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I scraped a boudler into a master square, here's my report

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  • I scraped a boudler into a master square, here's my report

    Is scraping a viable technique for making precision granite?

    Yes, no...kinda. Really depends on your tolerance. At 0.001” tolerance, scraping is absolutely a viable technique & not much more difficult than cast iron. Though most surface grinders could hit that, so its only useful if you don’t have a surface grinder to rough it out. But 0.001” tolerance is wide open for a gauge surface, finishing to close to 0.0001” requires lapping (as many of you said before I started).

    Here is the start of my master square (sorry I totally forgot to take a pic of the rock before I started):


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    From the tile saw rough cut, I took to it with a 40grit diamond wheel. That was rough as hell, but useful in taking bulk material down. I also tried to do the corner bevels—that was a mistake with such a course wheel. Chips in the corners would never actually get worked out. While using the cup wheel, I simply used calipers to roughly check parallel sides. I then used some soft lapping pads to do the last of the bulk removal.


    Notes on blades:

    I used carbide for all of this & it barley held up. I played with various tool angles between 85-95. beyond ~92, even a freshly sharpened blade wouldn’t bite. Under 90 –tool wear was mighty quick. I used my power scraper on the first few sides. Works really nice with a keen edge, but that disappears inside of one pass. With the manual scrapper, the edge held a little better, but I also have more control over down force, allowing me to continue to scrap even with a less than keen edge. Manual blades were sharpened before each side of my master square. I used a 1” x 60mm & a 1.25”x90mm blade for the roughing and then a 0.5” x 40mm to do the hook scraping. Hook scraping via a bump technique works well, though you’ll want a much more shallow angle.


    With the first few sides, I scraped until the geometry was inside a thou & then lapped the last little bit of geometry. I made two laps, one cast iron, one aluminum. Not sure the material really mattered much, but the smaller sized aluminum one was certainly useful in chasing a uniform surface finish. And yes, the cast iron one is the end of a dumbbell—it worked great (after I grooved it). 12-20 micron diamond charged both laps. If I were doing more of this work, I would like to have a set of courser laps for the early stages. After the first few sides, I realized the lap could be used like a stone during scraping to flatten the tiny peaks left by scrapping. This back & forth between scraping & lapping really saved a lot of time. It did, however, result in a different surface finish—a nicer looking one, but not as nice to work with. I started to find that the block was wringing to my surface plate causing a very sticky slide. My first side was hook scrapped much closer to final geometry, allowing me to stop lapping before I took out the bottoms of my scrap marks. The remaining marks are undetectable on my tenths dial, but that side slides over the table really smoothly. On most of the rest of the sides, I didn’t scrape close enough & all those marks lapped out. I am considering doing a half moon flaking on those super smooth sides, though I am hesitant to not want to screw it up.

    DOC:

    60r on power scraper (keen edge): 0.0005”

    60r on power scraper( dull edge): 0.0001”

    90r manual: 0.0001-0.0004” (variance from that of technique)

    40r manual half moon bump-hook: 0.0002” (0.0001” with a fresh blade)


    rubbing:

    for the most part, rubbings work the same as in CI. You’ll want to use light coats & little to no pressure so to not embed the pigment below the surface. The porous granite, especially in early stages, sucks the carrier out of the surface plate. So re-spreading to re-rub only works once or twice before the pigment to carrier ratio is way out. So clean & reapply every couple rubs. Its also essential that dust be kept away from the surface plate, nearly impossible in my small shop, making frequent cleanings even more necessary.





    The final product:

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    I had hoped to be able to say this was scraped inside 0.0001”, but I can’t confidently say its better than 0.0003” (maybe even 0.0005” in a few places). All the surface plate measurements lead me to think my surface plate needs some work. A week ago this was a boulder in my backyard & now its probably the most accurate thing in my shop. that’ll play.

    Here is a video proving it out & discussing the process for one of the sides.



    Conclusion / recommendations:

    Absent a surface grinder, scrapping can fill that void nicely, but lapping is needed for gauge quality finishes. Keep your blades sharp, trying to plow through with a dull blade is a fools errand. I think a(or many/various) diamond crested scrapper blades could make mighty fast work of the roughing. A factor of 10 wouldn’t surprise me, diamonds are haaarrrrddd. They’d need to be sintered (other methods?). Basically would amount to a scraper shaped diamond file. Might even be able to use one as a proof of concept. If I were to pursue making larger granite things that couldn’t be surface ground even if I had one, this course of actions might make sense. For now, it will remain in theory.

    Further experimentation/learning on lapping size, shape, material & technique needed.

    PS

    comparators are a really, really cool tool (self proving) that will likely cause me to have a mental break.

    Should those be on a 3pt contact system? Mine are ground flat & occasionally are a bit sticky.


    PSS

    lot of time for my mind to wander while I was doing this, got me thinking about the pyramids. Specifically, the precision fitment of the blocks.

    a) do you think (or know) they used something like a comparator to square out the blocks?

    b) people are often astonished by the fitment of the blocks “after so much time” – after just a little playing around with lapping, I wonder if time hasn’t improved the fitment...a bunch of heavy, fairly soft rocks stacked together, subject to vibration in the earth & settling. Wind might even help to clear or move debris. Maybe its lapped itself over the years?



    Attached Files
    "it is no measure of mental health to be well adjusted to a profoundly sick society." -- krishnamurti
    "look deep into nature, and then you will understand everything better." -- albert einstien
    "any intelligent fool can make things bigger and more complex...It takes a touch of genius - and a lot of courage to move in the opposite direction."

  • #2
    I'm impressed! I have a hunk of granite counter top that I thought of using as a poor man's surface plate, but a) I have yet do do anything that calls for a surface plate, and b) I haven't even checked it for flatness in any way, so I don't know how far off it might be. Also, it's only about 1 in. thick, so I don't know how that would compare for stability compared to a "real" surface plate.
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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    • #3
      WoW! Im impressed also. You have been showing up with all great projects lately. I for one appreciate it. Thank you. JR

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      • #4
        Great writeup! I'm going to read it again for the hints when I'm less nap'ful....

        On the type of base you use for items that slide around on a surface plate? On the round items I've done I relieve the center of the base so there's a roughly .09'ish wide ring. No issues with stickyness at all. If you leave the whole surface flat? I'm guessing that there's some risk of wringing the item to the plate?
        Chilliwack BC, Canada

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        • #5
          I am not surprised that scraping a stone is a difficult way to achieve precision. I salute you for the effort.

          But if I were going to produce flat surfaces in stone, I think I would use abrasives, not scraping. Some years ago I brought a piece of Pyrex very close to a parabolic mirror. This was not done with scraping or any machine tools. Only manual methods were used and most of the work was done with abrasive grains of decreasing sizes. This was not a lapping technique as the tool was worn down into the opposite curve with the loose abrasive grains acting on both surfaces. The final steps would have used a pitch lap which would be able to confirm itself to the developing surface by resting it in contact between actual work sessions.

          I guess my point is that abrasive processes are much better suited to producing a surface on stone. And the age old, three plate method, PROPERLY APPLIED, can generate flat surfaces of any degree of accuracy. The flatness is guaranteed by the method and all that is needed is successively finer grades of abrasive.
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

          Comment


          • #6
            Originally posted by mickeyf View Post
            I'm impressed! I have a hunk of granite counter top that I thought of using as a poor man's surface plate, but a) I have yet do do anything that calls for a surface plate, and b) I haven't even checked it for flatness in any way, so I don't know how far off it might be. Also, it's only about 1 in. thick, so I don't know how that would compare for stability compared to a "real" surface plate.
            I have one of those too! I remember when my father had granite countertops put it, the guys were walking away with the drop from the sink as my father yelled, "where you going with my granite?!?!" I've had that laying around for 20+ years, in the last 5 I've been thinking about making it into surface plate. They aren't particularly flat, especially if they've been manually buffed. But you could certainly lap one flat--the trouble comes in measuring. need something like a repeat o meter for local flatness & an autocollider or e levels for overall flatness...both very expensive.

            There is also the problem of thickness you mentioned, my piece is a mere 1.5 inches thick, much less substantial than a proper surface plate. Maybe you could set it in concrete, but I dont know how effective that would be. Lately, I've been thinking its best use would be as straight edges / prisms. I could cut 6, 8, 10" rips to make some nice granite edges, though they'd still only be 1.5" thick.


            Originally posted by BCRider View Post
            Great writeup! I'm going to read it again for the hints when I'm less nap'ful....

            On the type of base you use for items that slide around on a surface plate? On the round items I've done I relieve the center of the base so there's a roughly .09'ish wide ring. No issues with stickyness at all. If you leave the whole surface flat? I'm guessing that there's some risk of wringing the item to the plate?
            thanks. and yah you're spot on with the stickiness & flat bottom. all 3 of my surface plate bases are starret, they're hard & i dont have surface grinder to relieve them. They've been a real pain to set up as a comparator, hell my front bumper is PA glued on! I think I'll make my own, probably round like you described & not hardened so I can do things like the relief you described.


            Originally posted by Paul Alciatore View Post
            I am not surprised that scraping a stone is a difficult way to achieve precision. I salute you for the effort.

            But if I were going to produce flat surfaces in stone, I think I would use abrasives, not scraping. Some years ago I brought a piece of Pyrex very close to a parabolic mirror. This was not done with scraping or any machine tools. Only manual methods were used and most of the work was done with abrasive grains of decreasing sizes. This was not a lapping technique as the tool was worn down into the opposite curve with the loose abrasive grains acting on both surfaces. The final steps would have used a pitch lap which would be able to confirm itself to the developing surface by resting it in contact between actual work sessions.

            I guess my point is that abrasive processes are much better suited to producing a surface on stone. And the age old, three plate method, PROPERLY APPLIED, can generate flat surfaces of any degree of accuracy. The flatness is guaranteed by the method and all that is needed is successively finer grades of abrasive.

            thats more or less the conclusion I reached, this was my first venture into lapping so I had pretty much no tooling & even less experience. Though I still think the combo of scraping & lapping each round is a very effective way to removing material in the early roughing rounds. Serving as a substitute for surface grinding. I also just finished building a power scraper & new honed blades, so I was eager to use them.

            pyrex? like the lab glass pyrex? My god that must be hard! like way harder than granite! maybe i'm thinking about the wrong material, isn't it clear? wouldn't that make it a lens vs a mirror?

            thanks for the "pitch lap" term, searched it briefly, they sound interesting.

            and I really want to do the 3 plate method someday...just gotta find the right stock for it & order up half a ton of diamond dust.
            "it is no measure of mental health to be well adjusted to a profoundly sick society." -- krishnamurti
            "look deep into nature, and then you will understand everything better." -- albert einstien
            "any intelligent fool can make things bigger and more complex...It takes a touch of genius - and a lot of courage to move in the opposite direction."

            Comment


            • #7
              Originally posted by JRouche View Post
              WoW! Im impressed also. You have been showing up with all great projects lately. I for one appreciate it. Thank you. JR
              thanks, I'm glad. For years I've lurked around here in the shadows & benefited from everyone's experience. Never really felt like I had anything to contribute, now I do. just trying to pay it forward.

              plus, ya'll can understand the value of something like this. Try explaining to a friend how accurate 0.0001" is & why you need it...i did, it was disappointing to say the least.
              "it is no measure of mental health to be well adjusted to a profoundly sick society." -- krishnamurti
              "look deep into nature, and then you will understand everything better." -- albert einstien
              "any intelligent fool can make things bigger and more complex...It takes a touch of genius - and a lot of courage to move in the opposite direction."

              Comment


              • #8
                You rock, dood !

                Comment


                • #9
                  Originally posted by mtraven View Post

                  thanks, I'm glad. For years I've lurked around here in the shadows & benefited from everyone's experience. Never really felt like I had anything to contribute, now I do. just trying to pay it forward.

                  plus, ya'll can understand the value of something like this. Try explaining to a friend how accurate 0.0001" is & why you need it...i did, it was disappointing to say the least.
                  Right. I dont have a neighbor or friend that knows what a tenth is. say la v...

                  To be real, I only go to tenths with engines. I dont see any reason to look at that for regular machine work. Its very tight around those numbers. Talking about oil clearances at this point. Tenths are very tight. Difficult to manage sometimes, for me anyway. JR

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                  • #10
                    Damn! now that's mighty impressive and creative, well done. Having done a bit of scraping on machine castings and masters it never occurred to me to cut and scrape a boulder. Would you do it again? Being an inventor at heart you got my inventive juices flowing after being dormant for better than a decade. Great write up, you're certainly paying it forward.

                    Ron

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                    • #11
                      Yes, my mirror blank is the clear Pyrex. It is used because of it's superior temperature coefficient as compared to the various optical glasses that are also used. And yes, it is quite hard. A hardness tester that produces a dimple in the surface would shatter it. I understand that it is harder to get it today but there are other glasses that can be substituted. I suppose it can be used for lenses, but they usually need specific refraction indexes. I have not heard of Pyrex being used for lenses.

                      As far as the amount of abrasive needed, I purchased an assortment of various grades in small amounts, just a few ounces each. The coarsest grade was the largest amount but only a fraction of a pound and less was needed as they got finer. I used no diamond abrasive, just Silicon Carbide or Aluminum Oxide. Back then the polishing is done with red rouge, a form of ferric oxide. Today they use things like Cerium Oxide which acts faster. These are used with the pitch lap which conforms to the shape of the mirror. Again, mere ounces are all that is needed and you probably won't need all of that. But my telescope mirror was small, 4.25" diameter. I do not know how much diamond would be needed for a three plate process. I would order just a pound or so.

                      For small mirrors as most amateur scope makers would make, the pitch lap is used only after the general form of the mirror has been reached. This is a spherical surface. That is turned into a parabola of revolution in the polishing phase using the pitch lap. First it is polished and then the length of the strokes are varied to produce the parabola by some extra removal at the edge. This is where the ability of the pitch lap to conform to the shape of the mirror comes into play.



                      Originally posted by mtraven View Post

                      ...<snip>...

                      thats more or less the conclusion I reached, this was my first venture into lapping so I had pretty much no tooling & even less experience. Though I still think the combo of scraping & lapping each round is a very effective way to removing material in the early roughing rounds. Serving as a substitute for surface grinding. I also just finished building a power scraper & new honed blades, so I was eager to use them.

                      pyrex? like the lab glass pyrex? My god that must be hard! like way harder than granite! maybe i'm thinking about the wrong material, isn't it clear? wouldn't that make it a lens vs a mirror?

                      thanks for the "pitch lap" term, searched it briefly, they sound interesting.

                      and I really want to do the 3 plate method someday...just gotta find the right stock for it & order up half a ton of diamond dust.
                      Paul A.
                      SE Texas

                      And if you look REAL close at an analog signal,
                      You will find that it has discrete steps.

                      Comment


                      • #12
                        Originally posted by nc5a View Post
                        Damn! now that's mighty impressive and creative, well done. Having done a bit of scraping on machine castings and masters it never occurred to me to cut and scrape a boulder. Would you do it again? Being an inventor at heart you got my inventive juices flowing after being dormant for better than a decade. Great write up, you're certainly paying it forward.
                        Ron
                        I had been scraping a bunch of steel & CI...had a granite tile on my bench, thought, what the hell, give it a try. To my surprise it felt a lot like cast iron & if it didn't dull so quickly, granite would actually scrap faster than CI (imo). I needed master square, durabar is pricey, rocks are free, the rest is history.

                        and yes I will do it again, though likely with a heavier use of laps. I expect scraping with take a smaller, but crucial role in future granite projects. I'm gearing up to scrap my lathe, still need a good long straight edge, a prism or two & maybe some measure tools...granite seems like a good candidate for all that.

                        @Paul Alciatore thats interesting stuff, seems like you know a lot about making lenses...I might need to bounce some ideas off you as I contemplate the viability of an autocollimator build. I had assumed the lens(es) & mirrors would need to be bought (costly, I imagine), but reading your description of the process makes me wonder if I couldn't just rub one out. I don't mean to underestimate the task, I expect it would be a significant challenge.

                        you touched on "proper" 3 plate technique, is there any such thing for a set of straight edges? or as I suspect, does the technique rely on the plates having a roughly 1:1 length to width ratio?
                        ----------------


                        no one wants to talk about the pyramids, eh?
                        "it is no measure of mental health to be well adjusted to a profoundly sick society." -- krishnamurti
                        "look deep into nature, and then you will understand everything better." -- albert einstien
                        "any intelligent fool can make things bigger and more complex...It takes a touch of genius - and a lot of courage to move in the opposite direction."

                        Comment


                        • #13
                          Hopefully you didn't toss out the outer offcuts. You're thinking of a three surface setup for flattening the inner square? If you use one or two of the offcuts and flatten the cut face to the same degree as your inner cubic square then those offcuts when used on all the faces of the square will give you want you are after.
                          Chilliwack BC, Canada

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                          • #14
                            If this is an April fools joke, then har har har.
                            But if you actually made a flat granite square
                            from a rock in the yard, then Very cool Indeed !
                            Not time well spent, but maybe you have more
                            time than money or live in a remote part of the
                            world. Interesting anyhow.

                            -D
                            DZER

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                            • #15
                              I just watched your power scraper video.
                              Very legit.
                              Subscribed!

                              -Doozer
                              DZER

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