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Surface grinder question, and a bit of a rant.

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  • Surface grinder question, and a bit of a rant.

    Being somewhat of a newb, and not having full experience on all the machines in a well equipped shop, a question popped up this morning.

    Assuming a smallish manually operated surface grinder, how does one compensate for the wear of the wheel that will ineviteably occur when grinding larger pieces, say 4" x 8".

    Many people people are of the impression that a "surface ground" part to be the holy grail in flatness, yet if the piece is largish at all it would be near impossible that it would be truely flat.


    So what brings this on??
    Being more in need of a rudimetary hydraulic press than long on time to source the parts and build my own, I recently bought an off-shore press. A bit better than the average HF model, but a far cry from a Dake.

    As usual, the press came with a "pair" of free press plates, you know, the 4x6x1 or so plates of steel with a V notch cut in one side.

    I mention "pair" because these plates were laughable. A "set" maybe, but certainly not a "pair". Even the Chinese should realize that a "pair" of anything tool related, especially items that are specifically intended to be used together, at the same time, should probably be a relatively matched "pair".

    I seem to have gotten a mismatched set, more like a couple, of plates.
    Lenghth, width, height, and thickness were not anywhere close. The thickness was reasonably close, depending on whether you measured the high or low spots. I realize they don't need to be as close as gauge blocks, or even a pair of V-blocks, but they should be in the same neighborhood with each other.

    The worst(dangerous) part was the fact that the edges had horrible angles cut by a dull flame wrench. Had these plates been stood on edge and any pressure applied they would have rolled over and shot across the shop.

    So, while I was standing there watching my mill carve them down into a somewhat matched pair, I thought "these sure would look cool with a ground finish", and then realized how much work it would be on the machine that I have access to, a 6"x12" with a 1/2" wide wheel. Also realized that they probably wouldn't be flat when done.

  • #2
    Well, for one thing as the surface grinder is doing it work it is only cutting on the edge of the wheel and it moves across the work. The rest of the wheel is just dusting off the surface. That means that you can make several passes back and forth across the work before it needs dressing and the sound of the wheel on the work will tell you to dress it plus the excess sparking.

    I seldom take more than .002-.003" off the surface on each pass. If you have coolant on it you can take more. After you get the surface cleaned up the you take another pass without moving the wheel down and just dust the surface to get the surface flat.

    You have to be carefull when using the mag table that you don't pull the warp out of the plate and then grind it because when you release the magnet the warp will suddenly reappear.
    It's only ink and paper

    Comment


    • #3
      Well, for one thing as the surface grinder is doing it work it is only cutting on the edge of the wheel and it moves across the work. The rest of the wheel is just dusting off the surface. That means that you can make several passes back and forth across the work before it needs dressing and the sound of the wheel on the work will tell you to dress it plus the excess sparking.

      I seldom take more than .002-.003" off the surface on each pass. If you have coolant on it you can take more. After you get the surface cleaned up the you take another pass without moving the wheel down and just dust the surface to get the surface flat.

      You have to be carefull when using the mag table that you don't pull the warp out of the plate and then grind it because when you release the magnet the warp will suddenly reappear.
      It's only ink and paper

      Comment


      • #4
        Well, for one thing as the surface grinder is doing it work it is only cutting on the edge of the wheel and it moves across the work. The rest of the wheel is just dusting off the surface. That means that you can make several passes back and forth across the work before it needs dressing and the sound of the wheel on the work will tell you to dress it plus the excess sparking.

        I seldom take more than .002-.003" off the surface on each pass. If you have coolant on it you can take more. After you get the surface cleaned up the you take another pass without moving the wheel down and just dust the surface to get the surface flat.

        You have to be carefull when using the mag table that you don't pull the warp out of the plate and then grind it because when you release the magnet the warp will suddenly reappear.
        It's only ink and paper

        Comment


        • #5
          Actually, surface ground parts are quite flat if done with the usual grinding practice. After dressing the wheel, left-right traverse is started slightly longer than the length of the piece. Bring the wheel down slowly over the part, and the highest point if you can find it so it just touches. Back the table out so the wheel clears the part and lower the wheel by just a couple thousandths if you're not sure what the whole surface is like. Then start crossfeed, (continuing the left/right constant movement) and when you begin to cut on the piece move the part in toward the column about .030" for each back and forth cycle. This way you'll gradually cover the width of the part. The wheel will break down, but it will primarily just round off the front corner so as you're moving across the piece the area of the wheel behind the advancing edge will still be at the original dressed diameter. Finally, when you have the surface ground (redressing the wheel as needed) take an additional downfeed of just a half to a thousandth and cover the whole thing again with perhaps a larger crossfeed increment this time. The wheel diameter will not have broken down significantly during this last cycle and the piece will be as flat as the machine's ways can produce.

          <edit> Carl beat me to it (three times). Must be my slow day.
          Last edited by TGTool; 03-30-2009, 03:07 PM.
          .
          "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

          Comment


          • #6
            Well, for one thing this site was loading so slow I posted it three times.
            Last edited by Carld; 03-30-2009, 03:12 PM.
            It's only ink and paper

            Comment


            • #7
              .001" is an average DOC for a surface grinder,even that requires either oil or coolant to keep the wheel from glazing.

              Now,about those press plates.First thing I would do is see if those plates are even steel.A drill bit is the easiest way to determine this.If you get powder or easily crumbled up curls it's cast iron or malleable iron.At that point throw them in the scrap they will hurt you.Cast plates will bust unexpectedly in a press and turn into shrapnel.Just a heads up.
              I just need one more tool,just one!

              Comment


              • #8
                i guess my first question is just how precise of work you expect to do on a hydraulic press? the plates on mine certainly are not straight any more. if you want the plates to be flat i would start out on the mill or planer. surface grinders are for finish work. as far as standing plates on their sides in a press that is a dangerous practice and i would recommend that you make up some upside down "T" to support things if you need an edge.

                Comment


                • #9
                  Originally posted by Carld
                  Well, for one thing this site was loading so slow I posted it three times.

                  that will certainly teach it a lesson.

                  Comment


                  • #10
                    Originally posted by wierdscience
                    Now,about those press plates.First thing I would do is see if those plates are even steel.A drill bit is the easiest way to determine this.If you get powder or easily crumbled up curls it's cast iron or malleable iron.At that point throw them in the scrap they will hurt you.Cast plates will bust unexpectedly in a press and turn into shrapnel.Just a heads up.

                    Most definitely steel, and seemingly pretty decent stuff that machined rather nicely. Didn't find any nasty stuff hidden just below the surface or anything like that. Am well aware of what a cast plate would do in a press.
                    The biggest issue was just the goofy angles on the edges, didn't actually measure the angle but had to take more than .060" off one side to square it up.

                    No, I don't expect to do hugely accurate work, but it would be nice to be able to setup stuff with some semblence of flatness. Having parts rocking back and forth on the press makes me a bit nervous.
                    The surface grinding was just an idea that popped into my head for appearance sake.

                    Comment


                    • #11
                      Do you actually have a surface grinder then? That would be a good way to go to level up your blocks since they could probably be done together to be assured of the same dimensions. There ought to be enough tool shops still around Cedar Rapids to have them done, or stop out at Kirkwood and see if whoever heads the machinist program now would take pity on you and straighten them up.
                      .
                      "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

                      Comment


                      • #12
                        Originally posted by tdkkart
                        Assuming a smallish manually operated surface grinder, how does one compensate for the wear of the wheel that will ineviteably occur when grinding larger pieces, say 4" x 8".

                        Many people people are of the impression that a "surface ground" part to be the holy grail in flatness, yet if the piece is largish at all it would be near impossible that it would be truely flat.

                        .
                        so what is flat? there is no truly or perfectly flat, something can only be flat to a tolerance......and the smaller the tolerance, bigger the surface (especially if its thin) the more difficult it is to do.

                        the surface grinder is no silver bullet - really flat is difficult, especially if you're talking a 4x8" piece that is comparatively small in section, say 1/2" thick. if you are trying for surface grinding accuracy, tenths, the mag chuck itself will distort the work so when its released the work is no longer flat. How do you over come this? tediously mapping out the surface with an indicator and shimming the work until its not distorted. the other way, less perfect, is to turn the mag off for the final passes usually the residual mag power to hold the work

                        see the point though? the challenge isn't wheel wear, its how to hold the work so its not being distorted.

                        Wheel wear first off not as fast as might think, secondly if you've been doing heavy roughing you can redress before finishing.....but the way you avoid wear affecting accuracy is overlapping strokes. If you start with a fresh dressed 1/2" wheel and each stroke is advanced say .100", one side of the wheel does most of the work while the other barely makes a cut - this side will not really wear at all.
                        .

                        Comment


                        • #13
                          Originally posted by TGTool
                          Do you actually have a surface grinder then?
                          Not in my shop, but have access to a couple that I use on occasion, just never done anything of this size before. Normal stuff is maybe 2" in diameter max, run a couple sets of vise jaws, that kind of stuff.

                          I doubt that a 6x12" magnet will have enough umpf to warp a 1" thick plate??

                          Comment


                          • #14
                            Originally posted by tdkkart
                            I doubt that a 6x12" magnet will have enough umpf to warp a 1" thick plate??
                            Hi,

                            Wanna bet? I've spent many hours grinding plates for die sets to +/- .0005" corner to corner. Those chucks get a good grip, and can bend some pretty heavy thicknesses. Fortunately, most of the surface grinders I've run have vari-chucks that allow the operator to dial down the strength of the magnetic field as you spark out.

                            Last grinding I did was a Blanchard with a 6' diameter chuck. I was blanching ASTM 633, 3 1/2"x60"x92" plates. It was a miserable job had to keep everything to +/- .002 corner to corner. It was a bear to hit the numbers.

                            dalee
                            If you think you understand what is going on, you haven't been paying attention.

                            Comment


                            • #15
                              Originally posted by tdkkart
                              Not in my shop, but have access to a couple that I use on occasion, just never done anything of this size before. Normal stuff is maybe 2" in diameter max, run a couple sets of vise jaws, that kind of stuff.

                              I doubt that a 6x12" magnet will have enough umpf to warp a 1" thick plate??
                              my other hobby is grinding telescope mirrors.
                              I can promise you everything we think of as solid is more
                              like jello than our intuition will tell us.
                              --
                              Tom C
                              ... nice weather eh?

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