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IndustryArena Forum > MetalWorking > MetalWork Discussion > Facing and getting a FLAT surface
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  1. #1
    Join Date
    Mar 2004
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    368

    Facing and getting a FLAT surface

    We have a block of 6061 AL that is about 3.5" long and about 2.5" wide, about 1" thick.... mounted in soft jaws on a vise. Using a 4-flute 3/4" DIA endmill, making 5 passes or so on the top, you can feel a prominent ridge between passes. I want to finish these parts in a vibratory bowl so the flatter I can get the top, the better. The final pass is only about 1/8" depth of cut.

    I figured using the 3/4 mill would reduce deflection but we still get pretty big ridges between passes. The max tool diameter on the holder is 3.5" I believe, so I can't go using some huge flycutter, and while 3.5" would help on this part, it wouldn't help as much on other parts in the future.

    I'm thinking the problem is maybe due to the head not being 100% square to the table, or possibly deflection within the part (we do pocket it so it loses a lot of it's mass before the final finish pass on the back). But I'm not sure those problems are entirely fix-able.

    Any other suggestions to getting a better finish? If a flycutter, anyone suggest a brand/model for 6061? What about doing finish passes front-to-back in Y instead of side-to-size in X? That would perhaps make a smooth transition if the head is not 100% square rather than the more ridged X-axis passes, no?

    Any other suggestions are appreciated. We're not talking big deep gouges here or anything, just you can easily feel it with your finger if your rub it back and forth over the part.

  2. #2
    Join Date
    Mar 2005
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    1136
    the head needs tramming, but don’t take my guess as gospel, put an indicator in and check. Most mills get knocked about enough to come slightly out of square over time – using a tenth’s indicator is my preference…not that anyone mills to tenths, it’s just that the error is magnified and easy to see.

    you say you're not sure adjusting the head is possible??? there is no way to adjust it? if not you've got to find way to shim to square. fly cutting might avoid the ridges, but if the head is out of sq so will the cut be, the has to be a way to fix that

    while .125 isn't much of a cut, it could be enough to cause some deflection (what type of machine), try like 10 thou - what happens. like i say, my bet is the head is out of tram but if you are worried that it might be deflection, try a lighter cut.

    ps if really can't square the mill and you're not making millions, why not just cleaning it up with a file?

  3. #3
    Join Date
    Jul 2005
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    12177
    What Mcgyver says is most likely the case, the head is not perfectly true. And sometimes depending on the machine it is very difficult to get it true enough that you do not get detectable ridges as you describe.

    As a side comment it can be surprising how sensitive fingetips are, you can probably feel a 0.0005" ridge maybe even less.

    So try to go around the problem. Buy or make yourself a flycutter large enough to take a finishing cut spanning the full 2.5" width. You can hand sharpen a high speed tool with a good rake, Mcgyver may remember a picture he posted a while back and give you the link. Take off only a few thou and you should be able to get a very nice finish.

  4. #4
    Join Date
    Mar 2004
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    368
    Thanks for the feedback. I agree that the fingers can detect very slight variations - I don't think it's THAT far off square, but it is a little off. I am sure it's technically possible to fix, I just don't know how off I am from truly square, and I am not sure it's something I'd be able to do myself (and if I couldn't, I'm not sure it's enough of an issue to justify calling in a repair guy). The machine is a V-40 (40"x20" mill) so it's a pretty big and beefy machine.

    The main issue is that I was hoping to automate the finishing of the parts in a vibratory bowl, and if I leave them in the bowl long enough to remove the ridges, it will also break the nice crisp corners we've machined and soften the lines up more than I'd like. If I could reduce the ridges, I could probably just toss them in the vibratory bowl for a shorter time and the flat portion would be smooth without breaking all my corners too much.

    I will be doing these in batches of about 50 at a time (50 fronts, 50 backs, so say 100). Thats about once every couple of weeks. Its not a ton of effort to manually remove the ridges but better if I could avoid it

    I use a palm sander now with 220 grit paper to remove the larger machining marks, then sometimes go to 320 grit for a smoother (brushed-look) finish. Any other/better tools to quickly remove the ridges than a palm sander? I have a random orbital sander too thats a big bigger than the palm, and I have a pretty industrial strength belt sander.

  5. #5
    Join Date
    Dec 2006
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    242
    I would never expect a perfectly flat surface when taking .125" depth on the final pass. I would try .005" like suggested earlier. I think that is your only problem. Higher helix endmills work better in aluminum and other materials as well. If you are currently using a 30 degree general purpose endmill, you'll love a 3 flute 45 degree helix endmill.

    Dave

  6. #6
    Join Date
    Mar 2007
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    22
    I doubt you will ever get a perfectly smooth surface using a endmill and multiple passes, at least I've never seen it done, like said above make very small finish passes, I saw some parts once that were finished that way but they used a S shaped finish pass to break up the straight lines and give a cool look to the part.

  7. #7
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    Mar 2006
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    68
    Another alternative - although a lengthy process - use a ball mill and spiral out from the center. Control your finish with your step over. I do it sometimes if I have another job I'm working on another machine. If I'm not there the machine sits idle anyway, so I reclaim the time in a nice finish.

  8. #8
    Join Date
    Jan 2006
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    67
    Why not use a flycutter? Use a good sharp tool with positive lift and good clearance. Leave .005 t0 .010 for a finish pass with the flycutter. We make our own flycutters. 3/4 or 1" shank with a 3/4 or 1" body-dia. to suit. Cut a slot for 1/2 tool like an AR8 or a D (V shape). Tap for a couple of set screws and you're in business. We finish similar parts this way time. If your head is not square it may not be flat or parallel but you wont have cutter mismatch and the finish should look real nice.

    Dave


    Edit: I guess Geof pretty much said this already. In that case "What Geof said":cheers:

  9. #9
    Join Date
    Oct 2005
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    672
    No fancy repair guy needed to square up the machine. It can be done with a wrench, a good .0001" indicator, and an understanding of what is happening. A lot of folks don't realize VMCs have to be installed correctly to achieve the advertised accuracies. Having the riggers set it in place and bubble level is okay for some work but if you need precision, you need to spend another hour doing the detail work.

    The quick check is put the indicator in the spindle and hold it such that you can sweep a fairly large circle like 6". Bring the head down and sweep the indicator on the table (not a vise, not a fixture). On a decent VMC, you should see less than .0005" variation across the 6" sweep. If you get more than .0005", the head/column is not square to the table/saddle/base.

    Use the screw jacks in the base of the machine to control the flex. Start will the machine sitting on only the corner screws and perform a bubble leveling of the machine. Once it's square to Mother Earth, then you have to adjust for the flatness of the floor it's sitting on. Based on where the indicator sweeps high or low, use the middle screws to control the deflection of the base of the machine. You'd be amazed how little changes in the screws will affect the squareness of the machine.

    See attached illustration which I've greatly exaggerated for visualization. The bigger the machine, the more pronounced the effects of an uneven or settling floor. On big HMCs like Niigata and Makino, there are like 20 screw jacks that have an effect on the column's perpindicularity to the workpiece.
    Attached Thumbnails Attached Thumbnails flex.JPG  

  10. #10
    Join Date
    Sep 2004
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    145
    I've had good luck on a machine thats slightly out of square, doing a spiral cut from outside in, taking .010" or so finish pass, with 50 or 60% stepover.
    HTH
    Insanity "doing the same thing and expecting a different result"
    Mark

    www.mcoates.com

  11. #11
    Join Date
    Dec 2006
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    247
    What kind of feeds and speeds and tool are you using? We do what your talking about everyday the only diffference is we leave .010" for finish pass.
    You can hog off as much material as your machine will allow then leave .010"
    for finish. If this still doesn't work go with a flycutter or shell mill.
    Joe

    oh we also use a DA with 240 grit paper for a real nice blend.

  12. #12
    Join Date
    Mar 2004
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    368
    Thanks for the suggestions! I have the machine perfectly level (used a machinists level) - the thought of adjusting the screws to try to control the flex makes me cringe after how long we took to get it perfectly level The floor in the shop is pretty damn flat and it's poured in squares - we made sure to keep it all within one square.

    I will try the S-shaped finish pass - sounds like a neat idea. Sure I can use a flycutter, but the only issue is that this particular part is juuuust small enough to cut with one pass from the largest flycutter my machine can hold, but there could be cases in the future where I need a larger part. I will try dropping the finish pass to .005 or .010 and see how I do. I will also try some different bits to see how I do. I am thinking of those Dataflutes mentioned in other threads and see if they are as good as claimed

    I've seen folks get practically a mirror finish on a DMG with a 30k spindle. Maybe I am expecting too much (I have a 6k spindle).

  13. #13
    Join Date
    Jul 2004
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    374
    Level doesn't matter! Head perpendicularity to the table DOES matter!

    I'm going to restate what Capriris said...that is really the proper technique, plus use a large facemill or flycutter.

    The proper technique is to first, level the machine on the corners. (or, preferably on three points like a tripod) If possible, bolt the machine to the floor at these points...you will have more authority for the next steps.

    Then use the other jack screws to tweak the table to that the head sweeps perpendicular to the table (with an indicator). Use gage blocks between your indicator and the table, rather than sweep the table.

    If you have a small machine (< 30"), it may be a ***** to get the base to flex, but a larger machine, you should be able to get the head to sweep at least as good as 0.0005"...but shoot for 0.0002".

  14. #14
    Join Date
    Mar 2003
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    4826
    If the table is level, and the spindle does not tram in square, then you need to shim the spindle cartridge, assuming that you can do this on your machine. Much as I didn't believe at first that this was necessary, when I rebuilt my Haas, I was very careful to sweep the table and shim in such a way as to get 3 point support (approximately equal spaces between shims or to the unshimmed contact point) around the flange of the spindle cartridge. Once I had the shims figured out, I put a light smear of loctite around the flange face, and then tightened it up. That was supposed to 'bed' the spindle to improve its bearing against the head casting. Maybe overkill, but then I frequently do that whenever I tinker with my toys.

    I also am of the opinion that a quite light finish cut is needed to prevent the tool flexure effect from tipping the tool tip out of the level plane.
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  15. #15
    Join Date
    Oct 2005
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    672
    Hu,

    How sure are you that your column is square to the table? If you look at the silly doodles I made above, you'll see that shimming the spindle can give you a nice flat sweep with the indicator but doesn't really address the potential problem. For example, different length tools (like a centerdrill and long drill) will touch Z0 on the workpiece at different XY values because the head is moving a small XY amount depending on the position in Z.

    I think you could do a quick check by putting an indicator in the spindle and sweeping a known round part. Then, without removing the round part from the machine and without moving the table/saddle, place the indicator in a different tool holder that is roughly 2" longer or shorter so the head is higher or lower in the Z axis and see if you get the same sweep. Any shift in the center between tool holders would mean that the column is moving relative to the XY as it travels up and down.

  16. #16
    Join Date
    Mar 2003
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    4826
    That is true, that is another possible source of error, but one that is difficult to correct on a bed mill. Even if that is the problem, the simplest makeshift fix for a facing problem is to shim the cartridge so that the spindle trams square, even if it does not rise in perfect perpendicularity.

    Now if a guy is having trouble with bored holes being measurably elliptical and out of square with the table, then he's going to have to fix the column perpendicularity problem.

    On a turret type manual mill, there is of course ample adjustment available to square up the spindle. But I think Mike is running a vmc.
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  17. #17
    Join Date
    Jan 2007
    Posts
    103
    there is a great article on this in last months issue of cutting tool engineering
    www.ctemag.com

  18. #18
    Join Date
    Oct 2005
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    251
    Go in two directions. Rough in x and y then finsh in x and y. Do not move depth when changing direction on finish pass. Flat is a relative term and you will never get a flat surface on a mill or lathe like you get from grinding and lapping. Which makes think you can take a piece of fine sandpaper on a surface plate and hand dress the ridges off. Yes, I know some will be offended by the thought of putting sandpaper on a plate but sometimes you gotta do what you gotta do.

  19. #19
    Join Date
    Dec 2006
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    247
    ctate2000.
    When you're talking flat what do you mean, If I can't hold .0003" flatness on my mills then I call the service man to come down and tell me what the he** is wrong, And I haven't called him yet. Lapping is always a viable solution but rarely needed. I have a lapping machine for a part number I need to hold to .0002" but I found if I clamp on the part right I don't need the table.
    Joe

  20. #20
    Join Date
    Oct 2005
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    251
    Flat is .000035. Everything is relative. I would like to check one of the parts you say is .0003 flat. I am not saying it can't be done I just won't believe it until I see it.

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