How does one measure reamers?

WobblyHand

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The reamer set I got yesterday don't seem to be on size. I just tested them, but maybe I am measuring them wrong. I was using my QuantuMike across the widest point (cutting edge) near the main cutting tip. What is the right way to measure them?

The under sizes are supposed to be 0.001" under the "nominal" dimension. One reamer measures 2.5 tenths under the spec. (That's a pass to me.) All others are at least 0.001" over the marking on the reamers. Can't seem to win at this game, at least not playing the cheapskate card...

0.4990 measures 0.500x, 0.3740 measures 0.375x, 0.2490 measures 0.250x, etc.

Are my expectations valid that if a reamer is marked 0.2490 that it ought to be within a 2-3 tenths of that? 5 tenths, worst case? What the heck is the 4th digit for, if it isn't held?
 
first, are these chucking reamers or hand reamers?
it matters because chucking reamers are straight, while hand reamers have a small amount of taper in the beginning to help get them aligned and cutting.

it seems yours regardless are over, and not measuring what they should. across the highs they should be measuring marked size.
 
first, are these chucking reamers or hand reamers?
it matters because chucking reamers are straight, while hand reamers have a small amount of taper in the beginning to help get them aligned and cutting.

it seems yours regardless are over, and not measuring what they should. across the highs they should be measuring marked size.
Chucking reamers. Think these reamers are going back.
 
Probably worth checking your mic calibration.
Probably, however, I measure the pins with that mic and they are 0.2495". Also checked the QuantuMike against my cheapo vernier mike and it reads the same on the pin diameter to the tenth. I'll see if I find the mic standard and check it.

The first bad reamer I got, I didn't measure it first, reamed the hole and my pin fell though the hole. Measured the reamer afterwards and it was 0.2503" and the pin 0.2495", which make sense, considering the pin just slid right though the hole.
 
I'm with @mmcmdl, ream some test holes. The hard part is measuring them.
 
Drill and ream some test holes , then measure the holes . The reamer size is not what you're after , the hole size is . :)
that's all well and good IF he is on center, if he is using a floating holder yea, but if he is off by 1 thou his cut will be 2 though bigger.
And it appears he already tested by drilling and reaming.
IF his reamer is bigger than stated by measuring, he is not going to get the 1 thou under. If you were 2 thou under you might get a hole 1 thou under.. but a bigger reamer will never give you a smaller hole.

MY TWO CENTS and I'm sticking to it... btw, it's only worth about 0.10 cents today.
 
It is interesting they are mostly measuring oversize. I could believe cutting action would involve some elasticity, but I would guess that would depend a lot on material. I seem to remember Stefan Gotteswinter on youtube commenting that different lubricant mattered too. I can't remember which two lubricants he used, cutting oil and water soluble oil perhaps, but he claimed one definitely cut smaller than the other.
 
Of course my 1" standard is nowhere to be found. But making relative measurements of my manual mic and the Mit fancy pants one shows they agree to the tenth, at 0.2495" and at 0.5001" using pins. The 1/2" dowel pin is ground (sourced from McMaster) and I made a sharpie mark on it to measure the same way between the mics. Used a mic stand even, to make the measurements. Actually my vernier mic showed it was between 0.5001 and 0.5002. My QuantuMike measured 0.500150". So they both could be lying, but it seems they have already conferred with each other :)

I figure the hole will NEVER be smaller than the largest cutting edge to cutting edge. Can't figure out the voodoo it would take to be otherwise. (Maybe with plastic that might happen, but not aluminum.) As @woodchucker says, the hole could get bigger due to all kinds of reasons. But if the tool itself starts out too big, it won't get smaller by reaming, at least that's what my logic is telling me.
I'm with @mmcmdl, ream some test holes. The hard part is measuring them.
I did ream using my first NFG reamer, the pin fell through, although the part I bought was claimed to be 0.2490". The measured tip to tip on that reamer was 0.2503". I got my money back on that reamer.

I measured 0.250x on this latest reamer, (third one!) it was marked 0.2490". (I can measure it again, but that's what stuck in my mind.) Curiously, the one over size reamer I measured was as marked. Don't remember which reamer it was, as I really wanted the set for making interference fits.

I'll measure the 0.251 and the 0.249 side to side.
 
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