Hi all, one of the misconceptions of reaming is that if you have a floating reamer everything is OK.
Quite the opposite.
A number of factors are in force.
The reamer if it is rigidly held will BORE the hole like a boring bar, and can deflect as it gets deeper under the influence of the offset forces.
If the reamer is off centre in the horizontal plane it will pull to the side as it gets deeper.
The same happens if it is off centre in the vertical plane, that is too high or too low.
The floating reamer holder will resist the deflection to a degree, but with a sharp reamer the hole can get shaved at the entry or exit point.
Even a rubber or resilient bushed holder will get harder on the deflection and cause uneven parallelity.
One trick we used to use in the 60's, when I was on a horizontal borer, and when holes of smaller size were reamed instead of boring them , was to just rub the cutting edge along the reamer with a HSS tool bit to take the side cutting ability of the reamer away, not much, just a couple of rubs to dull it.
This made the reamer cut on the front angle and the sides just acted as a guide, keeping the tool straight into the cut hole, very much like the cutting action of a D bit.
There were some occasions when the reamer wouldn't cut to size, and that is when we rubbed a small burr on the side of the flutes for about a 1/4" at the end, and it cut "oversize", or to size.
I never used a floating reamer holder, because the spindle of the borer was maintained in good order, and also due to the fact that it was a live tool, that is, it rotated about it's axis as opposed to a lathe that had the tool usually held stationary in a socket that was "supposedly" in line wth the chuck.
In a drill press, (live tooling), the hole that is drilled is also in line with the hole that is reamed, it cannot be otherwise, which cannot be said for a tailstock or tool post in a lathe setup.
Rotating a tool is far more accurate than rotating the job.
You can bore a hole on a lathe and nine times out of ten the hole will be tapered, either due to misallignment or tool wear, but in a mill if the hole is bored from the spindle with a boring head and boring bar the hole is inherently accurate and can only be parallel, tool wear taken out of the equation.
I would make a test bar, and fit it into the holder that the reamer fits in, and run a dial indicator along it on top and on the side.
I wouldn't mind betting that if you are using a carbide reamer, the reamer is just too sharp on the flutes and is shaving the hole on it's sides.
Reamers should only cut on their ends and get guided by the side of the flutes.
Ian.