
Full Member
Full Member
 
Posts: 216
Joined: Sun Jul 10, 2011 3:04 pm

Re: Standoff size and side height

Postby dsm » Tue Jan 24, 2012 1:16 am

Scheme B example

The following is a quick & dirty example of Scheme B mentioned above.

SOB-DP5031-case-assembly-dsm-v1-perspective.jpg
SOB-DP5031-case-assembly-dsm-v1 (perspective)

In this context, quick & dirty means
    Side pieces not adjusted for laser cutter kerf.
    End pieces not adjusted for laser cutter kerf.
    Top and bottom pieces not adjusted for laser cutter kerf.
    4.0 mm top and bottom corner radius to echo board design language.
    10.0 mm box hex threaded standoffs.
    3.2 mm board hex threaded standoffs.
    3.2 mm board round swaged standoffs not shown.
    3.2 mm high essentially means 1/8" high in this context.
    5.0 mm (flat-to-flat) hex standoffs are 5.774 mm (vertex-to-vertex) (which fits within 6.0 mm mounting hole keepout).
    Screws not shown.
    1.5 mm acrylic.
    Not drawn with Google SketchUp.
    Top and bottom slot-to-edge width increased from 3.0 mm to 7.0 mm to allow box standoffs.
    50.0 mm x 31.0 mm board => 50.4 mm x 31.4 board+margin => 53.4 mm x 34.4 mm board+margin+acrylic.
    67.4 mm x 48.4 mm overall.
Thanks for your time.

dsm
Last edited by dsm on Wed Feb 08, 2012 2:40 pm, edited 1 time in total.
dsm
Full Member
Full Member
 
Posts: 216
Joined: Sun Jul 10, 2011 3:04 pm

Re: Standoff size and side height

Postby ian » Tue Jan 24, 2012 10:53 am

Thanks dsm,

I've had a really hard time with your questions because I'm jut not sure. I had planned to experiment hands on, and you point out how much a bit of planning might help :)

Many of the combinations are very hardware intensive. My goal is cheap, so whatever is cheapest will make me happy :)

I see several benefits to your 4 post plus interior mounting hardware approach (scheme B):
*Perhaps less hardware if the interior PCB mounting system is simple. For example the keystone nylon mounts (would need 1.5mm acrylic then).
*No concern about clearing posts on the top component layer
*Option to fit smaller PCBs with multiple drills at different standard sizes.
*Height adjustment with single part swap.

What is the PCB mounting hardware in this design?
3.2 mm board hex threaded standoffs.
3.2 mm board round swaged standoffs not shown.

To there are 4x3.2mm hex stand offs, and 8xscrews (4 top, 4 bottom)?
I'm sorry, I'm not familiar with what you mean by swaged standoffs. Are these the four corner PCB holders that you showed in a previous design?
User avatar
ian
Crew
Crew
 
Posts: 10803
Joined: Mon Jul 06, 2009 6:14 am

Re: Standoff size and side height

Postby arakis » Tue Jan 24, 2012 1:51 pm

I have a scheme F :) how about making the acrylic exact the same dimensions as the boards...and using two male/female standoffs ( with the board in between) and one screw that doesn't protrude too much(like the one I modeld above) The first standoff is screwid into the acrylic, while the top one has screws...

This loses the possibility of having any sides, but gains us more access to the edges of the board,( USB, etc....)
best regards FIlip.
arakis
Crew
Crew
 
Posts: 1044
Joined: Wed May 25, 2011 11:15 am
Location: Belgrade, Serbia

Next

Return to Sick of Beige