Is this available online? If so, please post a link, I'd like a read of
Basically, I'm making couplings to fit model electric motors which have
precision ground shafts. I made some for 4mm shafts and they were a good
fit, ie., no wobble. When I made some for 5mm shafts, these came
out with a
tiniest, and I mean tiniest, bit of wobble - enough to be
unacceptable at 40,000
I'm using chipping-grade aluminium and white spirit as the lube/cutting
drilling at 1000rpm and reaming at 500rpm. Perhaps 500rpm is
still too fast? The
reamer is a "Titex Plus" (not a brand I've heard of
before, I was given the
reamer) and I'm drilling 4.8mm.
I will play around with reaming at a lower speed.
Speed pay a large part, faster cuts larger.
Lubrication also plays a part, Oils tend to cut tighter than solvents.
It's possible to dial in a reamer to a set size and has been used for
may years by the old manual toolmakers.
Take a hardened dowel pin or piece of round HSS stock and slide it
once down each flute, end to shank, following to top land to reduce
A reamer that's cutting too tight can be nudged up by doing the same
but going shank to tip and run the dowel pin on the front edge of the
Here's my previous post from Dec 2008
wrote:>>I recently used some hardened and ground 4mm dowel pins to locate some
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.
Sorry Dave, you're still wrong on this one.
A two flute drill enlarging a hole will produce a three lobed profile. It's an
inevitable side effect of the geometry and stiffness of the drill.
You might not be able to see the out of roundness in most cases, but it can be
measured and plenty of learned papers and articles have been written on the
My copy (unknown date) of The Dormer Twist Drill and Reamer Information
handbook says the following:
"It is good practice to first use 50% of the drilling speed used on the
pre-forming of the holes, though in fact present ideas tend to reduce this
The first time I ever used a reamer that's what I did - used 50% of the
speed. It was to make bronze valve guides and it ended up as a complete
disaster. I spent ages turning, parting and drilling a batch and then ran an
8mm reamer through them all at several hundred rpm on the lathe. On the
assumption that a reamer could only ever generate the correct sized hole I
didn't measure any until I'd finished the batch. To my horror they were all
several thou oversize and consequently useless.
50% of the drill speed might be achievable on a totally rigid and perfectly
centered setup but for HSM you can chuck it straight out of the window. A
reamer running too fast on an old machine chatters its way through the job
cuts on the sides of the teeth as well as on the lead in. Ever since I've
about 80 to 150 rpm depending on the reamer size and never had the same
recur. I've even made pistons in house from castings which involved reaming
24mm gudgeon pin bore to very fine tolerances and managed that on a shagged
Bridgeport copy without too much difficulty at its slowest speed of 80 rpm.
an 8mm reamer on the lathe I'd maybe double that speed.
20 rpm is far too slow though. You need a decent feed rate to stop the
burnishing and blunting itself and that's hard to do at very slow speeds.
However, when in doubt start slow and build the speed up until a problem
So whatever Dormer's "present ideas" were when they wrote that guide I wish
they'd printed them before I made my valve guides instead of just saying
"50% of the drilling speed".
Also. If you want a reamer to cut on the small side slow it down and leave
1 use a floating reamer holder, Hemingway do a kit.
2 use a d bit to bring nearly to size
3 use a lubricated steel ball to finish, pushed through with smooth
positive action under a press.
This will give you a work hardenend mirror bore at low cost.
There's a very easy way to do this even if the holes are blind. Accurately
turn and polish a length of steel rod to the exact diameter you want the
finished hole. Hardened is nice but in alloy a bit of decent unhardened
steel will do. Chamfer the bottom end and leave the top 15mm or so longer
than the hole depth. Drill and tap a thread into the protruding part of the
rod. Now you can press or tap the rod into the hole to swage it to size and
withdraw it with a bolt, washer and a bit of hollow tube to pull against.
With a small bushing like 5mm you could make the swaging rod top hat shaped
with a larger diameter top you can get a decent sized thread into. Say a
10mm diameter top bit with a 6mm thread in it and a bottom section a bit
longer than the hole depth and 5mm diameter.
If you want to be clever you can polish the rod down a bit everywhere apart
from the very bottom part so it just swages on that short length and doesn't
lock into position on any part above that. Probably not necessary on
something as small as 5mm though.
You want to drill to within about 1 thou of target size for the swaging to
work and a nominal 5mm drill used slowly in a good concentric chuck and with
double drilling after say a 4.9mm drill should do that. Drills are always
sized to drill from +0.00 to - a tad. About -0.02mm in the case of a 5mm
drill which should be ideal.
If there is any springback in the swaging process you might need to play
with the rod size a bit to obtain the exact target size. i.e. a couple of
tenths over 5mm to get a hole that's exactly to 5mm.
Cheers Dave, I'll try that. I need these couplings to be absolutely
spot-knacker as I'm going to be turning them at 40,000 - 50,000 rpm.
I've just gone through my collection of brushless motors with a
micrometer, and not a single one has a shaft of exactly 5mm - they're
all 4.95, 4.97, 4.96 etc. - surely a precision ground shaft should be
the exact size, ie., 5mm?
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