face mill cuts little bit on back side: normal?
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    Default face mill cuts little bit on back side: normal?

    dear all,

    Large face mills on my Aciera milling machine cut a tiny little bit on the back of the mill. Most of the cut is done on the front of course, but on the back, the mill cuts a bit on the already milled surface. (i do not know how to describe it better). This is not good for surface quality.
    The spindle with the shank and the mill run reasonably true (runout = 1-2/100 of a millimeter).
    Perhaps i am malinformed, but is this normal?


    Many thanks!

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    Quote Originally Posted by michiel View Post
    dear all,

    Large face mills on my Aciera milling machine cut a tiny little bit on the back of the mill. Most of the cut is done on the front of course, but on the back, the mill cuts a bit on the already milled surface. (i do not know how to describe it better). This is not good for surface quality.
    The spindle with the shank and the mill run reasonably true (runout = 1-2/100 of a millimeter).
    Perhaps i am malinformed, but is this normal?


    Many thanks!
    Did you check tram on the mill ?

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    Yes, it is trammed perfectly (table is trammed square to the machine and spindle is trammed to the table)
    It appears to happen in both directions.

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    Face mills do deflect, no way around that. If using a honed insert, pressure is required to make the insert cut. If the front edge is lifted by the pressure, then the back side dips.

    The only way to not have that effect is to have the mill slightly out of square. So have it as close to perfect you can get it, then by trial cutting, determine if there is a 'best' direction to mill that does not back cut. Then always mill the final pass in that direction.

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    Quote Originally Posted by HuFlungDung View Post
    Face mills do deflect, no way around that. If using a honed insert, pressure is required to make the insert cut. If the front edge is lifted by the pressure, then the back side dips.

    The only way to not have that effect is to have the mill slightly out of square. So have it as close to perfect you can get it, then by trial cutting, determine if there is a 'best' direction to mill that does not back cut. Then always mill the final pass in that direction.
    or to have a cutter larger than the part.

    Perfect tram and the most solid machine would leave a crosshatch on the part with the follow making a slight hit.

    Plus the heavy cutting and taking stock pushes all the slack out of the fixture and the spindle..so the follw tends to come imto/towards the cutter a little.

    Canting tram if the very slight hollow in the part still makes spec.

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    What is this thing called perfect? Tram or anything else?

    L7

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    Quote Originally Posted by HuFlungDung View Post
    Face mills do deflect, no way around that. If using a honed insert, pressure is required to make the insert cut. If the front edge is lifted by the pressure, then the back side dips.

    The only way to not have that effect is to have the mill slightly out of square. So have it as close to perfect you can get it, then by trial cutting, determine if there is a 'best' direction to mill that does not back cut. Then always mill the final pass in that direction.
    Thank you!
    Deflection, how you describe it, it is completely logical
    Do you have any idea how much (degrees, minutes) the mill should be out of tram to begin to eliminate the back cut?
    The effect will be less if i use a smaller face mill (50mm (2") instead of 63mm(2.5")?

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    I would not concern yourself with how many degrees to cant the head. There are so many variables that this needs to be done in a holistic manner. Tilt it over until the back side doesn't drag and call it good.

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    In my humble experience the off-tram method was given as advice to me years ago.

    The surface does look better when the front side of the cutter is a bit higher than the rear side.

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    Perfect tram.... you guys are funny!

    Maybe look at a different insert, smaller radius or smaller wiper flat, more positive any of the usual things that reduce the cutting forces. And then bias you finish pass to feed direction were there is the least evidence of the cutter heeling.

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    I have been working on FUNCTIONAL tram vs Indicated perfect tram when I face mill castings that I sell. I have learned that perfect indicated tram will not result in a functional ideal tram where both the scratch marks made by the leading and trailing edge of a face mill are faintly visible in a light slowly advancing finish cut. It is relatively easy to get leading and trailing scratches when taking heavy cuts fast. But when you slow down and just remove a couple to 4 thou I have to tram by taking a test cut and adjusting a time or two or so. Then the tram is produces the "perfect" finish only in one direction of travel. This is on a fairly tight Brideport with a good spindle. But no bearings either in the dovetails of the table or spindle are perfect. So, I first indicate very close to ideal tram and then take a few test cuts making small adjustments until the cut looks right. Works for me and produces a very flat straight face.

    Denis

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    Quote Originally Posted by dgfoster View Post
    Then the tram is produces the "perfect" finish only in one direction of travel. This is on a fairly tight Brideport with a good spindle. But no bearings either in the dovetails of the table or spindle are perfect. So, I first indicate very close to ideal tram and then take a few test cuts making small adjustments until the cut looks right. Works for me and produces a very flat straight face.

    Denis
    If you are canting the head to eliminate the trailing side drag, the surface is not perfectly flat....it will be slightly concave...
    Cheers Ross

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    Having recently scraped one of Denis's machined straightedges, the concavity is not even as much as 0.0002".
    Last edited by sfriedberg; 06-25-2021 at 02:22 AM. Reason: rephrased for clarity

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    Near perfect tram is met on a Blanchard or Arter machine to a few tenths, The same can be done on a mill but machining pressure can affect the tram. Good that sfriedberg inspected the concave (.0002) to prove it is, or not within spec limits. On the other hand, one might see if the scratching or crosshatch is acceptable to surface needs.

    Ot Op [It appears to happen in both directions.] That might prove the ops mill is in near perfect tram.

    Canting tram will only alieve the crosshatch one way because the lead or heal will be slightly forward.

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    If you tilt the head to eliminate back drag or heeling on the face mill the concavity of the cut will be impossible to find with the inspection equipment someone who poses this question would own. The concavity is lost in the roughness of the surface finish.

    Swing the head so the chips are thrown to the back of the machine. We ran left hand face mills on one machine rather than rescrape for chip control.

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    Think about it. If your facemill starts parallel under no load, ideal tram, and then enters the cut and deflects one way or another enough to drag the cutters through the other side, that will make the face concave.

    If instead the cutter begins the cut biased and deflects back to parallel such that the cutters skim over but do not contact the face, that will be flat, evidently within tenths.

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    bigger question, does it even matter if the trailing edge is still touching yet it is still perfectly flat within tolerance?

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    Quote Originally Posted by HuFlungDung View Post
    Face mills do deflect, no way around that. If using a honed insert, pressure is required to make the insert cut. If the front edge is lifted by the pressure, then the back side dips.

    The only way to not have that effect is to have the mill slightly out of square. So have it as close to perfect you can get it, then by trial cutting, determine if there is a 'best' direction to mill that does not back cut. Then always mill the final pass in that direction.
    i think the front edge might as well be pushed back and lower. depends on the feed.

    op, what are you doing exactly. how big a face mill, how big the workpiece, what doc, climb/conventional (or both), what insert geometry? and what is the actual problem? getting a crosshatch or the trailing side digging in erratically (vibration)?

    imo a "perfect" surface is crooss hatched, when face grinding as well.
    Last edited by dian; 06-25-2021 at 12:21 PM.

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    Quote Originally Posted by michiel View Post
    Thank you!
    Deflection, how you describe it, it is completely logical
    Do you have any idea how much (degrees, minutes) the mill should be out of tram to begin to eliminate the back cut?
    The effect will be less if i use a smaller face mill (50mm (2") instead of 63mm(2.5")?
    Depends on machine rigidity, cutter dia, cutter lead, cut depth, even insert radius, wipers and edge prep as all these contribute to deflection.
    Very, very general is a thou or two but it needs to be found by trial and error.
    One added benefit is longer tool life since the trailing edge is not rubbing or trying to take a super light DOC which puts additional heat into the cutting tool.
    Downside is the tiny, tiny dip or concave and the fact that optimal cutting is only in one direction.
    Unlike a Blanchard a nice cross hatch is sometimes not the best choice.
    Then there is the cutter drop once the front is off the part, unloads and the trail hits. This says close but not enough as the part is now not flat but dips on the end.
    So many thoughts, theories and PHD papers out there on this subject.
    Bob

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    A friend of mine works on an Auto line, full out production. They have a schedule dialed in where every other shift or so the cutting heads/spindles are stopped and “trammed out” several tenths. Apparently their facing inserts last a very, very, very long time and when it is time to rotate them the cutter head is “tranmed in” to the starting point and the cycle starts all over again.

    I found that very interesting.


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