Hollow-Chisel-Mortiser Tune-Up

I didn’t know, originally, that virtually EVERY chisel that you buy needs to be sharpened, before you use it.

Neither did I extend that reasoning to my Grizzly Benchtop Mortiser’s hollow chisel mortiser bits.

But … after a LOT of use of the thing, and after always watching Norm Abram use what seems like SIGNIFICANTLY less effort, to make mortises with HIS Delta unit than my Grizzly seems to require …. I did some poking around.

I found an old test that compared like seven different hollow-chisel mortiser/bit sets, with the most expensive (Clico) being roughly TEN TIMES as expensive as the cheapest set.

The seven were:

...1) Woodcraft,
...2) Hartville,
...3) Lee Valley,
...4) Guildcraft,
...5) Clico,
...6) Shop Fox, and
...7) Fisch

Two clear conclusions:

...1) You really DO NOT get what you pay for, on this one.

In fact, the two MOST expensive sets were among the three considered “not recommended.” Shop Fox was the 3rd;

...2) Proper preparation of both chisel AND bit makes a WORLD of difference.

So … armed with this knowledge, and … thinking I should spend big $$$ and replace my OEM Grizzly bits … I, instead, went to the shop and did tune-up work.

Some of the instructions I worked from were found here. The rest was a combination of glancing at NUMEROUS sources, and … what I’d learned about sharpening, generally.

I ran all four outside faces of each chisel through the sandpaper gauntlet, starting with 150 grit, and—pretty much without skipping a grit—working my way through 1200 grit. What started as dull finishes, rough with milling and finishing marks … ended up as pretty much mirror-polished sides.

I then used the Rockler 220 and 600 grit diamond cones, made for sharpening the inside of the chisels.

The chisels have a 5/8” shank, so … using my Forstner bit … I drilled a 5/8” hole in a scrap of wood, and used that as the ‘stand/clamp’ for each chisel.

Centering it under the drill press chuck, I lowered the diamond cones, at the lowest speed my drill press will turn, and took a light pass at each of the chisel interiors for a few quick seconds.

I followed THAT with a quick de-burring process on a whetstone.

Look … I don’t have ANY before/after measurements, or ANY quantitative data to convincingly prove the difference that all this effort (maybe a total of an hour) made.

But here’s the deal.

The difference was staggering.

I had done a LOT of mortises in African Mahogany, so I knew THAT wood fairly well.

My newly-tuned bits flowed into and out of the wood MUCH faster, MUCH easier, without EVER getting “stuck” on the bit (so that the fence clamp had to ‘push the wood back off’).

The chip ejection was picture-perfect, smooth and steady, with smaller chips.

I made it a habit of holding my DC’s hose, with a reducer nozzle (ShopVac would work fine here, too), to keep the holes free of debris that would only slow the process.

After the mahogany, I mortised some SPF, some Red Oak, and some Maple—all with the same results.

To ME, the difference was night and day.

Truly.

The edges were cleaner, smoother, more even, and … the process took MUCH less time. It was FAR less of a wrestling match with the lever than before … and I HAD already greased the gears on which the head travels.

I did NOT take the time, yet, to properly sharpen the auger bits that come with the chisel sets.

That’s next.

If THAT makes ANY difference, then I’m going to be one tremendously happy mortise-making camper.

I also did NOT (yet) take a triangular file to work the inside corners of the hollow chisels, as the article linked above recommends. I will. I may also do what I can to polish the insides of the hollow chisels, figuring … can’t hurt, and may help chip ejection.

Two more learnings:

...1) The recommendation is that you point the OPEN face of the chisel _in the direction of the movement of the workpiece,” and

...2) Many sources recommend that you have the thickness of a nickel or a dime between the flat of the auger bit and the points of the chisel.

Testing revealed that MORE is better.

According to one test, a gap the thickness of a nickel—roughly 0.072” thick—results in operating temperatures roughly 40 or 50 degrees higher than if you increase that gap to 0.117” (the gap provided by the built-in spacer, on the Powermatic PM701 mortiser, used in their testing.

I have a 0.125” gage block that I will NOW use to set the gap between bit and chisel.

Going any further than that, obviously, creates fairly significant risk of breaking the bit.

Hope this helps.

-- -- Neil