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In the image, the table of dimensions only gives a resolution of 1mm; in the actual creation of such a sign, would these approximate values be used or their true values (within reason)?

For example, R is 3/40 of H which gives an exact value for some values for H (eg. 600) but not others (eg. 900)

Example

Of course, in real life I imagine the difference in negligible, but for things such as CAD are these rounded values used?

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3 Answers 3

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If you send the CAD file the CNC will do the best it can with the precision of the model's dimensions supplied.

With the drawing the machinist is free to use his/her wits and may just draw the base and construct an equilateral triangle from there and ignore all the height dimensions.

In general, giving higher precision on the drawing would be a sign to the manufacturer that tight tolerances are required. In this case there is no need for precision.

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    $\begingroup$ The tighter the tolerances, the higher the price... roadsigns made to 1/1000mm: really... $\endgroup$
    – Solar Mike
    Commented May 8, 2022 at 8:00
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When dealing with this type of information part of the answer will depend on what field of work you are dealing with. The tolerances for construction are not even in the same ball park as some one working with optics or computer circuitry.

Having said that I find personally try to draw as accurate as possible, but set visible dimensions to the degree of accuracy which is reasonable for what I am working on. This is also going to cause some grief at some point down the road where you will need to figure out with your experience what you will want to do or ask the designer how they want things to be displayed.

In my field a classic example of over dimensioning which is always done can be as follows:

The over all dimension is 9 mm. Dimensions are displayed to the nearest mm. You need to dimension to the mid point from both side for some reason (I did state this is classic over dimension in my field) You can easily draw the mid point in at 4.5 which is what you should do. When you go to subdivide the overall dimension you will either wind up with two dimension that show 4 or two that show 5 depending on how your system rounds. As a result, your subdimensions will not add up to your overall dimensions. You could cheat a bit and nudge the mid point 0.01 mm left or right and this would give you subdimension of 4 and 5 which add up correctly but does not position things truly in the middle. My field does not use the equal notation so if I am drafting for someone else I set the accuracy to 4.5 for those dimension then check with the designer when they review. If I am the designer and the drafter, I try to adjust the over all dimension to something that will subdivide nicely.

As Transistor stated in their answer. If you are sending it to a CNC machine A computer will just do whatever the numbers you supply it tell it to do until it finishes, or throws an error.

Below is how I would dimension and draw your 1200 sign. Not the alternate units [xx.xxx] is what is stored in the program and are shown for demonstration purposes only, I would not include them in a drawing.

enter image description here

This what I would wind up sending out on a hard copy.

enter image description here

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It's not about rounding, it's about tolerance. Specify tolerance. If you only write 1.00m then it will probably be interpreted as 1m +/- 0.005m. Don't make them guess.

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