Tuesday, September 22, 2015

Part 3 Where we actually get to finally use our bit in a brace...

We now have our bit sharpened and tuned up.
We are aware of the various types of braces and chuck`s types, which would fit our bit best.
And finally we have an understanding of why which braces is better than another depending on the type of job we want to do.
It is now time to put the metal to the wood...

So if those shell types are so old and primitives, why would we want to bother with them?

Old yes! Primitives?  Hardly, they have yet to invent a better bit for many operations.  They are just what you need to make strong chairs connection.

Drilling stop holes close to the outside surface.
A normal bit will have its center bit poking thru or nearly so.
The nose bit can get as close but not poke thru.
The outside surface near the end of the hole is rather fragile on the square hole.
Look how much stronger the same surface is in the round bottom hole.
Square surfaces at the bottom of the hole are in stress. Chip and break easy.
No such stress found on round surfaces.  

Here is another illustration from a different point of view :-)
History has recorded the British Comet as the first Jet liner to fly in 1949.

Comet Mk1A in RCAF livery, we had two in short lived service.
The RCAF was the first military transport to usher in jet transport  
Trans-Atlantic flight, before the USAF and the RAF.

It beat the Avro Canada Jet liner by a week, due to last minutes delays.

Avro Canada Jetliner. It never went into production, sidelined by 
the Korean war and the need for combat jets.

But the Comet also quickly developed a bad reputation for randomly blowing up in the air, for no apparent reasons.
It was also one of the first commercial aircraft to be pressurized for the passengers comfort. Well, either that or give them all oxygen masks, they had a cruising altitude of 35,000 feet!

It is probably to this day, the most studied aircraft air frame in the world!
Problem it turns out was simple. It was because the windows were the old fashioned square ones...and the plane was subject to compression and decompression stress.

Take a close look at the windows on this MK1A

Notice the round windows in later versions.
MK2,3, 4 etc

You guessed it, the compression/decompression cycles generated stress cracks at the window`s corner and the aircraft suddenly lost compression (rapid decompression, boom!) Making the windows round solved that problem. Next time you fly, take a look at your windows, the corners are never square.

So now you should be convinced that a round bottom hole is stronger than a square bottom one at any altitude :-)

Yes finally...

Depending on your nose version in whatever shell bit you have, the lack of a center pin, allow the bit to be started in any directions (angle). That is a big advantage for chair making, as an obvious example.

Bit easily started at that roughly 45 degrees angle

The resulting hole and "fur" near the top of the hole, 
it is the end grain fibers, there is nothing to cut them.

Because of that same nose design advantage, it can also be hard to start, tending to skate around.
Small inconvenience, easily overcome by making a small divot using a gouge, (the actual bit would do that just fine BTW) or a tad more pressure to start the cut should suffice in most case.
If centering or position is important to you, then just make a small gouge like cut where you want it.
Making a shallow cut with a gouge to establish a starting point.
Only need two shallow cut, one from each side, twist, done.

Nose bit rides inside the shallow cup at start true easily.

After a few turn of the brace, we have a good hole established.
The round bottom of the hole make a perfect glue reservoir.

One drawback of the shell family of bits is the lack of any mechanism to eject the chips, the deeper you go, the more likely you will jam.
Keeping the bit shiny helps, and stopping to remove the bit to clear the chips is what you need to do in deep holes. In my example, I as drilling thru roughly 3/4 in stock, it wasn't necessary to stop.

The following is true of any drill bits, following tune up:
Assessing the cut quality.
Look at the shavings formation.

Big thick shavings coming from the center bit. Their thickness and how tight they curl up is a function of the router angle. Also to cut thick shavings you need a long spur (side cutter). If your spur is barely longer than your router, it will tear up.

Not much chips to look at from the shell bit, however it should show signs of cutting not just making dust

Are they flowing easily or sticking (wet wood is sticky in case you did not noticed)
Looking at their shape we can see how it is produced and where it was on the bit that was responsible for the tears or scratches or whatever you can read into.
You are becoming mesmerized by looking at the chips,  you are becom....

Take a look at the hole left
Is the rim well defined
how rough or smooth is the hole

Same center bit before and after sharpening.

Can you tell which thru hole is the center bit and which one is the shell bit?
Notice what is left in the center of the hole started by the shell bit.

Now if you understand your bit(s) , you understand their limitations and should realize that some results cannot be bettered by more touch ups. You want better, change bits!

Because of its shape, roughly a half diameter, you cannot overlap its cut.  The best bit for that remains the Forstner. The shell requires to be fully buried, all around, in the wood to work (to turn).

So there you have it, from the history of making holes to the evolution of drilling implements to the invention of the brace, we can make quickly, efficiently, round bottom holes in wood, which allow us to make stronger joinery in less space. (can make holes deeper, closer to the bottom edge).

The tools I used for this demo.

And that my friends, is a wrap.
And all the brace`s and bits lived happily forever after...

Bob, feeling dizzy from all the spinning around of the bit.


  1. Truly educational series of posts. Thanks again.

    Would you recommend the Lee Valley spoon bits or are older ones superior in some way?

  2. Good question Paul. I never tried the LV spoon bits, but i'm sure they would be fine.
    Spoons bit are prized by chairmakers and old ones don't seems to come along often . I think chairmakers are hoarding them all :-)
    Old ones superiors? Don't think so, LV seems to have gotten the right form.

  3. Thanks Robert. I suppose hoarding is going to be a bigger and bigger problem. I always ask thrift shop owners about old tools but they rarely get them. One said recently that she thinks all the old guys are having their tools buried with them.