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Thread: Gd&t for a radial hole

  1. #1
    Associate Engineer
    Join Date
    Mar 2017

    Gd&t for a radial hole

    I have an application where a hole drilled radially into a tube needs to be positioned relative to another hole drilled radially into the tube. The radial offset of the two holes are critical, however, the axial position of the holes along the length of the tube have forgiveness as shown in the attached very simplified drawing.

    I wanted to use a multiple single segment positional tolerances to specify this to assure usable parts are not thrown away; even though I donít think it will be an issue where the axial position of the hole will be tougher to hold than the radial position. I believe my first GD&T callout underneath the ō.75 dimension satisfies my requirements of a tighter radial control on the hole while allowing leniency in the axial direction.But Iím wondering if dropped the tertiary datum C from the upper segment feature control frame, wouldnít this callout still be equivalent in this case?


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  2. #2
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    First, let's examine your design from a DFM point of view. Chances are that it will be CNC machined and the tightest tolerance indicated is dia.030 or simplified +/- .015. Assuming adequate wall thickness – this is easy for a CNC machining process. In fact these features will be as-built way better than position tolerance of dia. 030.

    Additionally, you might consider including a spot face on both hole features as manufacturing is going to put one on to facilitate the drilling operation. Drill bits need a starting surface perpendicular to the drill axis or a start drill point to drill correctly and not break the drill bit.

    Now let’s look at the specified tolerances.

    Datum A is the axis derived from the outer diameter, Datum B is the plane surface on the right side of the left view and Datum C is one of the holes.

    NOTE: Datum A is specified correctly if you want the outer diameter to be that datum feature, however I’m not a big fan of your drafting method. I would move that Datum and anchor and associate it to the size dimension for clarity. Personal preference…

    For the order of precedence A | B | C Datum A is an Axis + two perpendicular datum planes, Datum B is the last Datum Plane. Datum C stops the rotation of the two datum planes defined by Datum A. The defined datum origin is at the intersection of Datum A and Datum B – on the right side of the left view.

    All definitions provided on your engineering drawing work just fine if not cumbersome… Personally, knowing what CNC machining process capabilities are – I would simply define a single FCF of position tolerance Dia.030 relative to A | B | C. Then calculate the inner boundaries of the two hole features cumulative of size and geometric tolerance to determine if they meet your fit, function and form requirements.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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