Learning Curve: Fillet Fundamentals

9 Jun, 2006 By: Bill Fane

Using fillets properly can save you time.

Once upon a time, a long time ago, in a Learning Curve far, far, away, Captain LearnCurve wrote about the Fillet command. He decided to write about it again this month for three reasons: First, it's been 14 years since he last wrote about it; second, a few new tricks have been added to its capabilities in those intervening years; and third, his deadline is rapidly approaching, and he's desperate for a topic.

Let's start by clarifying a basic fundamental point. This month, we'll discuss AutoCAD's Fillet command, not the filet operation one performs on a fish.

In its simplest manifestation, Fillet automatically adds an arc between two lines or curves, or any combination thereof, and trims or extends them as necessary. To try this, draw two non-parallel lines as shown in the left side of figure 1.

Figure 1. The Fillet command inserts a tangent arc and trims the lines accordingly.

Fillet buttonNow start the Fillet command by entering it at the Command prompt, or by selecting Modify\Fillet, or by clicking the Fillet button on the Modify toolbar. In any case, it brings up the following prompt:

Current settings: Mode = TRIM, Radius = 0.0000

Let's start with a simple example. Press R, and then enter a suitable value for the radius of any fillets produced. A suitable value is anything less than the length of the shortest line. You also can click two points to show AutoCAD the distance to use.

Now, select the two lines in turn. AutoCAD creates the tangent arc of a specified radius and trims the two lines back as shown in the right half of figure 1.

These steps are pretty simple so far, but I have three significant points to note. First, the Fillet command creates a separate, independent arc. If you fillet the same two lines again with a larger radius, AutoCAD produces a new arc to the specified radius, trims the lines accordingly and leaves the original arc for you to delete manually.

Second, the fillet arc always is created on the current layer, regardless of the layer of the lines.

Third, AutoCAD remembers the last radius you used and retains it as the default. It displays this value in the Command prompt area the next time you use Fillet.

Newcomers often are startled to learn that a fillet of radius zero is valid, and in fact is the default. In this case, AutoCAD trims or extends the lines as required, but is smart enough to not actually draw the arc. It is actually one of the fastest ways to get two lines to meet at a single point.

A Shifty Character . . .
Okay, here's a cool trick that was added in AutoCAD 2006. In prior releases, you had to reset the radius every time you flipped back and forth between zero and a finite value. In AutoCAD 2006, all you need to do is to hold down a key while selecting the second object. AutoCAD ignores the current radius value and temporarily reverts to a radius of zero for this one fillet.

Now, let's look at a number of other clever filleting tricks.

  • The lines do not need to touch initially. AutoCAD extends or trims them as required to produce the desired fillet.
  • The fillet is created between two pick points, so four filleting solutions are possible where two objects cross. This ability is a vast improvement over very early releases where it always left the longest line segments regardless of the pick point locations.

There's No End to this . . .
You also can fillet between any cross-combination of lines, circles and arcs. If a fillet involves a circle, then the circle isn't trimmed but remains a full circle, because AutoCAD wouldn't know where to end the trim.

Poly Wanna Fillet . . .
Filleting can behave in several different ways when polylines (Pline command) are involved.

If you pick two pline segments, the fillet is applied between those two segments. If the two segments are not adjacent, then the intervening segments disappear.

You can fillet between a line and a pline segment, but the results are a little unpredictable.

You cannot fillet between a pline segment and an arc or circle.

Start the Fillet command and then enter a P to invoke the Polyline option. When you select a pline, a fillet is applied at every vertex between all the pline segments. Here is another quick trick: polygons and rectangles are really just special ways of constructing an ordinary closed pline. Filleting with the Pline option is a quick way of knocking all the sharp corners off these objects in one hit.

Your Results May Differ . . .
Now, let's look at the new options that have appeared in the last couple AutoCAD releases. If the Fillet command in your copy of AutoCAD doesn't have these options, it might be time to consider upgrading.

You can turn the Trim option on (the default) or off. When on, objects are trimmed or extended as necessary to just touch the ends of the tangent arc. When off, the exact same arc is created but the selected objects are neither trimmed nor extended but remain unscathed. Figure 2 demonstrates how this works.

Figure 2. Two lines (a) filleted with Trim (b) and No trim (c).

AutoCAD 2005 or so added the Multiple option. Actually, it was mUltiple in AutoCAD 2005 but changed to Multiple in AutoCAD 2006. What's the difference? In the earlier release you entered U to invoke it, but the later releases require M. Note that this may affect any menu macros or AutoLISP programming that you may have created.

In any case, the Multiple option repeats the command over and over again until you press to cancel it. The really cunning part here is that the other option switches remain available, so you can fillet several sets of objects, change the radius and fillet several more and then turn Trim on or off, all within one run of the Fillet command and without having to return to the Command prompt.

MUltiple was changed to Multiple because AutoCAD 2006 added the Undo option. This option reverses the action of the youngest fillet still existing from within the current run of the Fillet command, so you can add several fillets, undo a couple, apply several more, undo some more and so on.

A Really Solid Topic
You also can use the Fillet command to apply fillets to the corners of 3D solids (figure 3). A fillet can be applied to a single edge, or you can chain several together. Chained edges don't actually have to connect end-to-end like a chain. The Chain option means that you can select multiple edges anywhere on the solid, even if they don't connect or conversely even if several of them meet at one vertex.

Figure 3. Filleting also works on 3D solid models.

When you understand the details and power of the Fillet command, you should understand why I have very rarely invoked the Arc command in more than 20 years of using AutoCAD. It's almost always faster to draw lines and circles and then apply fillets to get the desired result.

And Now For Something Completely Different . . .
Be careful when buying things or booking travel arrangements online. Make sure you really know who you're dealing with. For example, we recently booked a rental car online. We found two different Web pages that offered the exact same car for the exact same price. The difference was that one was the home page of the actual rental company, whereas the second was a supposed discount booking agency. The latter wanted to tack on a $10 processing fee for the privilege of simply collecting our information and then passing it to the rental company.

AutoCAD Tips!

Lynn Allen


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