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Lapping A Granite Surface Plate

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  • #16
    I've done some lapping on straight edges and hand flats

    I agree that the Chinese surface plates are really cheap. After checking a few, they are also fairly accurate.

    If so, the why are granite straight edges and squares so darn expensive?

    I have used fine pink granite material to make hand flats up to about 6 x 6, straight edges, angled straight edges for dovetails and have worked some pieces for square edges. The measurement methods are the same as for hand scraping cast iron and the diamond abrasives are fairly effective in cutting.

    I use grinding wheels and pads intended for working granite countertops. (I paid for the tools by doing counter work for our kitchen and makig granite top end tables from the remnants.) and using the diamond plated files and hones that are now available for very little money. HF has a 4 sided hone with 2 x 6 surfaces in 200, 300, 400 and 800 grit that can be removed and glued to a more rigid backing. The final lapping for accuracy can be done with a single grit of 10 micron diamond lapping compound that I originally bought for lapidary work used on a cast iron or copper lap.

    I limit myself to 100 millionths and use a .00005 dial indicator on a base and measure per the federal standard. It's more than good enough for my work. For spotting, I use cadmium yellow pigment in light oil and spot just like with cast iron scraping.

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    • #17
      Originally posted by lazlo
      You could conceivably move the autocollimator around thousands of times, and map out the entire surface, but it takes awhile to setup and register each time
      And that's the general concept of what they do in the Starrett plant. The autocollimator is setup just off the corner of the plate parallel to the short axis, and a 45 degree mirror to reflect the view down the long axis. Then they map multiple points down the long axis, move the mirror, map another line, etc.

      Steve.

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      • #18
        Originally posted by SteveF
        And that's the general concept of what they do in the Starrett plant. The autocollimator is setup just off the corner of the plate parallel to the short axis, and a 45 degree mirror to reflect the view down the long axis. Then they map multiple points down the long axis, move the mirror, map another line, etc.

        Steve.
        I believe the mirror rides up against a straight edge. I think the autocollimator stays on the table. I'll check this as soon as I get a chance.

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        • #19
          Originally posted by dp
          Short of attending a class the video looks like it will provide a decent framework for learning. As I recall the class I took from Forrest the new material I picked up included work holding, tool usage, marking and analysis, and the need for discipline in tool usage including stroke length, stroke angle and pressure, posture, preparing the work for marking (India stone, remove all debris!).

          In addition I learned about the pace of progress which is surprisingly fast for the small blocks we were working on. Finally I became conscious of simultaneity. The object we were working on was intended to come to size, perpendicularity, and surface quality all at the same time, ending with a final stroke. Knowing when that final stroke has happened is elusive to say the least, but it is the most important thing to know - when to quit.

          Along with that is recognizing you are on or not on a path to successful completion. If you are not on a path to completion you are on a path to failure, so you need a mental map of where you are and where you are going, and what the lay of the land is, and that your progress is taking you toward success. The understanding of simultaneity is useful in this analysis.

          Not included was actual scraping processes such as frosting and working with dovetails and round work such as bearing surfaces.
          I'd like to be as prepared as possible before taking the class and I've been working up to this for some time now. You refer to this as a decent framework and I agree. I see this video as yet another step toward acquiring a decent framework.

          I just finished reading Robert Wade's The Art Of Hand Scraping .pdf. The pictures in the .pdf I was sent are horrible and I can't really make out what he's doing in many cases. One thing I wanted to ask was that Robert Wade seems to imply that you need to sharpen the carbide blades you use for scraping with coolant flowing. I don't see this happening in the pictures I've seen posted of homemade carbide grinders.

          I can't see really starting to scrape without taking a class. In my view you can read and watch videos all you want but without the actual hands on practice you can't really develop the skills of sharpening and honing a carbide blade, "the rock test", step cutting, properly holding a straight edge, etc. without apprenticing under as skilled mentor so as not to pick up lots of bad habits.

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          • #20
            One thing you can do is practice on a piece of sacrificial iron. If you have an old rusty faceplate around, give it a try. You can't hurt it but you can surely learn from it. Or get a starter piece of iron and make a 1-2-3 block. That is what Addy's class does, in fact. As you go you can ask questions and take notes of your scraping experience. Had I known then what I know now I'd not have hesitated to try a hand at scraping. Every stroke is a teacher.

            In our class we used diamond paste to sharpen the tools against a spinning flat iron faceplate on a motor shaft. There is clearly a better way to put an edge on a tool but that does a good enough job for the classroom.

            In the case of my own scraper it was sharp enough, but at a very magnified view it was ragged and that showed up in the scraping. Visible to me only with high resolution photographs, I should add. I doubt there's anything about scraping that requires unusual methods of tool sharpening, but in the case of any finish work you want the best edge possible. Getting that is already widely understood by anyone who grinds their own tools.

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            • #21
              Originally posted by Neil Jones
              I can't see really starting to scrape without taking a class. In my view you can read and watch videos all you want but without the actual hands on practice you can't really develop the skills of sharpening and honing a carbide blade, "the rock test", step cutting, properly holding a straight edge, etc. without apprenticing under as skilled mentor so as not to pick up lots of bad habits.
              Stephen Thomas, the co-instructor for the first scraping class, is a self-taught HSM'er.

              This is the table of a mortiser he recently posted on PM. Amazing!



              Just get a piece of cast iron, make a scraper, and start scraping -- the process is incredibly simple. It just takes lots and lots of practice
              Last edited by lazlo; 05-25-2009, 08:00 PM.
              "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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              • #22
                Originally posted by dp
                In our class we used diamond paste to sharpen the tools against a spinning flat iron faceplate on a motor shaft. There is clearly a better way to put an edge on a tool but that does a good enough job for the classroom.

                In the case of my own scraper it was sharp enough, but at a very magnified view it was ragged and that showed up in the scraping. Visible to me only with high resolution photographs, I should add. I doubt there's anything about scraping that requires unusual methods of tool sharpening, but in the case of any finish work you want the best edge possible. Getting that is already widely understood by anyone who grinds their own tools.
                Is there a reason that the Baldor 6" carbide tool grinder with two diamond wheels, 280 grit for roughing and shaping a new blade and 400 grit for finishing and sharpening with it's gravity feed wet system wouldn't be ideal? Wade suggests 80% kerosene and 20% light machine oil. The Baldor 6" carbide tool grinder spins at 3600 RPM.

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                • #23
                  Originally posted by Neil Jones
                  Is there a reason that the Baldor 6" carbide tool grinder with two diamond wheels, 280 grit for roughing and shaping a new blade and 400 grit for finishing and sharpening
                  For initial shaping that's great, but for sharpening a carbide scraping blade, you hone it with 9 to 12 micron diamond lapping compound.
                  "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                  • #24
                    Lazlo is correct - the grinder gets you to the shape, the diamond lap puts a nice finish on it, and I think a diamond sharpening steel can be used to refresh or strop the edge to remove any difficult to see irregularities thrown up by the grinder.

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                    • #25
                      Originally posted by Neil Jones
                      I believe the mirror rides up against a straight edge. I think the autocollimator stays on the table. I'll check this as soon as I get a chance.
                      That could be. I was there about 4 years ago and the old memory ain't what it used to be.

                      Steve.

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                      • #26
                        So Dennis and Lazlo you are saying you need both a carbide grinder (with rough and finish diamond wheels) and you also need to make a diamond lap tool?

                        I am struggling to grasp how the "rock test" that Robert Wade refers to works.
                        How are you suppose slide the part you are scraping and that has the red lead on it if the part you are scraping is convex, concave or flat so you don't get false readings?

                        Wade says that both red lead and bearing blue need to be prepared and must sit for a week. What a pain if you run out. You have to wait a week to scrape again? What's the shelf life of this stuff once prepared and aged for scraping?

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                        • #27
                          Originally posted by SteveF
                          That could be. I was there about 4 years ago and the old memory ain't what it used to be.

                          Steve.
                          Steve I saw it done at our shop 2 weeks ago and I can't remember. Feel better now?

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                          • #28
                            I don't know, what were we talking about?

                            Steve.

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                            • #29
                              Originally posted by SteveF
                              I don't know, what were we talking about?

                              Steve.
                              You win, Steve. Too funny. That made my day.

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                              • #30
                                Originally posted by Neil Jones
                                Wade says that both red lead and bearing blue need to be prepared and must sit for a week. What a pain if you run out. You have to wait a week to scrape again? What's the shelf life of this stuff once prepared and aged for scraping?
                                I think Wade is referring to mixed pigment in oil type spotting compound. Canode is available from Dapra (and probably others). It is ready to go and water soluble which makes clean up much easier. Otherwise there is the traditional blue spotting medium available from Dykem and Permatex and I'm sure others. Oil based and a bit messier.

                                Red lead is I think banned owing to its heavy metal poisoning effects.

                                One more advantage of Canode brand is that it also comes in Red and Yellow (although perversely its all called blue spotting medium-go figure). Having contrasting colours available is handy sometimes, and I find the red easier to see than the blue in any event.

                                Greg

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