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Scary moment / broken router cutter

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Scary moment / broken router cutter

Postby Wizard9999 » 07 Dec 2016, 16:44

Just come in from my workshop after a bit of a scary moment. I was cutting a groove (dado?) on my router table in the side of a piece of oak, one of four pieces that are intended to receive a panel to form the lid of my unphotographed and therefore imaginary box I am making. I was using a 6mm straight cutter which came as part of a set of six 1/2 inch cutters from Trend and the groove is a shade over 6mm deep. About a third of the way into the cut there was no resistance against the cut so I stopped the router and removed the piece I was working on. The cutter had sheared off the shank below the level of the table :o . I still haven't found the part of the cutter that sheared off and I suspect it has dropped into the router / table.

Clearly I am extremely glad the cutter did not have the scope to take off into the air at a possibly deadly velocity. Having gathered my composure I wanted to get some thoughts on the cause. My initial guess is that by cutting a groove the same depth as the router cutter is wide I pushing the cutter way too hard. Should this have been done in 2 or even 3 passes?

I'd like to learn my lesson and not repeat this!

Terry.
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Re: Scary moment / broken router cutter

Postby Pinch » 07 Dec 2016, 17:09

Hey Terry, that's an unfortunate incident. It could have been much worse if you were using the router over the work piece - there must have been a white feather in your shop somewhere. 8-)

Using a brand new 6mm flute to a depth of 6mm in one pass shouldn't be an issue, but this depends on what timber you're machining (any natural defects a long the line?) and which Trend cutter you've bought.

You're a lucky chap!

But... like they say, get back on the horse and all will be very cool indeed 8-)

:text-bravo:

PS EDIT: Any piccies yet chap? :|
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Re: Scary moment / broken router cutter

Postby Pinch » 07 Dec 2016, 17:22

Sorry Terry, me again :shock:

I wasn't that specific in my previous post and most likely, another one of our very fine chaps will come in and mention this too. I'm multitasking as I'm on cooking duties tonight.... Not that bit, but this bit to follow...... How old/dense is the oak? Does it feel dense or fairly light - for its size I mean. If it is dense or old, I would have gone with two passes at 3mm a pass - just for peace of mind. Could have been a slight defect in the bit of course.

Piccies will still be good though chap. 8-)

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Re: Scary moment / broken router cutter

Postby DaveL » 07 Dec 2016, 19:05

I think doing it in one pass was over stretching the cutter in oak, probably ok in pine.

I bought a set of grooving/slot cutters, it's an arbor with 7 different three winged cutters. It's much faster in use than using a straight bit.
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Re: Scary moment / broken router cutter

Postby Wizard9999 » 07 Dec 2016, 21:58

Thanks chaps. The oak is not dramatically heavier than I would expect, left over floor boards been drying for about 10 years. Yes, I think lesson learnt is slow and steady in future.

Problem with a slot cutter in this instance is that two of the dado cuts are stopped, so radius of slot cutter would mean having to stop way short and then chisel out the balance of the material.

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Re: Scary moment / broken router cutter

Postby Woodbloke » 07 Dec 2016, 23:39

I only ever take 2 or 3mm (max) when I'm cutting on the router table or even hand held. I also ensure that there's always plenty of shank gripped in the collet; it's a scary business when anything goes wrong - Rob
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Re: Scary moment / broken router cutter

Postby Wizard9999 » 08 Dec 2016, 22:35

Ordered a new solid carbide cutter from Wealden today, they appear to have a very good reputation. Will be making two or even three passes from now. First thing I need to do is find the broken bit of the cutter to make sure it is not lodged in the router somewhere :shock:

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Re: Scary moment / broken router cutter

Postby kirkpoore1 » 09 Dec 2016, 06:17

Terry:

I've busted several quarter inch straight router bits, including one where I was making the last pass of a through cut where I actually saw the bottom half of the bit fall off. This was in a hand held router, and the cut was suspended over my bench (each end was propped up so I could make the cut without hitting the bench top). The piece dropped, hit the bench top and stopped immediately. Although you might get cut by a bit like this, it would be minor (excepting a carbide fragment in the eye, of course). Here's why:

The tip speed of the bit is ((2 * pi * r * rpm) /12 in per ft) * 60 min per hour /5280 ft per mile
= (2*3.14*.125*20000/12)*60/5280 = 14.8 mph. Not that you want to be hit by sharp carbide even at that speed, but there is no way for the router bit to go sideways that fast--it's going to start slowing down as soon as it hits the first thing (hopefully not you). As a one shot deal in a direct line with a very low mass? It would be more dangerous for me to throw the bit at you. I've forgotten how to calculate angular momentum, but with a radius that small there just isn't that much there to translate into straight line motion.

Note that we're not talking a fragment of carbide here. I once it something with the tip of a cutter on my dado stack on my RAS and a tooth came off. It put a hole through the window behind my saw. But still, that was a 10" blade going 3600 rpm, or about 107 mph--5 times as fast. If a big diameter router bit comes apart on you, watch out.

So be cautious, but don't worry unduly.

Kirk
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Re: Scary moment / broken router cutter

Postby Phil » 09 Dec 2016, 07:11

Terry you are lucky. I have the greatest respect for the router in the table and also the RAS blade.

When routing I will only take 1mm off at a time, especially with hard timber. Even MDF.
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Re: Scary moment / broken router cutter

Postby tracerman » 09 Dec 2016, 09:49

Terry - yes its a scary business which is why I always wear my glasses , with my visor over the top . Sounds like you were unlucky , Trend are good . I bought 3 6mm straight cutters from Screwfix last year and all of them snapped off immediately even though I was only doing a 6mm deep slot in pine . In fairness to Screwfix they must have had a fault . Bought a Trend and its still in use . Its a bugger though - if you proceed too slowly it may burn the wood , if you rush it you may snap , best bet , mask up , go 3mm x 2 .
and wear ear protection also .
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Re: Scary moment / broken router cutter

Postby Wizard9999 » 12 Dec 2016, 10:16

New solid carbide Wealden cutter arrived Saturday and I had a bit of time yesterday before going to the panto... so straight back in the saddle. Having taken a fair bit of time to find the broken bit of the old cutter tucked up in the router I spent the rest of the time remaking the piece I had been working on when the cutter broke as when I looked at it closely it had widened the groove as it went. Made three passes of about 2mm each this time!

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