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  • Grinder Finish Question

    I had to grind a pin today on the T&C grinder. One end of the pin has a taper ground on it which fits into a cam shaft. I located off the taper to within .0001 in the 4 jaw chuck. I only had to take off .0007 to clean it up. It looks nice, but you can see a slight scalped finish. I didn't think much of it till I measured it with the calipers and rotated the pin between the caliper. I could feel the segmentation as I rotated it. Not sure what the cause is. The wheel not being dressed properly, vibration in the head?? Any clues???
    I don't know if I should try to clean it up again or just leave it. It runs inside a bronze bearing. I doubt that it would hurt the bearing.

    JL................

  • #2
    No pictures? How large is the pin? What was your setup? How agressive was the grind?

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    • #3
      I had a similar problem and traced it to fact that I was using a static phase converter on a 3 phase T&C grinder motor. Seems when I went to a rotary converter, the problem cleared itself up.

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      • #4
        Not exactly sure as there are many causes for this including a too heavy of a cut, a loaded wheel and too slow a traverse/rotation of the object being ground. Assuming you dressed the wheel before the grind and you mentioned only 0.0007 in total was removed, I would guess the rotation of the part was too slow.
        You need to move items under the wheel without hesitation or else a subtle scallop pattern will emerge. In my experience the scallops have been so small I could not measure them even with a tenth's DTI. You mentioned that you could feel it, so you might have something else going on. it could be possible you rotated it so slow you were basically indexing around and grinding flats every few degrees of rotation.
        I usually see this pattern on surface grinders where the work was ground with a too slow of a traverse speed on the table. Its very easy to go to slow; one has to focus to go fast enough.
        Assuming you were rotating the item under the wheel, try it again but with a much faster rotation. When rotating or traversing by hand, it's really-really hard to go too fast on a grinder.
        Last edited by jungle_geo; 02-18-2014, 07:32 PM.

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        • #5
          Lots of good tips and causes, but I can't isolate it down to any one of them yet. To answer some of the questions........... the dia. of the pin at the start was .615, the length of the portion to be ground is about an 1 1/4". The workhead was turning as fast it can go, I'm not sure what the SFP is, I'll check tomorrow. The wheel was dressed, I traversed the table by hand, maybe too slow??? not sure. I removed about .0002 per pass. I use a VFD and not a static phase converter.
          Pictures probably won't show the detail, but I'll try to take a few tomorrow. I'm also going to try a different wheel. The scallops are too shallow to
          measure with a tenths dial. You can see them under magnification. You can't feel them with your fingers, but you can feel the flats if you rotate the pin in the calipers. Be in touch tomorrow when I have more info.

          JL.................

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          • #6
            Something loose or the wheel not in sharp condition or the part movement or rotation too slow. Getting that same thing in a cylindrical grinder if some of these conditions exists, the most usual being too blunt wheel.
            Amount of experience is in direct proportion to the value of broken equipment.

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            • #7
              I have to ask this....... is there a rule of thumb on how fast you should traverse the table to move the part across the wheel?? I usually move it very slowly taking fine depth cuts of 2 to 3 tenths. Heat is never a problem with a small cut. I've tried faster traverse speeds but what I get is a spiral rub across the part and that usually results in traversing back and forth several times to get a clean finish or before it sparks out.

              JL..................

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              • #8
                Sounds like you did everything needed.either a bad dress or cold spindle bearings.

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                • #9
                  "is there a rule of thumb on how fast you should traverse the table to move the part across the wheel?"

                  it depends on wheelspeed and how much of the wheel you would like to do the cutting (the rest is finishing). simple example:

                  sped 6000 rpm (100 rps), 20 mm wheel, first 10% should cut

                  2 mm x 100 rps = 200 mm/s traverse speed.

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                  • #10
                    What dian said. I might add that the kind of speeds you get from the calc isn't always possible, as workpiece material, hardness and the grinding wheel grit, hardness, porosity etc. can lower it quite much.
                    Amount of experience is in direct proportion to the value of broken equipment.

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                    • #11
                      Do you see the same scalloping in the surface when grinding a flat? Could the finish be caused by slop (radial or axial) in whatever you are using to rotate the part?

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                      • #12
                        Originally posted by Rosco-P View Post
                        Do you see the same scalloping in the surface when grinding a flat? Could the finish be caused by slop (radial or axial) in whatever you are using to rotate the part?
                        I've never ground anything flat on my T&C grinder.

                        JL...................

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                        • #13
                          Here are some pics of the pin and my set up. This pin is actually part of the cam shaft for my Briggs engine I'm rebuilding. The end of the cam shaft has an internally ground taper, the pin as I've been calling it has a taper on each end, one side fits into the cam and the other taper is where the synchro balance counter weight gear is mounted. A long bolt runs through the center of the cam shaft and pulls it all together.
                          Back to the grinding........... I changed wheels today, I went from a 1/4" wide wheel to a 1/2" wide one. Norton 32A80JVBE.
                          I noticed vibration that didn't go away after dressing the wheel. What I did was lower the frequency on my VFD from 60 Hz to 52Hz as the RPM's dropped the vibration disappeared at around 52Hz. I don't know what the exact RPM's of the wheel are at that freq. The motor is 3450 RPM and I would imagine that is at 60 Hz. I'm running 1:1 with the pulleys from the motor to the spindle. Any way that cured the problem of the scalloped finish. The work head was actually set at the slower speed by belt to pulley ratio. The work head has two pulleys one speed is I think 250 RPM's and switching the belt over gives you 500 R's, I think or that's close, but as I dropped the freq. with the VFD the work head also drops in speed.
                          I've always run the work head at the lower speed, perhaps that is OK for larger dia. parts but for small stuff maybe I should be turning them faster.
                          The picture of the pin is the first grind, I don't think you can really see the scalloped pattern in the picture.

                          JL........................



                          Last edited by JoeLee; 02-19-2014, 07:12 PM.

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                          • #14
                            You've got a couple of things working against you with that setup. You have too much hangout for one, couldn't you have chucked it on the larger diameter closer to the taper you are grinding? Also, I ask if you've given consideration to work hardness vs. wheel hardness. The harder the work, the softer your wheel should be. That may be why I'm seeing burn/heat marks. When the abrasive is too hard it doesn't "break away" new cutting edges, but instead dulls and fills with chips.

                            While the 32A is a good general purpose abrasive grade, 38A is a little more friable. I think the bigger problem is in grit size. You'd do better with a 38A60-HVBE or similar. And yes, I would run the workhead at the faster speed for that diameter.

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                            • #15
                              I know that there is a little more of the pin hanging out than there should be but that entire larger surface is the bearing surface and had to be ground leaving the short shank the only place to chuck up on. I thought of putting a tail stock on the right hand side for stabilization but I figured it would pull the end of the pin off center and I couldn't afford ending up with the bearing surface tapering so I went with it as shown. This time I got pretty good results, much better than the previous. The problem with all these wheels of different hardness is a lot of time I really don't know what the hardness of the part to be ground is. This pin for example isn't hardened.

                              JL..............

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