Cast Iron Surface plate Checking and lapping

I have a medium sized (18''x24" eyeballed) cast iron surface plate. I can't find a name brand on it but I'm fairly sure it came from Xerox in the mid 70's and that was the inspected.
I'm going to check it tomorrow with a mag base and dial indicator (very similar to a repeat reading gauge) and see what I get.
I know it's off, but by how much is the question.
Are there still places I can get it lapped and re-calibrated and not let too many moths in my wallet fly out? Or am I better off getting a smaller Chinese tombstone?
Thanks!

imo better of with smaller tombstone based on the following logic. flat, really flat, is difficult to achieve.....mostly guys would scrape it flat ( you don't want to lap it) implying you need something flat (another high grade surface plate) to spot it wth. Since it sounds like you don't have one, and probably aren't going to surprise us by having an interferometer or autocollimator, so you'd have to hire someone with a plate to scrape it....ergo if a chinese tombstone is good enough it will be cheaper/easier than the foregoing

Go over the surface carefully looking for dings and scraches, if it looks clean and not overly abused, I'd just go by the certification label on the plate. And leave well alone.

If you want to go down the path of flattening it..

You need a reference surface. A known good plate to blue from, and then scrape it flat, some experience scraping is required. See if you can get into one of the classes that are advertised on a semi regular basis.

If it looks badly abused and has visible dings, you might want to take a lick off with a surface grinder before you scrape.

Ray

Thanks for the replies! I don't have a bigger surface plate to scrape in reference to.
As for surface grinding....don't have a surface grinder.
And as for the calibration, there is none. No name tag or anything. As far as I'm concerned it basically a flat piece of cast iron sitting on my bench.
How accurate would you think grade A plate from chinatown would be? Is what they say or are they bluffing?
https://www.shars.com/products/view/2339/Grade_A_12quot_x_18quot_Black_Granite_Surface_Plate

Spyderedge said:

Thanks for the replies! I don't have a bigger surface plate to scrape in reference to.
As for surface grinding....don't have a surface grinder.
And as for the calibration, there is none. No name tag or anything. As far as I'm concerned it basically a flat piece of cast iron sitting on my bench.
How accurate would you think grade A plate from chinatown would be? Is what they say or are they bluffing?
https://www.shars.com/products/view/2339/Grade_A_12quot_x_18quot_Black_Granite_Surface_Plate

Click to expand...

The one you linked to says it's Grade A, I'd think you would have to accept that it is Grade A, unless you have the means to prove it wrong.

I've checked one or two chinese surface plates in the past, never found one that was outside the claimed specs, but often the chart supplied didn't match the measurements.

The only thing I'd question, is 12"x18" going to be big enough...

Ray

My partner and I teach scraping classes and part of the class equipment is three 16 x 25 cheap import grainite surface plates. We sell them off at cost to students that want them to save haulng them (or shipping them $$$) home for storage.

When it's convenient I buy them from Grizzly as it is a 70mile daytrip and saves shipping. Out of the 15 or so we bought over the last few years only one failed to make the grade as being flat within specs. Since i had three together we could cross compare.

Based on my personal experiece I conclude that the import granite plates meet their specs. YMMV if you happened to buy a lemon but 1 in 15 aint bad odds. For that matter the dud plate I rejected was still good just not quite up to snuff for a scraping class where I preached accuracy and where more than a few students hauled in cast iron plates of their own to scrape. The dud was plenty good for general home shop use.

$70 for an 18 x 24 granite surface plate is pretty good if you can tolerate the shipping charge. If the plate turns out to be a dud the seller pays freight both ways (check to see if that is seller policy; it is with Grizzly).

Checking a surface plate from scratch requires some expensive or hard to borrow instrumentation. You need either an optical auto-comllimator or electronic differential level system and the smarts to use them. Checking from a known flat such as borrowed access to one is far more cvonvenient presuming a borrow-able plate is within driving distance.

So what to do if yiur cast iron plate is not flat? Surface plates are not lapped: they are scraped. Lapping plates are either selectively lapped flat or first ground and then lapped on a whole surface lapping machine such as a LapMaster - provided the LapMaser table is properly maintained.

I prefer a granite flat. They are flatter (usually) and much harder thus more wear resistant than cast iron. And cheap; you can't afford to calibrate a suspect plate when a replacement is so cheap. Simply replace. When your granite surface plate poops out donate it to a dedicated home baker or candy maker in need of a cool working surface and buy a new granite plate.

That said if you are drawn to restoring your plate to former glory by all means do so. Find a scraping class in your area and pack it along as a side project. If the plate is no more than 0.005 out and you scrape aggressively it may be a day and a half project to scrape an 18 x 24 plate in to 0.0005" of a plane overall. If you wish better accuracy, you have to be obsessive about temperature.

How flat do you need it? For 25 years I got along doing my layout and inspection work on a clean milling machine table. If you need a scraping reference for sub thousandths work you better get a real surface plate.

Below I refer to loose abrasive lapping. Abrasive sheet lapping is a diferent deal.

Lapping? Put that completely out of your head. Two classes of people lap to improve machined surfaces. People who don't know any better and people who know and have the skills, apparatus, and need to bring it off. Between these two extremes is a huge gulf.

Enco often has free shipping for orders over $50, you just can't beat that for getting a granite plate.

They had them on sale, plus 20% off, plus free shipping, a couple of months ago. i just couldn't resist and got an 12 x 18 for $37. I already had an 18 x 24 Starrett, didn't really need it, so when it arrived I came to my senses and gave it to my daughter's boyfriend as a Christmas gift.

Sound's like he's a good prospect.

Forrest, not only are your posts filled with useful, authoritative expertise, they're wonderfully clear and helfpul. Bravo.