Lofting….I don’t know about you, but this has been one of the things I learned to hate. Maybe because the application seems to have more control that I do during the process, or because I had some bad experiences with 3DS 10 years ago (pre MAX). Whatever the reason, I just never liked Lofts very much.
Lofting is an extremely useful tool. While you can reproduce the model by other means, the pain involved would be extensive. The Loft Feature has so much control over the process, by weighted interpolation, there is no reason not to use it. The trick is to control the interpolation to accomplish your goal.
Today, we’ll use Angles and Weights to create a transition between 2 other features, that would normally take a bit more time and planning to complete.
In the image below, you should see the part I needed to tie into another loft feature, with a specific angle at the transition.
While a Sweep around an extruded Feature might normally get a similar job done as easily, you would need more than 1 rail, adding complexity to process. The Right side view (below) shows the problem with fillets and sweeps. The rear edge is offset farther than the front, and the upper edge is not a typical fillet This is a perfect candidate for a Loft.
We start out with a lower and upper 2D sketch profile.
Using loft we select the outer profile from the lower sketch, and the upper sketch profile as well. Since this is a straight interpolation, we don’t need any centerline or rails.
In a Free state, the preview below shows the transition.
Now, if we proceed to the Conditions tab, we can tell the application more about what WE want to see.
Conditions
When you select the Condition pulldown, you will notice differing option depending on the profiles and guides selected previously. In this case, Free and Direction Conditions are available. (The upper icon is Free, and the lower red highlighted one is Direction; just look for the tool tips).
If you select Direction, the Angle and Weight fields activate.
Angle
The angle of the Direction condition is easy to understand, well once you do understand it. The Angle is calculated AT the edge of the Starting profile (first one selected), and measured FROM the inner area of the sketched plane, directed along a perpendicular plane, in the direction of the terminating profile (the last one selected). The Terminating profile is measured in a mirrored fashion, from the Terminating profile inner area, back towards the incoming Loft.
In the example above, we have an starting angle of 90°, or perpendicular to the sketch plane. We also have a Terminating angle of 180°, which will be tangent.
Weight
Weight is said to be a distance along the axis created by the Direction, however I try to think of it as a bias. The larger the number, the greater the bias applied.
*Note: The Inventor help says weights typically range from 1-20, however i have used them up to 95, and still received visible changes.
Here is the result of the above example:
The lower perpendicular beginning angle is barely there, and almost immediately starts into a curve. Lets say we want a bit more straightness. Add more Weight to the calculation.
in the example below, you should be able to see the impact that different weights had on the feature. Notice that using a weight of 5 forced the Loft beyond the intended path, causing it to grow above the terminating plane. Also note, however, that the feature still held its terminating tangent criteria.
Now I’ve had enough fun, and need to get my part completed. I set the beginning angle to 0 (which will be a sharp edge tangent), weighted at 2, and the Terminating angle of 150, weighted at 3.
The result is below:
Here the bottom edge is sharp and tangent, and the ending angle only went to 150° as we requested. The mating Loft feature in the part will tie at precisely the angle we want (and without a new sketch too).
After the profiles were created (and they were simple references to other features), it took me a whopping 25 seconds to create the specified part.
Addendum
If you mess with the conditions long enough, the operation will fail, as it tries to complete the feature using all the criteria specified. Usually the faces will intersect, and the nasty red note comes up, just when you think you have it made. Eventually you will have to back off on some of the conditions, ease the geometry, or add more space between the profiles.
This really is a great feature, and there are SO MANY options and possibilities beyond the scope of this discussion. Get in it and play around. You’ll probably come up with something new or interesting to make it worth your time. For any application where you need to transition between
2 profiles quickly, and with good control, this thing is awesome.
