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Creating a drawing for a part that will be CNC milled. Should all of my dimensions come from a baseline on the part, like an edge or is it ok to use Centerlines?

See yellow highlighted dimensions below for example. 1.775 is from the centerline of the part. Would it be better to dimension it from the edge of the material as shown with the red arrow? Same for the other yellow highlight, 0.100 X 4.

Thanks for any advice.

Example drawing

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When you are designing you use the dimensions that make sense for designing. So if the important dimension is from the centerline then measure from the centerline.

However, it is generally better if you measure something you can easily verify. So in many cases you wouldn't measure to the centerline but rather to the mirror entity, a bit like prefering to measure diameter rather than radius. This has some consequences for the design though, which may or may not be what you need.*

Be aware that your design may need to be flipped which may cause some tolerance issues. Or did you plan for the manufacturer to countersink manually? Also hard corner inner holes are a bit hard to make with the mill, so round the corners.

* So measuring half the distance may in some cases double up distances your general tolerance. But that may or may not come to play check your relevant standard. Remember your document is also your acceptance criteria in cases where something goes wrong.

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  • $\begingroup$ This is almost exactly what I would have written. This drawing is for a human, and should 1) be readable and 2) be useful for verifying that the computer made the part to spec. It should show any key functional inspection dimensions and their tolerances. $\endgroup$ – Jonathan R Swift Oct 14 '18 at 21:53
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I think it's okay to refer to the centrelines, however if you refer to the centreline, and you refer the centreline to the edge, then it's not a good idea at all, because you pile on the tolerances, i mean if you draw the dimensions subsequently one after the other.

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    $\begingroup$ If you refer the edge to the centerline (not the other way round) there are no tolerances to stack up. For some components (e.g. axisymmetric parts) the centerline is the "obvious" datum to work from. $\endgroup$ – alephzero Oct 14 '18 at 21:08
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It doesn't matter where you draw the dimension lines for humans to read. The software knows where everything is, otherwise it wouldn't be able to draw it on the computer screen.

The CNC machine is actually going to move from one point to another following the order in which it machines the various features, and you shouldn't be concerned about that level of detail while you are designing and drawing the part. Whether the machine's software will work out the best (i.e. quickest) order automatically, or whether it will need a bit of human guidance to avoid trying to do impossible things, is something to think about after you have reached the final design, not before.

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  • $\begingroup$ This fails to take into account machine tolerance buildup $\endgroup$ – Carl Witthoft Oct 15 '18 at 18:41
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The correct answer is: it depends.

If you want to have a bolt-hole line up perfectly with some other part referenced to the same baseline, then you must reference the baseline.

However, your sample part appears to be designed to mount a smaller part to a square set of four holes. In this case, you start by referencing two holes' distance, say holes 1&2 along one side, to the baseline to place the part correctly. The distance between the two holes (in the orthogonal axis) are referenced to the first hole's centerline. Similarly, set the distance from the orthogonal baseline to holes 1 and 4, along the next side, to the corresponding baselines in the same manner. The other holes references all deal with the centerlines thus produced, so that there is the minimum tolerance error for all four holes relative to the mounted part's boltholes (which presumably are toleranced in its own drawing).

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  • $\begingroup$ Thanks to you all for great comments. Setting the drawing dimensions so that the part can be inspected easily by a human makes a lot of sense. Also, avoiding tolerance stack-up. I just was not sure if when the CNC machinist runs the part, having the dimensions set to a physical point on the part was an advantage. I now see that that is not the case. thanks. $\endgroup$ – RickH Oct 15 '18 at 23:06

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