【转帖】basic gdt question

yang686526

basic gd&t question
anyone,
i'll do my best to describe my question.  is the primary reference in my "position" symbol gd&t box the plane on which the feature, "say, a hole", resides?  or is this place for the first reference of lateral or vertical position?  when is the residing plane included in the gd&t box?  i have a book on gd&t and i'm trying to learn but i see both as examples.
thanks
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the easiest way to re  
to expand on what looslib stated, the primary reference will normally be the surface that mates with the assembly or another part.  the second reference will be another plane or feature that you consider to be the most important in locating the part, and which would limit it's movement in one or more directions.  it could be an adjoining plane, a bolt hole, a pin, etc.  the third reference will be an additional feature required to keep the part in position by preventing it from sliding in another direction or rotating.
phil
but also, re  
it would be helpful if you could describe the particular geometry you are interested in.  a picture would be even more helpful.
what is important is order of engagement, regardless of geometry.  you can have a hole as a primary and plane as secondary if the part is firsty constrained on the hole and then touches off on the surface.
say i have an aluminum cube with a 1/4" thru hole in the front view.  does the surface that i actually drill on become the first reference in my gd&t?  am i telling the mechanic which surface/datum the feature is located on first?  i'm not stipulating a, b, c, etc...i am just wondering if the primary reference is the datum on which the hole originated.  or...is the primary reference a surface that locates the hole vertically and/or horizontally.
i haven't found a way to attach a picture...so i'm relying on my verbal skills.
depends on the situation.  do you have to clock or locate this hole to any other holes?  is it more important to maintain orientation to a different surface or feature than the face the hole is in?
you make datums however you like. for my parts, i make the resting surface (opposite from drill entry - "back" in your example) datum a. this is becuase i am always wanting perpendicular as most important. now i can make "bottom" and "right" the other two datums...
rookie,
may i suggest you look into a gd&t class?  the ability to use gd&t fluently in your designs can prevent costly mistakes and some embarrassment at staff meetings.  i know some community colleges offer classes in the design programs.
ah rookie, rookie, rookie!  i remember asking the same question.  looslib was pretty much right.  your datum reference frame (the 1, 2, or 3 datums referenced in the feature control frame (fcf) establishes how you locate the workpiece for inspection.  what you need to do is dissociate the two things in your mind...the location of the hole from the datum reference frame.  
using the cube example you gave, and assuming that you want to fully fix this part in space, you will need 3 datums.
the primary datum will correspond to a feature which mates with another component and most greatly controls the location and orientation.  if the cube is in its operating condition as described, then it will rest in use on the bottom face.  that makes the bottom face your primary datum feature; call it datum k.  affixing the workpiece to datum k (simulated by the inspection plate) locks three degrees of motion; up/down (along z-axis), and rotationally about two axis (x and y). oh yeah, 3 point minimum contact for a primary planar datum.
the next datum feature will further constrain two degrees of motion.  let's make the left face of the cube datum feature g.  now  two points (minimum) on the left face of the cube will abutt an angle plate mounted perpendicularly on the inspection plate.  that eliminates motion in the x-direction and rotation about the z-axis.  datum g is your secondary datum.
the final datum needs to eliminate the 6th degree of motion; motion in the y direction.  so for your cube, it will be the back face (opposite the drill face); datum a.  you would then push the workpiece into (min 1 pt) contact with another angle plate that is mutually perpendicular to the inspection plate and the other angle plate.  this keeps it from moving along the y-direction.  
your workpiece is now fully constrained with the k/g/a datum reference frame.  
i am not in complete agreement with looslib because of the reference to contacting two locating pins on the secondary datum feature and a locating pin on the third datum face.  this indicates either line or point contact at a predetermined location, which means datum targets rather than full datum features.  when you're starting out, keep it as simple as possible.
follow heckler's suggestion, and get some professional help (training that is).  be careful though, there's a lot of bad training out there, and some outstanding as well.  get references before you sign up.
jim sykes, p.eng, gdtp-s
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