deflection in router bits

[Electric Comet]

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

[Sonny]

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

[woodchucker]

+2

[dadiOH]

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

1. Bent bit

2. As sonny said, bit isn't properly seated in the collet

1. Collet is worn or not properly seated

2. The bearings - the router bearings, not the bit bearings - are worn.

[joeljcarver]

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

[krw]

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.

[Electric Comet]

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.

[Electric Comet]

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

[Electric Comet]

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

[Electric Comet]

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

[John McCoy]

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

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

[dadiOH]

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.

[Electric Comet]

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

[Electric Comet]

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

[Leon]

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.

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.

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.

[Electric Comet]

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.

true doesn't have gradations

i turned the light not entirely on

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

more adequate or more better

or once you start using bits that deflect under no load

[John McCoy]

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

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

[BenignBodger]

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.

[dadiOH]

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.

[dadiOH]

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