This is a useful routine that lets you easily create a fillet when the radius of the fillet is not not known. The point that you pick while defining the radius will lie along the fillet.

Here’s how:

• FP <enter> to start Fillet Point.lsp
• Select the first line
• Select the second line
• Pick a point that the fillet will lie upon

~enjoy

;;; By Irneb
;;; http://www.theswamp.org/index.php?topic=41775.0;all
;;; Creates a fillet but lets you pick a point that will lie on the arc.
;;; This is usefull when the radius is not known.
(defun c:FP (/ l1 l2 pt ed)
  (if (and (setq l1 (entsel "\nPick first line: "))
           (setq l2 (entsel "\nPick second line: "))
           (setq pt (getpoint "\nPick point passing through circle: ")))
    (progn (command "._circle" "_3P" "_tan" (cadr l1) "_tan" (cadr l2) "_non" pt)
           (setq ed (entget (entlast)))
           (entdel (entlast))
           (command "._fillet" "_radius" (cdr (assoc 40 ed)) "._fillet" l1 l2)))
  (princ)
)