Router Problem

[steve-norrell]

What is your router top base made out of? The white part? (Not the feather board but the part under it.) Could it be flexing any? I can see your T channel, but what kind of channel or attachment is under the featherboard? To me, it looks like the FB apparently has an attachment point under it rather than in the visible T track.

You are right. The table top is a Woodpecker 18 x 22. It came with the miter channel that you can see. I installed two T-tracks at opposite ends of the table top -- that is what the featherboard is attached to. Directly under the piece being cut is the JessEm phenolic plate that their lift is mounted on.

While it is possible that there could be some flexing, I doubt that there is enough flexion to double (or more) the depth of the cut. And, to repeat, this does not happen under any other circumstances. I have used the same bit, without difficulty, to cut similar dados, BUT NOT WITH THAT LONG FEATHERBOARD.

The bit I am using is Bosch #85279M, a 3/16" straight cut bit. However, the cutting depth is only 1/2" and the 1/4" shank is only 1" long. I usually am careful to make sure that all but about 1/8" of the shank is within the collet, but since the shank is quite short, less than one inch is held by the collet and there must be an air pocket under the bit shank.

I keep coming back to the hypothesis that a) I overlooked something in setting up the operation, and/or b) the long featherboard blocked enough air flow to allow excess heat to build up under the bit, causing it rise in the collet. When I stopped or when there was sufficient air flow, the heated air contracted and pulled the bit back down to its normal position. (OK, I did say it was only a hypothesis and not even rising to the level of a theory.)

In any case, the input is most welcome. It contributes to all of our knowledge base and makes us better (and safer) woodworkers.

Regards, Steve

[JimD]

The bit cannot expand that much from the sort of temperature rise you would experience. It will not grow 1/4 or even 1/8 inch when it is only 1 1/2 inches long to start with. I haven't done the calculations but I am sure the coefficient of thermal expansion for steel is not this high.

I also do not see how it could slip in the collet and then go back to the original position. It almost has to be the issue, however, IMHO. Air pressure should be at most a minor factor compared to the forces on the bit/collet from the cut. I rarely make dado cuts with the router or router table because of bad experiences - mostly getting wider dados than desired at some point in the cut or breaking the bit. When I make dados, I cut them very gradually. I would use at least two passes for a 3/16 x 3/16 dado. The motor of the router can do much more than this but can the collet hold onto the bit and the bit take the stress? The answer to the latter appears to be yes but perhaps negative with respect to the former. Still goofy that it goes back.

My bottom line is to try taking smaller bites. I suspect it will resolve the issue.

Jim

[steve-norrell]

The bit cannot expand that much from the sort of temperature rise you would experience. It will not grow 1/4 or even 1/8 inch when it is only 1 1/2 inches long to start with. I haven't done the calculations but I am sure the coefficient of thermal expansion for steel is not this high.

My bottom line is to try taking smaller bites. I suspect it will resolve the issue.

Jim

Jim, I did not mean to imply that the bit expanded that much. I meant that the air under it may have, and I did say that that was only a hypothesis, which as you probably know, is science-speak for a guess.

I keep coming back to the first premise -- I overlooked something. It is likely that I will never know what.

Your suggestion to make smaller cuts is probably the real solution. Since I was making a relatively shallow cut of only about 3/16" I had assumed that I could do it with one cut, but that assumption was probably wrong (as you suggested).

Is it written that one should never make a cut that is more than half the cutting diameter of the bit? Or is it some other maximum cut depth?

One lesson out of all of this is that in the future the maximum depth I will cut will be about 1/8" and I will make multiple cuts, as needed.

Thanks to all for the input. Regards, Steve

[LCHIEN]

...
Is it written that one should never make a cut that is more than half the cutting diameter of the bit? Or is it some other maximum cut depth?

One lesson out of all of this is that in the future the maximum depth I will cut will be about 1/8" and I will make multiple cuts, as needed.

Thanks to all for the input. Regards, Steve

I'm not sure I've read a rule on that.
Think about this: when you use twice as wide a bit and don't change the depth (e.g. 1/8") then the side force on the bit doubles since it's plowing away twice the material.

So if the bit is twice as strong you really can't cut any deeper.

So my theory is that you need to keep the cutting depth no more than about 1/8" per pass no matter what bit diameter you have.

If you look at the cross-section of a spiral bit, there's surprisingly not a lot of metal there, its been removed to make the flutes.

[RayintheUK]

The difference in depth is way more than can be explained by sagging plates ......

Not necessarily, although it would also require operator intervention.

Imagine your plate does sag. Initial feeding from a flat table wouldn't have much effect, due to the majority of the stock being on the flat. The leading part of the workpiece passes over the cutter and reaches the other side of the plate, where it is supported on the (coplanar) flat table surface. Any pressure in the center of the workpiece now would cause the symptoms you describe if your plate sagged.

I'm not saying that your plate does sag, because you said that it doesn't. All I'm suggesting is that it is possible to reproduce the symptoms in the method I described, so there may be something in there that will help you figure it all out.

Ray.

[LarryG]

Is it written that one should never make a cut that is more than half the cutting diameter of the bit? Or is it some other maximum cut depth?

Route guru Pat Warner recommends a maximum of 3/8" x 3/8" or the equivalent per pass ... for instance, with a 3/4" dia bit you'd cut no more than 3/16" of depth per pass. This assumes a well-sharpened bit and a router in the popular ~2HP class.

I usually follow this guideline and find it's just about right. One thing I almost always do, however, is make however many preliminary passes I need to waste the bulk of the material, and then finish up with a very light final pass, removing no more than 1/64" or so.

[LCHIEN]

Route guru Pat Warner recommends a maximum of 3/8" x 3/8" or the equivalent per pass ... for instance, with a 3/4" dia bit you'd cut no more than 3/16" of depth per pass. This assumes a well-sharpened bit and a router in the popular ~2HP class.

I usually follow this guideline and find it's just about right. One thing I almost always do, however, is make however many preliminary passes I need to waste the bulk of the material, and then finish up with a very light final pass, removing no more than 1/64" or so.

Larry - does that mean you can take a 3/4" pass with a 3/16" bit??? I don't think so.

Is that for regular carbide edged HSS bits, or is it for spiral up-cut bits?

Granted, as the bit diameter goes up the cross section of the metal increases by the square of the diameter. So doubling the diameter results in a quadrupling of bit strength. Likewise halving the diameter reults in a quartering of the cross section and a drastic drop in strength.
Plowing twice the depth puts at least twice the force on the bit, maybe more because of the angles invloved, it want to break at the beginning of the fluted area.

I have broken 1/4" Spiral carbide bits at 1/2" depth and would recommend
not exceeding 3/16" per pass at all diameters, myself. I start at about 1/8" per pass and go to 3/16ths if I feel comfortable about the bit, probably larger bits 1/2" cutting diameter and up.

If, as you suggested, you pregrooved it to remove bulk material, you can take a deeper bite. This is essential when grooving with say, dovetail bits or T-slot bits where you can't really start shallow and go deeper.

[LarryG]

No, Warner is talking here specifically about larger diameter bits ... his point is that as the bit diameter increases, you have to reduce the depth of cut to compensate. The usual rules still apply for the smaller straight and spiral bits.