Pushing a router bit to the breaking point

[David Nebenzahl]

... actually past the breaking point.

1/4" straight carbide bit (Craftsman), kinda long (more than 3/4"), being used to cut a slot in 3/4" pine, using my old Craftsman router (1/4", old-fashioned little one). I was cutting along, carefully regulating the feed rate so as to only slow down the motor a little bit, thinking how much stress that bit was taking. Sure 'nuff, after cutting about 6-7 slots about 15" long, the bit just broke off. Didn't go flying at all, but just landed on the table under the router in a pile of wood chips.

So obviously this is my bad. I was even thinking as I was making that last cut that maybe I should be doing this in two passes.

So what're the rules of thumb for how much stress is too much for bits like these? The bit seems well-made, so I don't think I can blame it. Now I'm thinking of making that same cut in two passes (~3/8" depth of cut each). Would that be un-stressful enough for the bit?

[Lew Hodgett]

RE: Subject

It's a "finishing", not a "hogging" tool.

Plan accordingly.

Lew

[Joe]

any single pass.

your opinions will probably vary (yowpv?)

jc

[Nova]

A general rule of thumb is to cut only as deep as the diameter of the router bit. For a 1/4" diameter bit the maximum depth os a single pass would be 1/4" cut.

[Robatoy]

Indeed. Wrong bit for the job. That is more a job for an 'upcut' spiral. I find that one can lean into those a lot more as it is all about chip-ejection. A straight bit can't get out of its own way because the chips have nowhere to go but towards the back of the cut.

[RicodJour]

I'd add another thumb to your rule, and not cut deeper than the smaller of the bit or shank diameter.

R

[Dave Balderstone]

With a straight cut bit I don't go more than 1/8" per pass.

Take more, shallower, cuts, and the job will go faster because you won't be fighting the tool.

[David Nebenzahl]

On 5/10/2009 5:04 PM Dave Balderstone spake thus:

Good suggestions so far, including the one about using a different tool (spiral cutter).

I notice one thing that happens is that there's a lot of fairly high-frequency chatter when you use a tool like this to "hog" wood, less with shallower cuts. It doesn't show so much on the cut, but I can sure hear it. I wouldn't be surprised if this vibration contributes to early tool mortality (all that flexing and stuff).

[Dave Balderstone]

Vibration is bad, period.

By taking lighter cuts, the cuts themselves are faster with less stress on the tool and on me.

Say I want to cut a fair number of 1/2" x 15" slots (let's say 10 for arguments' sake, or 150 linear inches). I can zip through those inches with a light cut (say six 1/8" passes) pretty damned fast with hardly any stress on the tool, the bit or my hands pressing the pieces against the fence.

Trying a deep cut, I'm not only stressing the router and the bit, but my hands and arms are going to get tired, and if I back/slack off at all, I just know the damned piece is going to come away from the fence, and I better hope that where I wanted that dado isn't visible in my finished project, because if it is, I've just made scrap wood.

In other words, the hurrier I go, the behinder I get.

[Tom Veatch]

I'm sure you're correct with that observation. Things don't vibrate without flexing and stress reversals from those cyclic deflections are prime contributors to fatigue cracking and failure.

Tom Veatch Wichita, KS USA

An armed society is a polite society. Manners are good when one may have to back up his acts with his life. Robert A. Heinlein

[Tom Veatch]

I've made more scrap as a result of impatience than from all other causes combined.

Tom Veatch Wichita, KS USA

An armed society is a polite society. Manners are good when one may have to back up his acts with his life. Robert A. Heinlein

[Dave Balderstone]

Amen.

[Lew Hodgett]

==================================== It wasn't until I started sailing single handed that I began to truly understand the virtue of patience.

A boat under sail has a rhythm that is insync with nature.

Try to rush things, you WILL make a mistake.

Make a mistake when single handing, chances are pretty good you ultimately end up DEAD.

Chances of finding any remains, SLIM and NONE, more likely NONE.

You quickly learn to be patient.

I find it is also a pleasant way to enjoy life while sailing.

YMMV.

Lew

[Joe]

the suggestion about the upcut bit is a good one because of the chip clearing. However, by taking much lighter passes with a straight bit, you may find that the chip clearing becomes moot. lighter passes produce less chips which are less likely to get jammed in a shallower grove. In other words, they become self clearing. And straight bits are cheaper!

[Lowell Holmes]

Lew,

It's hard to be patient when you are falling off the backsides of waves in a

10 foot sea at 2AM.

You had better be patient though. :-)

[Keith Nuttle]

I always thought that the ability to plan the future and patience were the things that differentiated a sailor from the common man.

[Lowell Holmes]

That may be the description for an aged sailor, but in their youth, it also has to do with proving they have hair on their chest.

[sweet sawdust]

that solid carbide bits tend to break/shatter when you get vibration, over torque on bit or heat build up. The bits are too brittle to handle much stress. Craftsman sometimes seems to have bits with case hardened shafts which act like solid carbide bits and break easily, any stress on the bit and you have a two piece bit. To do what you were doing I would use a HSS bit which seems to take stress better then carbide bits or bits with carbide cutters. You will change bits often though. Your best bet is as stated in other post is to use multiple cuts to reduce stress on the bit. If you wish to make only one router cut you could hog out the slot with a drill bit and use the router for clean up but I still believe that a decent quality bit with carbide cutters and multiple cuts would be the best way to go.

[Lew Hodgett]

I hear ya, but I'm of the school that gentlemen don't go to weather, that's part of being patient.

Lew

[Lowell Holmes]

I agree, but sometimes it just seems necessary to "harden up" and in those instances, my adam's apple would be in my mouth.