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Old 24th Jan 2017, 2:16 pm   #1
Al (astral highway)
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Default Advice on materials to 'box in' a sub-assembly

Hello,

I don't have a workshop other than a picnic table outside on the balcony and hence have limited equipment for the mechanical side of construction , so ingenuity is high on my list of resources. I need to box in a sub-assembly on my current home-brew project . It is a heavy brass panel mounted on the upper side of a chassis and contains three double pole switches and neon panel indicators carrying mains voltage and current. For safety reasons I want to box in the back and sides but wonder what the best way could be?

I have a small quantity of fibreglass board which is very light and hence ideal, but am uncertain as to how to fix this either to itself or to the brass panel. Any further metal is out of the question on weight grounds as the whole assembly of which this is part weighs well over 20kg now. So any tips on materials , methods or adhesives gratefully received. Robustness and safety are more important than looks as this will not be very easily visible in he greater scheme of things.

Thanks, folks!
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Old 24th Jan 2017, 2:20 pm   #2
Boater Sam
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Default Re: Advice on materials to 'box in' a sub-assembly

Varnished cardboard box? I don't know how big you want it but a wine box is excellent board, and preparing one, i.e. emptying one, is a grand start.
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Old 24th Jan 2017, 2:26 pm   #3
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Default Re: Advice on materials to 'box in' a sub-assembly

Can you post a picture of what needs boxing in?

Lawrence.
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Old 24th Jan 2017, 3:11 pm   #4
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Default Re: Advice on materials to 'box in' a sub-assembly

Find a suitable plastic enclosure with lid, large enough to contain all the back-of-panel parts that need enclosing. E.g.ABS box at RS

Method 1:
Remove the lid and use its screwholes as a pattern to drill corresponding holes in the panel. Cut slots in the side walls of the box corresponding with any cables that need to escape, then screw the box against the back of the panel without its lid. The screws might need changing from countersunk types to ones that look suitable on the face of the panel.

Method 2:
Remove the parts from the panel. hold the underside of the box against the inside of the panel and mark through all cutouts. Drill / cut out the box to match the panel, then refit the parts through the box and panel together, using them to hold the box against the panel. Make cable exits, refit the lid. This avoids extra screwholes in the panel but requires sufficient parts mounted through it to support the box, and long enough stems on the parts.
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Old 24th Jan 2017, 3:19 pm   #5
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Default Re: Advice on materials to 'box in' a sub-assembly

Plain copper clad PCB is a good material for the odd box, just cut and solder together. Best of three worlds, light, strong and screening. With a bit of care and paint it can look quite good as well.
 
Old 24th Jan 2017, 4:08 pm   #6
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Default Re: Advice on materials to 'box in' a sub-assembly

Boater Sam: thank you, I can think of times when this would be suitable, but in this case flammable materials are not suitable at all.

Lucien: good idea, I like it. Thank you. The only think that rules this out is that it doesn't offer RF screening (although I know I didn't mention this need in my OP). There is so much RF around that every wire needs either RF bypass measures or screening.

MerlinM: I think this is the winning idea, thank you. Ticks the RF screening nicely . I'm up to soldering theee pieces of copper clad board neatly to make a surround but any ideas on how to secure this to the back of the brass front panel? It is too heavy a grade of brass to solder to the narrow profile of the board.
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Old 24th Jan 2017, 5:50 pm   #7
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Default Re: Advice on materials to 'box in' a sub-assembly

Maybe use solder tags. In the past I've soldered 6BA cheesehead screws to the solder tag part and then used 4BA screws to fix the whole assembly. So in your case you could solder the tag part onto the copper-clad and then bolt the hole part onto your brass panel. Just use enough for the job.
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Old 24th Jan 2017, 6:05 pm   #8
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Default Re: Advice on materials to 'box in' a sub-assembly

A small flange of PCB with through screws. You can make it an internal flange and solder the nuts on.
 
Old 24th Jan 2017, 6:47 pm   #9
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Default Re: Advice on materials to 'box in' a sub-assembly

Thank you Graham and Merlin. Both great ideas that will work nicely! Pic attached now....

Actually, if I use double-sided board I could indeed solder the outside to the copper foil on my chassis , and then use one of your suggestions to secure to the front brass. Great progress here , thanks !!
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Old 24th Jan 2017, 7:22 pm   #10
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Default Re: Advice on materials to 'box in' a sub-assembly

Double-sided PCB is my suggestion too: solder it together and add triangular 'fillets' in the corners for additional bracing.

Lengths of brass studding soldered the entire length front-to-rear in the corners are a great way to add strength and to attach the front/rear panels; you can use chrome-plated 'dome nuts' on the ends of the studs where they poke through the pan4el to give a nice appearance.

Proper glass-reinforced PCB material is stiff and pretty fire-resistant (which can't be said for the older paxolin/SRBP type which I wouldn't use in this application) and is surprisingly strong too. I once built an entire 10-Watt 160/80Metre valve AM transmitter chassis and case using nothing but PCB material 'offcuts' and discarded pre-photo-sensitized boards that had been improperly stored so the photosensitive 'resist' had all turned.
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Old 24th Jan 2017, 7:27 pm   #11
Al (astral highway)
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Default Re: Advice on materials to 'box in' a sub-assembly

ah, trying those pix again ...
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Old 24th Jan 2017, 10:25 pm   #12
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Default Re: Advice on materials to 'box in' a sub-assembly

Would a metalclad surface box work? Your pael looks a bit large for a standard 2-gang one, and a bit small for a cooker switch type, but maybe you could cut it down.
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Old 24th Jan 2017, 11:15 pm   #13
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Default Re: Advice on materials to 'box in' a sub-assembly

Try a MK Grid switch surface pattress. These are made in std sizes up to 16 way. TLC electrical have these, you do not need a trade account to use them.
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