PLT OF A PIPER PA34 FAILED TO FOLLOW THE PUBLISHED SID DEP DUE TO MISUNDERSTANDING HIS CLRNC AND TURNED OFF THE RWY HDG BEFORE IT WAS TIME.

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I RECEIVED A COMPLETE WX BRIEFING FROM FSS BY PHONE FROM FBO'S PLT'S FLT PLANNING ROOM; AND I FILED AN IFR FLT PLAN FROM MEM TO TRI; TN. THE FBO ALSO HAD A RADIO TELEPHONE TO APCH CTL FROM WHICH I PICKED UP MY IFR CLRNC. I HAD FILED A RTE OF FLT OF DIRECT JACKS CREEK V16 TRI VOR DIRECT AT 5000 FT. WHEN I COPIED MY CLRNC; I COPIED AND READ BACK 'CLRED TO TRI VIA MEM 5 DEP; CLB AND MAINTAIN 3000 FT; DEP ON 124.15; SQUAWK XXXX.' AFTER PREFLTING THE ACFT; I READ THE SID; FIRST LINE; 'CLRED AS FILED.' SECOND; 'MAINTAIN RWY HDG OR ASSIGNED VECTOR.' THIRD; 'MAKE NO TURNS UNTIL AT LEAST 500 FT AGL.' WHEN I DEPARTED RWY 18C; I MAINTAINED RWY HDG UNTIL ABOVE 500 FT THEN BEGAN A L TURN TOWARD JACKS CREEK VOR. AS I WAS TURNING THROUGH ABOUT 080 DEGS; THE CTLR TOLD ME TO MAKE AN IMMEDIATE R TURN TO 180 DEGS. I BANKED TO 35 DEGS TO THE R AND TURNED TO 180 DEGS. THE CTLR GAVE ME A PHONE NUMBER AND ASKED ME TO CALL UPON LNDG; WHICH I DID. I WAS CONVINCED THAT ATC HAD FAILED TO ISSUE ME PROPER HDG INSTRUCTIONS. WHEN I LEVELED OFF; SET THE AUTOPLT AND RE- READ THE SID; I COULD SEE WHAT I WAS SUPPOSED TO HAVE DONE. EVEN NOW; I THINK THE WORDING COULD BE LESS CONFUSING IF THE FIRST 2 INSTRUCTIONS WERE REVERSED (MAINTAIN RWY HDG; CLRED AS FILED). I ALSO THINK; 'MAYBE IF I HAD TAKEN AN EXTRA MIN TO CAREFULLY STUDY THE SID; I WOULD HAVE AT LEAST BEEN CONFUSED ENOUGH TO ASK.' I'M NOT SURE.