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I recently started using my router table and read both here and on the net about router safety. That covered using the tool itself. Now reading the question about exceeding bit RPM I realize I never considered precautions about router bits.

Then looking at the link from this answer it brings other criteria to light:

  • use of an inappropriate router bit (in this case, a bit with wings too large for the shank)
  • use of a bit with a shank not long enough to be properly grasped in the collet
  • not marking the tool with the maximum permissible speed
  • not balancing the tool before use.

Inset photo from aforementioned link and article

Router-bit top view

Router bit’s wings are too large for the shank


Hopefully the answer is not just in the question itself, but what do I need to know about router bits to ensure they are safe for use with my router? To clarify when I say "my" I don't mean mine specifically.

A related query, stemming from the caption of the above image Router bit’s wings are too large for the shank, is how do I tell if a router bit was manufactured properly?

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    One would hope that the manufacturer tested the bits before shipping them to see if they're safe. Also, I have no idea how someone can look at a bit and say that the wings are too big for the shank. – grfrazee Dec 14 '15 at 15:36
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    Just seems like a judgment call to me. I mean, if you have a 1/4" shank and 5" wings, that's one thing. But the image you posted, I don't know you you can say those are too big. – grfrazee Dec 14 '15 at 15:53
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    The wings are 140mm, which is crazy big, but the shank seems rather thick too, over 2cm (might be 1''?). A 1'' shank is virtually undestroyable (a 12mm shank, half as thick, is already 2.25 times as strong as a 8mm shank -- which is strong enough to lift a mid-sized truck [ca 35kN]). However, the "no etching" part is what would immediately turn me away from this bit. No etching not only means "no knowing max speed", but also means no decent manufacturer. It is not even legal here to use such an "unknown" bit professionally (and given its size, it only fits in a professional machine). – Damon Dec 14 '15 at 18:01
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    There's at least 4 things you want to know about a bit, normally. Max speed, manual operation (or not), material, and shaft diameter (because 12mm and 1/2'' look nearly the same, but putting either one in the respective other brace results in catastrophic failure -- so you want to read to be 100% sure). All that is etched into the shaft, normally. – Damon Dec 14 '15 at 18:04
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    It's never safe. Always use a condom. Oh, wait, wrong forum. – Treow Wyrhta Dec 15 '15 at 14:47
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A router bit should have some information laser-engraved in its shank. Among these at the very least the shank diameter, maximum speed, and MAN (or not).
Some manufacturers (Trend, notably) have them written on the cardboard in the box, often in a "general" form such as 28,000RPM up to 30mm, 24,000RPM up to 30mm diameter, etc. (presumably they're laser-engraved too, but the writing on Trend bits is too small for me to decipher!).

A bit that doesn't provide such information should go right into the trash can as it is not possible to operate it safely. Seeing how my life depends on that safe operation, that's a must for me.

Shank diameter is not as obvious as you may believe. 12mm and 1/2'' shanks look quite similar, and 6mm and 1/4'' shanks are virtually indistinguishable just from holding them in your hand and looking at them.
However, clamping the respective slightly-too-small shank in the slightly-too-large collet will result in the bit slipping and possibly come hurling through the room. Clamping the respective slightly-too-large shank in the slightly-too-small collet will cause the collet to turn into shrapnels at an unpredictable time later. Neither one is a fun condition.

MAN is important if you intend to use the bit in a plunge router. It means the bit has a certain maximum clearance behind the edge, so it will only bite at most so and so deep into the material with each turn. That limits the amount of locking and kickback and determines whether it is reasonably safe to operate the tool manually (MAN). Lacking the inscription MAN, you may only ever use it in a router table (and forcibly controlled with a guide so the rotating bit won't kick the piece of wood out of your hands and into your genitals).

Regardless of whether a bit is "suitable" for manual operation, I consider diameters greater 20mm a "Meh, if I can't avoid" thing, and diameters greater 35mm a "Dude, you got death wish?" thing. Large bits become steadily more dangerous with increasing diameter, they belong in a router table.

Bits with approx. 140mm diameter like the one in the image probably don't fit into anything that you're likely to own anyway (they sure don't fit into my router, no 20mm collets available, that's an entirely different class of machine). But even if they did fit, I would think twice whether I'd want to use such a thing. 50-55mm is probably the largest diameter (profile cutters) that most people use regularly, though I've used 70mm, which is intimidating enough (and at least 89mm exists with 1/2'' shank, such as the T-C170AX1-2TC -- I've never used such a beast).

Consider that centrifugal force as well as cutting speed goes up with diameter, so quite possibly your router cannot even turn slow enough to operate safely (my router can just about rotate slow enough to satisfy 70mm, possibly 85mm profile cutters which usually allow 11,000 -- the ultra-large diameters often only allow 6,000RPM). Turning slower also means that your motor must be considerably stronger. If you have less than 2kW, you can pretty much forget running large bits at low speeds, it will be not a lot of fun.

  • @AstPace: You're right, it seems I've had a major episode of confusion writing that. Both increase linearly in respect of radius / diameter. Of course. What made me think something different... – Damon Feb 19 '16 at 22:36
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If a company sells a bit that is unsafe and you get hurt I am sure there's a few hundred lawyers that will take your case so you and them can retire. Just my opinion but have never had a problem with one coming apart on both soft wood and hard wood. Have even used carbide bits to shape aluminum with welding gloves and a face mask.

  • assuming the company even still exists and is located in a country where you can easily sue them. sorry but this is bad advice - assuming things are safe because otherwise they would have been sued already... – Steven Feb 19 '16 at 14:19

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