Here is a 3D model of a housing: 
How can I specify the position of the marked features ?
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$\begingroup$ Tip: crop your images before uploading. 80% of each is not needed for your question. $\endgroup$– TransistorAug 25, 2021 at 16:50
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$\begingroup$ Use ordinate dimensions $\endgroup$– jkoAug 25, 2021 at 17:22
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$\begingroup$ You need to tie the dimension "5.8" to the centerline or a reference line. You shall provide the vertical dimension for that feature too. $\endgroup$– r13Aug 25, 2021 at 17:37
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$\begingroup$ @r13 It's not clear to me, could you elaborate? $\endgroup$– Sam BAug 25, 2021 at 18:20
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3 Answers
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If you are talking about the position of the holes' centers, I'd just use the center lines of the holes with respect to the center line.
Regarding the 5.8, are you sure you need to give that one? Because in that scenario probably its best to give the centerline and countersink surface.
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$\begingroup$ Position of the holes and the depth of the pen hole, 5.8 I mean. Sorry I don't get what do you say $\endgroup$– Sam BAug 25, 2021 at 16:14
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$\begingroup$ I hope the sketch is making what I'm proposing clearer. I don't know if that's what you are asking. $\endgroup$– NMech ♦Aug 25, 2021 at 16:32
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This is what I meant in my previous comment.
All features need to be dimensioned and the dimensions tied to a reference line for measuring purposes. Otherwise, it is floating.
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2$\begingroup$ I don't like "c" here at all - should definitely be defining the hole axis not the surface. $\endgroup$ Aug 25, 2021 at 20:48
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1$\begingroup$ @JonathanRSwift This was to clarify the OP's question regarding my comment. There are many ways to dimension this element, my main point is, no matter what method, dimensions need to be tied to a reference line/point. A floating dimension does not meet the dimensioning standard. $\endgroup$– r13Aug 25, 2021 at 21:08
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@r13 and @NMech have given you great information on how to reference dimensions relative to main features of a part: like the centerline of a hole or relative to key surfaces.
When you design a part and have it built, it may not look exactly as you envisioned.
Machining leaves tool marks, surfaces may have groves or ridges (waviness), corners may be absolutely sharp or deburred, and holes may have grooves along the interior.
What you specify to control the accuracy of your part affects the cost to manufacture it. Polishing a surface to a mirror finish is labor intensive and very expensive.
Simple dimensioning of a part is useful. Consider a washer for a bolt, for example: make it so thick, with an outside diameter, and an inside diameter. How co-axial, or how close the centerlines of the two-diameters coincide may not be important. If it is, you need to reference one diameter relative to the other. Consider also the allowable variation in the thickness if it is bandsawed from rod stock.
There were (are) courses in college that taught (teach) Drafting and many books remain available: both in libraries and on-line. Simple dimensioning was the staple for many years and eventually got us to the Moon.
This link downloads a good article summarizing (GD&T) Geometric Dimensioning and Tolerancing. Just look at the pictures of the download as they will give you an idea of what your finished part may look like, given your design specifications. https://d2t1xqejof9utc.cloudfront.net/files/18904/Fundamentals_of_GDandT.pdf?1363288254
GD&T has it's advantages and disadvantages. I have encountered machinists (manufacturers) that only see dollar signs (when GD&T is used) as checking the part with only calipers makes it more difficult to verify the part meets all your specifications. However, the proper use of GD&T helps the designer to make sure components will properly fit together and function as intended given all variations on the tolerances of manufacture for all the various components. I found it to be a skill that is developed over time: from design a part to go here and do this to as you begin to design assemblies and components that all interact together.
Talking with coworkers and seeking ideas or direction will help guide your designs, talking with manufacturers will give you insight into their concerns, and learning through them all will help you grow as an engineer.
