I don't use a router often but recently I dusted both of mine off and
got a surprise. I was using a 1-1/4-inch x 1/4-inch shank flush trim
bit and I thought my router was broken.
But after some routing and closer inspection I found that the bit just
isn't stiff enough at top speed to remain straight. It is easy to see
it and when I tried to use it it was easy to feel it and hear it. It
caused the whole router to vibrate and caused a lot of chatter. I tried
in the other router and the same thing but to a lesser amount. I've got
a bit that has a 1/2-inch shank and finished what I needed.
Two things occurred to me.
1 the bit has two bearings and they both need to rest on the material
i wasn't doing that because the bit's longer than the material's
thick But even so a router bit needs to remain straight and stiff
and this doesn't at ~23,000/rpm
2 1/4-inch shank is not the way to go for a flush trim bit, needs
to be a 1/2-inch shank to maintain proper stiffness and avoid
the horrible deflection.
maybe it'd be ok at lower rpm but both routers are full on or
On Saturday, February 14, 2015 at 6:23:49 PM UTC-6, Electric Comet wrote:
But even so a router bit needs to remain straight and stiff
At the point the shaft is no longer straight, i.e., where it meets the bit
aspect, there is a slight curved "lip". If you sink your shaft all the wa
y down to the collet, the collet will/may catch on that lip, rather than fu
lly grasping onto the straight part of the shaft. If this happens, the re
sults will/could be a misaligned bit and, hence, vibration. When this hap
pens, your bit could possibly become loose from the collet's grasp, sending
your bit flying.
Try not to sink your bit's shaft all the way down to the collet, hence avoi
ding the collet grabbing onto that lip.
1/4" shank bits are fine. Possibilities...
1. Bent bit
2. As sonny said, bit isn't properly seated in the collet
3. Collet is worn or not properly seated
4. The bearings - the router bearings, not the bit bearings - are worn.
If neither the bit nor router is damaged, and the bit is seated properly, there is another possibility: The shaft flexes enough to produce a poor cut.
How big a bite is the bit taking? It might be too much for the router/bit combination.
Typically, machinists will make a "spring cut" or "spring pass" to compensate for any deflection of the cutting tool. They do this by using the same setting make a cut. Some will use a climb cut for this last pass.
You may want to test this on a sample and see what results you get.
That's deflection. I can see it happen as the router comes up to
max speed. lower revs it spins straight and true then it goes bad
at full rev on both routers the problem's inherent in the design
and going slower is the only solution
Put a 6-inch long 1/4-inch shank metal shaft in your router to see
an exagerrated example of deflection. I wouldn't really do this but
you can imagine what happens
it's a flush trim bit so the amount of material can be variable
in my case it was 1/8-inch but this irrelevant
This doesn't seem to make sense, so I must be reading it
wrong. If the bit is actually deflecting (and is not
permanently bent, or the router collet is what's actually
flexing) then it would flex more at slower rpm because
the loading per unit time is higher.
In general 1/4 shank bits are only suitable for light
cuts (and 1/8th, which is half the shank diameter, is
probably not "light"). Also, small routers (aka palm
routers or laminate trimmers) are only suitable for very
light cuts (not sure what sort of router you're using).
After reading Bodger's comment, I realized what the problem
is here: you don't know what the word "deflection" means.
from thefreedictionary.com: "deflection - the movement of a
structure or structural member when subjected to a load"
(the "structural element" in this case being the router bit).
What you are talking about is (as Bodger said) runout.
He explained the causes of runout. If it changes with the
speed of the router, it could be an unbalanced bit, or it
could be bad bearings - more likely the latter. If it
doesn't change with speed, it's most likely a bent bit.
I've stayed on the sidelines but this thread has gone on so long I
finally thought I'd see what was going on... :)
You're reading it wrong, yes methinks... :)
He's saying the shaft isn't stiff enough but what the mass on the end
causes flexure when at high speed. The case where he goes on to later
to mention the 6" long shaft is clearly what he's speaking of. Now
whether that's the actual mechanism can't tell w/o actually seeing it or
measurements, but seems clear what he thinks is happening.
Iff'en as he says it occurs in two separate routers I'd judge not the
collet/router runout but associated with the bit. It could be there's
sufficient imbalance to cause the problem if it were an inexpensive bit
that didn't muster quality control checks that a more expensive would
have failed at the manufacturer. Or, given the fact it has double
bearings it may just be that indeed the mass is too much for a 1/4"
shank and with _any_ imbalance is an issue. If this is an issue, I'd
put it back in the toolbox and never get it out again in anger--a flying
router bit head from a fractured shaft is too spooky to venture a chance...
I'd venture the solution would be to set the bit deeper into the collet
if have that much exposed shank that flexure is an issue and use the
router depth adjustment to control the depth. If the shank is so long
that even set a full depth it causes this, see above.
If it were a decent manufacturer and the bit is fully inserted in a
collet, I'd contact them with the symptoms; some Chinese import thru
eBay or whatever source, "not so much"...
ADDENDUM: Perhaps it's hitting a critical axial moment--at that point
bending resistance magically becomes much less and all kinds of bad
things can happen. Once't upon a time in a former life with centrifuges
for a casing of approx. tractor-trailer length and feet in diameter,
operating speed was at roughly router rpm's. To get up and down from
rest required passing through 5(yes, five, count 'em!!!) criticals;
early on the failure rates were quite the issue...
I can only say that I have been using 1/4" router bits since the late 60s
and I have never seen what you say is happening. (Half inch shanks are a
johnny come lately. In fact, I still have - and use - some 1/4" shanks
from the 60s and 70s. None of those have carbide, it wasn't available -
or at least not commonly so - back then.)
One of my favorite bits is a 1/4" x 2 1/2" (maybe 2 1/4) that I mostly use
with a mel board template for drilling shelf holes. I have drilled 100s
and 100s of holes, never deflected. Of course, there is no lateral push
when drilling holes but apparently you still get deflection even when
there is zero load.
'Tis a mystery but my money is still on one or more of the things I
It is a plain old straight bit. The presence of a bearing is not
significant unless the bearing is unbalanced or loose.
I've never even heard of - let alone seen - a 4" router bit with a 1/4"
shank. If such exists, I sure wouldn't want to use it. For that matter,
the longest cutting edge on my 1/2" shank bits is maybe 3".
Even a bit that long (4") should run true if it is round and if properly
seated in a collet that isn't screwed up.
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