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by Kendred Cooper,
MCAD Solutions Engineer
AutoCAD® 2010 2011 Can Constrain You…
Ok, so now AutoCAD 2011 has been released since our last
newsletter. And with it came some minor changes,
better known as improvements, to the constraint and
parametric workflow. In this edition, we’ll cover the
changes to the geometric constraint tools and then the
parametric dimensioning. Also take note that this
article is Part 2 of the topic. The first portion of
this topic can be found here:
http://newsletters.hagerman.com/newsletters/ebul85-Mech.htm.
So what changed with geometric constraints? One of the
hiccups with AutoCAD 2010’s constraints is that you had to
apply them to each object. The AutoCAD 2010 OSNAP’s
did not provide a constraint behavior so it was a little
extra work to build that behavior. With AutoCAD 2011
you now have constraint “inferencing.” When you apply
an OSNAP (with inferred recognition enabled) the OSNAP
choice, perpendicular for example, immediately generates a
constraint (perpendicular in this case). This control
can be enabled and disabled at will via the far left icon
across the bottom of your AutoCAD window’s status bar (hint:
it’s next to the Snap Mode button). With infer
constraints enabled, you now work similarly to Autodesk
Inventor.
Infer Constraints
Let’s now cover the parametric dimensions
and their behaviors. First off, parametric
dimensioning is a dimension that its value is changeable so
that the change will control the objects.
Historically, AutoCAD has been geometry driven CAD whereas
programs like Autodesk Inventor were dimension driven.
As an example, draw a line 3 units long and dimension it
with traditional AutoCAD tools. To change that line
length, you must stretch the line (or any number of other
editing methods that all apply to the line). With
Parametric dimensions, you would simply change the value to
something else and it would push the line to the new length.
All of the parametric dimension tools for AutoCAD are on the
Parametric tab of the ribbon interface, same as the object
constraints from part 1 of this article. For the
classic interface view, look under the Parametric menu or
simply turn on the Parametric toolbar.
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An additional feature of the parametric dimensions is that they can be linked. Place a dimension to control the length of a rectangle, then place a second dimension to control its height and for the second dimension value, select the first dimension and type “/2” (without the quotes). This syntax creates an equation in the second dimension. The second dimension now takes the length of the first dimension and divides by 2 (hence the /2) and uses that result for its value. You can also use the Parameters Manager, which behaves a little like a spreadsheet, to edit dimensional values to reference both other dimensions as well as user created parameter values. Also, the dimension names (d0, d1, d2, d3, etc) can be changed to better represent what the dimension is controlling, such as Length, Width, Height, Base_Angle, Wedge_Thickness. For naming, you cannot use spaces and the names are case sensitive when referencing.
When parametric dimensions are created, they default to only show in the CAD file and will not show on the printout. However, they can be set to show on prints in the properties window. When a parametric dimension is selected, activate Properties, under the Constraint section; change the “Constraint Form” from Dynamic to Annotational. The display of the parametric dimension will also default to the Name and Expression. This can be changed in the Constraint Settings dialog, Dimensional tab.
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