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  1. #1
    Join Date
    May 2003
    Location
    Sydney
    Posts
    12

    Default Power sanding aluminium angle - any hidden danger

    Hi. I have some aluminium angle which I wish to shape. These are 25mm x 25mm, 1mm thick, and about 600mm long. I need to grind off about 40% of the material. I was thinking of doing this by making a jig which will hold the workpiece squarely against my big boy sander (woodworking edge sander).

    Are there any hidden dangers in doing this (exploding swarf or some such??).

    Dust collector will be disconnected. Sacrificial sanding belt purchased.

    Perhaps later I might be doing the same with brass, so same question applies.

    Cheers
    Arron

  2. #2
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    3,537

    Default

    Brass is no problem but Al might gum up the belt if it is is a high speed (2880 rpm motor) belt.
    AT 1440 rpm the likelihood of gumming is much reduced especially if a coarse belt is used.
    Try it out and if it starts gumming, rub some hard wax onto the belt/disc.
    It sounds counter intuitive but it does work.
    Even ordinary candle wax works but not as well as the hard stuff.

    BTW I always connect my DC when sanding metal but I do use a chip catcher in between the sander and the DC.
    The reason for the catcher is not for the dust or sparks but so a hot metal lump won't be sucked into the DC bags. It won't be a problem with Al anyway.

    BTW you do know you can use a WW TS to cut excess Al off so you don't have to grind it all away?
    Ideally a negative raked toothed blade should be used but if you use a high tooth count WW blade and feed it slowly and also rub some wax along the line of the cut you can get a real nice cut that may not even need sanding. Watch out for the swarf - they are like little razor blades!

  3. #3
    Join Date
    Jan 2012
    Location
    Melbourne
    Posts
    80

    Default

    The exploding swarf thing is about thermite flashes occurring when grinding aluminium and steel on the same machine, without cleaning it in between:

    “…Injuries included deep second-degree burns to about 60% of the victim's left hand and 50% of his right hand and first degree burns to his neck, chin, cheeks, lips, and the end of his nose. ……he also lost half his moustache, one of his eyebrows….”

    From here; http://psc.tamu.edu/wp-content/uploads/doe2_0017.pdf

    Must blow my sander out in the morning!

    Neil

  4. #4
    Join Date
    Jan 2004
    Location
    Mackay Qld
    Posts
    3,872

    Default

    Aluminum dust from grinding is known to be a safety hazard.

    The amount of grinding as in number of units was not stated.That would affect the amount of dust.

    The ventilation conditions around your work area are not known, so its difficult to say.

    Damage to the lungs and an explosive condition are a potential determined by your job volume and ventilation of your work space.

    As with anything there are variables at play.
    the following is a cut and paste from the American Aluminium association

    | The Aluminum Association

    which states :

    General criteria for an aluminum dust explosion

    Several conditions must be met before a dust explosion can take place:
    The dust has to be combustible.
    1. It must be suspended in air.
    2. It must be fine enough to propagate flame
    3. The concentration of the suspended dust must be within the explosive range
    4. An ignition source contacting the dust suspension must have enough energy to initiate flame propagation, that is, combustion of the particles.
    5. Enough oxygen or other oxidizer must be available to support and sustain combustion of the dust suspension.


    I would certainly opt for a respirator and perform the grinding operation in a well ventilated area.
    There is a lot of reading via google which is available. In the end its your personal safety at risk so your need to choose.

    Cheers

    Grahame

  5. #5
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    3,537

    Default

    One good thing about metal dust is that for a DIY operation it is quite difficult to generate and keep enough of the dust suspended in the air to reach the minimum amount in air needed for an explosion.

    For Al the minimum concentration needed for an explosion is ~60g /m^3, whereas for petrol its about 12g/m^3.
    A dust extractor drawing 400 CFM (11 m^3 per min) that means grinding 660 g of Al per minute to get enough dust in the air flow for an explosion, I reckon you might be lucky to generate 6 grams a minute without gumming up a grinding belt belt.

    In an open shed the Al dust is dispersed and starts falling out of suspension as soon as it is made. I did some tests of metal suspension rates and found that the bulk of steel dust generated by an abrasive cutting wheel has a half residence time (time taken for half the dust to fall out of the air) in air of less than a minute. As Al is about 3x less dense than steel the bulk of the Al should have a half residence time in air of less than 3 minutes.

    This means you would have to grind up ~ 40g/minute and keep it restrained within 1m^3 of shed air to reach the explosive limit. However that does not happen in a normal shed and as we well know the metal dust is eventually dispersed throughout.

    About the only way to get enough Al dust into the air is to take a large salt shaker full of it and shake it around in the air, or tip over a bucket of Al dust onto the floor. The other situation would be grinding the Al in an enclosed space like a booth or a cupboard.

    More significant than dust explosions are the health implications of the dust.
    Al dust has a recommended OHS limit ranging from 15mg/m^3 in the US, 10 mg/m^3 in the UK to 0.5mg/m^3 in Japan (I wonder why theirs is so low?).
    This is from 4000 to 120000 times lower than the dust explosion limit.

    I have not done a lot of testing but what I have done shows it is possible for an operator to easily exceed these levels near the grinder/sander, even small pieces of Al. The problem is exacerbated by the way most grinders/sanders throw the dust towards the operator.

    This is why I use dust extraction, and for anyone that doesn't have dust extraction a respirator is recommended and as Graeme says good ventilation will help. Apparently not everyone is affected by Al in the same way, some people are more susceptible to Al dust but there's no way of know what you will be beforehand.

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