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Alignment to offset holes?

Stephen McMillan
Hi all. First post. Sorry if it's a rookie question.

I want to do a Plan-Circle-Circle alignment.
The first hole is the x0 y0 origin.
The second hole has some given coordinates, say x8.0, y4.0.

I don't want to have to calculate the angle and use the "Offset angle" in the "Rotate to" field.
I also don't want to use an iterative alignment.

Any ideas?

Thanks,
Steve
  • Both are correct depending on the nominals, basic or open. If the offset value is basic while the other axis is open dimension then your auto nerds are right. The print will define the datums and hence the proper way to build the gage or your alignment.

    TK
  • Both are correct depending on the nominals, basic or open. If the offset value is basic while the other axis is open dimension then your auto nerds are right. The print will define the datums and hence the proper way to build the gage or your alignment.

    TK


    Not to belabor a point, but both can't be correct if one method yields a good part but the other says the same part is bad. And the idea that both axes are valid for the construction of the datum since they're both basic I don't agree with - basic dimensions are also used to define exact locations, most notably when defining a gage line.
  • in reply to RandomJerk
    Exactly, if you rotate you'll be in, if you offset, you'll be out. That's why, your method is appropriate for a single axis basic, but not when the basics are in both axes, it's the same as calling for the angle. Because those 2 basics define two legs of a right triangle.
  • in reply to RandomJerk
    Each is correct, for different circumstances, is what he is saying.
  • 1) If you were to build a hard gage, the slot you would cut for the pin would be parallel, not slanted by the calculated angle.
    2) If the offset were to be to an angle, why wouldn't the offset be listed as a basic angle? Isn't the intent of the standard to be clear, and this is not clear.
    3) If the hole weren't offset, and actually in line, would you be suspicious if you reported X and Y and Y was not zero? But not suspicious when the basic wasn't the basic value?
    4) I'll bet a dozen doughnuts the intent is the datum hole is "perfect" in the perpendicular axis, and I'll be fatter than I am now.
    5) The figure I showed from the standard indicates the distance is exact, the diagram doesn't show it being rotated back.

    These are my arguments off the top of my head. I'm sincerely interested in factual based reasons as to why the rotation is the correct method.
  • in reply to RandomJerk
    RJ, If your part print shows the datum feature (tertiary) as a slot not at an angle the offset line is the correct method. That float your boat?
  • TK, what is your basis for your statement? And where is your angle? You're using an angle, but no angle is indicated. I'm very serious here, if I'm wrong I'll admit it again (I had to admit I was wrong when I was schooled on this topic many years ago and learned this method), that's why I'm asking for reasoning. What I've seen is that the rotation method is tribal knowledge, it's the method used by most programmers because they encountered it before they knew how to do an offset line, which many people still don't, and this is the method they figured out (me, many years ago), or they were trained that way by the same.

    Where did you learn that this is the correct method?
  • in reply to RandomJerk
    1) You wouldn't use a pin, you would use a diamond. For basic at both axes, the diamond pin would lock rotation perpendicular to line BC.
    2) The 2 basic dimensions define an angle, because they are the legs of a right triangle.
    3) Yes, Y would be zero, because it's inline with BC. Inline with BC allows the hole to deviate along line BC without affecting the rotation of the part.
    4) I would take that bet, if I ate donuts.
    5) The figure shows what the DRF looks like, not how you got there.
  • TK, what is your basis for your statement? And where is your angle? You're using an angle, but no angle is indicated. I'm very serious here, if I'm wrong I'll admit it again (I had to admit I was wrong when I was schooled on this topic many years ago and learned this method), that's why I'm asking for reasoning. What I've seen is that the rotation method is tribal knowledge, it's the method used by most programmers because they encountered it before they knew how to do an offset line, which many people still don't, and this is the method they figured out (me, many years ago), or they were trained that way by the same.

    Where did you learn that this is the correct method?

    An angle is used because it's basic in two axes. If it was basic only in Y, you'd have a diamond parallel to the DRF (like a slot parallel to the DRF). With two basics, you have a right triangle, (functionally equivalent to a slot that is parallel to line BC. The diamond would be parallel to line BC. Thus, offset rotation.

    Your method is entirely correct when the basic dimension is only controlling the offset of C in a single axis. Which would normally mean your C feature is a slot or a target point. It is sloppy to attempt to control a circular hole in a single axis, because a circle is a closed feature where every point is equidistant from the center point. A circle/cylinder should never be assigned a single axis (even though, it happens too often).
  • in reply to VinniUSMC
    Well said, Vinni.

    RJ, we're not saying you're wrong, only that it depends on the print. Tell you what, if I had to correlate my CMM readings with the gage you describe I'd be forcing the the axis (offset line) to do so. I haven't seen a print so I can't determine if the gage is correct to the print so we haven't been discussing a known hence the "it depends" answers. Vinni takes more time than I have to spend here hashing what I know (started CMMs 1980s) so read it again, he said it right.

    TK
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