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I Got That Wood Grain Pattern Again..............

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  • #31
    Originally posted by JoeLee View Post
    Is this supposed to be a joke?????? Maybe I should look for rift cut aluminum next time.LOL

    JL......................
    I do believe that would be straight faced sarcasm. An inside joke at best. A comment to be taken very seriously on lookhowmuchshinyoverpricedbilletcrapIcanboltonmysl owloudobnoxiousvehicle.com.

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    • #32
      I think i have figured out the answer to this "important" issue.
      And i can even replicate that.

      First of all i was wrong: it is not a product of vibration, but in fact a product of good tramming AND variable chipload.

      A cutting edge creates a circular groove. When the same edge goes over that groove it creates intersecting arcs that look like a grid.
      The problem is near the center of the tool grooves are parallel to each-other. Arcs simply dont intersect.
      So that creates a zone where there is no grid, but parallel lines.

      What causes this woodgrain pattern is variable chipload.
      Either because of uneven crwnking of the handle, or variations of spindle RPM spacing between grooves varies and it causes circles to intersect in different places.

      Here is a part flycut on a bridgeport.


      Here i replicated the pattern on a CNCMill simply by varying RPM withing 10%
      Last edited by Zero_Divide; 05-24-2013, 01:06 PM.
      FSWizard - Free Online Speed and Feed Calculator

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      • #33
        ZeroDivide, thanks for you help on the issue but I think you hit the nail on the head when you said to move the shell mill off center. I tried that and the pattern did not appear.
        My spindle speed was 500 RPM and the table feed was set to aprox. 15 inches per min. in all my photos.
        I did run the speed up to 1200 R's and still got the pattern however it changed slightly in size, more of a condensed version of all the previous ones. The intersecting arc is an interesting thought, however I think it's part of the pattern combined with vibration.
        I also checked my spindle for end play and I got zero, I knew that wasn't the problem. When I indicate the head to the vise and pull down with a few pounds of force I see about .0003 deflection. I know that the BP's aren't as solid as some mills are.
        Here are some pictures of the finish with the spindle off center.

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


        Center cut

        Off center cut

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        • #34
          Notes to above pictures.............
          I didn't make any difference which side I off set the cutter to but it had to be off set by a substantial amount or the pattern would still show up. I haven't tried putting a wiper insert in the cutter to see what effect that might have as far as wiping out the intersecting arcs.

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

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          • #35
            I can understand that part of it - but im still fixated on the progressive grain pattern, WTF is causing that?

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            • #36
              I'm not sure on the progressive part of the pattern either. It isn't the feed rate because I was using the power feed, not the hand crank.
              It must have something to do with cutter engagement across the surface??

              The progressive pattern kind of resembles the shock wave of a bullet in flight.

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

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              • #37
                I just tried this with a 1" 4 flute end mill, cutting down the center and I got no pattern at all, the finish is like glass.
                So I gues this pattern only shows up with a shell mill type cutter.

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

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                • #38
                  How about Zero_Divide's Moirre patterns as the cause of the optical effect, but the variation being produced by the shell mill and arbour flexing back and forth in the direction of feed? If the back teeth of the shell mill get pushed up a fraction as the rear of the mill comes on to the work, that would cause the front teeth to cut a fraction deeper in response, when the rear teeth get to that point the front teeth rise up etc. A bit like the patterns you sometimes get on bandsawed stock due to the set of the teeth 'echoing' along the cut.

                  The height differenced might be only a tenth or so, but still produce an obvious visual pattern.

                  There's got to be a market for it.
                  Location- Rugby, Warwickshire. UK

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                  • #39
                    Yeah it looks cool. And its obvious as a defect too.

                    To get it you need an insert with a radius and without a wiper.
                    As far as i know it even happens with 1 flute fly cutters.

                    The fact that OP was able to get rid of it shows that it has somethi.g to do with vibrations.

                    At the same time i was able to resemble that finish by varying chipload while in the cut.....

                    Wtf.?
                    FSWizard - Free Online Speed and Feed Calculator

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                    • #40
                      Highpower in post #27 has a photo with a pair of parallels being attached to long blue bars. Not sure what they are; but do those bars just "hang out"? Not saying they cause the patterning in JoeLee's case but bars hanging out will add to the vibration.

                      Sometimes when I use large g-clamps to hold stuff I find draping a wet towel over the ends of the clamp screws reduces vibration.

                      Cheers,
                      Norman

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                      • #41
                        Cutter flex and vibration propagation; cutting on center causes cutter flex, and vibrations propagate so much that they can be since in these very dense materials. Higher stiffness decreases propagation length. A carbide endmill would decrease them if it was single point as the same diameter as the flycutter. Small diameter tools don't have this vibration problem, because they are so small that vibrations cannot propagate noticeably. If you cut a wall with noticeable depth with an endmill, it will vibrate and produce this pattern VERTICALLY along the axis.

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                        • #42
                          I read somewhere that one should actually set the tram just a little off "perfect". And then let the low side do a trail cut.

                          Another item: I didn't notice in the posts if you have been locking the quill. After getting advice from a seasoned pro, I now always lock it and use the knee feed.

                          Geoff

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                          • #43
                            Originally posted by Zero_Divide View Post
                            Yeah it looks cool. And its obvious as a defect too.

                            To get it you need an insert with a radius and without a wiper.
                            As far as i know it even happens with 1 flute fly cutters.

                            The fact that OP was able to get rid of it shows that it has somethi.g to do with vibrations.

                            At the same time i was able to resemble that finish by varying chipload while in the cut.....

                            Wtf.?
                            Just because he got rid of it, doesn't mean it has to do with vibrations -- since maybe he covered it up somehow by recutting those edges, as would happen with a wiper. Of course in the general sense of everything, we can say it has to do with vibrations, but that's not saying much at all. It has to do with cutter flex and vibration propagation. Just because you can't bend your tool by beating the crap out of it doesn't mean vibrations can't propagate through it.

                            Chip recutting, or curling in the chip formation is a good way the average person might cover it up, thinking he dampened his tool by changing the cutting speed, which is wrong. If you changed your RPM by 2x doesn't mean you dampened your tool.

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                            • #44
                              That prevents recutting in non-rigid or elastic (they're not the same thing) of the back side as the mill is travelling. Better yet, that person should use a wiper insert so the recutting happens under the wiper edge, and remain flat.

                              In the lathe we can remain off center by raising/dropping the tool barely, since another cut can get to the desired dimension. In a milling machine, the 2D nature of a cut means that adjusting a size in one dimension can offset other. It's not a good idea to go by what you see with the eye. I can't think of any real world examples where this is a problem, since I lap by surfaces when I need to get good flatness. I'm sure it could cause seizing of butterfly dyes in the paper punching industry for example.

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                              • #45


                                Deflection is cyclical


                                Cutting thickness varies because of this


                                The result is a (simulated) vibration pattern along the direction of the profile in the material as a poincare plot.
                                Last edited by Elninio; 05-24-2013, 06:40 PM.

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