Painted granite surface plate

I just bought my first surface plate in black granite.

At least I thought it was black until I looked at the bottom which appears speckled grey bordering on white.

Knowing the Chinese proclivity for making something nice then destroying it at the last moment, I cleaned off a layer of paint using acetone, methylated spirit and Scotch-Brite.

It is still dark grey but a lot smoother. My height gauge slides around like Torville and Deane after a few bevvies.

Have I destroyed it?

No - repaired it if anything! The material is igneous rock and not many things dissolve that.

I wonder why it had been painted, unless perhaps as a surface-protection for transport.

Actually black granite surface-plates are not granite, but probably basalt - the masonry trade and tool-suppliers tend to classify hard rock that can be polished as "granite", and soft rock that can be polished as "marble", irrespective of geology.

Granite generally is very coarely crystalline and lighter-coloured. Basalt-class stones are finely-crystalline and very dark grey with a sparkly look.

Torville and Dean pickled on ice.... the mind boggles !

I suspect the cheaper plates are dyed to make them look black.
Because everyone “knows” black granite makes the best plates (unless it’s pink of course)

I wouldn’t advocate used of scotch brite - an abrasive- on a precision flat - it’s likely you haven’t done much damage to the precision, but it’s not easy to tell without proper measurements.

Dave

You won't affect the accuracy with a quick go-over of scotchbrite especially if it's the whole surface. I sometimes recondition granite plates and to remove 10 microns generates a lot of dust. You would certainly notice.

My experience is that the Chinese plates tend to be pretty damn good, certainly exceeding their certification. Much less risky than buying a used plate of any grade.Even the last 'in calibration' plate I bought was nowhere near spec.

That's a relief, I shall do the sides next.

Methylated spirit seems to dissolve the green colour of my Scotch-Brite so I will switch to Regina Blitz.

Depends what you bought it for. If accuracy is important, Robin's plate is compromised and it's now necessary for him to prove it's within specification by having it re-calibrated. Forum reassurances are worthless - the table has to go to a laboratory.

Fortunately, I doubt many Model Engineers have a formal requirement for accuracy. All a typical amateur needs is a reasonably flat surface. A kitchen worktop may be good enough, and can be improved by laying a sheet of plate glass on it. Quite easy to work to about a thou/0.02mm in an ordinary workshop.

Robin's Surface Table is still likely to be plenty 'good enough' for home use, but an apprentice who scotch-brighted a Tool Room Surface Plate would be lucky to get off with a serious b*ll*cking!

Dave

I have the coarse brown scotchbrite and have use it on adly gummed up granite. I couldn't raise any appreciable amount of dust with it. When I'm cutting a plate however it gets everywhere.

They do wear, that's a fact, but not from a single scotchbrite cleanup. Not so much that you could easily measure.

When we talk about "Scotch-brite" are we talking of wire- or plastic- wool?

Genuine basalt (the dark-grey, sparkly surface-plate stuff) and indeed granite (lighter-coloured, highly-mottled, kerb-stone stuff) are extremely hard. Basalt is even melted and cast into liners for industrial plant handling very abrasive slurries and powders (MP about that of cast-iron, but more viscous.)

So although I advise against making it a habit, I can't imagine a single, gentle cleaning with a plastic scouring-pad or even a wet, soapy 'Brillo' pad will really damage either rock beyond interferometry-test depth.

For normal cleaning use white spirit, meths or kitchen work-top cleanser, on cloth.

(Both rocks are made of silica, second in hardness to diamond; and metal-silicates, a bit softer but generally still harder than steel.)

'

Just don't use scourers on your shove-ha'penny slate though....

.

Similar problem of casual language to that with ‘Loctite’

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Scotch-brite is a trade name of 3M, and at least some of the products contain Aluminium Oxide particles bonded into the plastic.

**LINK**

https://www.scotch-brite.com/3M/en_US/scotch-brite/

A quick test is to rub the pad across-the-grain of a sheet of ‘brushed’ stainless steel.

The heavy duty ones will most certainly leave their mark. !!

MichaelG.

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Feast your eyes on the monochrome [SEM] image on this page:

 http://ijsrd.com/Sanding-Sheets/782016-VFN-3-Width-8-Length-Pack-of-40-3-Width-Nylon-Backing-3-x-8-A-VFN-Aluminum-Oxide/

Edited By Michael Gilligan on 20/10/2021 22:55:04

I use THESE on wood and metal they have a grit/grade to suit whatever I am doing. So they can be a little more gentle than some on the market. I have nothing to do with this company other than being a customer

For the record, Granite does indeed include a large proportion of silica - its the light crystals which can be milky or clear which are silica, and they wear least.

BUT - Basalt is finer grained, yes, and is made up of mixed plagioclase-feldspar and pyroxene, and perhaps some olivine. Very little silica or none. I'm sure Wiki will tell you more.

The main character they both have, as surface plate material, is inertness, stability, and readily available in fairly big lumps.

Cheers, Tim

I spent some time researching and then correctly mounting my granite surface plate, including a case and cover. It now sits with lots of stuff on it and I invariably use the cast iron table of my vertical bandsaw instead.

**LINK**

Which is why the cover I made for mine is this shape:

Dave