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I've read many times that you should avoid climb cutting at all costs on a router table. The router bit can grab the wood and pull it from your hands and possibly sending your digits right into the bit. The same article say that climb cutting using a handheld router is not as dangerous and even encouraged to for getting a chip free surface.

What is it about the router that makes it more dangeous upside down than right side up? If anything, a kickback on a hand held router should be more catasfrophic as the tool can be thrown from your hands and take a nice bite off your navel before landing on the floor.

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  • You flagged this and said, "I want a more technical explantiom from a physics point of view." Graphus' answer addresses the physics when he discusses the mass of the router vs. the workpiece. Does that answer your question?
    – rob ♦
    Feb 14 '17 at 18:27
  • I still can't see how mass plays a role.
    – user148298
    Feb 20 '17 at 18:43
  • Mass plays a critical role in Newton's laws of motion. Specifically, F=ma. Rebalance the equation and you get F/m=a. Something less massive will accelerate more given the same force--in this case, that imparted by kickback. Assuming the router table is much more massive than the workpiece during the operation and the workpiece+vise+workbench (or however you fix the workpiece) is much more massive than the router during hand routing, you can compare the masses of the workpiece and handheld router when each is guided by your hands. The less massive will accelerate more.
    – rob ♦
    Feb 20 '17 at 22:30
  • Yes, I understand, but the small piece is being held by clamps to presumably a bench or some fixture with a bigger mass than the router. Wouldn't you now have a small mass, the router, versus an even larger mass, the table and the small workpiece?
    – user148298
    Feb 21 '17 at 14:56
  • Yes, that is correct, but you are now not comparing what you want to compare in your original question. You want to compare router table routing vs. handheld routing. Scenario 1 (router table): suppose router table is 100 kg and workpiece is 1 kg. Scenario 2 (handheld routing): workpiece to 99 kg table, (effectively 100 kg) and router is 4 kg. Recall F=ma. Scenario 1 (handheld workpiece): a=F/1kg, Scenario 2 (handheld router): a=F/4kg. The handheld workpiece's acceleration relative to the router table will be greater than the handheld router's acceleration relative to the fixed workpiece.
    – rob ♦
    Feb 21 '17 at 16:30
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Kickback can definitely happen with a handheld router. It's important to use the correct feed direction (typically moving the router from left to right) to prevent it.

There are two reasons I can think of why it might be called out more in a router table than with a handheld router:

  1. You can take off more material and use a bigger bit with a router table. Both of these will increase the force of the kickback.
  2. If you use a router table fence like a tablesaw fence (i.e. pinching the workpiece between the fence and the bit) you are at an extreme risk for kickback. The corollary situation for a handheld router (cutting only the far edge with an edge guide) is also dangerous, but isn't as tempting because it's tougher to setup.
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Good Answer already from @SaSSafraS1232 but I wanted to address some additional points you raised.

If anything, a kickback on a router should be more catasfrophic as the tool can be thrown from your hand and take a nice bite off your navel.

Compared to routing a length of wood a router will generally have greater mass. And with routers where there's a notable risk of a dangerous kick (i.e. powerful enough) you have two hands on it, gripping handles specifically intended for controlling its movement.

So together, if thrown the router will tend to go less far, less quickly, compared to a piece of wood snatched from your grip.

Do a mime of how you would typically feed wood past the bit in a router table.... see how not under control the wood is compared to a router being gripped and controlled with care and attention? Also notice that one or both of your hands is near the level of the spinning bit, rather than much more safely positioned up on the handles of the router itself.

This is of course especially hazardous as most router tables don't have a safety guard (as they should)!

In the same articles, climb cutting usung a hand held router is not as dangerous and even encouraged to get a good edge.

I think it should be highlighted that climb-cutting is not the only way to get a really good edge.

IMO it's also something you shouldn't do without a full understanding of the motions involved and the potential risk, so if in doubt don't do it.

There are other ways to ensure a superior cut surface, none of which involve increased risk to the user, or to the workpiece, (e.g. clean, sharp bit and taking a final 'skimming' pass to remove a very small amount of wood).

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    Imagine trying to run a board through a table saw in reverse. Climb cutting on a router table with a fence is a smaller version of this. It is far more difficult to control the a board than it is to control a router in your hands with the board clamped to a workbench.
    – Jacob Edmond
    Feb 14 '17 at 11:34
  • Agreed about other ways to a good edge. I've logged thousands of of router hours building guitars and fine furniture. I almost never climb cut, exceptions being sometimes removing thousandths to widen a dado.
    – bpedit
    Feb 15 '17 at 4:44

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