Need advice on reconditioning a Cast Iron Surface Plate

[marka12161]

I recently picked up a Bridgeport and a round dial 10EE from the widow of an elderly gentlemen who passed away. Along with the machine came a pile of tooling and other assorted stuff. Among the pile was a 12 x 24 Brown & Sharpe Cast Iron Surface Plate. As you can see from the pics, there was some localized corrosion which I attacked using a razor blade followed up by some steel wool & kerosene. After this treatment, some pitting is apparent and i suspect the whole top of the plate needs to be rescraped.

I'm not terribly disappointed as I enjoy a good challenge. To date i have made a hand scraper and I think successfully scraped to bottom of a larg-ish cask iron V block to flat. It took me a few hours of practice to get the bluing just the right thickness but once i got the hang of it, all proceeded pretty well with uniformly distributed high points and proper hinging. Anyway, I also own a biax power scraper which I have yet to practice with and need to find the time to make a couple more scrapers with smaller radii blades.

My question is pretty simple, should i take a skim pass over the top of this with a fly cutter on the mill prior to scraping or should i just attack it with a scraper (hand or power)? In 2018 my 3' x 2' granite plate was inspected and certified to Grade A and it's seen little use since then so i'm pretty confident i have a good reference surface.

 

[ezduzit]

...should i take a skim pass over the top of this with a fly cutter on the mill...

You can only ruin it this way.

 

[sfriedberg]

I would not take a milling pass over this surface plate. I would also not try to scrape everything down to the bottom of the corrosion pits, either, as small pits or crevices won't interfere significantly with use of the surface plate. You do need to remove all the rust, even at the bottom of the pits, so I would continue razor blading then consider turning it face down in a shallow improvised basin of Evaporust, then scrubbing it with a toothbrush (or similar) and giving it a thorough rinse and dry. Then scrape it flat again.

 

[lucky7]

I’ve done a couple of similar sized plates that looked worse. It should scrape in fine without milling. I’d do 3-4 blind passes across whole plate to get rid of most rust then start printing with thick layer of blue, gradually getting thinner. Do you have lifting set up?

Have fun.

 

[marka12161]

Thanks guys. I'll stay away from the mill as everyone recommends. I do have a nice set up for handling the plate with a gantry that straddles my work benches so handling the plate isn't a hardship. I'll likely practice with the biax on the other V-block bottom before i get on the surface plate.

 

[Paolo_MD]

Personally, I would do the Evaporust treatment first, given that iron increases in volume when rusting.
Then, I would do a couple of even passes with the scraper (individual scraping marks, in individual lines, each spaced a scraping mark apart): you just want to clean up the surface and, at most, reinforce a possible already existing checkerboard pattern.

At that point you can start your evaluation. first, put the cast iron plate upside-down on the granite plate and "knock" each corner, to check for rocking. If it rocks, you can check how bad it is by trying to insert feeler gages at different locations. If is seems in solid contact, you should try to hinge it and get a feel if it is higher at the edges or at the center.

Based upon the results, you can organize your plan of attack.

Paolo

 

[Richard King]

Right on Paolo . King-Way rules of scraping!...Rule 1 & 2. Rule 3 is "Depth of those scrape marks", min off .0002" - Max .001" . Rule 4 - Hinge or pivot the part - 30% from each end when it's flat. Rule 5 - Wipe with a bare hand to feel for dirt or burrs - clean.

 

[marka12161]

Personally, I would do the Evaporust treatment first, given that iron increases in volume when rusting.
Then, I would do a couple of even passes with the scraper (individual scraping marks, in individual lines, each spaced a scraping mark apart): you just want to clean up the surface and, at most, reinforce a possible already existing checkerboard pattern.

At that point you can start your evaluation. first, put the cast iron plate upside-down on the granite plate and "knock" each corner, to check for rocking. If it rocks, you can check how bad it is by trying to insert feeler gages at different locations. If is seems in solid contact, you should try to hinge it and get a feel if it is higher at the edges or at the center.

Based upon the results, you can organize your plan of attack.

Paolo

Thanks Paolo. Good advise as was all the input i got on this thread. Coincidentally, i just picked up a fresh gallon of evaporust a couple of weeks ago. Your recommended strategy makes a lot of sense to me. I've actually got a very deep backlog of projects in the queue so it'll be a while before I get to this. I may do the evaporust soak sooner than later because it's farily passive work.

 

[Richard King]

Several years ago when I taught a class in the UK the host told me he bought citric acid for rust remover. I believe that is what Evaporust is. He owned a farm and said he bought it at a farm store in 20 kilo bags. A lot cheaper then a gallon of EV.

 

[lucky7]

Evaporust and similar use an organic polymer that chelates ferric oxide (rust) leaving a stabilized surface. Most of these processes use phosphoric acid to accelerate the process. Citric acid will also work but is slow.

On a simple planer surface like a surface plate, I wouldn’t bother. Just scrape the rust off.

 

[marka12161]

The level of knowledge on this forum is incredible and humbling. Thanks for all the input.