[Darc]

I'm turning a part that has a thread on the OD and a 3/32" bore.
The problem I'm having is I can't seem to get the bore to run perfectly true to the thread, it seems to be bell mouthing near the end of the job (see pic).
Currently I'm drilling then boring with a small boring tool, and it's bell mouthing, so I then tried to bore the hole then ream the bore, but it still seems to be bell mouthing.

[NC Cams]

What is the material???

What thread size???

How deep is hole???

[Kiwi]

Looks like your boring tool is deflecting. Try taking lighter cuts.

[Darc]

Thanks for the replies, the material is mild steel, the bore is 14mm deep and the thread is M6 x 1.

[Mcgyver]

that's strange. tool deflection should be more or less the same along the bore, almost looks like the headstock and tailstock axis aren't aligned, then again that doesn't explain bellmouthing with a bore bore - time to start indicating and eliminating potential sources. bit o dirt or chip in the collet or chuck? tailstock indicates to spindle, bar held in chuck indicates to ways, etc. do you get any taper when you cut the OD? this is all being done at one setting? the threads come out ok? boring bar isn't rubbing? what type of machine? threads done on lathe or die holder in the tailstock? hmmmm

[ViperTX]

I suspect that if you look closer you have more of a taper then an actual bellmouth situation, which would indicate that the headstock and tailstock are not aligned, of course you could have the problem that the cutting force is forcing the piece to deflect in the headstock attached chuck.

[cdlenterprises]

Personally, I wouldn't bother boring something that small if I could help it. The boring bar would have to be so small that if you sneezed the bar would deflect. I would drill it then ream it. I would however, take everyone's advice and check your alignments. Sounds like something could be off.