Climb-Cutting - Question of the Week
This week’s question comes from Paul-Marcel who says:
On play-day earlier this week (when I got my Bosch back from warranty repair -and- got the OF-1400!), I was using the 1400 to plow grooves for panels in this red oak cabinet. I was using a 3/8″ compression spiral bit to create a groove for 12mm plywood panels. Naturally this takes 2 passes. The second pass, I know to go the correct direction to avoid doing a climb-cut. But, I thought that when you plow the first pass, there really wasn’t a climb-cut or regular-cut direction since the bit is surrounded by wood (except in the wake). As it moves forward, one side of the bit is climb-cutting while the other is regular-cutting (hmm, not the right word, but you know…)
The thing is, the router got pulled and jerked exactly like I was doing a climb-cut when I was pushing the router (so the dust port was in the rear). If I changed directions on a different panel’s groove and pulled the router (dust port taking the lead), there was no climb-cut feeling. aWas it just something funky about this red oak or did I somehow miss an important footnote about climb-cutting? Turns out I preferred pulling the router because it was easier to give up slack on the vacuum hose than it was to pull the hose with you, but that still doesn’t explain this.
And this was my reply:
Hey dude. Good observation. I have made the same assumption about climb-cutting on that first pass. How can it be climb-cutting if the bit is surrounded, right? But as you saw, there is definitely a difference in force when you go one way or the other. Fortunately, this is not evil German black magic. There is a sensible explanation. Although the bit is surrounded by material, it is still rotating in a clockwise direction. So if you also push the router in a clockwise (or right to left) direction, you are still pushing in the direction that the bit naturally wants to go. So combining your hand force and the natural force of the bit, especially when you aren’t expecting it, can lead to disaster. Here’s a good analogy in case you can’t visualize the scenario. Think of a paddle boat with a fully submerged wheel (wheel has directional paddles on it). The wheel, even though its submerged, still has a natural directional quality that propels it forward. And if you are in the water pushing the boat in the same direction, your life will be much easier because you are both applying force in the same direction. Now think of that boat paddling away, and you are trying to pull it back. You are then pulling AGAINST the direction the paddle wants to go. No fun if you are the swimmer in the water. But if the boat is a router and the swimmer is your hand, that leads to a much more controlled cut. It all comes down to the direction and orientation of the blades. Make sense?
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In a situation like this, you are actually doing both. Under-cutting and climb-cutting at the same time. The bit is cutting on 180 degrees of the body. 90 degrees of it doing each. The under-cut takes some force from you, the climb-cut wants to self-feed, so the cut can feel like either is dominating. The wood itself has some effect on this too, as does the direction of the cut (with or across the grain) Depth of cut, number of flutes, shape of flutes, etc.
If you feed too quickly, the bit can chatter, which not only cuts a wider groove than the bit itself, it exaggerates any wandering feeling.
I have always prefered to “pull” when possible.
I’m not sure I followed with the paddle wheel there… But besides wood grain, depending on the direction you move the router, I guess the bit wants to either pull the router against the fence (or the piece against the router guide), or push it away. The former stabilizes everything, while the latter might be more prone to vibrations or following the grain…
Marc, either I don’t understand what you’re saying or I think you’re misunderstanding the forces involved.
When plowing a groove, the bit is surrounded by wood on all its front half. This will tend to make the router turn (anti-clockwise, against the bit) but will never pull away from you as long as the material is uniform and you have “ideal” dust collection.
If the material is not uniform, however, or if dust tends to accumulate on one side of the groove, one side of the bit might “catch” more than the other, causing the router to pull away from you or suddenly require more force to keep plowing.
Also, I don’t understand what you mean by a paddle wheel’s “natural directional quality”. If a paddle wheel is fully surrounded by equally dense liquid, spinning it won’t make it move in either direction, regardless of the shape of the paddes (unless they are hinged and their angle is changed dynamically). Note that if the wheel is just barely below the surface, there is almost no resistance in one direction (because water is being pushed up against air), but that has nothing to do with an intrinsic “directional” property of the wheel, it’s just an uneven medium (like routing a groove in non-uniform wood).
Well, this is starting to hurt my brain, lol. That was just the best analogy I could come up with. But that’s why we have comments here so that everyone can benefit from the collective knowledge. Thanks for the input everyone.
By the way, I am having trouble wrapping my brain around the last part of your explanation Mike. If a paddle wheel of some type were submerged underwater, and the paddles themselves had directionality, are you saying that the wheel would spin in place and never move? I can’t see that happening. I would think the wheel would have to move in a particular direction, no?
Obviously I am taking us way off the target topic here, but now you have me wondering. ;)
Depends on what you mean by “directionality”.
If you mean fixed paddles that just happen to be curved, then yes, it would just spin in place. Remember, the curvature is reversed when the paddles spin 180 degrees, so the amount of water pushed back by the paddles at the bottom will always be identical to the amount of water pushed forward by the paddles at the top. This is assuming a stand-alone wheel with no ducts, etc., around it, of course (not that those would help much, but then the forces would be more complex).
If by “directionality” you mean the paddles are hinged and their angle changes dynamically, then it’s possible to build a paddle wheel that pushes harder in one direction even at depth, but I doubt it would ever be energy-efficient. In fact, even a paddle wheel that is only 1/2 submerged will be virtually useless, since most power will be wasted pushing water up and down, and the wheel itself increases drag. For a paddle wheel to be even remotely efficient, you want only the tips of the paddles to actually go underwater (less than 20% of its radius submerged), so all the energy is pushing the water “back”.
OK. Let me take a stab at this. Two things: First, other than changes in the density of the wood, all forces are equal in a plunge cut where the bit is completely surrounded by wood. Second, the reason why there is more control when pulling toward yourself versus pushing is simply the musculature of the human body. When pulling you are more dominantly using the muscles on the inner circle you’re creating by holding the router with both hands.
Well, that’s my stab. The paddle wheel on the boat would need a pitch to the blades to propel in a particular direction if completely submerged.
I would say take lighter passes on the initial part of the groove or use guides on both sides of the router. The latter being the least likely to create error.
BTW, Damien has a point there, too. If you’re using a single fence, that fence should preferably be to the left of the groove (left relative to the direction the router is moving in), otherwise you’ll have to push the router against the fence as well as pushing it forwards (you should always push it slightly against the fence anyway, but if the fence is on the “bad” side you’ll have to push harder).
And if you’re using an edge guide, the guide should be on the right, or you’ll have to push it harder into the piece.
If you’re using a ruler and a slotted guide plate or if you’re using two fences or a dual edge guide (i.e., if the router can’t slide to either side, just backwards and forwards), then you don’t need to worry about any of that. By “backwards” and “forwards” I mean relative to the direction of motion, nothing to do with the base plate orientation, of course.
There should be a way to include drawings in the comments. :^)
I need to amend my response. “The paddle wheel on the boat would need a pitch to the blades to propel in a particular direction if completely submerged” A pitch in a “paddle wheel” surrounded by water (which should have the same density in all directions), would still not have propulsion, unless the pitch were similar to that of a regular propeller. Then it would propel sideways (or parallel with the blades) much like a turbine. Now, I think I got it right….who knows?, I’m tired. I’ll hang up and listen to your response:D
I think the key here is where the router bit cutting edges are located. Even if the bit is surrounded by wood, the cutting edges are going to create more force in one direction because they are biting into the wood.
Perhaps a wind speed indicator would be a good analogy. No matter what direction the wind blows, the indicator always spins in the same direction because one side has more “bite” into the wind.
I am thinking the torque of the motor may have some effect.
Vic: Your correction is correct. :-) In fact, the transition from paddle boats to submerged propellers came about by experimenting with “screw-like” paddles (propelling along the axis of rotation, like a turbine). The propeller is just a more efficient design for liquids (turbines are more efficient for gases, because gases can be compressed, while liquids can’t).
Germain: A wind speed indicator is a bad example, since it stays still as the air (a fluid) moves around it, and spins as a result of the air movement. A router bit rotates because it’s driven by a motor. If you pushed a router bit really hard with the motor turned off (so hard that you still managed to plow the groove), the bit would indeed spin of its own accord, much like a wind speed indicator (in the direction contrary to its normal, powered rotation). This is because the flat (front) face of the blades would offer slightly more resistance than the angled (side / back) face (assuming the wood had some deformability, however small).
Chris: The torque of the motor does have an effect, but it will tend to spin the router in place, not push it linearly in any direction.