We need a Video Tutorial on making fully defined sketches

[jaysinn]

Since I learned that I should be making my sketches Fully Defined, I've been doing a lot of experimentation and have learned a few tricks here and there - but it would be great if a real expert would do an authoritative video tutorial.

For example, why is it that sometimes I can have a section of a sketch all nice and fully defined, and then suddenly adding a line or arc makes the previously well-defined elements become undefined again? Or is it that they are over-defined? I don't know.

Some things I've learned:
(1) Using "Offset" creates a new line or arc, but Alibre does NOT remember the Offset; you have to go back and measure it.
(2) If you want to make a fully defined line tangent to two circles, the easiest way to do it is to draw a Reference line between the two centers, then Reference Lines from each center perpendicular to the line joining the centers, then draw your line from intersection to intersection.
(3) Sometimes you can see that the points on each end of a line are fully defined, but the line itself isn't. Delete the line and re-draw it.
(4) The priority seems to be on defining the nodes. If you have trouble getting your lines and arcs to behave, look at the nodes. Measure from fully defined nodes to nodes that are not yet fully defined, preferably along the axes.

 

[bigseb]

For example, why is it that sometimes I can have a section of a sketch all nice and fully defined, and then suddenly adding a line or arc makes the previously well-defined elements become undefined again? Or is it that they are over-defined? I don't know.

Experience will change this. All part of the learning curve.

(1) Using "Offset" creates a new line or arc, but Alibre does NOT remember the Offset; you have to go back and measure it.

True

(2) If you want to make a fully defined line tangent to two circles, the easiest way to do it is to draw a Reference line between the two centers, then Reference Lines from each center perpendicular to the line joining the centers, then draw your line from intersection to intersection.

Draw your two circles. Define. Then draw a line near the circle. Delete all constraints pertainingto the line (if any). Use double tangent constraint to constrain line to circles. Trim accordingly.

(3) Sometimes you can see that the points on each end of a line are fully defined, but the line itself isn't. Delete the line and re-draw it.

Impossible. Unless the points you are clicking aren't the end points of the line you want to contrain. Zooming in usually helps.

(4) The priority seems to be on defining the nodes. If you have trouble getting your lines and arcs to behave, look at the nodes. Measure from fully defined nodes to nodes that are not yet fully defined, preferably along the axes.

Not quite sure I get your meaning here.

 

[Dave H]

Properly dimensioning a sketch will usually fully define it.
The sketch MUST be related to the origin.
Those relations can be constraints or proper dimensions. If you change or add something that makes those dimensions or constraints no longer relevant to the origin, you must re-dimension or re-constrain.

 

[jaysinn]

Experience will change this. All part of the learning curve.


True

Draw your two circles. Define. Then draw a line near the circle. Delete all constraints pertainingto the line (if any). Use double tangent constraint to constrain line to circles. Trim accordingly.

Impossible. Unless the points you are clicking aren't the end points of the line you want to contrain. Zooming in usually helps.

Not quite sure I get your meaning here.

(1) Naturally I have been using the technique of drawing the circles and then making a line tangent to both circles, then trimming. Done that for years before I knew about "fully defined". But I swear to you, the simple and obvious method does not work as well as the technique I described.

(2) You may say it's impossible for a poorly-defined line to have two well-defined points at its ends, but I've seen it many times. This is in the context of a lot of cutting, trimming, etc. and often where there was a line that got shortened by adding fillets on both ends. I've rescued a situation that seemed impossible more than once by deleting a line and re-drawing it. (Apparently the line itself, though truncated, was originally defined by points that no longer exist.) I agree that it SHOULD be impossible, but it happens!

(3) I have been finding that looking closely at the nodes - seeing which nodes are undefined - seems to be a faster and more efficient way of getting the lines and arcs to be fully defined. Oftentimes the constraints that I think would serve to define the line are already defined (i.e. the value is grayed out in the Dimension box, and then the displayed value on the drawing is in parentheses). An advantage of looking at the nodes is that there is no halfway: nodes are either fully defined or they are not. Lines and arcs can be partly defined (length or radius) but still not fully defined. Once you have a nearby node fully defined, you can always easily fully define its neighbor with no more than two coordinates. If the node is actually where you want it (by previous construction) then this is a lifesaver.

 

[HaroldL]

(1) Using "Offset" creates a new line or arc, but Alibre does NOT remember the Offset; you have to go back and measure it.

I find it best to go back and place dimensions and constraints on the offset geometry right after closing the Offset dialog. It would be nice if Alibre did this for you.

(2) If you want to make a fully defined line tangent to two circles, the easiest way to do it is to draw a Reference line between the two centers, then Reference Lines from each center perpendicular to the line joining the centers, then draw your line from intersection to intersection.

This will only work if the two circles are the same size. If they are different diameters then the point of intersection on the arc of the Ref line that is perpendicular to the center-to-center Ref line will be at the Quad point of the circle and not the point of tangency.
If you are drawing Slots, another method with fewer steps may be to start with a Rectangle. Then draw a Tangent Arc at each end of the rectangle and either delete the end lines or convert them to Construction geometry. The last step in this scenario may be to apply any missing Tangent constraint.

(3) I have been finding that looking closely at the nodes - seeing which nodes are undefined - seems to be a faster and more efficient way of getting the lines and arcs to be fully defined.

How do you tell if the nodes are constrained? If you Toggle DOF Colors ON only the sketch figures display colors, all the nodes are black.

 

[DavidJ]

Occasionally, the DoF colours get a bit lost (don't keep up with true status). Closing the sketch and re-opening usually fixes it.

 

[oldfox]

Draw your two circles. Define. Then draw a line near the circle. Delete all constraints pertainingto the line (if any). Use double tangent constraint to constrain line to circles. Trim accordingly.

After fighting this bugaboo for quite a while, I found that this is the *surest* and *simplest* way to accomplish this scenario.
Hasn't failed me yet.

I always use "auto dimension" to get started with obtaining a "fully defined" sketch. Most of the time, auto dim leaves *1*
item not defined. That is when the *real* troubleshooting begins. I think a 2-3 word per item checklist should be included in any tutorial.

Lastly, when you dimension a line, node, arc, etc, *ALWAYS* make sure that it is dimensioned to 2 *AXES* and *NOT* any reference
lines that may be coincidental with an axis.

 

[oldfox]

Occasionally, the DoF colours get a bit lost (don't keep up with true status). Closing the sketch and re-opening usually fixes it.

Maybe this should be the final item on any future checklist. Good tip.

 

[jaysinn]

I find it best to go back and place dimensions and constraints on the offset geometry right after closing the Offset dialog. It would be nice if Alibre did this for you.

Of course you are right about the circles!

This will only work if the two circles are the same size. If they are different diameters then the point of intersection on the arc of the Ref line that is perpendicular to the center-to-center Ref line will be at the Quad point of the circle and not the point of tangency.
If you are drawing Slots, another method with fewer steps may be to start with a Rectangle. Then draw a Tangent Arc at each end of the rectangle and either delete the end lines or convert them to Construction geometry. The last step in this scenario may be to apply any missing Tangent constraint.

How do you tell if the nodes are constrained? If you Toggle DOF Colors ON only the sketch figures display colors, all the nodes are black.

View attachment 25621

I changed the color scheme to make the status more visible to me: Fully Defined is purple. Under this scheme, undefined nodes are black, but fully defined nodes are purple.

 

[bigseb]

Overall AD's sketching tool is not that bad. Yeah, it has the odd hiccup but in general it works well. You want pain? Try working with Creo Parametric. Now that's pain. Bring a gun to work the next day kind of pain.

 

[oldfox]

Overall AD's sketching tool is not that bad. Yeah, it has the odd hiccup but in general it works well.

+1

Try working with Creo Parametric. Now that's pain. Bring a gun to work the next day kind of pain.

To me, Creo is only a half-word i.e. Creole! Some fine vittles!

 

[oldfox]

Going back to one of my old posts called "What is this window?", here is an example of what made me post that.
Play with it and have fun. When you think it is ready to extrude and send to the CNC mill, go ahead and extrude it to 3/32 inches
and run through the simulate mode. To any of you folks already actively using Alibre to drive your machines, (not printers) will this example cut as it stands right now?

If so, then when and where is "Fully Defined" really needed? Not a test, I really don't know the answer.
Thanks.

Edit: OOPS.

 

[bigseb]

Looked at you file, wondered what the hell was that large node in the bottom left of the sketch. Turns out my screen needed a clean.

Extrudes just fine. And no, you don't need to have a fully defined sketch. Some industries require it though (aerospace and automotive for example).

 

[oldfox]

Some industries require it though (aerospace and automotive for example).

So this means that it doesn't have anything to do with the software, but rather just a directive to the engineering (drafting) department
that everything be "fully defined"?

 

[bigseb]

Yes. That said, a properly defined sketch means you can't accidentally move anything. But you don't need to do it.

 

[oldfox]

Got it. Thanks. That explains why I've gotten away with it all these years.