Surface Plate Resurfacing

[RaisedByWolves]

It looks like it was precision scraped, based on the full coverage of the markings. ScotchBrite (except White) is abrasive so apply it uniformly all over the surface. After cleaning, I would use a fine diamond lap, or equivalent, to search out high spots resulting from any dings,

They use cast iron for these as there will be no high spots from boo boos.

Cast is porous and one of the only materials that have this property.

I have a similar one I rehabbed on our surface grinder and it’s better than what 90% of most shops would need.

We have need using a beat to hell granite plate doing tool n die work since I was in my apprenticeship.

 

[projectnut]

Even if ground , wouldn’t it have to be scraped in also? Just asking.

Not necessarily. Challange offered both the machine ground and hand scraped version of almost all of their cast iron surface plates. The machine ground plates up to 12" x 18" were accurate to .0005". Hand scraped plates were accurate to .0002". Larger size machine ground plates were accurate to .001" and hand scraped were accurate to .0005".

There was a considerable difference in price. I have a 24" x 36" hand scrapped model. It was originally purchased by the company I worked for in the 1960's. At that time according to the 1964-65 catalog it listed for $670.00. The machine ground model listed for $321.00/
.
It's interesting that they offered both cast iron and granite plates. It's also interesting that the 24" x 36" cast iron model weighs 475 lbs. while the granite model weighs 470 lbs.

 

[Jake M]

That looks like a tool that's been around a while... Who knows what it is.

It appears to have been scraped at one time or another. Scraped for flatness? Scraped to look better? Scraped for a practice piece? Who knows. Probably not even the seller.

How flat do you need? My thought is that if you take any and every metrology tool in your shop, check it for flatness, and if it appears to be flat by any way you can test it- It's gonna be as accurate if not more accurate than the rest of your work.

Using it for layout? I use a granite plate for that. Why? Because it was the leg/side plate from a granite night stand that came unglued at it's joints and was being thrown away. It WAS 12X20 and pretty danged square cut. So I cut twelve inches off of that to make a 12X12 plate. Polished granite of that grade will inherently have a "dome" shape polished into it.
I can, to the best of my ability, "map" the curve in that. I've got the "center" of the dome near one edge, and I can map out the "sphere" from there. I dunno how accurate this is, but it's repeatable- That dome is 0.0025 to 0.0027 inches tall, and pretty accurately formed.
That's NOT flat, but it's a chore to measure it with what I've got. I use it for various things, including layout. My marks come out where I want 'em, to every bit as accurate as I can measure. Add a hand center punch here and there... So yeah.... Even though it's not flat, it's flat.

Or plan B, if the "not flat" bothers my OCD, I can flip it over. As best I can rig up to measure, the saw cut side... It's flat. Flat flat. It's not nice to work with, cleaning is a bear, it doesn't "slide" as smooth... But it's flat. Same results on either side.

Personal opinion- Chemically remove the rust, DON'T hit it with a power sander of any kind with ANY abrasive, INCLUDING scotch brite. Scotch brite will and does remove metal, all day long. You're not gonna get rust off of there without putting some "shape" into the plate. A hand touch in a tough spot, maybe... You'll still be making a hole, but a small enough one that you'll probably never find it.
Once it's cleaned up, check it for straight as best you can, and if YOU measure it with YOUR tools to be flat, or pretty close to it, it's gonna be every bit as useful to you as it would be if it were scraped to a millionth of an inch.

If you ever wanted to take up scraping... That'd be an AWESOME practice piece. Single plane, no corresponding features to "match up"... You could scrape that a thousand times until you got it just right, and not hurt it a bit.

Last I knew, a "one part" grinding setup for anything of that size cost about twice as much as what you could spend on a grade B plate of the same size. That's "shop grade". (That from not premium, but still reasonably trusted brands) That's accurate enough that it's errors are within the additional errors you'll see by not using a closed, controlled "laboratory" environment to keep it stabilized.

And while that is my opinion on the situation, I'll be the first to say that if I had a plate dialed in to half a microinch... It'd still put a smile on my face even thought I fully know that it moves out of grade every time I open the door. I wouldn't want do dissuade you from any project of that nature in any way. I certainly overdo things on occasion, that don't make sense, just because I want to......

 

[dwall174]

That looks like a tool that's been around a while... Who knows what it is.

Yeah, it's probably been bouncing around from shop to shop for a while, the stand was definitely shop made by someone.
At one time it was owned by some type of Government Faciality judging by the tag on it.

 

[RaisedByWolves]

That looks like a tool that's been around a while... Who knows what it is.

It appears to have been scraped at one time or another. Scraped for flatness? Scraped to look better? Scraped for a practice piece? Who knows. Probably not even the seller.

How flat do you need? My thought is that if you take any and every metrology tool in your shop, check it for flatness, and if it appears to be flat by any way you can test it- It's gonna be as accurate if not more accurate than the rest of your work.

Using it for layout? I use a granite plate for that. Why? Because it was the leg/side plate from a granite night stand that came unglued at it's joints and was being thrown away. It WAS 12X20 and pretty danged square cut. So I cut twelve inches off of that to make a 12X12 plate. Polished granite of that grade will inherently have a "dome" shape polished into it.
I can, to the best of my ability, "map" the curve in that. I've got the "center" of the dome near one edge, and I can map out the "sphere" from there. I dunno how accurate this is, but it's repeatable- That dome is 0.0025 to 0.0027 inches tall, and pretty accurately formed.
That's NOT flat, but it's a chore to measure it with what I've got. I use it for various things, including layout. My marks come out where I want 'em, to every bit as accurate as I can measure. Add a hand center punch here and there... So yeah.... Even though it's not flat, it's flat.

Or plan B, if the "not flat" bothers my OCD, I can flip it over. As best I can rig up to measure, the saw cut side... It's flat. Flat flat. It's not nice to work with, cleaning is a bear, it doesn't "slide" as smooth... But it's flat. Same results on either side.

Personal opinion- Chemically remove the rust, DON'T hit it with a power sander of any kind with ANY abrasive, INCLUDING scotch brite. Scotch brite will and does remove metal, all day long. You're not gonna get rust off of there without putting some "shape" into the plate. A hand touch in a tough spot, maybe... You'll still be making a hole, but a small enough one that you'll probably never find it.
Once it's cleaned up, check it for straight as best you can, and if YOU measure it with YOUR tools to be flat, or pretty close to it, it's gonna be every bit as useful to you as it would be if it were scraped to a millionth of an inch.

If you ever wanted to take up scraping... That'd be an AWESOME practice piece. Single plane, no corresponding features to "match up"... You could scrape that a thousand times until you got it just right, and not hurt it a bit.

Last I knew, a "one part" grinding setup for anything of that size cost about twice as much as what you could spend on a grade B plate of the same size. That's "shop grade". (That from not premium, but still reasonably trusted brands) That's accurate enough that it's errors are within the additional errors you'll see by not using a closed, controlled "laboratory" environment to keep it stabilized.

And while that is my opinion on the situation, I'll be the first to say that if I had a plate dialed in to half a microinch... It'd still put a smile on my face even thought I fully know that it moves out of grade every time I open the door. I wouldn't want do dissuade you from any project of that nature in any way. I certainly overdo things on occasion, that don't make sense, just because I want to......

This.

Without a climate controlled room (no, your air conditioned spare room or basement does not cut it) and expert level practices much of this is academic.

Scrape it to within whatever measurement you like and you will distort it while taking your reading or doing your setup by resting your wrist on the edge.

This is why granite is a thing, but even then your body heat is affecting every tool you touch.

The above mentioned dished surface plate we use at work is in an uncontrolled room right under a HVAC vent and we somehow get by.

 

[dwall174]

Without a climate controlled room (no, your air conditioned spare room or basement does not cut it) and expert level practices much of this is academic.

I agree!
Back when I worked in an actual machine shop, The quality control department was in a climate-controlled room along with all their measuring & testing equipment.

After all isn't this the "Hobby Machinist" forums.
I'm just a weekend worrier working in a normal 2-car garage, I'm not trying to make any kind of precision parts.

 

[twhite]

Just remove the rust as stated above. White Scotchbrite and remove any dings. Put that bad boy to work for you.


Cutting oil is my blood.

 

[extropic]

They use cast iron for these as there will be no high spots from boo boos.

Cast is porous and one of the only materials that have this property.

I have a similar one I rehabbed on our surface grinder and it’s better than what 90% of most shops would need.

We have need using a beat to hell granite plate doing tool n die work since I was in my apprenticeship.

Are you saying that, on cast iron, the impact of a hard, sharp point/corner won't raise a "donut" around the divot?
What is your definition of a "boo boo"?

 

[SouthernChap]

This is an apposite topic to have turned up.

I laid my hands on a 36"×24" "surface plate" pretty cheaply from Charter Auctions.

It was on this stand:



(The frame's inner 'shelf' is now a lot cleaner than it was when I first got it) and it's currently resting on some blocks of wood on the floor. Here it is before it was lowered onto the wood:


It has some surface corrosion and a couple of minor dinks close to a couple of the corners.

So the consensus is that a bit of evaporust and some delicate stoning to to take any high spots down is the way to go?

 

[Jake M]

This is an apposite topic to have turned up.

I laid my hands on a 36"×24" "surface plate" pretty cheaply from Charter Auctions.

It was on this stand:

If that were me, (and that's important, because I'm not sure if you're making parts for the space program yet), but if that were me- I don't like that stand, because it's currently supported at four points. Those being (apparently) the weld beads in the corner.

There are specific points you can use to minimize any deflection on a solid block (like a granite plate) and you could choose based on your use. A smaller one like in the original post, it doesn't matter. Yours is huge, but yours is a gusseted shape. I'm not really sure the same applies here, (and again, the "home shop thing", but that big and that heavy, were it mine, I'd have it on three points.. I would personally put tiny (oiled) soft wood shims on one of the short sides, at or near the corners of that stand, and let it sit on those. Just thin ones. If you've got an old wooden yardstick that needs to be gotten rid of, that's the "scale" of shim I'm thinking. They're pre marked so you can cut off just what you need and paint stores sell 'em cheap. And on the other short side, a single shim in the middle. Maybe a little longer to spread the weight over the unsupported angle. Maybe even six or eight inches. Wide enough to keep it from rocking (if that's an issue), but short enough to concentrate some weight so it'll "bed into" the shim, and find it's own happy place, without being influenced tangibly by the stand.

(The frame's inner 'shelf' is now a lot cleaner than it was when I first got it) and it's currently resting on some blocks of wood on the floor. Here it is before it was lowered onto the wood:
It has some surface corrosion and a couple of minor dinks close to a couple of the corners.
So the consensus is that a bit of evaporust and some delicate stoning to to take any high spots down is the way to go?

Yeah, I'd stand on that same advice for this larger plate. Chemically remove the rust. If you've got dings in it, there's probably a raised spot. That'd be an excellent use case for a "precision ground stone" if those have caught your interest, otherwise, great care to NOT remove what isn't raised. Remember that when you're "sanding", you're cutting EVRYWHERE under the paper, not just on the damaged part. Same with scotch brite.

Or maybe... Homebrewed gave me some really good advice once, he called this a "dead file". Take a file (any one really, if it's got a flat side). Well used is fine as you might have to break off a chunk to get access. Even a worn out one will work perfect, it just takes a couple of extra minutes (literally a couple, it's not bad). in the next step. Lay some sand paper on a flat surface, and run the file teeth across it (lap the teeth) to make the teeth have a flat top. Zero backrake cutters if you will, so they can't dig into ANYTHING that's not raised up. Test the file before you use it of course to make sure you're really done sanding/lapping it. Push it on to the damage, not off of it, and it's pretty darned good. IMO, those little dings will not bother you a bit, but the raised spots around them (if they're not already addressed by a previous owner) will cause you more grief than if the whole plate had bows or sags of the same measurement, across the whole thing.

That's a good score. I'm a little jealous.

Outside of that, I'd give you all the same advice I gave for the smaller plate