Far too many bits breaking

I’m trying to make door hangers for the family. All are out of 1/4" plywood, nothing fancy. I have a little over a dozen to make, I have broken 5 bits and only gotten 3 done. I have started off with a 1/16" downcut bit going at 60 in/min (broke), tried another at a different feed rate, (broke). 2mm bit @ 45 in/min (broke), took my last 2mm bit down to 30 in/min (broke). Ended it all with a big a** 1/8" bit going 25 in/min. It smoked like a brisket and eventually broke. Anything I’m missing here? I can share the file if need be. I’m at a loss.

What’s your depth of cut?
What speed is the router on? And which router do you have?
Simple version:
You need to take into account the speed (how fast the end mill is spinning… Should probably be as slow as you can), the feed (how fast the machine is moving through your stock) and the depth of cut (how deep the end mill is cutting each pass).
If the bit is smoking, you’re rubbing rather than cutting. Speed too high, feed too low.

Assuming a 1/8" bit (1F upcut) basic calculus on chip load indicate:

RPM 16k
Feed rate 25IPM
Depth per pass 0.06"

If you use a 2F bit double the feed rate

I’m using a DeWalt 611 set to the lowest setting, then tweaked it if I saw smoke. Depth of cut was .08, then I reduced it to the recommended .05 once I started having the issues.

What’s the bit cutting length? Are you sure it’s 1/4”? My 1/16” downcut only has a 3/16” cutting length so going to 1/4” would absolutely break the bit.

What bit are you using? This is an X-carve, right?

Yes. Well, not anymore haha

You say it is a simple design door hanger. you should use at least a 1/8 for cut outs (I actually prefer 1/4"). keep those tiny bits for detail work.

Depth of cut is usually half of the bit width

I wasn’t getting the clean corners i wanted with a larger bit. Ended u breaking a 1/8" bit anyway

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I just did about 6 hours of cutting this weekend with my 1/8 upcut bit on 1/4" ply (actually 0.22" but whatever). I had no breakage using the default settings, and indeed ran it sometimes at about 50 in/min with the router on 2 instead of 1. As such, I’m not sure how you’re getting down to 25 in/min with a 1/8 bit, especially if you’re causing smoke. This strongly suggests you’re just plunging way, way too deeply with your cuts. Can you post your project?


Smoke = rubbing = wood hardening = breakage.
Also, if the depth is too deep per pass your system may not be rigid enough causing the Z to appear “raked”. This in turn lead to a similar fate.

The number one factor to consider is chip thickness per tooth, if the bit is allowed to chew a optimum sized slice its happy. The machine stiffness then determine how deep you can go per pass.
In wood is is better to go too fast and shallow than the opposite.

Am I to assume I should just go back to the recommended settings when using a 1/16" downcut bit? I tried to speed up at lease the in/min rate because of the run time per piece. It was telling me about an hour and a half per piece and I have about a dozen more to go.

Physics is unaffected by your needs, I’m afraid. A given router bit/endmill is limited in the amount of material it can remove, per second, by such things as teeth shape/sharpness, the hardness and grain of the material being cut, the torgue applied by the router (and the strength of the bit itself), not to mention additional factors such as the heating of the material (as noted, smoking wood is harder than it would be otherwise, because you’ve compacted it with friction, pressing the cell-walls closer together.

Here is a well-regarded chip-load calculator for CNCs:

Here’s a more detailed explanation:

But basically, if you push your equipment beyond the means of the tool, the literally-inevitable result is failure of one sort or another.

If you want to learn how far you can push your machine, based on your bits and materials, the time to figure that out is not during a production run, but rather with experimental materials. Still, if you must do so, I’d suggest this simple workflow:

  1. Start the job with default settings (and with manual feed rate override enabled in Easel), with your router at 1.
  2. As the job starts, slowly increase your feed rate. If you see any bad behavior (e.g. chattering, bouncing, smoke), you can try to add a little speed on the router, but remember: that router wasn’t intended to run at maximum speed for hours on end. Most of us do our best to keep the router at 1 or 2 to try and get a year or two out of the brushes before failure. If you’re cranking it at max, you’re dropping that down by months at a time.