tomtomtom Posted April 8, 2012 Posted April 8, 2012 Hello all I am using autocad 2011 (fairly new to it). I am having trouble taking a dimension from a node to the edge of an arc. Attached is a diagram of what i am trying to achieve, i have also attached my actual dwg file. If you look into the dwg file and zoom in you will see that my arrows do not point to the edge of the arc. Can anyone shed any light on this? Many Thanks David. Dimension Problem.pdf Dimension Problem.dwg Quote
eldon Posted April 8, 2012 Posted April 8, 2012 If you look into the dwg file and zoom in you will see that my arrows do not point to the edge of the arc. After you zoom in, do a Regen. I am having trouble taking a dimension from a node to the edge of an arc. Use the Osnaps "Node" for the Point, and "Perpendicular" to the edge of the arc. Quote
tomtomtom Posted April 8, 2012 Author Posted April 8, 2012 From the pdf diagram you can see that there is a line that i have drawn from the centre of the arc to the node. This line intersects the arc. The dimension i require is the distance between the node and the point where the arc and line interects. How do i obtain this measurement. Sorry if i have not expressed myself clearly. Many Thanks David. Quote
eldon Posted April 8, 2012 Posted April 8, 2012 Why have you drawn the line? If you must draw the line, then the dimension (distance) is from the point (node) to the intersection of the line with the arc From the geometric properties of a circle, any line perpendicular to the circle must pass through the centre of the circle. So draw the dimension from the point (node), then perpendicular to the arc. This gives you the dimension (distance) that you want, without drawing any temporary lines. Quote
tomtomtom Posted April 8, 2012 Author Posted April 8, 2012 Your are correct. I have enabled Node and Perpendicular on Osnaps and then dimensioned from the node until i saw the perpendicular symbol. When i zoomed in as much as i could the arrow that should have been pointing to the arc was in fact pointing above - why is that. In the attachment (dwg file) i get a dimension of 0.0203 - But i think this is incorrect. Why does the arrow not point directly at the arc?? Thanks. Dimension Problem.dwg Quote
Dana W Posted April 8, 2012 Posted April 8, 2012 Because the arc is represented by a bunch of straight line segments on your screen, these segments may or may not be exactly on the true curve of the arc when you are zoomed way way in. A regen may bring the graphic representation of the arc closer to the dimension depending on your zoom level. Screen graphics software has to compensate for hardware pixels in your monitor screen. The dimension is pointing to the exact location of the arc mathmatically, and will plot fine. Quote
eldon Posted April 9, 2012 Posted April 9, 2012 When i zoomed in as much as i could the arrow that should have been pointing to the arc was in fact pointing above - why is that. In the attachment (dwg file) i get a dimension of 0.0203 - But i think this is incorrect. Why does the arrow not point directly at the arc?? To take your points in order: Did you do a Regen after you had zoomed in? The dimension is correct. You dimensioned to a different point to your first attached drawing. Did you do a Regen after you had zoomed in? Quote
tomtomtom Posted April 12, 2012 Author Posted April 12, 2012 Hi Eldon If we just focus on the first point on the left which has a dimension of 0.0190. When I dimension this point to the arc i get a value of 0.0190. Then if I set the precision to the max i.e. to 6 decimal places then this dimension becomes 0.018982. I then zoom in as far as I can, regen and able to zoom in further. Then if i drag the top of the arrow down towards the arc, the dimension then becomes 0.0193339. If what you say is true that is the dimension measured from the point to the arc at right angle then my dimension should be the same regardless of what the precision is? I hope this makes sense. Attached is my dwg showing my dimension value of 0.019339. Thanks for your input. Regards Dimension Problem.dwg Quote
JD Mather Posted April 12, 2012 Posted April 12, 2012 Then if i drag the top of the arrow down towards the arc... Sounds to me like you might be dragging the defpoint rather than (or with) the arrow. Quote
eldon Posted April 12, 2012 Posted April 12, 2012 I am not sure why you are moving the dimension after it has been drawn. You must trust the Osnaps accuracy. The dimension that you have dragged is no longer perpendicular to the arc, because you have eye-balled it in, therefore the dimension that it shows is nonsense. In fact if you draw a line from the upper end of the arrow line to the lower end of the arrow line, and then extend this line towards the centre of the arc, it now misses the centre point by nearly 2 units. In the picture, I have copied the one node and the arc, and dimensioned it showing 8 places of decimals. So the dimension of 0.0190 is correct when you take four decimal places. I think that your problem is with your usage, rather than with the dimension. DO NOT drag dimensions around willy-nilly to agree with something that you appear to see on the screen. Trust the Osnaps Quote
eldon Posted April 12, 2012 Posted April 12, 2012 I have just zoomed in to your dimension at the top end, and your dimension is no longer to the node. Quote
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