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Thread: Dimension Marking with Center Lines in Engineering Drawings

  1. #1
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    Question Dimension Marking with Center Lines in Engineering Drawings

    Hi,
    I am new to Engg Drawing and confused if I can Mark Dimension with Center Lines in Engineering Drawings like the attached file. Also, Can i add Positional Tolerance without any datum like mentioned in the diagram?

    center point.png

    Thank You.

  2. #2
    Administrator Kelly Bramble's Avatar
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    No... unless you specify how the center line is located on the part with dimensions and tolerances. Typically, this is done with datums.
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    Thanks for the Reply,

    If i remove the center-line marking ( 17.5mm ), How will the supplier understand the hole location, If i am not supposed to mark the Hole location from the edge.?

    also, Can i mark Datum on thin parts like this? the reference drawing supplier gave me do not have any datum markings

  4. #4
    Administrator Kelly Bramble's Avatar
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    Place a dimension from the edge (top and right side maybe) to one of the holes or identify the width and height dimensions as datums.

    Thin is not an issue... though if the part distorts in the free-state condition you might consider a note indicating the part is restrained flat.
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    Technical Fellow jboggs's Avatar
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    Your example is a very poorly done drawing in my opinion. Unfortunately it exemplifies a very common mistake, especially for inexperienced designers. The designer wants the hole pattern centered on the part, so he creates a centerline and adds the dimensions. The designer may want the holes centered, but it is a very rare case indeed in which the holes MUST actually be centered between two edges for the part to work correctly. In the great majority of cases, the only reason is pure aesthetics.

    And think of the guys in the shop. The only way they could accomplish what this drawing is asking for is to measure the actual distance between edges to create some theoretical centerline from which to then measure the hole pattern. A lot of wasted time.

    In my opinion, unless there is another feature to which the hole pattern functionally relates, the first hole should be dimensioned from a horizontal and a vertical edge. The rest of the holes are then dimensioned from the first.

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    Thanks For the Reply. I have made corrections in the diagram as per Your Suggestion. Please check it If i am Correct. It is a #rd Angle Projection.

    center point 2.png

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    Technical Fellow jboggs's Avatar
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    I will leave it to Kelly to comment on the symbology, but as far as I'm concerned the hole dimensions are much better.
    Two comments: (1) check your vertical dimensions. The holes are not centered.
    And (2), now there is no reason to even show a part centerline.

  8. #8
    Administrator Kelly Bramble's Avatar
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    Yes, the datum's specifications are functionally correct. Please note that the center line is not needed and the specifications are requiring a high degree of accuracy between the hole pattern and the left and bottom edge (datum B and C).

    There could be manufacturing capability issues.
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    Yes, The vertical dimension of holes are not center. So, the purpose of the centre lines are just to convey the shop people that it is symetric verticaly and horizontaly?

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by adirajck View Post
    Yes, The vertical dimension of holes are not center. So, the purpose of the centre lines are just to convey the shop people that it is symetric verticaly and horizontaly?
    No.. Center lines indicate that the feature associated is perpendicular to the given drawing view. Nothing more, common misconception.
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    Ok. So I got it. Center lines cannot be used as a Dimension referance and is used only if there is any featues at the middle of the part just to convey it is perpendicular to the centerline. Am i right?

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    I Have One More Doubt, For the below diagram i am only marking center to center distance and usual sizes. Is it acceptable not mark the distance from edge? How will the shop floor people intercept this Diagram?

    center point8.png
    Last edited by adirajck; 07-18-2019 at 01:02 AM.

  13. #13
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by adirajck View Post
    I Have One More Doubt, For the below diagram i am only marking center to center distance and usual sizes. Is it acceptable not mark the distance from edge? How will the shop floor people intercept this Diagram?

    center point8.png
    No, technically the holes to edges distances and tolerance variation is unknown. There is nothing to measure, verify nor reject should the holes be too close to the edge.
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    Technical Fellow jboggs's Avatar
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    You said the hole pattern is not centered vertically. It sure appears to be, which tells me that whoever has to handle it and install it will be confused and frustrated. And they will blame you. If a part can be installed incorrectly it will be. I would recommend that you either make the hole pattern centered in all directions, or make the offset large enough that it is visually obvious.

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    I discussed with My Supplier, they insist on marking the dimension from the center-line because the holes are not vertically centered. like the diagram I posted at the beginning of this thread.

    center point.png

  16. #16
    Technical Fellow jboggs's Avatar
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    So, you're going to let your supplier determine how to dimension a part they are making for you? I thought the customer was always right. What about the user? Don't they have a say here? Your part, as designed with the slightly off-center holes, is going to cause confusion somewhere. Be prepared for it, or find a way to prevent it. Enough on this.

  17. #17
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by adirajck View Post
    I discussed with My Supplier, they insist on marking the dimension from the center-line because the holes are not vertically centered. like the diagram I posted at the beginning of this thread.

    center point.png
    So, we've helped all we can.

    Most likely the part will get built but it is unknown how the fit, form and function will be based on the tight hole tolerances specified.

    Should your organization or supplier need dimensioning and tolerancing training I do provide training to ASME Y14.5-2009, ASME Y14.5-2018 and ISO 1101 current release standards. I'm certified by ASME at the senior GDTP level since 1997, have trained well over thirty thousand individuals, written and published nine books on dimensioning and tolerancing.

    Wish you the best,

    See: ASME and ISO GD&T Training
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