Reamer cutting oversize???

[donie]

Wow, along time ago, Forrest Addy I believe is talking about stoning a reamer with a fine stone for a cleaner-more accurate hole, reamers can be a little rough when new.
He was all about using sharp tools, stoning his drill bits, those little things do add up, not seen much these days.

 

[DMF_TomB]

Clearly I've screwed something up, but I just ran 2 test holes with the table locked and my .4990 reamer is definatly cutting .5 + Getting a sliding fit rather than the press fit that I'm looking for on a .5 shaft (actually .4995)

I thought that when reamers lost tolerance they were suppoed to start cutting undersize.

What did I do? I'd like to know so that I don't do it again!

B

.
1) due to runout or wobble most reamers make a bell mouth hole or larger hole as reamer 1st starts hole. oversize the first .1" depth can easily be over .001"
.
2) softer metal like aluminum needs more depth to bend reamer to lower runout bellmouth hole. reaming steel bellmouth is lower depth usually. many want reamer sticking out a long length so it bends easier
.
3) built up false cutting edge. if oil or tapping fluid not used, metal sticking to reamer can easily cause a oversized hole. coolant often not enough for reaming
.
4) metal hardness variations can cause a oversize hole and often a curved hole. that is long pin gage
wont go deep into hole cause hole not straight even when rereamed with new reamer many times. some
use spiral flute reamers cause straight flute reamers sometimes have vibration problems
.
5) many use a -.005 or -.004" reamer first. in old days when hand reaming its difficult to ream much
more by hand. hand reamers usually end of reamer is .001" smaller to go in hole and provide better support to
lower or prevent bellmouth shaped hole

 

[indianamoon]

I would often indicate the reamer just to be sure it was running true. Also made sure that it was starting in a chamfer