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gizmo2
05-26-2013, 11:38 PM
I was working on the connecting rod to a model steam engine today, and was drilling a 19/64" (about .297") hole before reaming to .302". Same as an N drill, but the drill would have made too big a hole. Turns out, so did the fractional, hole .303" diameter, and I thought I had plenty of room to spare. Don't trust 'em! Shoot a test hole first, before you have to make the sad face.

Zero_Divide
05-27-2013, 12:13 AM
Its yer own fault sir!
Most of the sources suggest a 3% smaller drill than the size of the reamed hole.
In your case drill should have been .293 or 0.009" smaller

gizmo2
05-27-2013, 01:00 AM
It's always my fault! No 'bout adoubt it. It just amazes me how many ways there are to get it wrong, and I seem to find 'em too. Not complaining, just didn't see this one coming. Very avoidable tho', you are right.

macona
05-27-2013, 01:04 AM
Twist drills are a roughing procedure. Never expect one to drill on size.

Black_Moons
05-27-2013, 01:09 AM
Yea its just annoying how much they drill oversize at times, almost never undersized. And no need to kick the clumsy bastard while hes down gizmo2, im sure hes going to be down there kicking himself for awhile looking for those couple thou that fell outta his hole. :P

Jaakko Fagerlund
05-27-2013, 04:35 AM
Best method is to get those fancy 3-flute "drills" (don't know their english name), they make the hole round(er) and straight(er) after a twist drill and leave the needed size for a reamer. For example, if I want 16H7 hole, I drill with 15 mm drill, then use the 3-flute to get to 15.75 mm and then shove a 16H7 reamer down the hole. Works everytime and provides expected results.

John Stevenson
05-27-2013, 04:43 AM
Called core drill here as their main use is in opening cored holes up in castings without grabbing.
Downside is, not saying they don't exist but I have not seen any under about 1/2"

The Artful Bodger
05-27-2013, 05:59 AM
OK then, I will not be finding anything except basic twist drills in my local suppliers' in .5mm sizes, anything more sophisticated would have to be ordered in for me, and of course I would have to know what to ask for!

So, supposing I need 6mm hole, how close will I be if I drill 5mm then 6mm? Maybe 5.5mm then 6mm?

Zero_Divide
05-27-2013, 06:04 AM
3% smaller than the size of the reamer
...
...

vpt
05-27-2013, 07:42 AM
I often wondered what the rule of thumb was for drill size before reamer size. I always assumed as long as the drilled hole was within the tapered part of the reamer I was good to go.

JoeLee
05-27-2013, 08:01 AM
Brownells has some 3 flute bits in small sizes.

JL.....................

Bob Fisher
05-27-2013, 12:58 PM
If I'm picky about a hole dia, I drill about .010 smaller and finish with the proper size. Checking the drill dia at the tip is a good idea to make sure the drill is actually the right size. Where ther are multiple users, you can never count on a drill being in it's proper storage slot. Bob.

Forrest Addy
05-27-2013, 05:04 PM
The way I understand it Macona is responsible not only for drills cutting oversize but global warming, sun spots, and flat beer. The man carrys a heavy burden. ;-)

michigan doug
05-27-2013, 05:06 PM
A new (good) drill, and/or a well sharpened drill, is less likely to drill oversized. Dull, poorly sharpened, a little bent, oversized every time.

doug

philbur
05-27-2013, 06:05 PM
Me too. It seems that with no center point for it to swing around it's more difficult for the second drill to drill oversize. It kinda acts like a "roughing" reamer.

Phil:)


If I'm picky about a hole dia, I drill about .010 smaller and finish with the proper size. Checking the drill dia at the tip is a good idea to make sure the drill is actually the right size. Where ther are multiple users, you can never count on a drill being in it's proper storage slot. Bob.

Juergenwt
05-27-2013, 06:16 PM
Having drilled and reamed thousands of hole in my life drilling minus 1/64 or metric minus 0.3mm has served me well. It is not easy to find a drill to be 3% under unless you have a metric drill set by 0.1mm. With a standard inch size you would need to look at three drill sets (fractions, numbers and letters).

Edwin Dirnbeck
05-27-2013, 08:35 PM
The ONLY time a drill will drill right on size is when you dont want it too.Example,I will just use a 3/8 drill so that this dowel pin wil slip right thru,nope tight as can be every time.

mickeyf
05-27-2013, 08:45 PM
The only time a drill will not drill over sized is when it it PERFECTLY sharpened. Some factories may come close enought to that that you don't notice. I'm pretty sure I never do.

Zero_Divide
05-27-2013, 09:10 PM
Having drilled and reamed thousands of hole in my life drilling minus 1/64 or metric minus 0.3mm has served me well. It is not easy to find a drill to be 3% under unless you have a metric drill set by 0.1mm. With a standard inch size you would need to look at three drill sets (fractions, numbers and letters).

1/64 may be a bit tight for smaller holes.

Besides there is nothing wrong with having all 3 sets of drills.
One is never enough.

quasi
05-27-2013, 11:25 PM
Screw machine length drill bits make holes closer to their designated size than jobber length drills do I.M.E.

J Tiers
05-27-2013, 11:42 PM
Having drilled and reamed thousands of hole in my life drilling minus 1/64 or metric minus 0.3mm has served me well. It is not easy to find a drill to be 3% under unless you have a metric drill set by 0.1mm. With a standard inch size you would need to look at three drill sets (fractions, numbers and letters).

Get one of those nifty charts showing fractional, metric, number, and letter size drills, in order of increasing size. No need to look at several sets, who wants to do that?

There is nearly always one to fit any size you want, and in many cases more than one are either the same or so close it makes no difference.