Redimensioning in AutoCAD

Is there an easy way to redimension something in AutoCAD? Will the redimension show up as a permanent change or is it just for blueprints?

I’m trying to make a solid model using a wax based rapid prototype machine, but wax has a tendency to not like press fitting. Therefore, I need to change the dimensions of a number of shafts and holes by a few hundredths of an inch.

Thanks,
indieFan

Most likely, you’re going to need to change the actual geometry, since dimensions aren’t linked to the geometry components of the drawing.

AutoCAD is unique in that it’s not a constraint based CAD package, every entity is independent… dimensions are set to points, not geometry. Once you change the geometry you’ll most likely have to redimension everything you changed.

Sorry for the bad news.

Matt

To really answer this question I would have to know what version of AutCAD you used, and how you drew it.

If you used AutoCAD 14 or below, then you have to do like Matt stated, and redraw the geometry.

Now if you used AutoCAD 2000 or above, or even a Mechanical Destop product - MDT5, MDT6 or MDT2004, and if you created the geometry correctly and made it 3d, and extruded it, then more than likely the dimensions are “parametric” and you can just change a dimension and the geometry will change.

AutoCAD products (with the exception of Inventor) are still on the edge in that way. They are not a true parametric product yet like Solidworks and it can be used to make just 2d sketches, or 3d models.

Side note:
What kind of rapid prototype system are you using? I used a Rapid Prototyping system from Z-Corp but that was basically starch and glucosed based to make the parts.

Well, I used AutoCAD 2002 to draw the part. Essentially, I drew a circle of 0.25" diameter and then extruded the circle. Whether or not that changes it to a 3D or not, I have no clue. (I’ll have to read the book as my prof. never discussed that stuff with us.)

As for the RP machine, it’s a 3D Systems ThermoJet. Easy removal of the support structures, but no so easy to mate two parts that are “identical” in size. (ie, 1/4" bearing to 1/4" shaft)

indieFan

this is y i love inventor :slight_smile:

The basic problem is probably two fold.

First, for rapid prototypes you need to plan your gaps just as you would when making metal parts. So if you want to put a bushing on a shaft, for example, you need to make sure that the hole in the bushing is slightly larger than the shaft. So if you have a shaft that is precision ground to be 0.250 + or - 0.001 then your minimum hole diameter should be something like 0.252. Press fits are a slightly different story, but I probably wouldn’t press fit anything made of wax.

If you accounted for this in the design, then you still have the issue of the .stl file used to build the prototype. STLs are approximations of the shape of you part composed entirely of planar triangles. Curved surfaces (like the inside of a hole) are approximated by a bunch of triangles connected together. That’s why holes produced by Rapid Prototyping machines aren’t round, they are polygons. So you also need to allow for the fact that round things will not be round, nor will their diameter be constant around the part

With some converters you can control how big the “chordal tolerance”, the distance between the triangle and the cad model surface , can get. Other converters use a fixed number for this and if it’s too big you get huge facets and an ugly looking part. For my work I like to use a maximum chordal tolerance of 0.003 in. Sometimes I have to take it down to 0.001 to get good results. But one of the CAD systems I use has a fixed value of 0.010". :mad:

So the answer to your problem may be to simply retranslate with a much smaller chordal tolerance. Also be aware that most rapid prototype systems have tolerances on the order of +or- 0.005in/in. So if you want things to fit together you often need to allow substantial gaps between parts, like 0.010-0.020 in. or more

On the other hand RP machines are pretty fun to work with. I like loading the file in the afternoon and having the part the next day, exactly as I designed it. Of course then there’s no opportunity for the machinist to point out your mistakes before the part is done either :o

OOPS time to go back downstairs and finish glueing the SLS (another rapid prototype method) mold I made together. The glue from the previous joint should be set by now.