Why are old surface plates scraped instead of polished?

Greetings all.
It has been a while since my last post. Been a bit busy with work, rather than just fun stuff.

Anyhow, I had a question I meant to ask, but havn't.

Why are the metal surface plates scraped instead of polished?

I understand the benefits of a scraped or flaked surface for oil retention on a load baring surface; and I realize that scraping is much faster than fine grit polishing.
However, the granite surface plates are polished... Why are most if not all metal surface plates are scraped instead of polished?
There is really no reason for surface plate to be a heavy load baring surface. No reason for oil retention as offered by scraping / flaking. Why not polish metal surface plates? Never seen one polished...

Not accurately flat. polishing can lead to waves. no good way to control flatness by polishing.
Bil lD.

Granite plates are lapped, not polished. Lapping is very slow compared to scraping. Scraping can go pretty quickly on soft cast iron, and obviously required rather low-tech tools for the day and age cast iron surface plates were popular. I remember going to some used machinery dealer in Pawtucket, RI in the early 80s. They had a literal heap of iron surface plates in the basement all jumbled together. Probably couldn't give them away.

Iron or steel on lapped granite is a low-friction interface, but smooth iron on smooth iron or steel on steel can be high friction to the point of wringing (like gauge blocks). The scraped surface gives you bearing points that allow contact, but not to the point of sticking together.

Coefficient of Friction Reference Table - Engineer's Handbook

What i am hearing is that scraping iron plates is good enough, but really the accuracy of the plate would suffer.
Terms girding, polishing and lapping are used interchangeably by many people, even though they have unique definitions.

In all honesty an electrical wheel (grinding), a rag in a hand (polishing) or a proper lapping plate (lapping) can all remove material from the target object. In all cases the rate of removal can be controlled by choosing the grit of the abrasive. I own abrasives as small as 125 nano-meters or as large as 60 grit.

I have a 9x12 cast iron surface plate which i am scraping flat using my 12x18 starret surface plate as a reference. From what i am hearing is that i will only continue to improve the flatness of it if I switch to abrading it with sand paper or buffing/polishing compounds and maketa multi-tool after i get it reasonably flat by scraping.

Milland said:

Iron or steel on lapped granite is a low-friction interface, but smooth iron on smooth iron or steel on steel can be high friction to the point of wringing (like gauge blocks). The scraped surface gives you bearing points that allow contact, but not to the point of sticking together.

Coefficient of Friction Reference Table - Engineer's Handbook

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Ahh, that makes sense. Saw your post after posing my last reply.
Thank you for sharing your knowledge.

My answer isn't complete, there's more complexity to material friction than one would think. For instance, friction can be very different between materials in atmospheric condition vs. in a vacuum, where the tiny amount of moisture almost always present on all surfaces is removed. Similar materials can stick together must faster when moisture is removed.

Milland said:

My answer isn't complete, there's more complexity to material friction than one would think. For instance, friction can be very different between materials in atmospheric condition vs. in a vacuum, where the tiny amount of moisture almost always present on all surfaces is removed. Similar materials can stick together must faster when moisture is removed.

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yeah, I realize the atmosphere vs vacuum effects. I just did not think that even the very well polished surface plate be THAT smooth. Looks like gauge blocks flatness from mitutoyo is about 2 uin (https://www.mitutoyo.co.jp/eng/support/service/catalog/09/E12014.pdf) and if we estimate the size of a gauge block to be 1 inch long we get gauge block flatness of 2uin/in. A grade AA surface plate from starret is flat to 35 uin for 30 inch diagonal plate, so also about 1 uin/in (http://www.starrett.com/docs/other-downloadable-resources/tru-stone---bulletin-807.pdf?sfvrsn=4) i guess it is comparable to flatness of a gauge block...

Didn't put the two and two together and did not think about wringing to cast iron surface plate, if it was polished...

pcm81 said:

In all honesty an electrical wheel (grinding), a rag in a hand (polishing) or a proper lapping plate (lapping) can all remove material from the target object. In all cases the rate of removal can be controlled by choosing the grit of the abrasive. I own abrasives as small as 125 nano-meters or as large as 60 grit.

I have a 9x12 cast iron surface plate which i am scraping flat using my 12x18 starret surface plate as a reference. From what i am hearing is that i will only continue to improve the flatness of it if I switch to abrading it with sand paper or buffing/polishing compounds and maketa multi-tool after i get it reasonably flat by scraping.

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Lapping is a slower means of materiel removal compared to scraping for the given surface finish achieved. Using course grit at fist requires more stages of lapping with ever finer grits, while increases the chances of grit contamination in the final stages.

For your second plate you are more or less done. You can't much improve on what you achieved by transfer from the the granite, unless you have a measuring means that would otherwise be suitable for calibrating the granite plate. I use my secondary cast iron plate when doing grinding setups as a "dirty plate". It's inevitable that by proximity and occasional carelessness that you are going to get surface grinder dust between the jig and the granite. So I use the cast iron plate knowing that I can simply do a scarping pass or two and the iron is back to flat. All the while sparing my granite of all but the bluing.

pcm81 said:

A grade AA surface plate from starret is flat to 35 uin for 30 inch diagonal plate, so also about 1 uin/in (http://www.starrett.com/docs/other-downloadable-resources/tru-stone---bulletin-807.pdf?sfvrsn=4) i guess it is comparable to flatness of a gauge block...

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An AA granite plate is not comparable to a hypothetical 30" gauge block. That chart at the end of the Starrett catalog is the Tolerance for repeat reading taken from the GGG-P-463C document. Where a repeat gauge having three feet in a ten inch line is slid all over the plate and there can not be a fluctuation of more than 35 uin. If say an 18"x18" plate is perfect except for having a 30uin deep pothole the size of your hand, it will pass both the overall flatness of 50uin and the reapeat as an AA plate but the ridge around this pothole will be steep enough not to pass as a gauge block.

To add another reason; many times, ie layout work and in process inspection, you aren’t going to be putting a finished, polished product on the plate. A ground and polished cast iron plate would get scratched and gouged pretty quickly. Especially when you consider the different hardness of materials to be inspected.

pcm81 said:

What i am hearing is that scraping iron plates is good enough, but really the accuracy of the plate would suffer.
Terms girding, polishing and lapping are used interchangeably by many people, even though they have unique definitions.

In all honesty an electrical wheel (grinding), a rag in a hand (polishing) or a proper lapping plate (lapping) can all remove material from the target object. In all cases the rate of removal can be controlled by choosing the grit of the abrasive. I own abrasives as small as 125 nano-meters or as large as 60 grit.

I have a 9x12 cast iron surface plate which i am scraping flat using my 12x18 starret surface plate as a reference. From what i am hearing is that i will only continue to improve the flatness of it if I switch to abrading it with sand paper or buffing/polishing compounds and maketa multi-tool after i get it reasonably flat by scraping.

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In reality The hand scraped plates are more accurate than those that are machine ground. Challenge 3 point iron surface plates can be purchased either machine ground or hand scraped. The machine ground ones have an accuracy of .0005" on the 12"x18" and smaller sizes. By comparison the hand scraped ones have an accuracy of .0002"

In the larger sizes the machine ground ones have an accuracy of .001" and the hand scraped ones have an accuracy of .0005". The last time I looked a hand scraped plate was nearly double the price of a machine ground one. A 24"x 36" Challenge 3 point hand scraped plate listed for $6,700.00 while a machine ground one was $3,700.00

pcm81 said:

Greetings all.
It has been a while since my last post. Been a bit busy with work, rather than just fun stuff.

Anyhow, I had a question I meant to ask, but havn't.

Why are the metal surface plates scraped instead of polished?

I understand the benefits of a scraped or flaked surface for oil retention on a load baring surface; and I realize that scraping is much faster than fine grit polishing.
However, the granite surface plates are polished... Why are most if not all metal surface plates are scraped instead of polished?
There is really no reason for surface plate to be a heavy load baring surface. No reason for oil retention as offered by scraping / flaking. Why not polish metal surface plates? Never seen one polished...

Click to expand...

.
you ever lap large parts ??...... many a time i have lapped medium small parts to remove .0001" takes a long time usually

What makes you think all cast iron surface plates are scraped? A shop I worked in had some fairly large (5' X 10') cast iron plates that were likely ground but looked polished. They were used 24-7 for as long as I remember for lay out and checking work. We used Bonami dry to clean them when the tools started sticking to them.
Really, I never gave that question a thought until now!
Dan

My understanding is that you can buy cast iron surface plates either way, scraped or ground/polished/lapped. The latter is technically more accurate when new in terms of the percentage of a square inch that's flat (overall flatness could be better or worse for both types of plate, depending on it's "grade" of accuracy), but a scraped surface is much more functional and the decrease in sq.-inch flatness is negated by the fact it won't scratch or wear as easily. They'll both do the same job the the same amount of time, but the scraped surface is going to be easier to use.

To add to Milland above, soft cast iron is sometimes used to make a lap. I have a few small CI flat laps and they have to be ground to get them back in shape as embedded abrasives makes the edge on cutting tools go away NOW.

You really wouldn't want the surface plate to be grinding away on your precious tools IMHO...

Matt

any mirror finished surface if you want to rough it up nylon abrasive will loose the mirror finish fast.
.
also lapping a mirror finish with 100 grit sand paper will loose the mirror finish. since it takes a long time to lap .0001" on larger parts just taking the mirror finish off only takes maybe .00001" to .00005" off surface to rough it up.
.
i often had to remove .00001" off larger surfaces and 600 grit takes a long time and often used 400 grit. most cast iron in a shop gets a tarnish or light rust after months. sure oil or rust preventive helps but unplated iron not going to stay mirror finish for long. it needs chrome or something other than iron if its not going to rust. thats why ring gages are often hard chrome plated then ground to size. steel ring gage will rust relatively fast.

Why are the metal surface plates scraped instead of polished?

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Often its a simple matter of practicality

Joe the experienced scraper hand - with his normal scraping equipment/supplies and a suitable master / masters can generate a usefully flat surface - to what ever degree of niceness needed - with nothing else other than his eye and mind and a certain amount of his time

No array of compounds needed

No maintained laps needed

No classy optics (or similar systems) to check your work

Galling...

It's not so much the wringing, but the galling that may obtain if any shifting occurs while two metal surfaces are in a wrung state. A wrung work is a happy work - as long as you can "un-wring that bell" (to fracture a metaphor).

Didn't put the two and two together and did not think about wringing to cast iron surface plate, if it was polished...[/QUOTE]

I am not aware of a polishing process being used to create a surface plate. Generally, after getting a good start with milling or a scraper or a lathe, the candidates for making a surface plate are scraping, grinding, and lapping.

You did say "metal" surface plates. Metal surface plates are a thing of the past: most today are granite or some other hard stone. The key here is "the past". One hundred years ago they did not have the modern machines for grinding and lapping. Those processes would have been a lot slower and a lot more expensive than scraping. Any power tools used would have been hand made for the process and probably not used for anything else. The tools for hand scraping are dead cheap. They can be made in almost any shop. And that is probably the key factor as to why the metal (cast iron) surface plates were scraped. Pure economics. Cheaper tools and less labor.

BTW, some of the most accurate surfaces in the world are and were made with a lapping style process followed by polishing. They are better than scraped surface plates ever were. They are optical surfaces, both flat and spherical. And they are made from glass or other, more exotic optical materials. The process is DEAD SLOW and therefore EXPENSIVE. And the people who do it call it grinding, not lapping. Read the story of the making of the 200 inch mirror for the Mount Palomar Observatory. Grinding and polishing of that surface took 13 years. And they DID use machines; they had to for something that size.

Hale Telescope - Wikipedia

Even a small optical surface can take hundreds of hours to grind and polish to it's final shape. And they DO use a POLISHING process that is even slower than the grinding/lapping to get the final surface. I made a 6" telescope mirror and it took many, many hours just for that small size. Those telescope mirrors are spherical or parabolic and are made with the mirror blank and one tool. Going for a flat surface would take longer because three surfaces would be involved instead of just two.

You can talk all you want about this advantage or that disadvantage or problem with the various methods, but the real reason why the metal surface plates were scraped was pure economics. It was the cheapest way.

pcm81 said:

Greetings all.
It has been a while since my last post. Been a bit busy with work, rather than just fun stuff.

Anyhow, I had a question I meant to ask, but havn't.

Why are the metal surface plates scraped instead of polished?

I understand the benefits of a scraped or flaked surface for oil retention on a load baring surface; and I realize that scraping is much faster than fine grit polishing.
However, the granite surface plates are polished... Why are most if not all metal surface plates are scraped instead of polished?
There is really no reason for surface plate to be a heavy load baring surface. No reason for oil retention as offered by scraping / flaking. Why not polish metal surface plates? Never seen one polished...

Click to expand...