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A lot of router bits say "maximum speed X rpm". What will happen if I run it faster? Will I die? Will a portal to another universe open? Will demonic creatures, neither man nor beast, burst forth from the bowels of hell and spread chaos and merciless destruction throughout the land as angels helplessly look down upon us and weep for our souls?

What if I have no choice but to run it faster, what precautions should I take and what technique should I use?

For example, I am looking at a 1/16" slot cutter with a stated maximum RPM of 16000, but the variable speed range of my router is 20000 RPM to 30000 RPM. Can I still use the bit to make the cut?

  • 3
    "What if I have no choice but to run it faster?" Unless armed men are threatening to kill you unless you use it, you always have the choice of not using it at all and either getting a slower (or variable speed) router or a bit with a higher speed rating – Johnny Dec 10 '15 at 6:06
  • Quote: > Routers are a common item of fixed plant used in wood machining. They > are used to cut, trim and shape materials such as wood, metal and > plastic. > > A 31-year-old worker in East Bentleigh, Melbourne, Victoria, > Australia, was operating an industrial router at a cabinetmaking > factory when the bit disintegrated and a piece of metal hit her in the > chest. > > Contributing factors may have included: > > • 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 coll – Treow Wyrhta Dec 11 '15 at 20:31
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In short: No.

Compared to 16,000 RPM, 20,000 RPM is 25% over the specified maximum. Manufacturers -- even cheapish ones -- always calculate in a bit of tolerance, but 25% over limit is a lot. I wouldn't mind running a 19,000 RPM bit (if that existed) at 20,000 if it was the only one I had available, but surely not a 16,000 one.

What may happen?

1/16'' (1.5mm) is pretty slim, these bits are not unlikely to break even when treated properly. Most reputable manufacturers here do not even sell such small bits except with very slim shafts (3mm), which are usually only available for "toy" routers (like a 175W Dremel or Proxxon) that are a lot weaker and usually allow speeds as low as 7,000, too. A "typical" router for a Real Scotsman normally cannot fit such slim shafts at all, and for good reason -- you can already predict with high likelihood what would happen if you clamped that tiny fragile bit into the router.

Indeed, when you do find such ultra-small bits here, you usually get them in packaging units of 10, which is tell-tale for their destiny. Or, to cite Robert Louis Stevenson:

"Prophetic," said the doctor, touching this picture with his finger. "And now, Master Billy Bones, if that be your name, we'll have a look at the colour of your blood."

What will happen?

Nobody knows! The two most likely things to happen will be either nothing at all ("works fine"), or the bit ripping off right away near the shaft as you plunge in.

It is usually much more serious to have a wide-diameter bit go off (that's more like standing next to an exploding hand grenade). But of course, even a slim bit may just as well go flying somewhere, including the direction of your stomach or your face. You have no way of telling whether it will just work fine or whether you will die from injury.

Of course, desastrous failure can always happen with any kind of bit, all the time, and for a wide variety of reasons. Among them pushing forward too hastily, tilting, buying low quality bits, not maintaining bits properly, or just... bad luck. And, of course, running 25% over the specified maximum. Life is dangerous and often lethal, and power tools -- routers in particular -- are dangerous beasts.

But there is reasonable risk and unreasonable risk. Reasonable risk is wearing a knife-resisting vest (I'm actually doing that) which will catch 99.9% of all sharp objects that aim for your heart and your liver, and a polycarbonate face mask while working with well-maintained tools at the indicated speed. Or, more traditional, protective glasses and a thick cow leather apron.
Unreasonable risk is... everything else.

What may also happen?

The bad thing about taking unreasonable risk (read as: being grossly negligent) is that not only is desaster a lot more likely to happen, but also that you are out alone in the dark with the consequences.

If the bit sends out shrapnels and you lose an eye or are otherwise injured in a non-trivial way, you may find that your casualty insurance tells you: "Why! You ran 25% over the specified maximum, this had to happen", and refuses to pay the bills. The same can (almost certainly will) happen with your indemnity insurance in case you injure a bystander.
If nothing else, that alone is a good reason not to go too much over the limits. The risks simply are not acceptable.

  • I ended up making the cuts with a sheet of 1/4" polycarbonate bolted to the table between me and the bit, and lots of feather boards and push sticks. Reading this answer I'm not going to do it again. A slower router isn't hard to buy. I like the vest idea. – Jason C Dec 14 '15 at 15:56
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Note: Bits generally have speed concerns because they have a large radus, which means the outer edge is moving at a much higher linear speed and that you have more mass trying to fly outward. Panel-raising bits are the most common example of this.

A better alternative may be to switch from wide to tall, reducing the radius and mass back to something more normal. Vertical panel-raising bits are available for exactly that purpose. Disadvantages are that you have to run the panel past it "on edge", which takes a bit more work to set up safely (or a horizontal-router table), and that you have to make the back-cut (if one is needed) as a separate pass.

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Yes, you can run the bit faster than the maximum. None of the fantastic options you mention will happen.

What may happen, is that the bit will fail, spectacularly!

Should a failure occur, you could find flying shrapnel headed your direction. This could cause other spectacular failures (e.g., destroying your work piece, router, your body parts).

That said, the higher quality of the bit, the more you can run it over its maximum stated rotations. Note: a high quality bit does not necessarily mean more expensive.

You could always use the Montgomery Scott rule of engineering: add 10% to the maximum safe level in the manual.

  • So, really, the only proper option is to buy a slower router, correct? Is there anything I can do to minimize the chance of the bit failing? Is there a place I can put important body parts to minimize the chance of being hit? I imagine that pieces would tend to want to fly out in the plane of rotation, assuming they don't bounce off of things? – Jason C Dec 10 '15 at 1:25
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    You could also procure a router speed control to make the router run slower - basically a rheostat that reduces the voltage. – Ast Pace Dec 10 '15 at 3:53
  • @JasonC, they do make variable-speed routers for just this purpose. – grfrazee Dec 10 '15 at 14:04
  • @grfrazee Mine is variable speed, from 20000 to 30000. – Jason C Dec 10 '15 at 14:35
  • @JasonC, ah, I see your issue, then. – grfrazee Dec 10 '15 at 14:35

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