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Discussion Starter #1
Terribly sorry about the length here, but lots of variables and after reading other threads/responses I want to try to answer some questions before their asked.

I’ve broken two Freud ¼” Up Spiral Router bits (¼” shaft) while trying to cut 1” deep x 1 ¼” wide mortise slots in ¾” x 2” Red Oak using a Craftsman Professional router (12.5 amp/2.25hp) with the plunge base at just below its highest speed of 25,000 rpm (making mission style drawer fronts for kitchen cabinets and attaching the frame pieces with floating tenons—the center will be plywood). Both Up Spiral bits were brand new. I’m using the router freehand on a Mortise jig I made with solid clamps, a fence and traverse stops (eye & ear pro and tight fitting gloves are being worn as well). I don’t have a drill press or mortiser.

The first bit cut very well making mortise slots on the both ends of 24 pieces (all of the pieces that requiring mortise slots in the ends for this project). The bit broke on the 5th piece that I cut the mortise slot on the side—so different grain direction. The first bit broke inside the router collet and not on the blade. Also, while using the first bit, I was plunging all the way down to 1” and traversing straight over to the stop—not making layered cuts. It still seemed to cut very smoothly and the blade seemed as sharp as it ever was when I inspected the cutting surfaces.

I exchanged the bit for a new one at Sears and went at it a second time, again making mortise slots on the side of the pieces, but this time making multiple plunges at both stops and a few in the center of the slot, not traversing, just up and down, then cleaning the rest out by traversing in 2 to 3 layered cuts as to relieve pressure on the blade. The 2nd bit broke in the blade just above the shank on the fourth piece to be cut using the new method.

Other than technique, router speed vs. bit, wood, grain direction and all of the other variables possible, both pieces in which bits broke off had a 1/4" x ¼” dado cut out of length of the board on the side I was cutting the mortises on, and both ends where the bits broke had 2 Kreg jig holes cut in to the rear and end of the piece (recycling some wood in places that won’t get much visibility—it’s for me anyway and will be rear facing and covered by the drawer). It seems that the bits broke just after where they broke into the Kreg hole. Also, when making these cuts the grain is running the direction of the cut. Not sure if that makes a difference.

The spiral bit worked so well for the end cuts I don’t want to give it up and I don’t have another option. It also seemed to work great on solid pieces (not recycled pieces with a few holes already in them). It seemed like when the spiral bit hit some of the groves in the wood running lengthwise it pulled strips of wood out and broke the bit but I still think it may be my technique.
 

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That was alot of reading so I may have missed it, but you are hogging out too much wood at once, for such a small bit. Carbide is many good things but it is also very brittle. It won't bend a little like HSS, plus once it gets hot that don't help. Then you run into a "wall" (the other side of the kreg hole) and that's all it takes.

Try taking two passes at it. If they still break try 3 passes. Takes longer but what's better, a little more time or more broken bits.
 

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Discussion Starter #3
Thanks for the tip. I'll get another bit and try shallow passes until it works great. Any comments on using 3/8" mortises on 3/4" lumber vs. 1/4" mortises?
 

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Two things

The two most likely culprits (when combined):

Cut Depth

Red Oak

Definately take shallow cuts with red oak
 

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STICK is correct...depth-of-cut is one of the culprits. The primary culprit, in your case, is the 1/4" shanks. If you move up to 1/2" shank bits, your problem will go away - if you reduce the depth-of-cut somewhat. A good rule-of-thumb is to use a depth-of-cut equal to 1/2 of the shank diameter. There can be good reasons to violate that rule, such as when cutting a dado in plywood. I use 3/4" plywood bits and run them 3/8" deep in one shot - with no problems. The up-cut spiral bits work really, really well.
 

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A 1" deep cut is a heck of a lot of wood for a 1/4" shank to buck against. I'm with everyone else on this in that you need to make multiple, shallower passes. Also, You gave no indication of your feed rate, but you might try pushing it through the wood a little more slowly.

As Jerry said, the rule of thumb is 1/3 the thickness for tenons. Going to 3/8" will reduce your mortise sides to 3/16", so you'd have to weigh the significance of that in your joint strength.
 

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Discussion Starter #9
Brilliant. Thanks for the gouge. The 3rd bit worked like a charm, slower feed rate, shallower cuts, although I ended up plunging the router to depth without traversing throughout the cut, then cleaning it up with shallow passes. The frame pieces fit together nicely. Thanks. I couldn't find an upspiral bit with a 1/2" shank at any local retailers but there's a great local sharpening company nearby that makes their own. It was more than double the price of a 1/4" shank, and that worked well for this project in the end once technique was corrected. Thanks again for the input.
 
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