Clearance between mortising chisel and bit. I'm not quite sure of this

[Don Morris]

I have a 20+ year old 4-piece mortising chisel & bit set made by FISCH, the Austrian Co. I tried to use them once when I got them but never really successfully. I have a project where it would be ideal to be able to use the set. The prior poor results I believe was because I didn't have it set properly. I don't think I understand and still may be missing the point (pun) of how to set the clearance between the chisel and bit. I may have misread/misunderstood the diagram that came with the set, showing a clearance required between the chisel and bit of 1/16" to 1/32". My understanding of that is: If you let the chisel and bit touch the piece at the same time, that won't work. The bit is supposed to touch the piece about 1/16" to 1/32" before the chisel touches the piece. So far that hasn't worked well. Where have I gone wrong? Thanks for any insight into this. Guidance from experienced users of these devices is what I should have looked for sooner.

[ChrisA Edwards]

Yes the drill part needs to be cutting out the core of the hole before the chisel starts shaving away the sides. I set mine about 1/16" - 3/32". You just don't want too much of drill hanging out the bottom.

Are you using this with a mortising machine or a drill press? Even with a mortising machine, the chiseling requires some good down pressure, especially on hard woods.

[Mel Fulks]

Agree with Chris. The bits need to be real tight or they will move and and damage the chisel. Used them a lot when making window sash.
They are fragile and expensive. They last about as long as a pencil.

[William Hodge]

The reason to make some room between the auger and the chisel is to prevent over heating. Different woods, and different moisture contents, require different clearences. The point is to not cook the tool tip, meaning don't turn it black. Leave just enough room for the chips to clear, and make it close.

If you are having a hard time, pre-bore the mortises with a smaller drill bit on a drill press. This takes a lot of burden off the auger. For smaller chisels that push the limits of the hollow chisel design, pre-boring is standard, for me. 1/4" mortises in Eastern White Pine will smell like a campfire if you don't pre-bore.

[Brian Holcombe]

I hang the bit out about 1/16”, I also grind the ramp of the auger to allow additional clearance.

[Brian Holcombe]

Here is a photo.

before

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after

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[Jeff Roltgen]

I've got the PM 719T free standing mortiser, as well as their chisels. Polished chisels on 1500 wet SC paper on glass, and used diamond cone mortise chisel sharpener from Lee Valley prior to chucking.
1/2" set has been sharpened, tuned, fixed in place and used on quite a few projects with zero attention for quite some time now.
No heating/bluing /smoldering, no re-sharpening. It just works. Every thing from QSwhite oak to plain old NWPine. Appears to be ~1/32" gap. Outer points of chisel are about dead-even with drill bit's flat auger tip - only the brad point extends beyond that depth. Slight rubbing-clicking sound as it spins idle, which is simply because nothing's perfectly straight - questioning the assertion that it needs to be tight to avoid chisel damage, and lean more towards keeping open chip-clearance, as it's clearly working fine set up this way. Really like that machine, and look forward to reasons to use it.


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[Don Morris]

I remember going back to where I bought the set and they gave me some sharpening stones. I'll make sure everything is sharp and then give it a go with all your added advice. I did notice some heat when I tried before, but no blackening or other color changes. Hopefully haven't ruined it already. Thanks for the input. MUCH appreciated!

[Aaron Rosenthal]

I seem to remember a Fine Woodworking video a few years ago about setting up hollow chisels for mortises.
I sharpen my chisels and bits every time I do a setup, I also set my bit about a 16th down down from the chisel end.

[Lee Schierer]

I have a mortising chisel set for my drill press. It works pretty well now, but didn't start out that way.

I honed the outside surfaces of my chisels to a mirror finish. I space the drill so it is about a 1/16th inch clearance between the wide part of the bit and the inside of the chisel. The first hole is the hardest and I move exactly one chisel with for all subsequent cuts. I use a fence to maintain alignment and a hold down on the pie e being cut to make extracting the chisel easier. My set is a cheap Chinese set so it fues get hot and sometimes clogs. When maki g deeper motives pull the bit up now and then in each cut to let the chips clear out of the interior of the chisel.

[Tom M King]

Old school is to put a dime under the shoulder of the chisel, slide the bit up until it touches, tighten the chuck for the bit, take the dime out, and slide the chisel all the way home.

The better chisel, and bit sets come with long bits, so you can cut them exactly the right length, so you can put it all the way home in the chuck. For my older, cheaper sets, I super-glued little washers on top of the bit to take up the space. I can put any one I own all the way home in the chuck, and not worry about it sliding up during work.

The quickest way to ruin a set is to let them rub. On long runs, with any machine I've ever used, the bit will work its way up. Having them the right length, so you can seat it all the way home, not only saves the set, but saves time fiddling with the initial setup.

[Dave Zellers]

If on a drill press make sure to slow the speed down.

[Malcolm McLeod]

Old school is to put a dime under the shoulder of the chisel, slide the bit up until it touches, tighten the chuck for the bit, take the dime out, and slide the chisel all the way home.. ...

I don't carry cash anymore; credit card is a good stand-in for me.;)

[Mel Fulks]

[QUOTE=Tom M King

The quickest way to ruin a set is to let them rub. On long runs, with any machine I've ever used, the bit will work its way up. Having them the right length, so you can seat it all the way home, not only saves the set, but saves time fiddling with the initial setup.[/QUOTE]

That is a good idea. You can use the machine for an hour and think all is well , then the bit moves and gets damaged.

[roger wiegand]

I used to use the dime, now I do it by guess and by golly. Seems to work in maple, cherry and walnut.

For me prep of the chisel and sharpening was key. I honed the outsides of the chisel on a fine water stone so that they had a mirror polish, then used a fine diamond hone to sharpen the points. They need to be sharpened with the care you'd give a plane blade or bench chisel. Polishing the faces made a big difference in how much effort it took to push the chisels into the wood. With the augur bit I've just touched up the cutting edge after every couple of uses. The original Chinese bits that came with my mortiser were pretty hopeless, the better Japanese bits from Lee Valley and the Fisch bits are much better in terms of staying sharp, but all need to be honed for use.

[Don Stephan]

I keep a dime, penny, nickel, and maybe a quarter with the mortising machine tools and use one, depending on the size of the hole being made. Don't know if I noted which to use with each chisel set size.

[William Chain]

Old school is to put a dime under the shoulder of the chisel, slide the bit up until it touches, tighten the chuck for the bit, take the dime out, and slide the chisel all the way home.



I do this idea but with a rare earth magnet about the thickness of a quarter. It holds all the pieces in place, I get the bit chucked and tight, then raise the chisel collar up by removing the magnet. This seems to work nicely each time. And make sure those chisels are honed.