8.4 Fillet & Chamfer

Real drawings are full of corners — where two walls meet, where a countertop turns, where a bracket's edges join. FILLET and CHAMFER are the two commands built specifically for cleaning up and shaping those corners, one with a rounded arc and the other with a straight bevel. Both commands share a nearly identical structure and both include a zero-value setting that produces a clean square corner instead of any curve or bevel at all, which makes them a natural pair to study together.

FILLET: Rounded Corners

FILLET connects two selected objects — lines, arcs, circles, polylines, splines, rays, or xlines — with a smooth arc at a radius you specify. Running the command shows a set of bracketed options — [Undo/Polyline/Radius/Trim/Multiple] — before the first object-selection prompt.

• Radius sets (or changes) the fillet radius that subsequent fillet operations will use; once set, this value persists as the default for future FILLET commands until deliberately changed again.
• Polyline applies the current radius to every vertex of an entire selected polyline in one operation, rounding every corner at once instead of one pair of segments at a time.
• Trim / No trim controls whether the two selected objects are automatically trimmed or extended to end exactly at the fillet arc's endpoints (the default, Trim) or left at their original full length while the arc is simply added as a separate connecting piece (No trim) — which can leave the arc floating with small gaps or overlaps relative to the original lines' endpoints.
• Multiple keeps the FILLET command running after completing one fillet, letting you fillet several different corner pairs in a row without restarting the command each time.
• Undo reverses the most recently completed fillet operation within the same command session.

Fillet Radius 0: The Square-Corner Shortcut

Setting the Radius option to 0 and then selecting two non-parallel lines (or other objects) produces no visible arc whatsoever — FILLET instead simply trims or extends both objects until they meet precisely at a sharp, clean corner. This is an extremely common real-world cleanup move: two wall lines that were drawn slightly past each other, or two lines that stop just short of touching, become one tidy square corner instantly with a 0-radius fillet, without manually running TRIM or EXTEND on either line individually.

The Parallel-Lines Exception

One quirky, testable exception to how FILLET normally works: selecting two parallel lines with FILLET ignores whatever radius value is currently set. Instead, AutoCAD automatically creates a semicircular end cap using exactly half the distance between the two lines as the radius, regardless of what radius was stored from any previous fillet operation. Two parallel lines 4 feet apart, filleted together, always produce a 2-foot-radius semicircular cap connecting them — typing a different radius value beforehand has no effect on this particular case.

CHAMFER: Beveled Corners

CHAMFER produces a straight angled corner between two selected lines instead of a curved arc, and its bracketed options — [Undo/Polyline/Distance/Angle/Trim/Method/Multiple] — mirror FILLET's structure closely, with two extra options (Distance and Angle) replacing FILLET's single Radius setting.

• Distance method sets Dist1 and Dist2: Dist1 is measured along the first line selected, back from the point where the two lines would intersect; Dist2 is measured the same way along the second line selected. Equal Dist1 and Dist2 values create a symmetric bevel relative to the two lines, though the resulting bevel angle itself depends on the angle at which the two original lines meet — equal distances on a 90° corner produce a 45°-angled bevel, but equal distances on a non-square corner produce a bevel at a different angle.
• Angle method instead sets one distance along the first selected line plus an angle measured from that first line to the chamfer line itself, giving a second way to define the same kind of bevel without specifying a second distance directly.
• Method toggles which of these two input styles (Distance or Angle) the next chamfer operation uses, remembering whichever values were most recently entered for that mode so you don't need to retype them just to switch approaches.
• Trim / No trim, Multiple, Polyline, and Undo all behave identically to their FILLET counterparts described above.

Chamfer Distances of 0: Matching Fillet's Square Corner

Setting both Dist1 and Dist2 to 0 (or, under the Angle method, setting the distance to 0) produces the exact same result as a 0-radius fillet: the two selected lines are simply trimmed or extended to meet at a clean square corner, with no bevel added. FILLET and CHAMFER converge on this identical square-corner behavior at their respective zero settings, which is a detail the exam likes to test directly by asking what a 0-radius fillet and a 0-distance chamfer have in common.

Worked Example

Two perpendicular wall lines meet at a corner that needs a rounded exterior edge. Running FILLET, setting Radius to 0.5 (6 inches), and selecting both lines produces a 6-inch-radius arc connecting them, with each line trimmed back to the arc's tangent points.

The same corner, treated instead with CHAMFER using the Distance method and Dist1 = Dist2 = 1 foot, bevels the corner with a straight diagonal cut exactly 1 foot back along each wall line from where the original corner would have been — a common treatment for clipping a sharp cabinet or countertop corner in furniture and casework drafting. Because these two walls meet at exactly 90°, switching to the Angle method with a distance of 1 foot along the first line and an angle of 45° produces that identical bevel shape, since a 45° angle from a distance of 1 foot along a 90° corner works out to the same 1-foot offset on the second line as well.

On the Exam

Expect questions asking what a 0-radius fillet or a 0-distance chamfer produces (a clean square corner, not an error and not a visible arc or bevel), questions on the parallel-line fillet exception that ignores the stored radius in favor of half the gap distance, and questions distinguishing No Trim's full-length original objects from Trim's default behavior of cutting both objects back to meet the new corner geometry exactly.