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Millrite DRO Part II

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  • Millrite DRO Part II

    This is the final part of the 3-axis DRO that I installed on a Millrite knee mill. The first half of the thread, showing the x- and y-axis, is here:

    http://bbs.homeshopmachinist.net/showthread.php?t=33062

    The Z-axis install was the biggest challenge of all. Again, this is a mill that was designed before DRO's were even imagined, so Burke apparently thought it elegant to design the mill's base as a conical section, sloping away from the Z-axis ways, and tapering inward up toward the turret base. You can see the extent of the curved casting here:



    The problem is that you need to mount a 2" wide glass scale flat against the casting somehow. I was hoping to use an endmill in the magnetic drill press and bore/mill a flat on each end of the casting where the scale would mount, but not only was the curve so severe that I couldn't get the endmill started, but when I measured it out, the glass scale was going to sit backwards about 1/4". Considering that the reader head (which is going to travel on the knee) has to be flat and parallel within 4 thou across the entire length (16"), that's just not going to work.

    So I ended up scraping a strip on the casting that was flat enough to keep the Z-axis scale within the constraints of the reader head.

    A the time, I had been doing a lot of hand scraping, so I started with a fresh scraper blade. But it turned out that scraping vertically, below and behind the mill table, just wasn't going to happen. I had bought a Biax power scraper earlier last Summer, but my few attempts taught me that power scraping was a much different skill than hand scraping: the Biax is like a hand-held jack-hammer, and if the angle of the carbide blade is too shallow, it just skips off the work. If it's too deep, the blade will shatter. But two hours of trying to manually scrape vertically up convinced me that this would be a good time to learn to power scrape, albeit not under the best possible conditions.

    In any event, with some perseverance, a lot of Dapra Canode Blue, and a couple of coffee cups of cast iron later, I got a reference plane scraped along the Z-axis like this:



    After another 20 hours of so of scraping, I actually started to get some spotting

    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  • #2
    At one point during the scraping process, I saw a miniature reference master on Ebay, and thought it would make my life easier. It was advertised as "beautifully hand-scraped." Look at how it blued-out



    The reason this job was especially time-consuming was that I was scraping in at the bottom by about 1/8" -- that's a lot of metal to remove. But my strategy was to get the bottom planar, and work it up towards the top, where it leaned inward:



    ... and eventually, several coffee cups of Meehanite later, the reference plane blued like this:

    "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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    • #3
      It's getting there looking good so far.

      Simpler way to deal with the tapered castings though is to fab a drill jig that locates and clamps on the dovetail. Use it to drill the mounting holes and use a counter bore to spotface the mounting pads.Then make standoffs to follow the landscape.That saves the finish and paint along with a lot of work
      I just need one more tool,just one!

      Comment


      • #4
        It was kind of sad that the reference plane was going to be hidden, because it really did have that pretty glittery appearance that people love on freshly scraped surfaces:



        Forrest, McGyver and the other scraping experts might notice that I don't have the cross-hatching that you normally want on a scraped surface. Because I was scraping next to the Z-dovetails, I could only change the orientation of the scraper by around 20° each way. Didn't really matter -- I just needed to get the reference plan flat enough for the reader head to ride up the Z-axis without pulling off the glass scale.

        Now that the Z reference was done, the last part was committing to the final mounting points. The general idea is that this bracket was going to be mounted on the knee, and the mounting ears would tap into the 6 mm screws on the Z-axis reader head:



        This involved a lot of trial and error: you can't let the knee cause the reader head to bottom-out at the ends of the glass scale -- it will ruin it. So I had to try a bunch of potential scale and bracket mounting points. I ended up with the glass scale further down, and the bracket at the very bottom of the knee.

        The final part was dropping the mounting holes determined by the knee travel. The magnetic drill press was great for this:



        It made me a little sad tapping through that scraped surface:

        Last edited by lazlo; 02-03-2009, 12:00 AM.
        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

        Comment


        • #5
          ...and the final product:



          By a convoluted set of Machinery Karma events, I ended up stumbling across a pristine Excello mill right as I was finishing the Jenix install on the Millrite, so the Bridgeport J-Head servo retrofit I posted here was the only time I got to use it

          http://bbs.homeshopmachinist.net/showthread.php?t=31628
          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

          Comment


          • #6
            Originally posted by wierdscience
            Simpler way to deal with the tapered castings though is to fab a drill jig that locates and clamps on the dovetail. Use it to drill the mounting holes and use a counter bore to spotface the mounting pads.
            That's a great idea Darin. The problem is that the casting drafts inwards a lot too. After I got that section roughed in, I tried making some spacers to get the glass scale up over the compound (backward and inward) curve, but I would have needed a 4-axis CNC machine to make them. But the biggest issue is that I needed enough clearance for the Z-axis scale to fit behind the Y-axis reader head, with the table wound all the way back. This is a 3/4 sized Bridgeport, so everything is shrunk down -- it was a tight fit!

            The other approach I've seen is to mount flat beams across the Z-axis dovetails, above and below the knee travel, and make an outrigger for the Z-axis scale. But there's no extra room on the Millrite to do that
            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

            Comment


            • #7
              Originally posted by lazlo
              That's a great idea Darin. The problem is that the casting drafts inwards a lot too. After I got that section roughed in, I tried making some spacers to get the glass scale up over the compound (backward and inward) curve, but I would have needed a 4-axis CNC machine to make them. But the biggest issue is that I needed enough clearance for the Z-axis scale to fit behind the Y-axis reader head, with the table wound all the way back. This is a 3/4 sized Bridgeport, so everything is shrunk down -- it was a tight fit!

              The other approach I've seen is to mount flat beams across the Z-axis dovetails, above and below the knee travel, and make an outrigger for the Z-axis scale. But there's no extra room on the Millrite to do that
              That's where a slim line scale comes in handy.Trouble is getting a DRO mfg to substitute a single slim line into a mill package.They all want you to buy separate scales and monitor.
              I just need one more tool,just one!

              Comment

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