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Custom aluminum chassis

Discussion in 'Electronic Components' started by EBG, May 22, 2004.

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  1. EBG

    EBG Guest

    I've used a few local fabricators to do some aluminum chassis for me in the
    past but was wondering if there are any really big names that do business
    like some of the big PCB houses.

    For example, I have alot of boards done at PCB express and it's great the
    way you can just email the files and get the boards in such a quick
    turnaround.

    I realize fabricated aluminum chassis are more complex, but if I email the
    DXF drawing and complete info to them, it (should be) everything they need.

    Are there any places like this that are high volume, do quotes online, have
    quick turnaround and good prices?

    Thanks.
     
  2. Joerg

    Joerg Guest

    I don't have online fab addresses but one thing I found in the past is that
    this task gets a lot easier when using steel. I try to convince my clients to
    use steel whenever it is feasible. It can be welded, tapped, sheet metal
    screwed and so on. That's not so easy with aluminum where there will be some
    assembly work required. Steel is a lot sturdier as well even if the metal is
    much thinner.

    So if you look at steel you may even find a local shop that can take AutoCAD or
    SolidWorks files and turn out chassis. Going local has the advantage that you
    can drive over there to discuss an interference or tolerance issue face to
    face, with the "corpus delicti" in hand.

    To be fair I must mention one downside: After assembling a steel chassis (shops
    often call that a "weldment") it usually needs to be plated. With ever tougher
    environmental rules that may become an issue in some states. Aluminum usually
    is treated but that can have its own caveats. Anodized material may lead to
    some serious EMC headaches and alodine treatment is expensive.

    Regards, Joerg
     
  3. Activ8

    Activ8 Guest

    ISTM that a welder's job is to weld the material without changing
    the dimensions. He'd clamp the workpieces in position and do the
    weld. In fact, my stepdad had to do this for a test at work. He took
    a tube, laid two wires on the end to allow space for the weld and
    get his length dimension - no bevel allowed. Then he put the end cap
    on the wires and tacked it to the tube in a few places then he just
    filled in the rest without heating thing up so much that he'd loose
    his dimension. Simple shit. 0.1mm tolerance? I'd have to ask him.
     
  4. R.Legg

    R.Legg Guest

    High volume and low price mechanical detail generally involves earlier
    design effort of hard tooling.

    If you want good prices without this effort, then expect delays and
    uncertainty while the vendor tools up, as best he can.

    You should ask for early budgetary options, that include itemized
    tooling options as a variable, from your vendors. Some consider this
    proprietary and will only quote volume, with the tooling costs
    absorbed, based on mechanical dwg. Finding a vendor that knows how to
    do what you want, cheaply, can take time.

    RL
     
  5. Terry Given

    Terry Given Guest

    And the great thing about weldments is that welding changes physical
    dimensions. If, like me, you design a 840h x 630v x 140mm frame, with 0.1mm
    tolerance on h,v dimensions and critical hole positioning, you will quickly
    discover this fact (if, that is, you bother to measure the stuff you
    bought). We used Aluminium, because weight was a critical factor - these
    units clip together (big clips), 15 high and as many wide as you want, and
    its structural integrity is maintained in 100km/hr winds.....LED video
    screens for football stadiums/concerts etc

    Terry

    Cheers
    Terry
     
  6. Joerg

    Joerg Guest

    Hi Terry,

    That really depends on which shop does it. Tolerances need to be spec'd and the
    vendor must agree to that spec. We never had problems. Mostly it was much larger
    gear such as medical ultrasound systems. But there were some really critical
    areas in them such as the card cages. The front of each circuit board panel was
    screwed to the weldment for securing. The DIN connectors were almost a foot
    behind that and had to really be within fractions of a millimeter. They always
    were.

    Regards, Joerg
     
  7. Rich Grise

    Rich Grise Guest

    I design weld fixtures for CRES & Alloy tubing. I draw to the
    spec - the boss, who is also the chief weldor, specifically
    told me not to do _anything_ about shrinkage - just put the final
    dims on the print, and let the weldor take care of shrinkage.

    IOW, if they deliver a part that's out of print, and blame
    weld shrinkage, get a different vendor.

    I also worked for a guy who got chassis bent & punched locally,
    and they came in already powder coated, fwiw. You have to remember
    to scrape the paint under the ground lug.

    Good Luck!
    Rich
     
  8. Hey, same problem here, and it was hundreds....
    JP
     
  9. Joerg

    Joerg Guest

    Hi Terry,

    That is indeed a really tough spec to build to. I wonder if there is another way
    to join the units together without relying on tolerance. But I bet you guys
    looked into that and decided it just had to be done the way it is now. Anyways,
    it looks like even a slightly rough landing of a freighter plane might give
    these units enough deformation to run out of spec.

    The Romans already had some bigshot mechanical engineers. I remember from a
    story that they made huge caliper-like structures from marble (the expensive
    Italian kind no less) for such purposes. Don't know if it worked but much of
    their stuff is over 1500 years old and still holds up.

    I wonder if aluminum lives up to these specs over time. There usually is some
    creepage and sag. With steel you have more options, they can cook up an alloy
    for you that is pretty stiff and temp stable.

    Regards, Joerg
     
  10. Terry Given

    Terry Given Guest

    Hi Joerg,

    our biggest problem to date is measuring things accurately. We have a number
    of safety-critical dimensions (these clip together to form a structure, that
    is placed near large crowds of people). The solution is post-welding
    machining of the critical holes/faces, which allows us to soak up the
    inevitable manufacturing tolerances providing the NC machine is suitably
    accurate (and calibrated regularly). Dont forget this thing is almost a
    meter long, and we need guaranteed 0.1mm accuracy (ie around 0.01%).

    Actually I lied - our biggest problem was that initially we didnt measure
    anything at all, and just relied on the vendor to give us what we asked for,
    which of course they didnt. So we bought a 1200mm vernier caliper (its a
    2-man job using that thing!), straight-edges, engineers squares etc. and
    promptly discovered how far off everything was. We hired a mechanical
    black-belt (im just a lowly electron-pusher), and he has developed highly
    accurate jigs that provide the manufacturer, and our inwards goods guy, with
    a simple, easy-to-use go/no-go test.

    Apparently, we independantly discovered "nasa theory" - closing your eyes
    and crossing your fingers is a poor way to see if problems exist. Then
    hurling them at mars makes it a costly approach too.

    Terry
     
  11. Terry Given

    Terry Given Guest

    amen on the powder coat (I just had to explain to my dipshit boss that
    polyester resin is about 1200x worse conductor of heat than Al, so dont
    powder coat my heatsink!)

    as far as who is to blame - moot point, it doesnt help me meet my deadlines
    if I send everything back, even if they pay. which is why we are working
    with the vendor to identify the root cause(s) and remove them. 0.1mm aint
    much, especially over 840mm. A major improvement over our measuring nothing,
    then wondering why they are hard to assemble in the feild.

    Cheers
    Terry
     
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