DIY surface plate (out of curiosity)

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felix0685

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Not that I intend to do it (yet) but mostly for my personal education.

I have seen the suburban tool you tube video where they talked about refurbishing machines and had these large steel tables which they had scraped as reference. from my understanding you would need a larger surface as a reference, but they had 4x8 and larger tables. So their reference tables would be even larger, and they need calibrating as well, which would require even larger tables...

OK here the question, assuming one would not be able (lack of space, money, etc.) to buy a large granite table as a reference. Is there a way to scrape a steel plate flat with a reference smaller than the plate I want to scrape? For example, would it be possible to buy a 12" x 12" granite stone which is calibrated and use this to scrape an 24"x24" steel plate?

If so how would one go about it? I am aware of the three surface lapping method. but well then you end up with three plates and not one...

I am perfectly aware that this is far from economical, it just boggles my mind. In woodworking I use straight edges and winding sticks, which is mostly by eye and I don't seek perfect flatness.

If this is a stupid question, please feel free to tell me, but I am really curious
 
Firstly, surface plates are made of cast iron, not steel, and are heavily ribbed to withstand deflection, the three plate method of generating flat surfaces is by scraping, and not lapping. Trying to use a smaller plate to generate a larger plate would not be practical and would be unlikely to produce an accurate result.
If you are interested in the process of scraping, buy an accurate granite surface plate, a carbide tipped scraper and the means to sharpen it (diamond wheel), marking medium such as prussian blue or Canode and find a piece of cast iron to practice on, videos may be helpful in learning to process.
Richard King holds classes on scraping all over the country, they are not cheap, but there is a lot to be learned.
 
benmychree said:
Firstly, surface plates are made of cast iron, not steel, and are heavily ribbed to withstand deflection, the three plate method of generating flat surfaces is by scraping, and not lapping. Trying to use a smaller plate to generate a larger plate would not be practical and would be unlikely to produce an accurate result.
If you are interested in the process of scraping, buy an accurate granite surface plate, a carbide tipped scraper and the means to sharpen it (diamond wheel), marking medium such as prussian blue or Canode and find a piece of cast iron to practice on, videos may be helpful in learning to process.
Richard King holds classes on scraping all over the country, they are not cheap, but there is a lot to be learned.
Click to expand...

thanks for the response. I am aware that you would want to use cast iron, but it is not so much what I am curious about. my question is mostly whether or not I can use a smaller reference? because there is obviously a limit to how large a reference can be.

I know about Richard's classes but I it is either wrong timing or the wrong site of the country.
 
In the trade, the reference surfaces will nearly always be larger than the object being worked on, while it may be possible to use a smaller reference surface to generate a larger one, I thing a factor of two times larger would make it quite difficult or impossible to achieve an accurate result.
I have participated in three of Richard's classes when they were held in the San Francisco Bay area.
 
You can scrape a large surface plate without an ever larger plate. You just need to be ever more diligent with your marking method. See the video below, never do they use a larger plate. Only a straight edge long enough in each axis to rub off the die then they scrape, turn 90 degrees and repeat. Fast forward to 17min 40sec. Then later they use an autocolimator to check absolutely before sending it out the door.

Its important realize that they are not trying for absolute flatness, doing so would drive one mad. What they are doing is achieving a superb localized flatness then a more forgiving over all flatness. For a 48"x96" granite plate to the GGG-P-453c standard in AA tolerance, they are permitted .000075in in localized repeat error, and .000500in in over all flatness. For their cast iron plate they are working towards effectively the same result.

 
Yes, the straightedge is the necessary tool to achieve reasonable overall flatness on surfaces larger than the surface plate used, that is how I rescraped a #2 B&S mill column back in the 1960s, using a 48" B&S bridge straightedge and a 18" square surface plate.
 
Alexander is right. I’m surely no expert at all on this subject but I remember Ox Tools had a video of when he got all his surface plates checked.....resurfaced and certified. The guy used that visual thing Alex mentioned to check the plates.

So it seems one could surely use the same visual testing device to check a larger surface.
 
That’s it. autocollimator.....ie.....that visual thing.
 
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