Hollow Chisel Mortising

[Cody Colston]

I've been building garden armchairs out of Eastern Red Cedar. The joinery is all M&T and I've been having a time with the 1/2" mortising bit and chisel clogging right at the bottom before the chips get to the exit slot in the chisel.

I'm using a Delta Model 14-651 benchtop mortiser. I've varied the bit stick-out but it makes no difference. About every other mortise, it will clog and instead of cutting will just start smoking when the wood gets hot from the friction. I then have to stop and dig the packed-in wood out with a scratch awl.

I'm still using the bit & chisel sets that came with the mortiser and I suspect they are not all that great. What brand is recommended to replace those that I have now?

[George Bregar]

Cody, I have a Jet and the same deal on the 1/2" bit. I find that if I only go 1/2 or 2/3 of the chisel over (that is, overlap) that it reduces the incidences of the bit getting jambed up. Even in QSWO, which is really a coarse grain. HTH

[harry strasil]

I use a foot mortiser and wax the cutter to make it cut easier and release from the cut easier, have you tried waxing the inside of the chisel and the flutes of the bit, spray silicon or just plain household pledge might make the chips slide up the flutes or just rubbing the surfaces with small candles.

[Larry Edgerton]

I take my auger bits to the buffing wheel and make sure all of the flutes are polished to perfection. The better bits are already pretty good in this respect, but a little additional polish seems to make the difference in chip extraction, plus it keeps the temp down for better bit life.

I have a air blower rigged up with a brake line ran through the bit collar that blows the chips out of the hole with a foot pedal. I have not had any problems with plugging up since I did these two things, no matter what the wood.

The bits I like are Fische and, well, I am drawing a blank.......

Someone will mention the other one, but the big difference is the quality of the machining of the flutes on the auger. That and the Tiawan bits tend to bend the tips of the chisel outward on something like Jatoba, and then they split.

[Cody Colston]

Thanks for the tips, guys. I did use some spray silicon in an attempt to help out with chip extraction but it didn't have effect. I'll try buffing the flutes on the bit, also.

[Alan Schwabacher]

Try smoothing the inside of the hollow chisel with a chainsaw file.

Roland Johnson has an excellent video called "sharpen a mortiser bit" on the Fine Woodworking site showing all the different ways to tune up a hollow chisel and bit. His conclusion was that all of the hollow chisels could be made to work well, and the differences were in how much work it took. He shows how. If it's one of the videos they charge for, you can get a free short-term subscription to see it.

If you do buy new ones, note that the chisels come in various sized shanks -- usually 5/8" or 3/4" for smaller mortisers. Not all chisel vendors mention which size they are selling.

[Floyd Mah]

My observation on hollow chisel mortising is that there is an inherently poor design to the technique. The drills are not designed to aggressively remove wood, like an ordinary brad point or Forstner bit. The tolerances are not loose enough to avoid a lot of downtime to trouble-shoot each hang-up. I don't have enough time to fine-tune each bit-chisel combination. With that in mind, the last time that I tackled a project requiring making mortises, I changed my approach and was very successful in producing adequate mortises, and with little wasted time.

The hollow-chisel mortiser is really most efficient at producing straight sides. The chisel points bite in early and will insure that the chisel progresses vertically. The bits work best when they have light work, i.e., removing the chiselled wood. They function poorly when required to drill large holes (like the 1/2" holes) and then removing that waste. What I did was approach the mortising operation in two steps. I pre-drilled holes where the mortise was going to be located. The holes did not have to overlap, otherwise I would be faced with the drill trying to go into the adjacent void. This was quick work that I could do on the drill press. Next, I took the work to the hollow-chisel mortiser and used it to create the straight sides. No burning, light effort, accurately placed cuts. The chisel did not wander due to the pre-cut holes.

To some extent, I had not zipped through the task like Norm on the New Yankee Workshop. His mortise chiseller always works like a charm, leaving me with a bit of hollow-chisel mortising envy. However, I got the work done, and very efficiently. Anyway, if you use the machine as you would use a paring chisel to fit any of the other joints that you make, you can get a lot more done without the frustration.

[Julian Nicks]

I use a multico mortiser with fische chisels. I have never had a problem with the chips clogging up in the bit. Are your chisels and bit sharp? What method do you use to sharpen them? I keep mine very sharp and have no issues in any type of wood.

[steve swantee]

Try the Lee Valley premium mortising chisel and bit sets. I have found them head and shoulders above the chisels and bits that come with the machines.

Steve

[Scott Rollins]

I second the Lee Valey Premium chisels. They are very good and required very little effort to tune for use.

[Larry Edgerton]

The bits I like are Fische and, well, I am drawing a blank.......

.

I remembered! Clico. Thats the other brand. What a memory! :(

[Gary Herrmann]

Another vote for the LV chisels. I also lapped the outside of the bits and briefly touched up the inside with the cone sharpener they sell before I used them the first time. No clogging at all.