I recently used some hardened and ground 4mm dowel pins to locate some
As an experiment I drilled some with a 1.5mm pilot hole and them
drilled 4mm and the second lot I took to 3.8mm and then 4mm.
The first resulted in a easy fit that I push by finger where as the
second required the pin to be tapped in with a hammer (which is what
I would imagine that the effects get more pronounced as the size
That agrees with what I've found over the years and which I'm hoping I'm not
going to have to question myself too hard on based on other comment. Double
drilling with the 2nd drill close to the 1st in size ought to lead to a very
tight tolerance on the drill size because it rubs as much as it cuts.
There's an element of plastic deformation in the workpiece which springs
back after the cutting edge has passed. This is aided by a slow speed on the
2nd drilling which helps preserve alignment by cutting down on chatter and
wander of the drill bit and also leads to a coarser cut with more high spots
With very rigid equipment and constant machine feeds I suppose you might do
almost as well with single drilling at a slow speed compared to double
drilling but I've not found it to be the case drilling by hand on a
Colchester Student or a Bridgeport clone. Also taking all the stock out at a
slower than ideal speed is not good for the tool life. There are usually
good reasons for why we do what we do but in my case they're so lost in the
mists of time I'm more tempted to question them again than perhaps I ought
to be because I've forgotten the hundreds of tests I've already done to
establish why I ended up with this viewpoint in the first place.
I always rather assumed that a normal drill would want to 'unwind' a bit
when taking a heavier cut and that would be part of the reason for the
hole to be over nominal size.
A machinist of my acquaintance once suggested rounding off the end of
the drill a bit - like a ball end-mill but only slightly. This also
seems to help them cut to size.
I have always believed there were two main causes:
(1) If the drill lips are slightly unequal in length, the drill will
rotate slightly off centre, and thus give an oversized hole.
(2) Pushing on too far without clearing the flutes will result in their
packing with swarf (as someone mentioned earlier). This is almost
certain to occur more on one side than the other, which would push the
drill slightly off centre. This would result in (a) a slightly oversized
hole, and (b) a slightly off-centre hole.
In really critical cases the first can be corrected by drilling
undersize, then using a small boring tool down to at least 1 diameter
deep, then using the correct sized drill or reamer. (If the hole is too
small for your smallest boring tool - and they come pretty tiny - you
can in a pinch use a slot drill, as this will cut straight down without
being noticeably deflected by the off-centre walls.)
The cure for the second is fairly obvious.
I think GHT had a good write up about long hole drilling in one of his
Interesting. Anyone else heard this, or have any idea why it might work?
The above are indeed factors but the single biggest cause of oversize holes
when drilling or reaming is chatter from excessive speed. The tool literally
bounces from side to side in the hole removing material with the lands as
well as the cutting edges. You can do an easy experiment with a small
reamer, say about 8mm, in the lathe. Drill a hole to 7.8mm and then run an
8mm reamer through it at drilling speed, 800 to 1000 rpm and see what size
hole you end up with. Then try again at 250 rpm.
I found this out the hard way when I was trying to make some 8mm bronze
valve guides for the first time about 15 years ago. I turned and drilled a
bunch of blanks without any problem and then reamed them all without
stopping to check one first. Every one came out not just a bit oversize but
several thou over and completely useless for their intended purpose. At
least bronze wasn't so bloody expensive to scrap in those days. I was well
miffed because I'd followed the advice in the Dormer twist drill and reamer
handbook which said use half the drilling speed for reaming and for 8mm in
bronze this was in the range 1000 to 2000 rpm for drilling and 500 to 1000
for reaming. On rigid machines you might get away with this but on an old
Student you clearly couldn't. I tried again at 250 rpm and achieved the
perfect size and finish.
Same scenario with drilling which is why I started double drilling with the
2nd pass at a slow speed. It cuts down on chatter because of both the speed
and the minimal stock removal which also helps the drill cut straight
instead of veering off to one side.
Anything that makes the drill rub instead of cut will help but it's the
wrong way to go about it. Also rounding off a drill by hand is going to lead
to some eccentricity which might promote wander instead of cutting in a
straight line. The correct speed is the key and no special measures are
needed to get a good hole size tolerance other than this.
Somta may not be a familiar name to you guys in the UK - from the
Somta site -
"Somta Tools was founded in 1954 by Samual Osborn Ltd of Sheffield,
England, through its subsidiaries Osborn Steels and Osborn Mushet
Tools. The name Somta was chosen as the acronym for these companies
operating in South Africa."
It is commonly used in industry here.
On Wed, 3 Dec 2008 04:17:32 -0800 (PST), "Tim Nash (aka TMN)"
It is a feature of twin flute twist drills that if you enlarge a hole with
them or drill through thin stock, they will tend to drill a three lobed hole
rather than a round one. Similarly, three flute drills will tend to cut a four
lobed hole in similar circumstances etc. By pre-drilling the hole, you remove
the ability for the conical end of the drill to stabilize the drill. This then
allows the flutes of the drill to try to move in the shape of a Reuleaux
Unless you hand drill through thin sheet with a large bit, you are unlikely to
see it by eye, but a Talyrond or similar equipment will show the
out-of-roundness of the hole.
Note that perfectly symmetrical cutting edges are not necessarily required for
accurate, to-size, holes. Gun drill bits specifically make use of the
asymmetric end to press the bit to one side, which supports the bit and avoids
You don't have to believe this explanation, but don't dismiss it without
measuring roundness of the hole!
Cutting fluid also plays a part in the size of the finished hole.
Drill and ream a hole using soluble oil then do the same on another
part using neat cutting oil.
The hole done with oil will be a tighter fit.
It's also possible to play with reamer sizes over about a 2 tenth's
range to dial in a hole.
If the reamer is cutting oversize and all else is correct take a
hardened dowel pin and stroke if down the top lands of the reamer,
once per tooth.
If the reamer is too tight then again use the dowel pin but stroke it
on the face of the lands and this will take the reamer back up to
It's a good tip to remember if you have to ream aluminium bronze at
any time as this stuff is a total bitch at blunting cutting tools.
I'm sorry but that's really nonsense. Drills have a cutting tolerance in
correct usage which is always +0.00 to -something. They are intended to make
holes that are never bigger than the nominal size. The Dormer tolerance on a
4mm drill is -0.018mm or about 7/10ths of a thou which is a light tap fit on
a nominal sized dowel. Running too fast or not double drilling will use this
up, and perhaps more, leading to a nominal sized hole or an oversized hole.
Reamers by contrast have a tighter tolerance but which is slightly oversize.
They are intended to make holes which are not smaller than the nominal size
so that the mating part, say a gudgeon pin or shaft, is a running fit. The
tolerance on a 4mm reamer is +0.004mm to +0.012mm or about bugger all to
+5/10ths. Again running too fast will also lead to an oversized hole.
If you want a press fit on a job then you're better off using a drill at a
slow speed than a reamer of the same nominal size. Tim's results were
exactly what you'd predict from correct usage of a decent quality 4mm drill.
No reaon at all to suppose that triangularity played any part in it.
It's commonly thought that reamers are intended to cut to exact size and
that drills cut oversize but this is invariably caused by incorrect use
rather than design. In fact the whole raison d'etre behind how things worked
out this way is you have a choice between a press fit hole or a sliding fit
hole using readily available nominal sized tooling by either just drilling
or drilling then reaming. If both drills and reamers were designed to cut
oversize you'd have to have special tooling made to get press fit holes.
That also explains why it's drills that cut undersize and reamers oversize.
If you want a press fit you're not so worried about the quality of the
finish. If you want a running fit, say for a bearing, you are. If reamers
were designed to cut undersize and drills oversize this would end up the
wrong way round.
Now that explains a lot. Now I see why instructions for making holes for
bearings say drill then ream. I could never work out how a bearing was
supposed to work if the hole was the exact size of the shaft. The common
belief that drills always cut oversize simply implied that the reamer
just gave a tighter tolerance. Thank you.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.