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I am quite new to mechanical engineering, but currently working on a project which I eventually want to sent to a CNC service for machining. Many services I found can manufacture to, e.g., ISO 2768-m tolerances, (+/- 0.1mm from 3mm to 6mm, +/- 0.2mm from 6mm to 30mm nominal sizes).

This range of tolerances are sufficient for most of my part, but for some bore diameters, locating features, and their relative position I require a tighter tolerance, which I should specify in a drawing (wherever those requirements are tighter than ISO 2768-m).

However, I really struggle to understand how to interpret the general tolerances (e.g., from ISO 2768). I attached a sketch of a simplified geometry I want to tolerance properly. On the left there is the ideal CAD model (no tolerances) and on the right is how I understand that ISO-2768-m general tolerances would apply to my part, i.e., whenever the produced parts is within the boundary on the right side, the manufactured part would be considered okay. I assume I might be very wrong about this interpretation and would highly appreciate if somebody could help to clarify if/where I am wrong. Thanks a lot!

enter image description here

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In general your idea behind tolerance are ok, but for the part you've drawn it's already way more complicated then you would think.

For example, you have a tolerance on the left hole diameter of +/- 0.1mm. However, there is also a +/- 0.2mm tolerance on the location of the hole (distance from the center of the part to the center of the hole). These tolerance can stack, therefore the location of the edge of the hole can vary with +/- 0.3mm horizontally and only +/-0.1 mm vertically. Moreover, there are also tolerance that can indicate how round your hole should be such that is not an oval, i.e. one side that is -0.1mm while the other is +0.1mm.

So again, the basic idea is good, but you should be careful with how tolerance can stack and how it can affect relative positions. For example the left hole can be placed with a tolerance of +/-0.2mm from the center line and the right hole with +/- 0.1mm. Hence, the distance between the two holes can vary with +/- 0.3mm. If you choose to define the distance between the two holes, the tolerance on this dimension is only +/- 0.2mm.

Hence, you should carefully consider which dimensions and tolerances are important for the final part/assembly.

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  • $\begingroup$ Thanks you! Regarding "[...] therefore the location of the edge of the hole can vary with +/- 0.3mm horizontally and only +/-0.1 mm vertically." Why is there a difference between the tolerance on the vertical and horizontal hole position? $\endgroup$
    – raisyn
    Nov 14, 2022 at 23:09
  • $\begingroup$ The hole is directly on the center line, this dimension is thus 0. The tolerance in your post are defined as +/- 0.1mm from 3mm to 6mm, +/- 0.2mm from 6mm to 30mm. You did not specify the tolerance for dimensions smaller than 3mm. $\endgroup$ Nov 14, 2022 at 23:35
  • $\begingroup$ @raisyn, You are missing a few dimensions in your drawing, for example: the left radius, how far is it from the right side; give me one number, don't make me calculate. Similarly, the position of the holes involves a calculation from three dimensions, as does the top and bottom radiuses. --- Finally, you don't need to specify the tolerances for each dimension, as that complicates the drawing, if you simply provide a note that all dimensions are to ISO 2768-m. --- The machine has its own tolerances, accuracy, which it should meet unless it's a very old machine. Use "M" for all tolerances. $\endgroup$
    – Rob
    Nov 14, 2022 at 23:39
  • $\begingroup$ Mind that ISO 2768 is replaced by DIN EN ISO 22081. Probably the reason was to get rid of the interpretation difficulties. For new drawings using ISO 2769 and 22081 is suggested. $\endgroup$
    – Tawhiri
    Nov 23, 2022 at 8:48

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