Tolerances


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Thread: Tolerances

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    Default Tolerances

    Heya all. Im struggling with this one. Whats going on is i cut two parts that are both 15x20 out of mdf. When i put them to next to each other, they are not the same size. One is wider by .5mm or .019". Is this within tolerance?

    In my motor tuning and setup screen i have turned my velocity to 300. I have also turned my acceleration to 5. This was recommended by the manufacture help desk. It did not help.

    They say this is is within normal cnc tolerances. I am having a hard time believing this. Any thought would be appreciated.

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    Default Re: Tolerances

    Could cutter flex be the issue?

    cheers, Ian

    It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!


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    Quote Originally Posted by aarggh View Post
    Could cutter flex be the issue?

    cheers, Ian
    Its a 3/8 carbide compression bit.



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    Registered Jim Dawson's Avatar
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    Default Re: Tolerances

    You state that there is 0.019 difference, but is either part actually on size? That amount of error seems to be a bit excessive, but it depends on your machine. The accuracy is dependent on the stiffness and positioning ability of the machine. It may very well be all your machine is capable of. Maybe some mechanical and motor tuning adjustments would be in order.



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    Default Re: Tolerances

    Tolerances are something you need to set depending on the use you plan to make of your parts. It might be that a half-mm is well within a reasonable tolerance for something like a picture frame, where you expect the picture to fit rather loosely and budget for a little space around it. On the other hand, if you're making a clock out of MDF, then that same half-mm could be a big deal. It's up to you to establish reasonable tolerances for each project you do, then use machines which can achieve them.

    That said, I'd expect a well-made CNC router to do a little better than that when cutting two identical parts from the same G-code file. Establish a zero point you can check for the axis that seems the worst, run the part again, and see if it returns to that exact same point when commanded to. If not, you're either losing steps or there's some mechanical problem with your router, like a coupler that's slipping. If it does return exactly to zero, it could be a hold-down that's not holding the part firmly enough.

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    Default Re: Tolerances

    It can also depend on how the parts were run. For example if 2 "identical" parts are run from the same piece of wood, but nested with one rotated 90 degrees, then on one part X is responsible for height, but on the other X would be responsible for width details.



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    Default Re: Tolerances

    Still fighting this issue. It seems to be something with the stepper motors and setting my steps per unit and setting up the gantry square. Squarenesshas been quite a bear to do. It seems good now. Ill keep pligging away. I will also post any findings.

    To answer awerby, im cutting cabinet parts. If this machine can not cut duplicate parts then it is useless. I need it to be fairly accurate.

    The machine seems to be a well made machine from a reputable company. It is a 5x10 with vac table. It is a cost effective machine compaired to the other machines in its class. The onlything it does not have is a tool changer, which i would LOVE to get. (Any advice on that would be amazing). But one thing at a time.

    Thanks again



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    Registered Jim Dawson's Avatar
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    Default Re: Tolerances

    As an experiment you might try cutting the parts in two passes, make a rough cut leaving about 0.030'' for a finish cut. If the two parts come out the same or at least closer, this might give you an indication if the machine a ''springing'' a bit. I would disagree with the vendor help desk, 0.019'' is not in the tolerance range of good machine IMHO. I would be very disappointed if my 4x8 didn't hold +/- 0.001''



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    I will try this. I cant do much for a few days. Im in charge of a photo shoot of our products. Eeew



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