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In my project I have to choose a form tolerances ( parallelism ) between the handle ( purple part ) and the part that sits inside it ( the grey part ), where the two parts slide on each other. But I don't know how to choose the right reference line on my technical drawing ( drafting ). enter image description here

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You have to show two reference lines for tolerance measurements. One is the longitudinal axis of the linked parts. The other one is the line through the center of the pin as shown below.

enter image description here

Does this answer your question?

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  • $\begingroup$ Kind of answers, I get it, you mean the two parallel planes should be referred to the centreline, but I don't get it how the other centreline comes into play ? the one that goes through the holes ? Usually we can choose only one reference line, but here you chose 2 ! $\endgroup$
    – Sam B
    Apr 11 at 19:52
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    $\begingroup$ @Sam B The simple answer is because you are drawing a 2D diagram The construction and offset tolerance in each direction must be measured to/from a reference plane. If you draw a 3D diagram, then you'll need 3 reference lines to tie the subject in space to eliminate the confusion on where the indicated measurements start. By convention, a measurement always starts from the reference point defined by the reference axes. $\endgroup$
    – r13
    Apr 11 at 20:35
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    $\begingroup$ @Sam B From the reference line through the hole, you can control the size of the hole and pin, the edge distances (28, 32), and work out the omitted dimension (from the reference line to the left end) if the overall length is provided. $\endgroup$
    – r13
    Apr 11 at 20:45

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