Saving a Surface Plate

[Cro]

Hey guys, 

Has anyone ever managed to salvage a surface plate and if so, how?

I have been given a 12" x 15" Cast Iron plate that's sadly been neglected and under the simple surface rust is a fair bit of pitting and I wondered if anyone had ever managed to restore one of these. 

Untitled by Adam Cro, on Flickr

I imagine I can clean up most of the surface with fine paper/scotch brite but it will never be as good as it should and it almost feels wrong to do it this way. 

Thanks,
Adam

[ettingtonliam]

Scotchbrite soaked in oil doesn't do any harm and is good for getting the rust off. Providing its still flat, I don't see that some rust pitting does any harm. See what it looks like after you've cleaned it.

Otherwise its planing grinding or milling, and thats quite a big surface.

[timb]

I echo Liam, the finishing touch would be to scrape it flat using another surface plate for reference. It is a big surface though, maybe need one of the electric scrapers. If nothing else your scraping skills would be honed (forgive the pun) by the time you are finished.

Tim

[Cro]

Thanks chaps, I'll give it a clean and see how it looks after that.

As you say as long as its flat that's all that matters, I had planned to use the face for lapping valve faces but an alternative method has been proposed which I think will be more suited.

I'll post some photos when I have a go at it.

Adam

[jem]

A piece of good flat glass might be better for lapping valve faces, as it is easily replaced, rather than a valuable surface plate.

best wishes

Jem

[RGR 60130]

I used a small mirror as lapping plate for years. They do have a finite life though.

Reg

[jackrae]

Provided the plate hasn't been used as a "chiseler's" work bench, a derust as suggested and a light rub over with the smooth side of either an indian oil stone or a 4-sided diamond lap block will identify any 'pimples' (ie high spots). These can then be scraped off and you should end up with a plate that's more than good enough for model engineering work. Aiming for perfection may well cost just too much in time.

[lesstoneuk]

Right, you'll need some thick glass plate, a sheet of 600 wetndry, a flat file ground at the end to a slightly curved top profile and square side profile and a tube of blueing paste.
Scotch rite the crap off and get a tidy finish with the wetndry and some oil by laying the sheet on the glass and rubbing in a figure 8.
Clean off everything then apply a thin layer of blue to the glass.
Gently rub the plate on the glass. The high spots will now show up. Gently scrape with the scraper. Wipe the plate and redo the blue rubbing to find the new high spots. If you are lucky, you'll only need to do this a few times. You'll know when to stop when you see a light even mottled finish with the blue.

[simplyloco]

Nov 17, 2020 17:58:45 GMT lesstoneuk said:

Right, you'll need some thick glass plate, a sheet of 600 wetndry, a flat file ground at the end to a slightly curved top profile and square side profile and a tube of blueing paste.
Scotch rite the crap off and get a tidy finish with the wetndry and some oil by laying the sheet on the glass and rubbing in a figure 8.
Clean off everything then apply a thin layer of blue to the glass.
Gently rub the plate on the glass. The high spots will now show up. Gently scrape with the scraper. Wipe the plate and redo the blue rubbing to find the new high spots. If you are lucky, you'll only need to do this a few times. You'll know when to stop when you see a light even mottled finish with the blue.

You will know when to stop when the glass won't leave the surface! I know...

[92220]

Ideally the glass should be a piece of plate glass. Plate glass flatness is a few tenths of a thou, whereas standard glass flatness is in thous. Actually a piece of 25mm plate glass makes a very good surface plate.The way it can be supported, is on a bed of dry sand. You just have to be careful not to drop a large piece of steel, like a chuck, on it!!

Bob.