A B767-300 RETURNED TO THE DEP ARPT WITH ENG VIBRATIONS AFTER MAINT DETERMINED A FAN BLADE BAL WAS NECESSARY.

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IMMEDIATELY AFTER TKOF OUT OF ZZZ NOTICED L ENG FAN/N1 VIBRATION THROUGHOUT ACFT. LEVELED OFF AT 5000 FT MSL AND APCHING LOW ENG RPM NOTICED HVY ENG VIBRATION WORSENED. REDUCED CLB PWR SETTING TO A RANGE OF SLOW CRUISE PWR PRODUCED ACCEPTABLE ENG VIBRATION LEVELS WITH ALL OTHER ENG INDICATIONS NORMAL. AT CRUISE ALT OF FL350; L ENG VIBRATION LEVEL WAS JUST PERCEPTIBLE. NOTIFIED DISPATCH THEN CONFERENCED CALL WITH MAINT. A REAL-TIME ENG MONITOR LOG GAVE A CLR PICTURE TO STATUS OF THE L ENG FAN VIBRATION PROGRESSIVELY BECOMING WORSE. THE FAN SECTION NEEDED A BLADE BAL PROC. THE RECOURSES AT OUR DEST WOULD BE INADEQUATE SO MAINT RECOMMENDED WE RETURN TO ZZZ TO AVOID AN EXTENSIVE DOWN TIME OFF LINE OF THE ACFT. FOR OPERATIONAL REASONS WE RETURNED TO ZZZ. A COORD EFFORT WITH CTR AND ZZZ EFFECTED A SAFE RETURN. ATC ASKED FOR INFO NORMALLY COLLECTED WHEN YOU DECLARE AN EMER. I INQUIRED AND CTR STATED THEY DECLARED AN EMER FOR OUR FLT FOR SPECIAL OPERATIONAL REQUEST. OUR REQUESTS WERE ADDITIONAL SPACING BTWN OUR ACFT AND OTHERS ARRIVING ZZZ; TO REDUCE THE CHANCE OF A GAR WHICH MIGHT REQUIRE A HIGH PWR SETTING; ALONG WITH A SPECIFIC RWY; THE ARPT'S LONGEST RWY. IN SUMMARY; WE LANDED ON AND TAXIED UNEVENTFULLY TO OUR ASSIGNED GATE. WE CHANGED ACFT TO ANOTHER SHIP. UPON ARR ZZZ1; OPS AND MAINT THANKED US FOR NOT FLYING THE ORIGINAL SHIP TO ZZZ1; STATING THE TECHNICAL WORK REQUIRED WOULD HAVE REMOVED THE ACFT FROM SVC FOR AT LEAST 3 DAYS.