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
off

On Saturday, February 14, 2015 at 6:23:49 PM UTC-6, Electric Comet wrote:

But after some routing and closer inspection I found that the bit just
isn't stiff enough at top speed to remain straight.
Two things occurred to me.

But even so a router bit needs to remain straight and stiff
and this doesn't at ~23,000/rpm


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 way down to the collet, the collet will/may catch on that lip, rather than fully grasping onto the straight part of the shaft. If this happens, the results will/could be a misaligned bit and, hence, vibration. When this happens, 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 avoiding the collet grabbing onto that lip.

Sonny

On 2/14/2015 8:26 PM, Sonny wrote:
On Saturday, February 14, 2015 at 6:23:49 PM UTC-6, Electric Comet wrote:

But after some routing and closer inspection I found that the bit just
isn't stiff enough at top speed to remain straight.
Two things occurred to me.

But even so a router bit needs to remain straight and stiff
and this doesn't at ~23,000/rpm


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 way down to the collet, the collet will/may catch on that lip, rather than fully grasping onto the straight part of the shaft. If this happens, the results will/could be a misaligned bit and, hence, vibration. When this happens, 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 avoiding the collet grabbing onto that lip.

Sonny

+2



--
Jeff

"Electric Comet" wrote in message

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
off

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.


--

dadiOH
____________________________

Winters getting colder? Tired of the rat race?
Taxes out of hand? Maybe just ready for a change?
Check it out... http://www.floridaloghouse.net

... 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.

Joel

On Sun, 15 Feb 2015 06:41:23 -0500, "dadiOH"
wrote:

"Electric Comet" wrote in message

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
off

1/4" shank bits are fine. Possibilities...

I wouldn't use them for any major material removal. In fact, I don't
have any (that I use, anyway).

1. Bent bit

My bet. At some point, the bit got too hot.

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.

On Sat, 14 Feb 2015 17:26:02 -0800 (PST)
Sonny wrote:

Try not to sink your bit's shaft all the way down to the collet,
hence avoiding the collet grabbing onto that lip.

No lip on the bit. It's pure deflection due to the length
of the bit, 1/4-inch shank and the bearing and allen screw at the
end.

On Sun, 15 Feb 2015 06:41:23 -0500
"dadiOH" wrote:

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.

No to all those, including 1-1/4-inch long, 1/4-inch shank bits
are not fine and I've found that some attach a rheostat so they
can lower the router rpm

On Sun, 15 Feb 2015 07:18:56 -0800 (PST)
" wrote:

properly, there is another possibility: The shaft flexes enough to
produce a poor cut.

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


How big a bite is the bit taking? It might be too much for the
router/bit combination.

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



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.

Not sure what this means

On Sun, 15 Feb 2015 11:14:40 -0500
wrote:

I wouldn't use them for any major material removal. In fact, I don't
have any (that I use, anyway).

right, won't be buying any more long 1/4 shank bits, short ones are
fine if price is right

Electric Comet wrote in news:mbqihi$pl8$3
@dont-email.me:

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

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).

John

"Electric Comet" wrote in message

On Sun, 15 Feb 2015 06:41:23 -0500
"dadiOH" wrote:

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.

No to all those, including 1-1/4-inch long, 1/4-inch shank bits
are not fine and I've found that some attach a rheostat so they
can lower the router rpm

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
mentioned.

--

dadiOH
____________________________

Winters getting colder? Tired of the rat race?
Taxes out of hand? Maybe just ready for a change?
Check it out... http://www.floridaloghouse.net

On Sun, 15 Feb 2015 15:50:46 -0500
"dadiOH" wrote:

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

So there's no bearing out at the end since it's a drill bit
this is significant

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.

correct zero-load
put a long 1/4-inch shank in a router, say 4-inch or more and you'll
get to see it

I'm going to take the end bearing and allen screw off and see how
it looks

On Sun, 15 Feb 2015 17:15:58 +0000 (UTC)
John McCoy wrote:

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

it's a flush trim bit so what you're saying doesn't apply

the material thickness is not a factor either as said in original
post

load or not the bit deflects

On 2/14/2015 6:19 PM, Electric Comet wrote:
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

A two bearing bit, top and bottom, is a convenience thing. It is some
times easier to use one or the other depending on the orientation of the
bit to the work and or pattern.
Typically a straight cut bit with a bearing is a flush cut bit.


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.

Well that is not entirely true. If you are flush cutting veneer or
laminate a 1/4" flush cut bit is more than adequate. and those are
typically bottom bearing bits. In thicker cuts if is best to not cut so
much in a pass that you hear the chatter. At that point your bit is
over loaded with what it can easily handle and does not deliver the
smoothest cut.



maybe it'd be ok at lower rpm but both routers are full on or
off

The smaller the diameter of the cutter portion of the bit the faster you
need to run. Once you start using bits that have a 1" or greater cutter
you need to slow down the router speed.

A 1/2" shank bit will typically deliver less chatter however it does not
prevent you from removing too much material. You can still over load a
small diameter cutter on a 1/2" shank bit.

On Mon, 16 Feb 2015 10:40:19 -0600
Leon lcb11211@swbelldotnet wrote:

A two bearing bit, top and bottom, is a convenience thing. It is
some times easier to use one or the other depending on the
orientation of the bit to the work and or pattern.
Typically a straight cut bit with a bearing is a flush cut bit.

I've not seen a straight cut bit with a bearing at top only so
the definition should be a straight cut bit with a bottom bearing
and may also have a top bearing with a collar to secure it.

Well that is not entirely true. If you are flush cutting veneer or

true doesn't have gradations

i turned the light not entirely on

laminate a 1/4" flush cut bit is more than adequate. and those are

a laminate 1/4-inch shank flush cut bit that doesn't deflect under
no load is adequate

more adequate or more better

The smaller the diameter of the cutter portion of the bit the faster
you need to run. Once you start using bits that have a 1" or greater
cutter you need to slow down the router speed.

or once you start using bits that deflect under no load

Electric Comet wrote in news:mbt6cc$1k0$2
@dont-email.me:

On Sun, 15 Feb 2015 17:15:58 +0000 (UTC)
John McCoy wrote:

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

it's a flush trim bit so what you're saying doesn't apply

How the hell not? If you're cutting material off, how much
you're cutting always makes a difference, regardless of
whether you're flush-trimming or cutting a profile.

load or not the bit deflects

Then either your bit is bent or your router is bad. If
it happens unloaded then it's not deflecting.

John

On 2/14/2015 7:19 PM, Electric Comet wrote:
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
off




I'd just put a dial gauge on it and check for runout (powered off of
course). You could have anything from true bending of the bit, a slightly
dirty collet or sleeve, dodgy bearings, bad seating, or some combination.
2-1/4" for such a bit sounds more like a pattern cutting bit rather than a
standard flush-trim bit but I guess that the difference is more a matter of
degree. I've never seen flexing of even a 1/4"-shank router bit used at
normal loading.

"Electric Comet" wrote in message

On Sun, 15 Feb 2015 15:50:46 -0500
"dadiOH" wrote:

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

So there's no bearing out at the end since it's a drill bit
this is significant


It is a plain old straight bit. The presence of a bearing is not
significant unless the bearing is unbalanced or loose.

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.

correct zero-load
put a long 1/4-inch shank in a router, say 4-inch or more and you'll
get to see it

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.

--

dadiOH
____________________________

Winters getting colder? Tired of the rat race?
Taxes out of hand? Maybe just ready for a change?
Check it out... http://www.floridaloghouse.net

"Electric Comet" wrote in message

On Mon, 16 Feb 2015 10:40:19 -0600
Leon lcb11211@swbelldotnet wrote:

A two bearing bit, top and bottom, is a convenience thing. It is
some times easier to use one or the other depending on the
orientation of the bit to the work and or pattern.
Typically a straight cut bit with a bearing is a flush cut bit.

I've not seen a straight cut bit with a bearing at top only

They are common, handy for routing around a template that is on top of
what is being cut.

--

dadiOH
____________________________

Winters getting colder? Tired of the rat race?
Taxes out of hand? Maybe just ready for a change?
Check it out... http://www.floridaloghouse.net

On 2/16/2015 10:53 AM, Electric Comet wrote:
On Mon, 16 Feb 2015 10:40:19 -0600
Leon lcb11211@swbelldotnet wrote:

A two bearing bit, top and bottom, is a convenience thing. It is
some times easier to use one or the other depending on the
orientation of the bit to the work and or pattern.
Typically a straight cut bit with a bearing is a flush cut bit.

I've not seen a straight cut bit with a bearing at top only so
the definition should be a straight cut bit with a bottom bearing
and may also have a top bearing with a collar to secure it.

Great cor cutting dado,s with a router and template/straight edge guide.
http://www.cabinetmakerwarehouse.com...6hQRoCu9Tw_wcB





Well that is not entirely true. If you are flush cutting veneer or

true doesn't have gradations

Not always true.





laminate a 1/4" flush cut bit is more than adequate. and those are

a laminate 1/4-inch shank flush cut bit that doesn't deflect under
no load is adequate

more adequate or more better

Look, I am only trying to help you out. If yo want to get into a
****ing contest, you win.




The smaller the diameter of the cutter portion of the bit the faster
you need to run. Once you start using bits that have a 1" or greater
cutter you need to slow down the router speed.

or once you start using bits that deflect under no load

Then you have a defective bit or it is not properly mounted in the router.

On Mon, 16 Feb 2015 17:19:37 +0000 (UTC)
John McCoy wrote:

How the hell not? If you're cutting material off, how much
you're cutting always makes a difference, regardless of
whether you're flush-trimming or cutting a profile.

the most wood you can cut is the diameter of the bit plus the depth
depth is limited by top and bottom bearings

material was 1/8x1/8 a smidgeon

Then either your bit is bent or your router is bad. If
it happens unloaded then it's not deflecting.

no to both I tried in two routers I still need to try it with
the end bearing and allen screw removed

On Mon, 16 Feb 2015 13:18:50 -0500
BenignBodger wrote:

I'd just put a dial gauge on it and check for runout (powered off of
course). You could have anything from true bending of the bit, a
slightly dirty collet or sleeve, dodgy bearings, bad seating, or some
combination. 2-1/4" for such a bit sounds more like a pattern cutting

it's a 1-1/4-inch x 1/4-inch shank

bit rather than a standard flush-trim bit but I guess that the
difference is more a matter of degree. I've never seen flexing of
even a 1/4"-shank router bit used at normal loading.

I've never used any bits longer than 3/4-inch with 1/4-inch shank
so first time I'd seen it


as mentioned will remove bottom bearing & allen screw just to
see of there's an imbalance

On Mon, 16 Feb 2015 14:19:48 -0500
"dadiOH" wrote:

It is a plain old straight bit. The presence of a bearing is not
significant unless the bearing is unbalanced or loose.

right, that's the point

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".

right that was the point, more an exercise or example that I
intentionally exagerrated to make the point


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.

you say should but I say wouldn't
find some 4-inch x 1/4-inch diameter stiff rod and rev it up

On Mon, 16 Feb 2015 15:31:40 -0600
Leon lcb11211@swbelldotnet wrote:


Great cor cutting dado,s with a router and template/straight edge
guide.

now I know, just hadn't seen them as another poster pointed out too

Not always true.

it is, a partial truth is a full lie but I just figured your reply
was meant to be polite instead saying I was full of it


Look, I am only trying to help you out. If yo want to get into a
****ing contest, you win.

Yay! wait what'd i win

settle down is rec.woodwroking

Then you have a defective bit or it is not properly mounted in the
router.

I'm leaning toward a poorly engineered bit but I still have a couple
of tests to do with it

so possibly defective by design but still a chance that it's defective
in materials although it appears fine

Electric Comet wrote in news:mc22gu$k7o$2
@dont-email.me:

On Mon, 16 Feb 2015 17:19:37 +0000 (UTC)
John McCoy wrote:

load or not the bit deflects

Then either your bit is bent or your router is bad. If
it happens unloaded then it's not deflecting.

no to both I tried in two routers I still need to try it with
the end bearing and allen screw removed

After reading Bodger's comment, I realized what the problem
is he 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.

John

On 02/15/2015 11:15 AM, John McCoy wrote:
Electric wrote in news:mbqihi$pl8$3
@dont-email.me:

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

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.

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 would demand and expect my 1/4" drill bit to not wobble in my drill
press. I realize that the RPM is higher on the router, but the
principle holds true. Maybe I'm just lucky as I've not ever experienced
what you reference. I still suspect that the bit is bent as it doesn't
take much as you move the router around, lay it on its side, etc. I had
a buddy buy some cheap router bits - total waste of time and money. I
have mostly 1/4" shank carbide bits, I expect to pay $20 -- $30 per bit,
I have them resharpened, some are over 20 years old and still work fine.

On 2/18/2015 7:10 AM, Electric Comet wrote:
On Mon, 16 Feb 2015 14:19:48 -0500
"dadiOH" wrote:

It is a plain old straight bit. The presence of a bearing is not
significant unless the bearing is unbalanced or loose.

right, that's the point

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".

right that was the point, more an exercise or example that I
intentionally exagerrated to make the point


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.

you say should but I say wouldn't
find some 4-inch x 1/4-inch diameter stiff rod and rev it up