# Thick glass can substitute for granite



## Syaminab

Just to mention, on hobby works, light parts, you can use a glass as a granite for measuring parts.


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## Tony Wells

One of my early mentors used a tombstone as a surface plate, but I suppose a piece of tempered safety glass would be cheaper and easier to get. I'd stay away from plain window glass. It's wavy and not flat.


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## Ray C

+1 on Tony's thoughts about thin glass.  For grins, I mic'd a small 6x8 pane and it was off several thou from end to end.   Optical quality glass is probably dead-on but would cost about as much as a B-grade slab -I'd go for the slab and a B-grade is cut better than anyone in a home shop could dream of measuring (something like +/- 0.00005").  The granite at Shars is pretty inexpensive but shipping (naturally) is costly due to weight.

As far as they go, I never thought I'd use one much but it ends-up getting used a lot...


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## Syaminab

Ray C said:


> +1 on Tony's thoughts about thin glass.  For grins, I mic'd a small 6x8 pane and it was off several thou from end to end.   Optical quality glass is probably dead-on but would cost about as much as a B-grade slab -I'd go for the slab and a B-grade is cut better than anyone in a home shop could dream of measuring (something like +/- 0.00005").  The granite at Shars is pretty inexpensive but shipping (naturally) is costly due to weight.
> 
> As far as they go, I never thought I'd use one much but it ends-up getting used a lot...


What I like most, you can put it up in a drawer.


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## 7HC

How about employing actual granite as is used for counter tops in kitchens and bathrooms.
Just for fun I put a straight edge on the one in my bathroom and couldn't get the thinnest of feeler blades under it, nor could I see any light.

It's certainly not as thick as a 'proper' granite block but it does seem pretty flat.  How flat does it have to be, to be useable in any serious way?


M


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## Tony Wells

All depends on what you want to use it for. For qualifying straightedges and scraped flat surfaces, most likely a counter top would not do, but if you wanted to invest the time and money, and for the learning experience, you could start with three equal pieces and trade lap them as flat as you wanted. But as thin as they are, their use would be limited to work in the 0.0050-0.0100 range at best off the shelf, if I had to guess.  For layout work, drilling holes, sawing, etc., plenty good I'd think. Or for working in a small area, like a height gage comparison to a Jo block stack, I'd trust it to 0.0010/0.0020 if within a few inches of the reference.


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## FarFar

Maybe one could glue two or three sheets of granite counter top scrap pieces together?


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## Daver

So let me make sure I get this.  There are two considerations for a plate. The first of course is a truly flat surface. Not eye the edge flat like determining if a 2x4 is twistd, but measured flat to some Nth degree.  The second consideration is how thick it is. My assumption here is, the thicker the better. Thicker means less flex, less movement, and less plasticity/flow.

am I correct?
am I missing anything?

so back to point, on OP, would a sink cut-out from a thick granite counter top be a good starting place?


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## Syaminab

I find shears granite very cheap. Granite in Mexico is ten times more expensive. I liked the Idea of getting a 8" x 12". More than enough for most common hobby work.

   Comming back to thick glass, if you dont have granite stone, you need to have a measure and you cant wait for a deliver, go find yourself a piece of thick glass, run the mic over it and get it as straight as possible and finish the job. Its a handy tip only, not the most precise. Mirror glass is quite stright, give it a try.
  Now, for the science, can anyone describe a process we can do in a universal grinder or similar machine, to resurface a top counter granite to make it straight under 0.0001" more than enough for my work and assemblies.
regards


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## Ray C

Not only very, very flat but with true granite the corners are very, very square.  Nice thing about that is you can check all your t-squares.  Granite slabs are very thick too and do not twist.  I don't believe they guarantee that the bottom is flat or even -just the top and corners.  Side thickness might vary a little.  Thin glass twists a lot and typical stuff you get at the hardware store is not very even.  Thick glass is probably a little better but, if you can see optical distortions (almost all glass optically distorts) it's because of variations in thickness.  It can be used for some hobby purposes but not for critical measurements or checking for flatness (such as in checking chuck backplates).  If you took a strip of window glass, put it on a granite and slid a TDI over it (referencing off the granite) it would see it right away.  In the thread about "D1-4 spindle and chuck mesurement" I showed how to use granite to check for back flatness.  In that case, you're looking for 0.0001" accuracy -and that won't happen with glass.

I don't know about counter top material.  I guess it depends on the quality of it.  That and glass are certainly a handy tool for some homeshop uses but, if you're looking for +/- 0.001 tolerances or better, it would not be a good choice.




Daver said:


> So let me make sure I get this.  There are two considerations for a plate. The first of course is a truly flat surface. Not eye the edge flat like determining if a 2x4 is twistd, but measured flat to some Nth degree.  The second consideration is how thick it is. My assumption here is, the thicker the better. Thicker means less flex, less movement, and less plasticity/flow.
> 
> am I correct?
> am I missing anything?
> 
> so back to point, on OP, would a sink cut-out from a thick granite counter top be a good starting place?


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## hermetic

Here's my two penn'orth
 Plate glass, as in shop window or glass display shelving is ground and polished till optically flat. I have a slab about 3/8" thick which sits atop a thick baize wad on top of a slab of chipboard worktop. With the tools I have I cannot detect any area which is not perfectly flat, it therefore suits me far beyond the accuracy I wish to obtain. I use another similar set up for a lapping plate. If you drop something on a steel or cast surface plate, it needs scraping, do the same on plate glass, and it is probably in the bin! I have a close friend, ex career precision engineer, who used to make templates for a granite counter top company, and also inspect the finished work Their granite was ground flat and polished to "optical standards" as it was possible to detect defects in reflections from the granite, which used to annoy some of the wealthy clients as there seems to be a race on amongst the mega rich to have the largest single piece of granite worktop in their kitchen! Cheaper granite worktop is ground flat, but then "mop" polished which puts on a beautiful shine, but doesn't guarantee "flatness" You could of course use some bluing or lapping compound to check a piece of granite with plate glass to see if you got an even, all over "touch" but for the purposes of HM I think we are as near perfect as makes no difference. Don't sweat it, git er done.


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## dian

my kitchen granite plate is true to 0.015 mm over 20" x 40". if you have a 00 (din) beveled straight edge, you can find a line on the plate, that is almost perfectly flat. glass will "wrap" around the surface ist resting on. either you support it by some kind of foam or you put it on the mill table, to average out small differences.


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## darkzero

I'm not sure if they still go on sale but I got my import grade A 12x18 granite plate for $37 & free shipping from Enco. I was thinking that I wanted something bigger but so far I haven't had a need for a larger one. It's got ledges on the sides for easier transport but mine just sits on the bench & I keep it covered with a desk mat that was the perfect size for it.








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## srgtherasta

Here in lreland shipping of granite slabs is a joke so my way around  this was to find a old photocopier and pinch the glass from it. Very flat but needs some support under it, cost free!


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## Hawkeye

I paid $20 for a 1 1/4" x 25 1/2" x 14 1/2" piece of black granite. Suits my purposes just fine.


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## 7HC

darkzero said:


> I'm not sure if they still go on sale but I got my import grade A 12x18 granite plate for $37 & free shipping from Enco. I was thinking that I wanted something bigger but so far I haven't had a need for a larger one. It's got ledges on the sides for easier transport but mine just sits on the bench & I keep it covered with a desk mat that was the perfect size for it.
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That was one heck of a deal @ $37. 
Thought I'd grab one if the price was still in that ballpark 
Not only is it out of that ballpark, it's not even in the same state! 
I just checked the current price and it's now risen to $113.54  :nuts:



M


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## fastback

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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## darkzero

7HC said:


> That was one heck of a deal @ $37.
> Thought I'd grab one if the price was still in that ballpark
> Not only is it out of that ballpark, it's not even in the same state!
> I just checked the current price and it's now risen to $113.54 :nuts:
> 
> 
> 
> M



Holy crap! Luckily I got when I did. Just checked the latest Enco mailer & the cheapest they have now is grade B, same size with no ledges for $46. When I got mine they used to run the free shipping off $25 almost every month. Haven't seen that offer in a long time, now it's like min $50 now.

Shars offers a similar one to mine for $48, grade A, but I'm not sure how much shipping is for one. 

I remember when I got mine from Enco it barely made it with a 1/4" of the plate sticking out the box & gave me a big surprise when I went to pick up the box as I had not expected how much it weighed. Luckily it did not get damaged. Someone else that ordered one around that time was not so luckily & it arrived with a piece broken off.


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## george wilson

Wait for Enco free shipping. Order a granite plate + whatever else you need to buy to get the minimum cost for free shipping. I've been getting heavy brass bar stock shipped for free. They may have a $100.00 min. by now. I can't blame them!!


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## cazclocker

Just my two cents, if you want a FLAT surface, get a granite surface plate. I got mine from WoodCraft for $36.00. But having said that, I was recently made aware of a type of glass called "Float Glass". It's manufactured by pouring molten glass on top of a bed of molten tin. The glass floats on top of the tin, and when it's cooled the glass is of a uniform thickness and EXTREMELY flat, maybe flatter than the +/- 0.0005" that my surface plate is rated.


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## 7HC

george wilson said:


> Wait for Enco free shipping. Order a granite plate + whatever else you need to buy to get the minimum cost for free shipping. I've been getting heavy brass bar stock shipped for free. They may have a $100.00 min. by now. I can't blame them!!



Shipping wasn't the issue, it was the jump in the item price (the granite plate) from $37 to $113.

M


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## joe kozak

so how would one go about testing the "flatness" of a home hobby granite sink cutout reference plate or a tempered glass plate / granite sink pancake layering plate for true flatness?   

ideas:

a)  tram "flat" then test concentric circles?
    1.  an lever could be used to amplify the sensitivity of a dial indicator
    2.  glass is very smooth, if not flat..., tramming might work well with amplified sensitivity

b)  ink another "flat" and compare the touch patterns under different orientations.  (reverse, iterative 
    1.  would not give clear indication of which direction the test surface is not true
    2.  cant scrape glass flat
    3.  hard to see ink on granite
    4.  relies on flatness of second flat however as a test of uniformity, would be useful.


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## Eddyde

Years ago, I used to use a piece of 1" thick tempered glass as a surface plate. I worked well as I wasn't doing super accurate work at the time. Back then, I didn't even have the instruments to determine how inaccurate it may have been. If that's all you got, it will work.


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## Andre

http://www.myheap.com/the-notebook/homemade-surface-plate.html


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## kd4gij

I have a 24"x24" x1 1/4" high end granite counter top. It is flat as I can mesure. As a hobbyest it's only use is for layout witch I use a phase II hight gauge for. It works fine for what I need. When I was younger and messin with 2 cycle motor cycles we used an old temperd oven window for laping the heads and it worked great.


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## ranch23

Tony Wells said:


> One of my early mentors used a tombstone as a surface plate, but I suppose a piece of tempered safety glass would be cheaper and easier to get. I'd stay away from plain window glass. It's wavy and not flat.



I guess a tombstone could be pretty cheap... unless you get caught!


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## Tony Wells

I guess you could buy yourself one,leave it blank and get some use out of it in the shop until it was needed elsewhere. Might be an interesting epitaph on it at that time between the Dykem stains.


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## Mark in Indiana

I lap surfaces by attaching a sheet of very fine grit sand paper to a piece of glass, and moving the workpiece in a figure 8 motion on the sand paper.


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## Mark in Indiana

ranch23 said:


> I guess a tombstone could be pretty cheap... unless you get caught!



Ths grave yard is a good source for flowers around Valentines day.:roflmao:


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## John Hasler

Tony Wells said:


> One of my early mentors used a tombstone as a surface plate, but I suppose a piece of tempered safety glass would be cheaper and easier to get. I'd stay away from plain window glass. It's wavy and not flat.



Tempered glass is also not flat:
http://en.wikipedia.org/wiki/Toughened_glass#Disadvantages
It also cannot be cut to size.

I have some chunks of float glass that were the surfaces of coffee tables.  Not being tempered they can be cut and are far flatter than I can measure (which isn't saying much).  They're only 1/4" though, so they are too flexible.


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## David Kirtley

Most work doesn't require accuracy of less than 0.001" +- 0.0005". Within that range, a surface plate is not that expensive but sheets of glass or cut granite tiles are usable. If they are not flat enough, get three of them and get busy and make them flat enough. It doesn't take that long because you are taking off very little material. Just get some fine grades of diamond paste and a little water.  We would flatten stuff that way when I took Optical Mineralogy in college using 1/2" thick glass plates. Glass is a bit easier as you can see the interference figures in the cutting fluid when you put them together. It is pretty amazing how well the plates can stick together.

Don't get too hung up over the accuracy of a shop surface plate anyway.  There are just too many sources of contamination. Just because an instrument is calibrated to 5 or 6 digits, doesn't mean they are meaningful digits. Without really good technique and a very clean environment, measurements in tenths (0.0001") and beyond are not really reliable in general shop conditions. That's why they make clean rooms.


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## David Kirtley

Syaminab said:


> I find shears granite very cheap. Granite in Mexico is ten times more expensive.



If you order from Rex Supply in Pharr and let them wait for it to come in regular shipment, you can have it picked up in the store without paying shipping. That's how I bought mine. Lots of people come here from Monterrey for shopping that could pick it up and take it back. Just tell the Aduanales that it is a metate.


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## joe kozak

I'm thinking not so much high tolerance within a small dimensions but rather a reasonable tolerance over a wide x or y axis.


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## Ebel440

A few months ago I bought a black granite floor tile 12x12 just to check out to see if it would be useable. I was in the aisle and it was a loose one so I just grabbed it didn't check it at all in the store.  I set it up on a real surface plate to check it for flatness and parrelelism,  I stopped checking after I measured one corner +.01. It rocked on the surface plate as well. It wasn't really useable for anything. Its probable some are better and some worse I suppose you would need to check a few to make any real conclusions. But if you don't have a larger surface plate to check it I'd say forget the tiles. I have been meaning to get a sink cut out to check.


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## Keith Foor

I am curious to know how the sink cutout checks.  I have one myself that I use to sharpen and lap on.  I haven't built a table for it yet, due to not wanting to waste the effort if it wasn't really  usable.  I assume that it's about flat, but I don't really have a method of checking it to any real degree of accuracy at this point.   The other thing I use for lapping is a mirror.  I wet it and put fine grit wet dry paper on it for sharpening as well.  I can typically get a pocket knife far sharper than it would ever reasonably need to be with this method and the nice thing about it is unlike a stone that will wear in the middle, glass with paper on it stays as flat as it was to start with.


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## Wizard69

Daver said:


> So let me make sure I get this.  There are two considerations for a plate. The first of course is a truly flat surface. Not eye the edge flat like determining if a 2x4 is twistd, but measured flat to some Nth degree.  The second consideration is how thick it is. My assumption here is, the thicker the better. Thicker means less flex, less movement, and less plasticity/flow.
> 
> am I correct?
> am I missing anything?
> 
> so back to point, on OP, would a sink cut-out from a thick granite counter top be a good starting place?



It depends upon what you want to do with it, how much the parts and tools sitting on it weighs and what type of granite it is.   A surface plate also needs proper support Under it.    Beyond the issues you outlined above some granites are far more wear resistant than others.  

A one inch thick piece of granite has its uses but thicker is even better.   Just consider huge variety of squares one can buy some are more suitable than others for the job at hand.


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## TOOLMASTER

I has used glass for many years...thick table tops are great...the last one I bought cost me 5 bucks at salv army....24x24x 1/2


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## Wizard69

Keith Foor said:


> I am curious to know how the sink cutout checks.  I have one myself that I use to sharpen and lap on.  I haven't built a table for it yet, due to not wanting to waste the effort if it wasn't really  usable.


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


> I assume that it's about flat, but I don't really have a method of checking it to any real degree of accuracy at this point.


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.  


> The other thing I use for lapping is a mirror.  I wet it and put fine grit wet dry paper on it for sharpening as well.  I can typically get a pocket knife far sharper than it would ever reasonably need to be with this method and the nice thing about it is unlike a stone that will wear in the middle, glass with paper on it stays as flat as it was to start with.


 
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.


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## Wizard69

Syaminab said:


> I find shears granite very cheap. Granite in Mexico is ten times more expensive. I liked the Idea of getting a 8" x 12". More than enough for most common hobby work.
> 
> Comming back to thick glass, if you dont have granite stone, you need to have a measure and you cant wait for a deliver, go find yourself a piece of thick glass, run the mic over it and get it as straight as possible and finish the job. Its a handy tip only, not the most precise. Mirror glass is quite stright, give it a try.
> Now, for the science, can anyone describe a process we can do in a universal grinder or similar machine, to resurface a top counter granite to make it straight under 0.0001" more than enough for my work and assemblies.
> regards



Your best bet here is to look at the armature telescope making forums.   Normally they are making very precise curves in glass but there is also a need to make flats.   I know it can be done, at least in smaller sizes, but you would likely need to build some optical instrumentation to do so.  Telescope makers have been DIYing optical parts since Galileo so something should be floating about the net.  

If you don't want to go that route you can always hand scrap in surface plates using the three plate method.  Back in my youth I meant a guy at a place I was working that did this for a plant he worked in in the 1940's and 50's.   This was a machine tool making business and hand scraped surface plates where suitable for the shop floor.  I do believe he was talking iron plates at the time but you can scrape in granite.  It is possible to get very accurate references this way even if you can't measure them against standards or optically.


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## Wizard69

Ray C said:


> Not only very, very flat but with true granite the corners are very, very square.  Nice thing about that is you can check all your t-squares.  Granite slabs are very thick too and do not twist.  I don't believe they guarantee that the bottom is flat or even -just the top and corners.  Side thickness might vary a little.  Thin glass twists a lot and typical stuff you get at the hardware store is not very even.  Thick glass is probably a little better but, if you can see optical distortions (almost all glass optically distorts) it's because of variations in thickness.  It can be used for some hobby purposes but not for critical measurements or checking for flatness (such as in checking chuck backplates).  If you took a strip of window glass, put it on a granite and slid a TDI over it (referencing off the granite) it would see it right away.  In the thread about "D1-4 spindle and chuck mesurement" I showed how to use granite to check for back flatness.  In that case, you're looking for 0.0001" accuracy -and that won't happen with glass.
> 
> I don't know about counter top material.  I guess it depends on the quality of it.  That and glass are certainly a handy tool for some homeshop uses but, if you're looking for +/- 0.001 tolerances or better, it would not be a good choice.


You can also see optical distortions cause be the lack of flatness on highly polished surfaces.  This is a contributor to optical distortions seen in plate glass.  If you happen to come across a really old house you will find glass panes that are actually wavy on the surface distorting you view of outside. 

For a hobbiests the real question how good do you need and over how Wide of an area.  A surface plate accurate to 50 million of an inch isn't going to be all that useful for a hit and miss engine.   This especially when you consider that these engines, in commercial guise, where the high precision devices to begin with.  The same can be said for steam engines.


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## Wizard69

fastback said:


> 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.
> 
> View attachment 45880



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.


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## Andre

Wizard69 said:


> 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


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## 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.


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## Keith Foor

Wizard69 said:


> 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.


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## 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.


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## John Hasler

Keith Foor said:


> 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.


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## 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!


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## Keith Foor

Ulma Doctor said:


> 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.


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## 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!


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## 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.


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## Ulma Doctor

i'd say grind within .001" and scrape the rest,
of course you can go finer but you have wiggle room at a thou, and you can correct a mistake and still have meat.
there will be lots of checking flatness and parallel, you may even devise a holder that keeps both pieces in alignment while scraping.
it could speed the operation and assist in supplying symmetry to both blocks


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## John Hasler

Keith Foor said:


> 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.


What's the accuracy of your grinder?  You'll need to leave that much plus however much you plan to scrape off.

How do you plan to check the dimensions while scraping?


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## Tony Wells

If it were me, I'd trust my grinding long before I'd trust my scraping. On size, squareness, and finish.  And besides that, on keeping them identical.


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## Keith Foor

Plan is to grind them and turn them while grinding them once I get them in close.  I saw this technique used by Don Bailey of Suburban Tool YouTube fame.  The machine I am using to grind on is a T&C grinder and not a surface grinder.  So again, this might not turn out too well because the grinder may not hold the level of tolerance needed to do it.  And again, this is why I want to try doing all this with a couple chunks of cast iron and not a cross slide or other part of my lathe or mill.  

I already know that I have to retram my mill as it's cutting .003 deeper towards the frame across 2 inches.  So I am already tapered slightly.  I still have material to remove with the mill so I will be tramming the head before I do any more cutting.  I have considered making one of those dual gauge tramming setups but without knowing I can get everything squared up and even it seems pointless to try at this point.  So that is a project for later.  

I figure I can get the head trammed in to .001 or so and then square up the blocks and get them down to 1.030 or so for grinding and then leave some for scraping.  

I will try to get some pictures and video of all of this and keep everyone in the loop.  I will be starting a thread that I will post a link to in here once I get on the project.  Renter moved out of my other house.  Doing some work on it and getting it ready to occupy again so the machining is on the back burner.  

I will also be doing a full build of a 15 HP rotary phase convertor from start to finish with part numbers / specs of all parts for duplication in your shop.  Working on the idea of starting a youtube channel for projects like this.  And linking it back here to the forum.  Will need to discuss that with the powers that be first to make sure it's ok.


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## John Hasler

Keith Foor said:


> I figure I can get the head trammed in to .001 or so and then square up the blocks and get them down to 1.030 or so for grinding and then leave some for scraping.


How are you going to maintain accuracy while scraping?    Seems to me that with the usual process you could end up with perfectly flat but non-parallel sides.


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## Keith Foor

I was under the assumption that the grinder would bring it into parallel.  Now that might not be the case but it is what I was thinking


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## John Hasler

Keith Foor said:


> I was under the assumption that the grinder would bring it into parallel.  Now that might not be the case but it is what I was thinking


I thought you were going to scrape as the last step.


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## Keith Foor

Yes, grind first then scrape.  Assuming that the grinder will make them parallel but not quite flat.  Am I not thinking right here?


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## Tony Wells

When you scrape, odds are that you will lose parallelism.


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## Ulma Doctor

you can scrape to parallel


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## John Hasler

Ulma Doctor said:


> you can scrape to parallel


How?  I'm quite interested as I don't have access to anything remotely resembling a surface grinder.  In this case it is also necessary to scrape to an exact thickness.


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## Ulma Doctor

it will not be easy, that's why i suggested an easier target to the writer of the question.
you would be measuring the whole process of making the 123 blocks
bring them to .001" flatness and parallel with the surface grinder, then measure and scrape to specifications.
there is no magic, just very diligent measurement and scraping.
scraping can remove the smallest amount of metal you can't with a SG
to expand,
by scraping one surface parallel to the surface plate, you can indicate the other side to see it's parallelism by direct measurement.
any high spots can be recorded and scraped to parallel with the surface plate.
you'll be scraping to the final dimensions.
if the process is repeated on all 6 sides, you'l have a parallel and square device


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## Tony Wells

I didn't mean to sound like it was impossible, Mike, just for a first project, to hit size, parallelism, and squareness is going to be quite the challenge. And without knowing what instruments are available to verify each phase in the process.......difficult may be a little bit understated.
Plus, comparing a grinder with scraping, I dunno. Scraping is at best a series of high spots. Granted, they can be very close together, but I don't think you could scrape to a 4µ finish, but you surely can grind one. And those high points are much, much closer than you can ever get hand scraping. And then there is lapping, beyond grinding. I know there is a lot of deserved respect for the art of scraping, but there is a reason there is so much more grinding and lapping in the modern world. For the sake of nostalgia, or originality, or even just because a person doesn't own or have access to grinding and lapping machinery, scraping is a very viable option. But I'll never be convinced it is equal to or superior to grinding.


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## Ulma Doctor

you can grind a 4u finish, granted- but not on your home machine- but you can scrape and verify to a tenth
most people don't have lapping stations and the ability to even grind or verify a 4u finish.
for a lot of people scraping is the best option, in due respect.

it's my belief that amazing things can be done by those willing to make the mistakes others may fear to make.


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