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Thread: The art of metal polishing

  1. #1
    NSRA member langysrodshop's Avatar
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    The art of metal polishing

    Thought this may of been of interest to someone, its an article done for Custom Car a while ago, I have others if its thought to be worth posting.

    Can a Mod move it to tech archives (copied to tech archive)


    POLISHING

    What is polishing all about really ???

    Polishing is the process of removing surface imperfections so that it reflects light rather than absorbs it. The more light it reflects the better it is polished.

    When a piece is polished, what we do is to change its surface. We remove surface imperfections by scratching them off with various grits.
    We start with a coarse grit and gradually reduce it to finer grits. Each grit has a scratch factor. We work our way from a coarse scratch to one that cannot be seen by the eye. The small particles on the surface become polarized. They line up like soldiers and the surface becomes slightly harder, and more resistant to oxidization, acid and water stains. Water runs off easier, as it is can’t get a grip on the polished surface. It is also more resistant to corrosion, just because of its smoothness and its hardness. Of course it will dull with time due to handling and chemicals in the atmosphere.

    Corrosion sets in when the ions no longer line up like soldiers. The surface gets attacked by chemicals in the air and the little soldiers start falling by the wayside one by one.
    Once the oxidization starts, if left unattended the polished surface dulls and can discolour.

    Safety

    Polishing machines are dangerous in themselves so be careful of loose clothing etc etc.
    Wear a decent quality face mask as the dust you will making is not good to breathe, also wear overalls as polishing is a real dirty process.

    Machines

    Polishing machines come on a variety of forms, some have a pigtail out of each side while some only have a pigtail on one side (these usually have a grinding wheel on the other end)
    All the best machines are 3 phase 3/4HP basically as the 3 phase gives more grunt to the machine, obviously most home workshops do not have a 3 phase supply but these days Digital phase converters give the home polisher a far better range of machines they can buy.
    The single phase machines are still a good choice as is a 3 phase machine converted to single phase but neither will have the grunt of the 3 phase; this means polishing will just take a little longer.
    If your looking for a 3 phase machine to convert to single phase you will need a machine that has the motor mounted in the base and is driven by a belt, the top mounted motor type cannot be converted as the motor shaft is the actual polishing spindle.

    There are also bench grinders that can have polishing mops mounted to them for very light work, these are not ideal as the motors are generally low horse power.

    Maximum Thickness of Polishing Wheels

    Motor Size Wheel Diameter
    4” 6” 8” 10” 12”
    1/6 hp 1” 5” - - -
    1/4 hp 1.5” 1” 5” - -
    1/3 hp 2.5” 2” 1” - -
    1/2 hp 3” 2.5” 2” 1” -
    3/4 hp 4.5” 3.5” 2.5” 2” 1”
    2 hp 4.5” 4.5” 4.5” 3” 1.5”

    Mop running speeds
    For best results your wheel should maintain a surface speed of between 3600 & 7500 Surface Feet Per Minute. (SFPM). The higher your speed, the better and quicker your results.
    Formula for calculating surface speed of wheel in SFPM.
    SFPM = 1/4 x diameter of Wheel x RPM (revs of spindle per min.)

    DETERMINING GRIT SIZES and BUFFING COMPOUNDS

    The question often asked is, 'Which grit should I use first?” Unfortunately, there is no cut and dried answer to this, because it really depends on the condition of the part. So, let’s discuss some examples:
    1) Aluminum Cover with one deep scratch.
    To successfully remove the scratch, ALL of the material around the scratch has to be disposed of. In our example, this means a lot of aluminium! Initially, we are concerned with getting as much aluminium sanded off as quickly as possible, so we would use the coarsest abrasive available, say a 40 grit sandpaper on a flat block. It would be of NO USE to try to buff out this deep scratch with a buffing wheel, because the wheel would remove more material from the hole, as well as the surrounding areas. By keeping the abrasive on a flat block, no further material can be removed from the hole.
    As the scratch is lessened, the grit size of paper is reduced from 40, to 80, 120, 240, 320 and finally 600. Once the surrounding material is removed, then the actual polishing can be started.
    Buffing compounds will be determined by the size of the scratches. If you have used a 600 grit paper, you may like to proceed directly to a white buffing compound. If fine scratches are visible, then you'll need to 'back off' and go to the Grey compound before retrying the white.
    2. Aluminum Side Cover - just dull.
    Obviously, it would be a step backwards to start treating this piece by using the technique in our first example. As there is no large metal content to be removed, you could virtually dispense with the sandpapers and move directly to the polishing aspect of the part. The Brown buffing compound with a Sisal wheel could be employed to see if the shine is good enough. If not, then use a grey compound, and then rework the part using the brown compound.
    3. Aluminum Wheel Casting.
    Many wheels are prepainted, directly over a rough sand casting. Remove the paint using Nitromors, to expose the aluminium. The 'pimples' caused by the sand cast have to be removed to flatten the metal ready for polishing. Because of the difficult contours of the wheel, it is virtually impossible to sand with a flat block, so this is where Greaseless Compound comes in. Start off with the 80 grit, using a spiral sewn wheel or, for the awkward areas, a felt bob. Then proceed through the various grit sizes of Greaseless, until the metal is smooth , all 'pimples' are removed and an even 'flat' finish is achieved. The polishing should only begin once all traces of 'pimples' are gone. Use Grey Compound for most of the polishing work, and finally, on a fresh or cleaned wheel, buff lightly with the Brown compound.





    Initial rough cutting

    Professional polishers do their roughing out on a belt on the end of a 3 phase machine, obviously the home workshop isn’t going to have this feature but there are large diameter flap wheels to mount to a pigtail available in grits from 40g to 400g
    Also mops can be turned into good cutting mops by the addition of Red Head Wheel cement & aluminium oxide grit, the mop is liberally coated in the water based glue and then the mop is rolled through the oxide so that the surface is coated this is then left to dry. The oxide grit is available in grits from 40g to 320g
    Another useful wheel is the felt mop which can also be glued with aluminium oxide the same way.
    Often to tone down the cut of the wheels a Grease compound is used on the wheel, this reduces the finish cut even more.


    Polishing Mops


    Where polishing mops are concerned the world is your oyster, there are many types of mops available for specific jobs, the ones that interest us mostly are Sisal, Cotton Stitched or White Close Stitch Mops & Loose Finishing Mops. Mops are available in several diameters from 5” to 12” and usual widths are 1” or 2”


    Loose on left, cotton centre & sisal on right





    Mops need to be cleaned out regularly with a rake, a rake is a double handled piece of wood with spikes on one side that is pressed against the revolving mop, a piece of wood with nails banged through it does an inexpensive and excellent job.








    THE SISAL BUFFING WHEEL
    Sisal is a slender, hard, cellular strand of fiber that has demonstrated its great strength and tough resiliency in the form of binder twine, cord & rope for many years. These qualities, along with its natural abrading and grease absorbing characteristics, provide an ideal buffing wheel fabric.
    Sisal buffing wheels will provide both polishing and cutting action. They will remove stretcher strains, orange peel, polishing wheel grit lines, light die marks, etc. To effect a fast cut, use a sisal wheel




    Polishing Compounds/Soaps

    There are many manufacturers of soaps but Canning & Menzerna are about the best, Soaps are rated by the cut & gloss they give, some have a aggressive cut but low gloss while others have very little cut but have a high gloss.
    A quick word about colour coding of soaps is needed as not all manufacturers use the same, It’s always best to ask your supplier which soap will give you the results you seek.



    .
    Any advice or help given is actually based on having done the job, not read about doing it or Googling it.

    www.langysrodshop.co.uk Our parts are air freighted so 5-7 day delivery, The best GRP Willys body available/Rebel Wirings only UK dealer/Speedway Motors authorised dealer/Summit racing/Jegs/Hotrod parts supplied, MAC Autos, We deal with all the US hotrod suppliers even non car related stuff.
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  2. #2
    NSRA member Flamin Black A's Avatar
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    great article steve thanks for posting

  3. #3
    NSRA member crumble's Avatar
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    That was worth reading.
    Real steel

  4. #4
    NSRA member langysrodshop's Avatar
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    Quote Originally Posted by Flamin Black A View Post
    great article steve thanks for posting
    Thanks, it should help anyone polishing, I know not everyone reads custard cart
    Any advice or help given is actually based on having done the job, not read about doing it or Googling it.

    www.langysrodshop.co.uk Our parts are air freighted so 5-7 day delivery, The best GRP Willys body available/Rebel Wirings only UK dealer/Speedway Motors authorised dealer/Summit racing/Jegs/Hotrod parts supplied, MAC Autos, We deal with all the US hotrod suppliers even non car related stuff.
    Brake,Oil & Fuel etc plumbing stockist/Totally Stainless fastener dealer/Dolphin Instrument dealer, LMC & Brothers Trucks,
    Stainless Exhaust tube & mandrel bends stockist

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