Cycle 84 tap breaking and surface problems 3d milling

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Thread: Cycle 84 tap breaking and surface problems 3d milling

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    Default Cycle 84 tap breaking and surface problems 3d milling

    Hello friends from the forum.
    I bought in april this year a 5 axis cnc machine. the machine is build in 2002 with only 780 hours on the counter.
    It has a Siemens 840d controller with NCU 573.3.

    Only one of this type is build, the company who has made this machine also has gone bankrupt on this one.
    Because the bankrupcy the plc never completed like the way it must be completed.
    The last tweaks are done in a hurry so there where some problems.
    I had a Siemens service engineer for more then a week (lot of money) to complete it as good as possible.
    I made the first chips last weeks and the machine gives a nice surface and is fun to work with.

    But there are also 2 issues i can't solve.
    Rigid tapping with Cycle 84 breaks the thread.
    The machine does have a encoder on the spindle so Cycle 84 must be no problem.
    I try the tapping cycle in plastic so the tap won't break.
    But the thread isn't like it must be.
    I think it goes wrong when the spindle goes from CW to CCW then the Z axis doesn't slow down i think.
    I use the tapping cycle so the programming part can't be wrong the machine also doesn't come with a error.
    Are there some things i can change in the parameters from the machine?

    And when i program a large Cam file with a lot of smal lines there are also problems.
    When i use G60 then the machine starts Jerking, but that is normal i think because it is a exact stop.
    So then i try G64 then the jerking stops but the surface is very bad.
    There are differences in hight more then 0.5mm
    Also tried G641 Adis with different numbers but also no jerking but a very bad surface.
    I'm stuck with this one because i have only worked with Heidenhain and Fanuc.
    Siemens is new for me so i hope someone can help me with this problems.
    In fanuc i can use ai nano but in siemens you have a lot of different things to do.
    Also don't know if there is a parameter that must be changed.
    I don't have Cycle 832 on my machine is there a possibilty to install it?

    Sorry for my bad English hope some one can help, here in Holland the CNC forum is very tiny.

    Kind regards Branko

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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Rigid tapping with Cycle 84 breaks the thread.
    The machine does have a encoder on the spindle so Cycle 84 must be no problem.

    Interesting logic, but probably completely wrong.
    Sounds to me as though the drive to the spindle can NOT suport rigid tapping. That is very likely. Perhaps the mfr had hopes of getting it to work - and failed.
    Can you use thread milling instead?

    Cheers
    Roger



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Thanks for your reply Roger.
    Thread milling could also be a solution.
    But why doesn't it go like it supposed to be.
    The machine has normal Simodrives with a normal spindle motor.
    But like you said thread milling is also a solution.

    Because the thread cycle isn't the number one problem.
    If somebody has the solution for the 3d problem please let me know.



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    To run rigid tapping you MUST be able to drive the spindle very precisely. This can be done with a stepper motor quite easily, but then the spindle will be very slow. You can get reasonable sync with the spindle on a lathe for thread-turning with one encoder pulse per rev, but remember that there is all the momentum of the chuck assisting. I gather you have a conventional spindle motor - a Simodrive, not a stepper drive.

    To do rigid tapping the spindle must not only turn very stably at quite low revs, it must also be able to stop very abruptly and reverse very abruptly. An ordinary servo motor system with one encoder pulse per rev cannot do this in practice. So the Z axis motion and the spindle do not stay in sync, and either the thread is wrecked or the tap is broken.

    I imagine the company was hoping they could solve the problem, but found they could not. Hardly surprising: no-one else has managed to do it either - not with a single sync pulse/rev. My own opinion is that you would be wasting your time trying to make it work, and that you would be much better off accepting the lack of rigid tapping. I come from the (very large) Mach3 world, and none of us have rigid tapping either. But i do a lot of thread milling with very nice results.

    The jerking WILL be due to the exact stop. But it will also be due to the machine trying to follow very high acceleration parameters. That may be why goping into Constant Velocity mode is giving you such a bumpy surface: the motors cannot keep up with the required acceleration. We see a fair bit of that too. Drop the acceleration paramaters to 1/4 of what they are now and try again. If the finish is better, keep going with the adjustments. Yes, the machine WILL run slower - but the parts might become acceptible.

    Cheers
    Roger



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Thanks again Roger, so i guess i have to live with it.
    I have taken some pictures from the surface and the program.
    Maybe there are some people here on the forum with any suggestions.

    Here you see the part in my cam program Onecnc XR6 expert.


    Here you have the toolpath:


    Here is the beginning of the program:


    And here is the part, but as you can see it looks like crap!
    Feedrate only 5000 mm per minute (sorry don't know the inch rate)
    5000 isn't slow but also not extreme fast.




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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Hi Bensa

    Um ... It's a bit hard to see exactly what shape you have there. From one angle it looks as though there are bumps and ridges at the corners which are totally inexplicable, but from another angle those may simple be corners in the milling pattern. That is really hard to tell. I gather you were running a ball mill?

    However, there does seem to be a thudding big groove across the middle and out the back. That has got to be a serious bug in the program. No way could I explain that in terms of a hardware problem! If you had been machining mold steel instead of wax/plastic?, that might have been a broken cutter.

    Cheers
    Roger



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Yes i was running a ballmill and what you see is a height difference of 0.5mm (0.019inch i think) at the corners.
    So when the machine slows down from a x to a y motion then the height difference occured.
    I grabbed a piece of plastics and do a test program so don't look at the outher corners.
    But do i have to do some changes to my program, what do you think is my program correct?



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    So when the machine slows down from a x to a y motion then the height difference occured.
    The mind boggles. (That is a very technical engineering term ... meaning I am totally puzzled!)

    Ah - what direction is the cutter going at the front - from left to right or right to left? CW or CCW loops?

    Could you provide a hi-res photo of just a front corner showing the bump?

    Could you provide that bit of the program where the direction of travel changes from X to Y? 10 - 20 lines would be enough.

    Some ideas come to mind, but I need more data.
    Cheers
    Roger



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Hello Roger,

    Today it is sunday so i'm not at my shop at the moment.
    Tomorrow i have some work to do for customers, but when i have some time over then i make some pictures and provide you some more information.
    It can't be the program because when i run the same program on my other VMC with Fanuc 18m controller then there is no problem at all.
    But because the Fanuc is a little bit outdated the controller can't run faster feedrates then 1500mm per minute and that is realy slow.
    Thank you for helping me so far Roger.



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Hi Bensa

    In RL I am a research scientist: 'solving problems' is what I do. :-)

    One thing you could try, if you want, is to rerun that program on your 5-axis machine but at 1/4 the speed. Yeah, sure, really slow, but you only have to do it once, for the experiment.

    Cheers
    Roger



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    There where some issues here on the forum but finally i can post an update.
    The motion is in CCW and you see a picture with the red arrow.
    That is the point where the loop is round and he goes to the next loop.
    So the axis are breaking at that point and that is what you can see at the surface.
    The black loop i made with a marker inside the black loop is the toolpath.

    This is still the same part as you have seen before but now with (i hope) better quality to see.


    Here you have another one


    Just like you have asked try to slow down the feedrate.
    I have tried with feedrate 1000mm per minute.
    That is even slower then my fanuc does.
    There are some improvements but still not a good surface.
    I can't get it better then this.

    another one

    and this is the last one


    The last 2 pictures are from the program because the X to Y motion can't fit on one picture i made two.



    I hope you can do something with this information Roger, i guess it have to do something with the machine parameters.
    I also don't have the cycle 832 but that is probably because my machine is from 2002.



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Tricky, very tricky. Photos are limiting of course.
    Now, I am just guessing but for what it worth, looking at the three photos taken of a part machined at slow speed:

    It looks to me as though there is a definite dip in Z at the front left hand corner, where the motion goes from X to XY (circle) to Y. If this is so, it suggests that the Z axis is not keeping up until you hit the corner, where the steps are much smaller. This is very strange, unless there is a whole lot of backlash in the Z axis or the Z axis motor parameters are really, really wrong.

    There is at least a serious groove down the middle of the thing, but the bottom of the groove seems to match the Z height for the Y movement part of the cut. That suggests that the cutter went away from the camera up the middle and then came back on itself, cutting a little deeper on the retrace.

    There is a groove going off to the left from the middle. That looks as though it was done when the main loop program finished and the cutter was supposed to go off to the left out of the way - assuming that the left hand side had been machined to the correct Z height. But since it was too high, the cutter was dragged through the surface.

    When you machine at higher speed (1st 2 photos), the problems with the Z axis seems to be a lot worse.

    Just how free to move is the Z axis? Can it drill 6 mm holes OK?

    Does this match what you see?

    Cheers
    Roger



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Seems as if there is backlash in the Z axis


    - use a dial indicator onto the spindle nose face
    - wind the Z axis down to the zero mark on the indicator
    - wind up slowly........ how far does the readout count before any movement ?

    then do the movements in reverse

    Another area of concern could be the leadscrew end play ( not secured or adjusted correctly ), or the leadscrew nut
    with the amount of play....I'm leaning toward the leadscrew having end movement, thrust bearing etc )

    Last edited by Superman; 10-25-2016 at 08:19 AM.


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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Okay i have done some tests.
    But there is no backslash for sure.
    Also not any problems with the brakes on the Z axis.
    I disconnected the Z axis motor cable and put 24volts on the brake connector pins.
    Then i have to hold de ballscrew by hand otherwise the spindels goes downwards by gravity.
    So there is no problem with the braking mechanism.
    And the z axis also isn't stuck, it moves very smoothly.

    Then i make a test to measure the backlash in the picture below is my setup.


    The youtube video link let you see how the test goes.

    First i manual jog 0.01mm back and forwards and repeat it several times.
    This goes from 0 to -0.01 to 0 and to 0.01.
    You also can see this on the dial indicator.
    When i manual jog 0.1mm then the dial goes to -0.09 but i think this is because the dial indicator isn't line up straight with the z axis.
    In normal (non 3d) mode the machine is very accurate so i don't think this is a problem.

    I still think it has to do something with the parameters but wich one must be adjusted?
    The machine has only 780 hours on the counter so it is like new.



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    That's the sort of problem which requires a CNC expert on site for several days. Someone who can write test programs in g-code directly. Service technicians may not have the range of skills needed for this: they usually are only trained on the electronics.
    Dunno - sorry. Keep asking around for someone local. I am on the wrong continent!
    Cheers
    Roger



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Okay Roger, thank you for trying to help me.
    I think it have to do with some servo tuning.
    But thats a thing for a Siemens guy.
    If there is still someone with suggestions you are welcome.



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    OK, one last suggestion: check the PID controller for the Z axis: has the I term been adjusted? That's what looks after the slow 'catching up' in the servo driver. Tuning the PID controls often starts by setting the I term to zero while P and D are adjusted. Then the I term is wound up until follower accuracy is acceptable - without going into oscillations. If 'they' forgot to adjust the I term ... you could get those symptoms. If the I adjustment potentiometer is faulty - ditto!

    Cheers
    Roger



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    Do you have a simple documentation for that?
    Pid controller is a therm i never heard from before.
    So that's something new for me.
    I have a little bit knowledge of cnc electronics but Pid not.



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    Default Re: Cycle 84 tap breaking and surface problems 3d milling

    re PID control: this is a VERY standard terminology for any servo mechanism. The web must have scores of good references.
    I don't have any references to hand, but that is because I have been using the theory since the start of the 70s. Yes, it really is that old.

    Cheers
    Roger



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Cycle 84 tap breaking and surface problems 3d milling

Cycle 84 tap breaking and surface problems 3d milling