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    Default Whitworth-three plate method

    I know that it makes no sense to use the three late method to generate a precision plate. Still I'm curious about it.
    What would someone use for the plates? In other words what do you start with? Where does one look for three slabs of CI plate of similar sizes?
    Thanks for the patience. Whitworth-three plate method

    Galaxy S4, Slimkat
    If I wasn't married I'd quit fishing

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    Quote Originally Posted by challenger View Post
    I know that it makes no sense to use the three late method to generate a precision plate. Still I'm curious about it.
    What would someone use for the plates? In other words what do you start with? Where does one look for three slabs of CI plate of similar sizes?
    Thanks for the patience. Whitworth-three plate method

    Galaxy S4, Slimkat
    If I wasn't married I'd quit fishing
    You buy machined castings of surface plates and go from there. Whether you get them ground, milled, or planned to start out with is your choice. Probably from Busch as they're the last ones in the US that would still make them. Next is measuring how flat plate is and to know when to stop. If I were even going to mess with this. I would get castings made like the master plates either from Dixi, Moore, or SIP. Large enough to do real work on them.

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    One of the moore precision books (stored away) has pictures of what they used to derive masters. They seemed to be large purpose built castings, and yes they were machined/ground before the three way process started.

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    The Moore book ( Foundation of Mechanical Accuracy ) Goes into detail on how to do the three plate method. The one detail that is rarely mentioned is that for it to work you have to use a square or round plate. If you try to do it with a rectangular plate you won't be able to spot the plate when you turn it 90 degrees. This will cause the plate to have a twist in it. The other point is that it's not necessary to start out scraping the three plates at first. Scrape to a surface plate then start spotting the three plates together. This saves a lot of work wrestling three plates around.

    I am working on making a pattern and corebox to make a 24" square Moore style surface plate. I hope to have castings poured by this summer.

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    Quote Originally Posted by SIP6A View Post
    The Moore book ( Foundation of Mechanical Accuracy ) Goes into detail on how to do the three plate method. The one detail that is rarely mentioned is that for it to work you have to use a square or round plate. If you try to do it with a rectangular plate you won't be able to spot the plate when you turn it 90 degrees. This will cause the plate to have a twist in it. The other point is that it's not necessary to start out scraping the three plates at first. Scrape to a surface plate then start spotting the three plates together. This saves a lot of work wrestling three plates around.

    I am working on making a pattern and corebox to make a 24" square Moore style surface plate. I hope to have castings poured by this summer.
    Nice, You picked up on some really good details I forgot to mention.

    Will you sell castings of the small plates?

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    Quote Originally Posted by SIP6A View Post
    The Moore book ( Foundation of Mechanical Accuracy ) Goes into detail on how to do the three plate method. The one detail that is rarely mentioned is that for it to work you have to use a square or round plate. If you try to do it with a rectangular plate you won't be able to spot the plate when you turn it 90 degrees. This will cause the plate to have a twist in it. The other point is that it's not necessary to start out scraping the three plates at first. Scrape to a surface plate then start spotting the three plates together. This saves a lot of work wrestling three plates around.

    I am working on making a pattern and corebox to make a 24" square Moore style surface plate. I hope to have castings poured by this summer.
    Yep. Minor point, but not being able to rotate the plates 90 degrees won't CAUSE a twist, it's just that it's possible to HAVE a twist in the plates and you won't be able to see that. They may also be perfectly flat ... or somewhere in between. The point is you'll never know unless you can rotate them 90 degrees and again match them.

    That book is essential reading for anyone interested in this type of thing. That point raised is something that most sources overlook, but is critical in a proper understanding of what you're trying to achieve. In this day of inexpensive surface plates, the 3 plate method seems to be a monumental waste of time and effort as far as I'm concerned, but some people enjoy train spotting too, so there you go!

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    I did not know that plates used were cast for this purpose. This seems to make it even less logical IF trying to get a precision plate on "a budget". I was envisioning a few scrap pieces of CI from the local scrap yard.
    Its true that there are granite plates to be had that are cheap enough for most. Just not any that are cheap enough for me. Whitworth-three plate method
    On a side note I thought that castings had to age before they were ready to become precision plates?
    I'll wait it out and find one at some point. The time, effort and costs put the three plate method in the impractical column IMO.

    Galaxy S4, Slimkat
    If I wasn't married I'd quit fishing

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    I've thought about doing the 3 plate method but if I have that much free time I need to be getting my butt out and making some sales!

    I don't know how much of a bottom feeder you are but I just got two 48 x 72 granites for just over $100.

    Precision castings are usually heat treated before final scraping.

    Dan

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    Yikes! WTF man? I just checked the "Internet" and found out you don't need two plates! Whitworth-three plate method.
    Mail one to me and I'll give you $50.00Whitworth-three plate method
    Don't know if I like the bottom feeder term but maybe it applies??? More about my conscience being uncomfortable with spending a whole lot of coin on something that s strictly for pleasure. Sure I could tell myself I'll be learning, it will come in handy, I can resell it blah blah blah. Truth is I'm blessed enough to be able to afford a new granite plate of any practical size. Still I know it's purely for luxury so???
    There are very few around me but I'm sure one will come up.
    Nice finds BTW.

    Galaxy S4, Slimkat
    If I wasn't married I'd quit fishing

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    Sure I need two plates. One can be a sweet kitchen island top...

    I take the bottom feeder comment as a compliment. It's about the same as "tight fisted German".

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    Forgive me if I come off a little pompous here. I've done considerable research on the three plate process and I think I have it pretty well mastered is not well practiced. I scraped two groups of three cast iron surface plates an one group of three 8 and 10 ft straight edges employing a modified Whitworth three plate scraping process. I was younger then and heedless of the amount of hard work and excruciating care involved but eventually I achieved the goal of scraping flat references in all three cases..

    There's a mistaken impression the process works by one scraping the plates in the group in strict rotation. This is not the case. While you can eventually attain flat references by scraping three plates in strict rotation, I estimate it takes three times as long to accomplish the same job as described by Whitworth in the latter paragraphs of "On Plane Metallic Surfaces and the Proper Mode of Preparing Them" (1840) and to a lesser extent in his letter in "The Practical Mechanical Journal" (date unknown by me). Also Charles Porter "Engineering Reminiscences" in Chapter 22.

    A wealth of practical scraping information may be found in the following link including PDF documents of the material I cited:

    Hand Scraping (For Precision Surfaces)

    Discussion of the three plate scraping process may more expeditiously proceed once participants have assimilated the relevant portions of these resources..The process is tricky to learn in concept but once it has been labored through the scraper hand, surveying his finished work of three generated flatness references would likely say to himself: "That was a lot of work but I wonder why I thought it would be so hard to do."

    Square or round or rectangular plate makes no difference in the final flatness provided the work is rotated a RANDOM amount. The object of rotation of the plates in successive cuts is to prevent "radial lobing" of the surface. If the plates are rotated exactly 90 degrees, 72 degrees, 60 degrees, or any other angle corresponding to a regular polygon, there is the hazard of radial "lobing" to appear in the surface. In an extreme case the surface would have the shape of the starched ruffled doily your great-grandmother may have crocheted for her dining room table. If a specific regular polygonal angle of rotation is pursued it is very possible to scrape a set of plated that print perfect but are not flat because of the radial ruffling of their surface. Thus this angle of relative rotation should be randomized from cut to cut.

    I disagree with the Moore assertion of only square or round plates being capable of generating truly flat surfaces. Square or round plate shapes may be helpful but not essential. I suspect that assertion was a bit of salesmanship via mystification. Think about it. Whitworth, Porter, Naismith, and the other luminaries of precision stimulated by the Industrial Revolution made no particular mention of square and round surface plates Vs rectangular in their writings. Neither has the lore passed down through the generations of gifted workers to us the in the present..

    I suggest we allow Wayne Moore a little BS in this one instance given his enormous contribution to mechanical precision in his lifetime.

    All is moot in these days of very acceptable granite surface plates (in compliance with the provisions of Fed Spec GGG-P-463b and current revisions) available on the used market and from import seller for low cost. The is no need to scrape three references to generate true flats when a hundred bucks will buy you an 18 x 24 granite plate flat within 0.0001" in any square foot and 0.0002" in the whole surface (YMMV depending on grade.).

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    Quote Originally Posted by Forrest Addy View Post
    ...
    I disagree with the Moore assertion of only square or round plates being capable of generating truly flat surfaces Square or round plate shaped may be helpful but not essential. I suspect that assertion was a but of salesmanship via mystification. Think about it. Whitworth, Porter,Naismitn and the other luminaries of precision fostered by the Industrial Revolution made no mention of square and round surface plates Vs rectangular in their writings nor the lore pass down through the generations of gifted workers to us the in the present.
    ...
    Forrest,
    I think that Moore was (rightfully) concerned about the plate used as master deforming (and therefore making false prints) if a significant part of it had to overhang beyond the surface of the spotted plate. Similarly, the spotted plate could be deformed by an uneven distribution of weight on it. When we're talking about less than 20 millions, I suspect that, thise issues matter.

    Paolo

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    Quote Originally Posted by Paolo_MD View Post
    Forrest,
    I think that Moore was (rightfully) concerned about the plate used as master deforming (and therefore making false prints) if a significant part of it had to overhang beyond the surface of the spotted plate. Similarly, the spotted plate could be deformed by an uneven distribution of weight on it. When we're talking about less than 20 millions, I suspect that, thise issues matter.

    Paolo
    I looked at that and thought about it before I formed my conclusions. A good cast iron plate with its triangular webbing has three support points cast into the stiffening. When set on these three points and the surface plate loaded to its ratings it should not deflect out of its flatness tolerance. That is if it's a "good plate" such as made fifty years ago by Browne and Sharp, Pratt & Whitney, Challenge, etc. These were deep, stiff, and heavy castings to start with and when scraped to validated masters were the equal of the granite flats that now prevail.

    I've seen on eBay and in posts here and there where bench plates and lapping plates are offered and accepted as surface plates. Anyone having access to a foundry and a notion of how to design a surface plate produced some real gems along with a few wimpy creations inadequate for serious precision. When properly scraped and cared for these expedients can give good service but they have limitations in stiffness - and stability if the metallurgy is dodgey..

    Rectangular plates rarely have a surface aspect ratio over 50%. If 25% overhangs one side and the 25% over hangs the opposite side there is an increase in unit loading but negligible deflection as a result in overhang. The increased loading can be compensated for by manually loading the opposite side by an amount roughly equal to the estimated weight of the overhang. I was aware of this back when but never noticed a serious effect in the final print with the rectangular outline matched Vs rotated 90 and slid back and forth to generate a print. You see differences in density in the print on the overhung portions but the bearing points are unchanged in area or distribution in the final scraper product. It does take more time.

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    Quote Originally Posted by Iridium77 View Post
    You buy machined castings of surface plates and go from there. Whether you get them ground, milled, or planned to start out with is your choice. Probably from Busch as they're the last ones in the US that would still make them. Next is measuring how flat plate is and to know when to stop. If I were even going to mess with this. I would get castings made like the master plates either from Dixi, Moore, or SIP. Large enough to do real work on them.
    Iridium77,

    What kind of surface plates did SIP make?

    As for selling casting of the Moore plate that I am making. I will be willing to sell rough casting when I have some but it's looks like the price will be at least $800.00 ea.

    I haven't gotten a price from the foundry yet but will post updates when available.

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    Now just to move slightly to the side of this discussion, If I have three 8" carborundum stones, apply the three plate method to them will I achieve the equivalent of precision ground stones? I don't have access to a surface grinder with a diamond wheel.

    Any thoughts.

    Ian Mac.

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    Quote Originally Posted by multimc22 View Post
    Now just to move slightly to the side of this discussion, If I have three 8" carborundum stones, apply the three plate method to them will I achieve the equivalent of precision ground stones? I don't have access to a surface grinder with a diamond wheel.

    Any thoughts.

    Ian Mac.
    No. ( extra characters...)

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    Oxtoolco has a multipart youtube series on making lapping plates with the three plate method. I'm not qualified to judge the work, but I found it interesting. Here's the first part.

    Making flat lapping plates 1 - YouTube

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    Quote Originally Posted by multimc22 View Post
    Now just to move slightly to the side of this discussion, If I have three 8" carborundum stones, apply the three plate method to them will I achieve the equivalent of precision ground stones? I don't have access to a surface grinder with a diamond wheel.

    Any thoughts.

    Ian Mac.
    Ian,
    In theory, yes. Practically, I'm afraid it's not worth the effort: stones are rather porous and it will be a challenge to keep the grinding compound on the surface using a method that would not interfere with the stone, once you're done with it.
    A second problem I see is that you are more likely to dislodge the whole grain of carborundum, instead of cutting it flat, as it happens when diamond grinding the stones.
    Third, due to dislodged grains rolling around gauging both stones, I'd expect the process to become a real mess and waste of time. But I could be completely wrong and that somebody has a magic bullet for this.

    Paolo

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    Again read the Moore book.

    One thing not yet mentioned is that they only start with the three plate trick. They would continue on with
    other tests and adjustments after the limit of that process was reached.

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    Thanks Paolo,

    I've given it a try tonight, not using any lapping compound just water with a bit of detergent in it and lapping them against each other, 3 plate method. And the results......well three very sad, misused and abused stones from the work shop, gummed up with years of crap, are all very clean and "flat" and looking like new! Worked on 'em for about hour and half. I won't try to define how "flat" but I can certainly take all of the high spots/damage marks off the drill press table without actually marking up the original machined surface. I surprise myself, and yes I was inspired to try this after having a look at Tom Lipton's vid.

    Thanks for the replies guys,

    Ian Mac.


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