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unit system question

unit system question

(OP)
Hi All,

I'm pretty new to draftsight and was wondering a few things that I was hoping could be clarified.

My task is to paste my design onto another design. However I am having some problems with scaling/dimensions. I have made a design in with blocks unit format in millimeters. Its a 6.35mm x 6.35mm square with a design in it. I want to then paste it into another design in another file with the blocks unit format in inches. However, the design in inches seems to be much larger than the design in mm.

I was given the design in inches by my professor, its supposed to be a silicon wafer. When I tried measuring the radius of the wafer, it says 38100. Is this really inches? Doesn't make sense to me since wafers are not 38100 inches in radius.

Am I reading something wrong? Or am I misunderstanding something with the scale? Any help would be highly appreciated! I'm really confused here.

Kevin

RE: unit system question

CAD does not know units. It knows one of something, anything will do - angstroms, inches, miles, fathoms, etc. One unit in CAD is simply one unit - it has no associated length until you dream it one. So, if you draw something that is one unit long, you can think of it as one inch, one meter, one millimeter, one light year, etc. The CAD program does not know or care. If you use a dimension style that assumes that one unit is one inch, and have drawn the object thinking that one unit was one inch, it will correctly dimension the object without a scale factor needed. You have drawn something thinking that one unit was one millimeter. Your professor has given you a drawing that he drew using the assumption that one thousand CAD units was one millimeter, or something else that you will have to figure out. Once you know what he drew his in, you can scale yours as you insert it so the two human scale assumptions match.

RE: unit system question

Ok, you're dealing with a units problem, just as you correctly state.

If its simple in/mm, recall you'll need to scale one or the other either up by 25.4 or down by the inverse (1/25.4) However, 31800 seems quite large in mm or inches, so I'd get some input from the Prof about what scale he used ( whether he drew to 1:1 in ANY units) and whether he was thinking (post above (IFRS) is spot-on) in mm ? Its possible he had values set in feet, and you're getting large mm's trying to get feet down to mm. If the program he used (was it native ACAD or something he converted out to dwg?) assigns units to its output, he may have to re-write the output to unitless, after re-sizing to either mm or inches (depending on what you need) .

I face this frequently, but on a much larger scale. The client is thinking in inches, even tho his inputs are in values he thinks of as "feet". He'll input 120 for the length of an offshore platform leg, write out stuff we are told is in inches, it's in feet, and we have to do the conversions. What we'll do is scale down by 25.4, and if necessary, again by 12 (remember, 12" = a foot). Again, just as the post above (IFRS) states, its all in the units you're thinking of at the time of input. Then things like DIMSTYLES and LTSCALES are set to compensate, and when the drawing is shared, this has to be accounted for by rescaling inserts, etc.

31800mm = 1499.995in, and = 124.9996 (125) ft.

Encouraging note, you have a clear idea of how big you're working with, (6.35mm x 6.35mm) so you can use that reference to scale the inserts up and down by 25.4, 12x or .0394, depending on scaling the insert bigger or smaller when bringing it in to your file. One thing that might help- if you know the INTENDED length of an item in the insert file, use that as a reference for your scaling. Since you've correctly defined a 6.35mm x 6.35mm square, stay with that and scale (possibly "reference" command option) the insert accordingly, using the clues I've shared.

Seems clear as mud, but once you've run the drill, you'll see how it all works.

Good luck, and post back with results ?

-C.

RE: unit system question

Remember, in ACAD, you don't multiply by 1/25.4, you multiply by 10/254


RE: unit system question

I use 1.0/25.4 - actually it's (/ 1.0 25.4 )

RE: unit system question

Buggar-
winky smile
Yes, nice point. Goes to my basic message - scale up or down, as needed. Remember too, tho, that sometimes you have to handle feet-inch-mm's ...
Thanks !

RE: unit system question

Just to add more to the mix, but first a disclaimer... This is info from Autocad, I have never used Draftsight so I don't know if it handles it the same or not.

Also, this may or may not be helpful, but is a good cautionary item, if nothing else.

In Acad, when you specify units, you can indicate what one unit is to be considered, and Autocad will adjust automatically, based upon that info.

For instance, if you have a drawing that you create, and specify inches, and draw something 10 units square, it counts that as 10".

If you block that, and then insert that component into another drawing where the settings are that 1 unit is 1mm, it automatically scales the first, to account for the conversion factor, inches to mm. When it works, it's a time saver. But, just to pull an instance out of the air....

If you get a plan from a subcontractor, that they have drawn in feet/inches, but the units are set as meters (because their drawings are all set the same, the discrepancy works for them)... If you don't catch that setting in their file, but your file is set to feet and inches, when you insert theirs, it automatically scales to be helpful and everything hits the fan. Even better if it's an odd sized component that all you have is a general idea of size, and it's scaling yards to meters, because it's close enough you may not catch it right away.

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