RyanMark

Polishing Mill Finish Sheet

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I buy copper alloy sheet (bronze, brass, nickel-silver) and make things out of it.  The metal usually comes with a mill finish.  That is, kind of shiny but with marks and light scratches (sometimes not so light).  I'd like to polish it to a mirror finish before I start working it.  For me, this would make more sense than polishing/buffing afterwards.  I know this has been asked probably a million times, but what would be the most economical way to achieve this, given that I currently have no specialized equipment for such a task, and taking time into consideration as well?  I've wet-sanded up to 1500 grit a few times, and have never quite got a mark-free surface.  I'd be happy to start at the Harbor Freight level.  Thanks!

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17 hours ago, RyanMark said:

 I'd like to polish it to a mirror finish before I start working it.  For me, this would make more sense than polishing/buffing afterwards.  

Forgive me, but I don't see or experience the logic in this train of thought, 

Any working is going to produce scarring of some sort and degree, the careful use of handling and forming techniques along with highly polished forming tools will help keep surface damage to a minimum so less finish polishing of the material is required, and if you are annealing the copper before forming, then the highly pre polished surface will suffer in the process.

Buffing is my preferred way of achieving different degrees of a polished surface, depending on the compound used.

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Very difficult to advise without knowing what you are making...every project has a "most efficacious" approach.

In general terms I agree with the advise of the posters above.

I certainly would not polish it twice, once before fabrication and once after, unless the form produced made it difficult for access to polish post production.

My school metalwork teacher had a saying which has informed the last 40 odd years of my metalwork practice..."the best way take marks out of metal is not to put them in" 

So as John says careful handling and forming techniques and mirror polished tools will save hours of finishing time...and leave a much fresher crisper end result.

Buying mirror polished stock and make sure you have specified that it be protected with low tack sheets on either side and preferable in a hardboard sandwich and maybe even strapped to its own pallet, so that it is not scratched by the stockholder/transport people is a good start if you are not having to radically manipulate the material. Buying one sheet at a time is always at risk of scratches from when the stockholder drags it and the one on top of it off the stack and then slides it onto and off the truck.

There are some amazing polishing compounds on the market now. Expensive  at £20 a bar instead of the general £5 but easily worth it for the finish they can give you. A coarse cut followed by one of these finishing compounds can deal with heavy scratches and take a third of the time that lustre/tripoli and rouge/crocus compounds used to, whilst producing a much better brilliant mirror finish than rouge/crocus ever could.

Alan

Text below from http://www.thepolishingshop.co.uk catalogue, there will be an equivalent supplier/ product near you.

Menzerna P175 Yellow Super Finish 1.30kg Bar

In stock

 

What is this compound? Best finishing compound I have seen in my 15 years of polishing. Need to get metal to a pretty good standard before you can use this compound. Use as a last stage mirror finisher.
What metal is it for? All metals 
What mop can I use? WDR loose, G loose, R loose
Stats: Cut factor 1 Gloss Factor 10

Menzerna M5 Super Finishing Compound 1.3kg Bar

In stock

 

What is this compound? New Super Finishing compound. 
Need to get metal to a pretty good standard before you can use this compound. Use as a last stage mirror finisher. Slightly more dry and finer than the P175 Yellow.
What metal is it for? All metals, lacquers and plastics.
What mop can I use? WDR loose, G loose, R loose
Stats: Cut factor 1 Gloss Factor 10

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One note about the plastic protection on sheets with the better finishes--try not to let them sit in the sun for long periods or they can be a lot harder to remove.

You didn't mention sheet size (unless I missed it).  Big difference on handling a 12" square than a 5' x 12' sheet.  A hint about initial thickness might help also.

On a commercial scale, one would run the sheet through a "grainer" which is nothing but a rigid wide-belt sander for metal sheet that puts that nice "all one way" finish on.  That'd give you the proper starting place to go further.  You might be able to find a way to emulate that if the sheets aren't too big.  

There might be a way to electro-polish your stuff also.  I've seen amazing results on stainless (mirror results) and I assume there might be a similar process for copper alloys.  

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51 minutes ago, Kozzy said:

One note about the plastic protection on sheets with the better finishes--try not to let them sit in the sun for long periods or they can be a lot harder to remove.

You didn't mention sheet size (unless I missed it).  Big difference on handling a 12" square than a 5' x 12' sheet.  A hint about initial thickness might help also.

On a commercial scale, one would run the sheet through a "grainer" which is nothing but a rigid wide-belt sander for metal sheet that puts that nice "all one way" finish on.  That'd give you the proper starting place to go further.  You might be able to find a way to emulate that if the sheets aren't too big.  

There might be a way to electro-polish your stuff also.  I've seen amazing results on stainless (mirror results) and I assume there might be a similar process for copper alloys.  

Prolonged exposure to sun is not something I have to worry about over here unfortunately!

I have used electro polishing on most of my stainless projects. Very effective. Great thing is that you get the mirror reflection and easy clean resilient surface whilst retaining the forged surface. Abrasive polishing processes always blur and soften details.

Point about electro polishing for the OP's problem of removing scratches, is that it does not remove scratches at all, every little surface detail is preserved but shiny polished! Because both sides of the score mark are now mirror reflective they do tend not to show up as much...but they are still there.

This is the blurb from the company I use, and yes they mention copper, but I don't know what that would come out like,  I have only had stainless done.

Alan

 

 

ELECTROPOLISHING IS IN EFFECT ELECTROPLATING IN REVERSE. INSTEAD OF DEPOSITING A COATING OF ANOTHER MATERIAL ON A SURFACE THE PROCESS OF ELECTROPOLISHING IS TO REMOVE A SURFACE LAYER, TYPICALLY 20-40 MICRO-METRES IN DEPTH IN THE CASE OF STAINLESS STEEL.

Electropolishing requires a source of electrical current and a rectifier to convert from alternating current (a.c.) to direct current (d.c.). With the aid of bus-bars, the d.c. is transmitted to the job and cathode bars suspended over a tank containing an electrolyte solution, normally a mixture of acids. Metal cathode (negative) sheets are suspended from the cathode bars into the electrolyte. The job to be electropolished is suspended from the anode (positive) bar into the electrolyte and adjacent to the cathode sheets.

When the current is switched on metal removal from the component surfaces takes place in a controlled manner with micro-peaks being eroded preferentially. This results in a smoothing of the affected surfaces, whilst the macro-profile of the surfaces remains unaltered.

Careful selection of the process electrolyte solution allows electropolishing to be carried out on metals such as aluminium, copper and brass, but it is in the treatment of stainless steel that it has found its greatest commercial application. Hastelloy ©, Inconel © and other nickel-chromium alloys have proved suitable for electropolishing.

In the majority of cases, electropolished stainless steel surfaces are bright and reflective. This results from the removal of an often contaminated surface layer and the electrochemical action of micro-smoothing. The advantageous features resulting from such a bright and smooth surface are many. The passive oxide layer, which is essential to prevent stainless steel from corroding, cannot be improved upon following electropolishing.

By reducing the total surface area, a result of micro-smoothing, unwanted products are less likely to adhere to an electropolished finish. In the same way, surfaces can be cleaned and kept clean more readily. Friction is also reduced and the mechanism of preferentially removing surface high spots makes electropolishing suitable for eliminating fine burrs.  The highly reflective bright finish is also suitable for a number of decorative applications, particularly where the shape of the item requiring polishing is extremely complex.

 

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Ditto to all of the above!  however some additional info- you can do DIY electro Polishing, albeit on a smaller scale than an 8 x 4 sheet and a Makita 6032 power file with the right belts will certainly help you get closer to a good finish!

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One thing not mentioned thus far is the use of a good random orbit sander.  With fine micron papers (usually they actually have a cloth backer) you can get right up to the point of fine buffing!  Pretty fast too, though the abrasives are not real cheap!  There are commercial versions sold that are keyed to produce standard stainless finishes at various levels.  For big flat sheets the random orbit sanders are pretty effective!  This is where they show off their potential!  

You do need a soft pad for this type work... often sold with  a selection of fine grits.

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Thank you everyone!  Sorry to take so long in replying.

So it looks like I need to use a car buffer after sanding to a high grit.  First with a course compound with a "cutting" wheel, and then the buffing wheel and compound, correct?

Bigfoot - Where do you get the fine grit sanding discs for the orbital sanders?  I can only ever find up to 320.  I find a random orbit sander to be quite fast, although the discs don't last very long.  But if I buy in bulk then it's worth it.

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And to answer some questions:  I mostly make gongs, as in the musical instrument.  Recently I had an opportunity to get some material from Germany.  It came already cut into circles (80cm diameter) and with a mirror finish.  Really nice stuff.  I have to cut my own circles and I can only get mill finish from my US suppliers.  The discoloration from heating is desirable, therefore it must be polished beforehand.  Keeping it from getting scratched during production is something I always have to be conscious of, but it is not that difficult.  With summer coming, I'm more worried about my sweat dripping onto it!

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