i am currently machining a modified O.D. 3.5"-5 buttress in DOM tubing. by modified, i mean that the customer has changed the major Ø and minor Ø to accomodate their customers mating part. the mating part is a plastic cap that is used on a hydraulic tank for the fill tube. the parts are subject to below freezing temps, so that is where the modification came in. anyway...having trouble with a nasty burr at the first thread - can not get rid of the darn thing. i've tried to re-machine the lead in angle after threading 18 passes, and then re-cutting with 4, 6, or 8 passes to attempt to clean out the threads. i've even gone as far as trying to machine a linear step at the start to minimize the lead in chamfer size, but to no success. result is always the same - a razor sharp edge on the first thread. any one have any ideas on how i can resolve this matter? also, i'm using a kennametal nt4rb insert. they recommend that i use a negative 15° modified flank infeed. anyone heard of this? i've done modified infeed angles, but i'm a bit confused on the -15° infeed...any input would be greatly appreciated.