Thick glass can substitute for granite

[Wizard69]

Well I use granite that 12 x 18 and it has worked out well. I got mine for free from a cousin with a machine shop. This has been a good size for what I do. I also have a cast iron one I got when I purchase some machines. This one is about 18 x 24. I made wooden covers for both plates. The covers are lined with felt. As you can see I also use it to display projects I have made.

It is interesting that granite only became the material for plates in World War II when steel was in short supply because of the war effort. Because of its stability and resistance to moisture it is now the choice for plates.

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The problem with granite is that it is harder to maintain the plates. Assuming you have a set, iron plates are easy to keep accurate and maintain By scraping. You can scrape granite of course but the high spots can be harder do see and hard spots can give the scraper a lot of grief.

I guess that in the age of dirt cheap granite reference plates they have effectively become disposable. Kinda sad really as another art and talent bits the dust. I have to wonder how many people right now could go a step further and hand polish and lap a surface plate to a high standard of flatness.

 

[Andre]

The problem with granite is that it is harder to maintain the plates. Assuming you have a set, iron plates are easy to keep accurate and maintain By scraping. You can scrape granite of course but the high spots can be harder do see and hard spots can give the scraper a lot of grief.

I guess that in the age of dirt cheap granite reference plates they have effectively become disposable. Kinda sad really as another art and talent bits the dust. I have to wonder how many people right now could go a step further and hand polish and lap a surface plate to a high standard of flatness.

Scraping isn't lost, nor has it ever been lost. Many people and many shops still rebuild and scrape machines by hand. It's not hard to learn at all.

Buy the piece of granite, then have it resurfaced to a high degree of flatness using a cast iron lap and diamond abrasive. Measure the plate with a autocollimator and a repeat gauge.

Sent from my XT1053 using Tapatalk

 

[Tony Wells]

There is a world of difference in cast iron and granite surface plates and the way they are cared for and reconditioned. No one I know of actually scrapes granite. If it's taken care of between maintenance intervals, it may need nothing, or it may need a minor lapping to bring to spec. If it's abused and has a "hole" in it, then it can be sent back to the factory for regrinding, then lapped in. Larger, more expensive plates are handled that way in a commercial environment. Smaller plates (and in some cases larger ones) are merely downgraded and kept in use, provided there are sufficient "good" plates in the house to fill the need. Those smaller plates, even after downgrading, are useful for many things. Even after some use in the downgraded condition, they are sufficient for home shop use and if you have an in, you might be able to score one for cheap or free. It's generally not worth the money to send a small plate back for refurb, so they get set aside.....or sent to the welding shop. Eventually they are disposed of, so being at the right place at the right time can mean everything.

Before granite became the defacto standard surface plate, CI was it, and generally in those days shops had people who knew how to take care of lapping or scraping them as needed. Instrument verification has come a long way since granite became standard, and methods of in-shop resurfacing and certification have made granite more practical in almost every sense, hence the increasing rarity of seeing CI plates in use. This especially true in cases where large plates are needed. It's just not as practical to keep 3 60 x 120" CI plates, nor spend the time working all 3 to achieve the required planar requirements.

 

[Keith Foor]

You are using it already so obviously it's usable.

In a home shop at best you can do the three plate method and scrape them in. That can get you very accurate plates but you still won't be able to measure flatness effectively. That would likely require setting up some optical instrumentation and hardware to lap the plates to perfection.


Obviously the granite is useful to you already. The real question is how flat do you need in a home shop. I wouldn't be surprised to find some of these granit table tops to be very flat over a limited area. If they where the you would see it via optical distortions in the surface reflections.

Well, I am doing it, but that don't mean it's correct. I am still really only getting my feet wet and the only thing I have been lapping is crap like cylinder heads that have a gasket seal. So while I am getting them as flat as surface of the granite if still may not be all that flat.

I think as a test I am going to make a couple 123 blocks and try to scrape them in on the sink cutout. I figure if they stick together they are pretty well flat. so out of curiosity does anyone know how flat something has to be to start sticking together Are we talking a couple tenths, less than a tenth? Or significantly more? I have a big slab of 1.5 thick steel that is destine to be tool holders on a feeler second operation lathe. But it's 3 foot long and that's A LOT of tool holders. I have never done any scraping, so I am thinking that making some 123 blocks would be good to learn with.

 

[Ulma Doctor]

I can't say when metal starts wringing together, but it's pretty finely ground.
I've heard that it's actually molecular attraction, the molecules are sharing electrons in a weak bond.
if you have never scraped,
i don't want to discourage- but you may wish to start with something a little simpler like an angle plate or the t-nut slot and faces of your lathe's compound rest.
in due respect, you'll learn more completing several small projects to a high degree of accuracy, than go hunting for bigger game without proper understanding.

 

[John Hasler]

out of curiosity does anyone know how flat something has to be to start sticking together Are we talking a couple tenths, less than a tenth?

Much less than a tenth. One microinch finish and five microinch flatness.

 

[hermetic]

Plate glass is ground and polished to optical flatness so that it does not distort images, so an old piece of shop window glass or display shelf glass will be as flat as a hobby machinist will ever need, but the thicker the better, as it will distort if put under pressure. Nothing less than 1/4 inch, and preferably 3/8+ thickness. The same applies to good quality granite worktop, also ground and polished to optical flatness. Glass and granite are much more temperature stable than cast iron, but you need to take care not to chip them. A piece of plate glass can be supported on a flat or carefully folded towel on a flat surface in order to support it evenly, stop it sliding around, and minimise any distortion. You can go one stage further and use a thick felt lining over smooth kitchen worktop with a frame to stop sideways movement. You can also lap on them to very fine degrees of flatness for gasket faces, and using wet/dry paper it is quite possible to bring a cylinder head back into tolerance in quite a short time. If you work for NASA, let them provide the expensive flat surfaces, and pay for the yearly certification, the above methods and materials far exceed the need for accuracy found in the hobby shop, no experience with granite tiles but as has been said above, with three, you can produce a perfect surface. theoretically. and little point in marking out to superb levels of accuracy, only to put the work in a 3 jaw chuck which was probably at nearest about 3 thou concentric when new!

 

[Keith Foor]

I can't say when metal starts wringing together, but it's pretty finely ground.
I've heard that it's actually molecular attraction, the molecules are sharing electrons in a weak bond.
if you have never scraped,
i don't want to discourage- but you may wish to start with something a little simpler like an angle plate or the t-nut slot and faces of your lathe's compound rest.
in due respect, you'll learn more completing several small projects to a high degree of accuracy, than go hunting for bigger game without proper understanding.

The blocks I am making are from a couple chunks of cast I had around that were 1.25 by 3 by 3. I was going to cut them down, machine them then grind and finally scrape them in. I figure that if I screw them up I have lost nothing but time. And since they are within the readability of my micrometer set I could at least verify I got them to 1 by 2 by 3. These first two are not even going to have setup holes in them as I just want to see if I can get down to that sort of accuracy in my garage with the gear I have. Honestly I don't believe I ca, but it seems like fun to give it a try. If I can get two of them the same size I can at least use them to prop up large pieces I am clamping down on the mill.

 

[Ulma Doctor]

it never hurts to try, you can do anything! anything you set your mind and spirit to!
i wish the best of luck, please keep us in the loop!

 

[Keith Foor]

OK, so here is a question.... and I hate to continue hijacking this thread. But i think it sort of applies. What dimension should I grind to on a 123 block that I am going to scrape in? Obviously I am going to take off some more metal as I scrape. So where do I stop grinding to do that? Should I leave 2 tenths, or 5 tenths,,, more,,, less I just don't know. I was thinking 5 which gives my 2.5 ten thousands per side to scrape but if that's not enough I will end up small on them. Now of course I can scrape them BOTH down until they are the same, but smaller than 1 by 2 by 3 but I would rather try to actually hit the mark dead on. Or at least as dead on as I can with what I am working with.