deflection in router bits

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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
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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.
Sonny
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On 2/14/2015 8:26 PM, Sonny wrote:

+2
--
Jeff

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

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

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

Not sure what this means
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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
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On Sun, 15 Feb 2015 17:15:58 +0000 (UTC)

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
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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.

Then either your bit is bent or your router is bad. If it happens unloaded then it's not deflecting.
John
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On Mon, 16 Feb 2015 17:19:37 +0000 (UTC)

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

no to both I tried in two routers I still need to try it with the end bearing and allen screw removed
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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.
John
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On 02/15/2015 11:15 AM, John McCoy wrote:

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...
--


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wrote:

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

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

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On Sun, 15 Feb 2015 11:14:40 -0500 snipped-for-privacy@attt.bizz wrote:

right, won't be buying any more long 1/4 shank bits, short ones are fine if price is right
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On Sun, 15 Feb 2015 06:41:23 -0500

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
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wrote:

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
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On Sun, 15 Feb 2015 15:50:46 -0500

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

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
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wrote:

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

right, that's the point

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

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