face mill cuts little bit on back side: normal? - Page 2
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  1. #21
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    Perhaps I am wrong, but assuming the tram is "perfect" and "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.", according to HuFlungDung, then I would think that the front side of the mill would cut a bit high and then the back side would cut almost dead flat. And that would leave a better surface even if the surface finish is a bit questionable due to opposite arcs.

    I keep my mill well trammed and always like to continue a facing cut until the rear edge of the cutter has cleared the surface being cut. That way I seem to get the best flatness. And yes, there are arc marks going both ways; at least when the tram is good.

    If the tram is off, you are almost certain to get a concave surface either from the leading edge or from the trailing one.

    In theory the depth of a concavity is given by two factors: the width of the cutter and the angle of tram error.

    w = cutter diameter
    a = angular tram error in degrees
    d = depth of concavity at it's center

    d = 0.5 x w x sine(a)

    For a 1" diameter cutter and a 1 degree tram error the theoretical depth would be

    d = 0.5 x 1 x sine(1)
    d = 0.0087"

    Or perhaps a more practical example would be with a 3" diameter cutter and a 5 minute tram error (1/12 degree):

    d = 0.5 x 3 x sine(1/12)
    d = 0.0022"

    I don't know about you, but those numbers tell me that I want to keep the tram error as close to zero as possible. Even a smaller tram error can easily produce a concavity depth of a full thousandth of an inch.

  2. #22
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    You can switch to horizontal milling. Large helical cutter in full contact with work, no angles. Perhaps angels, if you do something wrong. Face milling is sooo overrated.

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    you mean side milling? there the problem is chatter as soon as you exeed some shalow doc.

    i still dont see how a square insert will produce much up-force. imo it rather makes the front dig in. and the quention remains: what is the problem? is it "real" (can you feel it) or is it "optical" and maybe can be seen only at a certain angle (like very often happens on aluminum)?

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    Some Mills have the tram built in the machine with gibbs adjusting the needed clearance to the machines built to the right angle to the table.

    Note that he uses a positive insert cutter.

    First Chips On The Giant Cincinnati Milling Machine. - YouTube

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    Quote Originally Posted by dian View Post
    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.
    What I am doing exactly: running a face mill of 2.5" over a piece of C45 steel (that is carbonised) of 2 inches wide, using a face mill with SEKT carbide inserts which is suited for light mills (double positive, it is said). Cutting speed is 100m/min, slow feed, cutting depth of 0,5mm (sorry, difficult in inches, i think this is 20 thou) My mill is a Aciera f3 machine in fairy good condition with the gibs pretty tight.
    The problem is a crosshatch pattern which does not look appealing. Or, on the back stroke it is rubbing instead of cutting which leaves a dull instead of shiny finish.

    Thank you all for your answers, this was very informative.

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    Quote Originally Posted by dian View Post
    you mean side milling? there the problem is chatter as soon as you exeed some shalow doc.

    i still dont see how a square insert will produce much up-force. imo it rather makes the front dig in. and the quention remains: what is the problem? is it "real" (can you feel it) or is it "optical" and maybe can be seen only at a certain angle (like very often happens on aluminum)?
    If what you propose is true, then a properly trammed spindle would never exhibit a back cut. They do, so therefore I think your theory is incorrect.

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    well, the last piece of 7075 i milled with a square insert didnt have any crosshatch. (→ insert, feed, speed.) at least thats how it apears to me. i sure do get crosshatch on other occasions.

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    Quote Originally Posted by michiel View Post
    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.

    Perhaps the only way to solve this problem is to get a mill cutter that is large than the part so the following side of the cutter will not hit.

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    Quote Originally Posted by michiel View Post
    What I am doing exactly: running a face mill of 2.5" over a piece of C45 steel (that is carbonised) of 2 inches wide, using a face mill with SEKT carbide inserts which is suited for light mills (double positive, it is said). Cutting speed is 100m/min, slow feed, cutting depth of 0,5mm (sorry, difficult in inches, i think this is 20 thou) My mill is a Aciera f3 machine in fairy good condition with the gibs pretty tight.
    The problem is a crosshatch pattern which does not look appealing. Or, on the back stroke it is rubbing instead of cutting which leaves a dull instead of shiny finish.

    Thank you all for your answers, this was very informative.
    Post a few pix of the cut face.

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    I've never seen tram done in degrees.
    It is done in toe to heel rise and to the machine slides, not the tabletop.
    IMO tramming anything to a table top is asking for problems and tolerance stack up. Machines cut by moving on their slides.

    Along with deflection one has to consider surface finish generated by the front only working.
    The greater the peak to valley produced the more tilt one will need so as to not hit on the trailing so feed per tooth and insert corner geometry come into play.
    Measured static toe to heel will never be what is happening during cutting so one has to try different settings if one wants to dial this in.
    Too little is bad looking or torn, too much and concave which is just bad or maybe worse even though it "looks" nice.
    Added here is that bottom flat on a SEKT and cutter pocket, insert indexing. These are famous for works one time and then not the next and then back to good with a new index.
    One can see this in the surface finish traces but how many have a big dollar gauge to do this. Not a Ra checker but showing the map of generated surface.

    On a normal cnc mill one is going to do both directions so as equal as possible in both directions under load is the only working setup.
    Bob

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    another strategy might be to let the trailing edge do the finishing. the effect of the mill comming off the work will still be visible (sometimes). actually the face mill is too smal.

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    This seems to be about cosmetics so how about a finishing/burnishing tool that's like a face mill but instead of cutters it has hardened balls or rollers that mash down the surface irregularities?

    Might even give a structural benefit like shot peening, building in compressive surface stresses.

    Perhaps it could actually peen if it had springs and ramps to create impacts.

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    Quote Originally Posted by michiel View Post
    What I am doing exactly: running a face mill of 2.5" over a piece of C45 steel (that is carbonised) of 2 inches wide, using a face mill with SEKT carbide inserts which is suited for light mills (double positive, it is said). Cutting speed is 100m/min, slow feed, cutting depth of 0,5mm (sorry, difficult in inches, i think this is 20 thou) My mill is a Aciera f3 machine in fairy good condition with the gibs pretty tight.
    The problem is a crosshatch pattern which does not look appealing. Or, on the back stroke it is rubbing instead of cutting which leaves a dull instead of shiny finish.

    Thank you all for your answers, this was very informative.
    Steel is certainly going to deflect the cutter. If this is the Aciera, it does not seem especially more rigid than a bridgeport format mill.

    I've got lots of experience with aluminum, but hardly enough with steel to "know" much other than it is definitely going to require some force. Things I don't "know" but would think about are: offsetting the cutter to one side or another to see how that impacts finish; try different depths of final cuts at varying speed/rpm to see how that affects the finish; if you aren't using cooling, try pointing a blow tip at the cut to see how that affects the outcome. I've got a mag base with a thin Cu tube on a valve that I sometimes use to cool an operation. It was indispensable when I had to work some Ti. I personally wouldn't consider 0.020" / 0,5 mm to be a final cut on a part I'm trying to make attractive.


    Quote Originally Posted by Noah Katz View Post
    This seems to be about cosmetics so how about a finishing/burnishing tool that's like a face mill but instead of cutters it has hardened balls or rollers that mash down the surface irregularities?
    From this, I read "buying tools, tool change, setup, more operations". If it's a small run without chance of landing big contract, probably just looking to get something coming off the mill that looks better.

    I get it ... I hate handing things to customers which I think could be done better. I rehabbed an electric motor for a print shop and cleaned the thing off. It looks awesome, makes them think I really care, and best off: doesn't make my wife's car get dirty.

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    Quote Originally Posted by technerd_chris View Post
    From this, I read "buying tools, tool change, setup, more operations". If it's a small run without chance of landing big contract, probably just looking to get something coming off the mill that looks better.

    Are you saying that what I propose would take too much time?

    If such a burnishing tool did exist, I'd think it would be a lot quicker to throw it in and make a pass than messing with the tram.

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    and even quicker to take it to the sanding plate.

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    Quote Originally Posted by technerd_chris View Post
    I get it ... I hate handing things to customers which I think could be done better. I rehabbed an electric motor for a print shop and cleaned the thing off. It looks awesome, makes them think I really care, and best off: doesn't make my wife's car get dirty.
    This something I had to emphasize all the time. "It takes a trained person with precision tools to tell if a part is right. The kid who sweeps the floor can tell if it looks like sh*t."

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