Scraping in a surface plate.

Hi all,
I’m here looking for some advice on how to hand scrape in my grandfathers old 2 foot by 2 foot crown surface plate. Having a small workshop of my own the surface plate will be really useful, however during the years of no use whilst my grandfather was ill it suffered some rust damage.
Now I’ve had some experience scraping in and I have also watched YouTube videos of the leading lights on scraping in, but they all use the surface plate as the point of reference, I’m wondering if there is another method out there that doesn’t involve me buying another surface plate to scrape in surface plate?

In the absence of a master surface plate large enough to print yours to, If it was me, I'd get a straight edge of length equal to or greater than the diagonal across the plate and have it scraped and checked against another known flat surface (such as a friends straight edge or surface plate). With that you can print the surface plate you are working on in sections for local flatness, then print it diagonally and along the ends to check for twist and bowing.

Similar process to how they check granite plates for flatness. Look for overall flatness and local flatness, separately, and compare those readings to see where the low spots really are. Be extra careful of how the straight edge hinges and watch how the blue on the high points transfers. Because you are using a standard smaller than the printed surface, you have to be extra sure you are not getting a false reading. Even then, I'd take it to a meterology lab where they can check it's flatness and confirm your results. I imagine you plan to use this plate as a master once's it's done(?) so you don't want to assume something and unknowingly transfer it's defects to everything it prints.

I scraped in a 8x16 surface plate awhile back, which took some time, but bigger stuff is going to take longer no matter how you do it. If you did not have another standard to print against it or some other way of measuring precision flatness over a larger area (like an autocoluminator or a CMM), the only method I'm aware of would be how they created the first flat surfaces, but that would require three matching plates and lots of time...

Pug2000 said:

Hi all,
I’m here looking for some advice on how to hand scrape in my grandfathers old 2 foot by 2 foot crown surface plate. Having a small workshop of my own the surface plate will be really useful, however during the years of no use whilst my grandfather was ill it suffered some rust damage.
Now I’ve had some experience scraping in and I have also watched YouTube videos of the leading lights on scraping in, but they all use the surface plate as the point of reference, I’m wondering if there is another method out there that doesn’t involve me buying another surface plate to scrape in surface plate?

Click to expand...

Why not take it to a calibration shop? They will lap it in as necessary and provide you with calibration papers also. Yes, it will cost some money but a truly flat surface is useful in many ways.

metalmagpie

metalmagpie said:

Why not take it to a calibration shop? They will lap it in as necessary and provide you with calibration papers also. Yes, it will cost some money but a truly flat surface is useful in many ways.

metalmagpie

Click to expand...

Having a lab check it isn't a bad idea. In the ideal situation they'd tell you it's good as it is. You can take the surface rust off with something like a brass brush and mineral spirits.

If they tell you it needs work, they're not going to lap a cast iron plate for you, I don't think. I could be wrong, maybe they do that these days.

<edit> You'll probably get more specific help for scraping in the machine rebuilding forum, and help with measurement in the metrology forum.

Pug2000 said:

Hi all,
I’m here looking for some advice on how to hand scrape in my grandfathers old 2 foot by 2 foot crown surface plate. Having a small workshop of my own the surface plate will be really useful, however during the years of no use whilst my grandfather was ill it suffered some rust damage.
Now I’ve had some experience scraping in and I have also watched YouTube videos of the leading lights on scraping in, but they all use the surface plate as the point of reference, I’m wondering if there is another method out there that doesn’t involve me buying another surface plate to scrape in surface plate?

Click to expand...

What you should do is figure out what is your ultimate aim? Is it to renovate this iron plate in the memory of your grandfather? To have a 24" square reference surface? To do the first in order to achieve the second?

I would give serious consideration to buying a suitable-sized new granite reference for your scraping duties and cleaning/de-rusting your grandad's old plate. Trying to scrape-in a large plate like that as a novice is quite the challenge.

Pug2000 said:

.. how to hand scrape in my grandfathers old 2 foot by 2 foot crown surface plate.

Click to expand...

"At a later date, if at all", is "how."

At ony 24"x 24" by - what? Ribs & skirt to around 8" tall, overall?

You can de-rust it in a Big Box few-use plastic mortar-mixing tray, kid's plastic wading pool, or a tray/tub shop-fabbed with wood, plastic and tarp liner.

Do that first.

You might not even have any significant rust pitting. You might find it is still rather decent as to flatness, too.

You WILL find a new OR used GRANITE plate, and in larger size, can be had rather inexpensively.

A small CI plate, and "small" it is, will VERY useful for layout, measuring, basic inspection, trial fit-ups and the like.

So yes, to "really useful".

But even in a light-machines-only shop, at least one dimension of a Surface Plate meant to vet straightedges against nearly-always needs to be 36", better-yet 48".

Otherwise, if neither space nor budget, find he who has such a plate and periodically vet your second-level reference surfaces off of his one.

OTOH? One can do a LOT with even a 12" square CI bench/layout/inspection handy WORKING plate.

24" is MUCH nicer. And you have that. Your G'Dad was no dummy. Practical guy who knew his stuff, more obviously.

But it is too small to do much with as a reference for other references.

So I'd suggest just an Evap-o-Rust chelating or reverse-electrolysis de-rust - then try to avoid scraping it for at least a year.

Not because it needs to age, like a fine wine.

Until you know more as to direction. Where and how you are getting your best/most use out of it, and where your other options are taking your future needs for your shop. THEN you might be well-served to re-scrape it. You want to make it better. Never WORSE.

My B&S 18" X 24" Grade B Black granite HAD to be upgraded to a Herman 30" X 48" Grade A to get much of anything DONE, for example.

Even the Herman was a tad short on size. But it can be rolled through a 36" wide door, so I make it do.

And black granite - whilst a good surface plate - wudda been too easily and often damaged if used as a Daily-driver shop layout/inspection plate.

CI is better for that. Wish I had your plate, actually! Especially as I would still have the larger Granite SP, too!


I did say your G'Dad was no dummy?

what pecision do you have in mind and how bad is it now?