Sunday, September 20, 2015

Tuning and using shell bits part 1, the nose and the sides

Now we get to the fun part. Tuning and using the bits in a brace.

The shell bit...

And its variance, are simply sharpened at the cutting lip. Now as with any drill bit, if we touch the outside diameter, we effectively reduced its nominal size. So we will do most of our work on the nose, both faces, and mostly the insides of the side's flutes, IF required.

Think about sharpening a turner or carver gouge. We only do the tip...
Yes, we will eventually have to touch up the inside & outside edges to refine them.
Because...Sharp is the meeting of two flat surface with zero thickness.
As soon as I say flat surface, someone always question: But what about the gouge, they are not flat but curved?

Good question, simple answer: At every point in the curve are two flat surfaces meeting. Take a flat sheet of paper, imagine one edge is at the zero thickness, now curve it. Every point on it are still sharp, because every meeting point at that edge are......still flat. We are talking of course at a very small level, the size and distribution of the atoms of iron carbide surrounded by nickel, carbon, manganese etc.
You cannot go flatter (hence sharper) than that... Hint, that's why Stainless steel can never be brought to the same sharpness levels as High Carbon Steel (HCS).

Back up to our more manageable level.
IF we want to touch up the sides; We would prefer the whole surface of the shell edges to be in the same plane. Because it is easier to maintain that way.  Do a light sanding as required. And I mean light sanding! I would rather tolerate less flat than to remove too much material.

Here's the little secret to sharpening any drill bits.
You need to understand how they cut, so you can quickly troubleshoot a bit, to know if it is salvageable or if it is now worn out past its usefulness.
Once you know that, you can quickly see where you need to touch up to restore it.

Secret No 2: LESS IS MORE. Only take out as little as you need to make it cut with file, etc. You can quickly ruin a bit if you take out too much, you been warned!

The various shells family bits (Nose, Spoon, etc) have the major difference
at the nose. Make sure you do not mess up the end when flattening the sides.

Because of their construction, they have possibly two cutting edges.
The nose edges are what is doing the actual cutting of he hole.
The two long parallel edges on the side, are not really cutting.
You read right!

The nose edge is cutting, but look how rounded the long side edges are.

See the shape of the sides on the bit?

Leave a nice hole. If I try to go much deeper, 
the sides would now enter in play, no?

Lets make a through hole to find out...

It just requires light, constant down pressure to keep cutting.
Next thing you know, you just start to poke on the other side.
I could sense when the bottom changed density and stopped before blowing thru.

Hole on L is with my big nose bit, hole on R is from Center bit.
Both on backside.

We are through... The sides did not contributed
 to anything as far as cutting. It was all nose action, by a nose :-)

And BTW I uses that saw bench also as a place to pound chisels into the work piece, or drilled through into a 3/4 in bench dog hole. That's why I wanted it super heavy stout. Made of 2X6 construction lumber, dovetailed in the corner and braced by glued and screwed skirts.  Thanks to my bench dogs holes and removable fence , I can hold all kind of material to work on comfortably while straddling my bench.

My saw bench is also a mortising bench and a drilling bench.
It uses the same holdfast and bench dogs as my main bench.

Again hole on L is that very rounded sides, nose bit.
And on R Center bit. The sides are smoother from the nose bit.

And here is the big secret of that nose bit, 
it is really a small tanged turning gouge.
Not carving gouge, because it it was, it's pretty thick and
 heavy for no good reason. Yet it drilled effortlessly right thru...

Just to illustrated that the nose action of the shell bit's family, can be duplicated with a simple gouge, without having two cutting edges along its side. Exactly like a turning or carving gouge.

So what about the sides? Well, as long as they do not become lower than half the diameter of the bit, they would be fine. What happen if you lower the diameter of the bit? It wobble in the hole, becomes harder for the bit to track true in the hole. And chances are the diameter of the nose would be reduced too, if so, everything is fine, you are just drilling smaller holes :-)

As a matter of fact, the outer edges of a good bit should be slightly proud of the bit's shaft.

A good nose bit. The bit overall has been brighten, the outside very lightly polished, the sides are clean, no burr, the nose only is sharpened.

Now, since they are riding on the outside edges of the hole created, they could cut or scrape, or... do nothing, just keep the bit in the hole.

When they cut, they can becomes too aggressive and tear out more than they shave when encountering end grain. When you drill a hole on the face of a board, you get two end's grain and two surfaces going with the grain, and a bit of both in between as you go around.

1- going against the grain
2- going with the grain    
3- going against the grain
4- going with the grain     

If the side edges are either dressed to cut or scrape, they should not be reversed and only turned in the direction of the cut to protect that edge.

If you put a thin sharp arris on the edge, it would quickly crumble and cause problems, jamming etc. Remember that when tackling the hollow taper bit.

A small negative relief angle on the trailing edge of the cutting edge is all that is required. We are after more of a scraping cut than cutting. Hence the cutting edge is rather stout. There are a lot of force exerted on the side walls of the bit while cutting.

Notice how quickly you could reduce the diameter of the cut.
Unless it is nicked or damaged, don't touch it.

Look at this modern tapering bit from Lee Valley. It uses a rose head pattern.
Notice how the edges are sharpened. More like a scrapping action than a cutting one. With just enough clearances behind the scraping edge

It work perfectly good in wood, but it would be fine in soft metal.

Imagine trying to touch up every scraping surfaces at the same angle on all of them, with only a file? Forget it, you will quickly muck it up.

HINT to sharpen at home, using a previously shaped hole in a piece of wood
(Made by the tool you want to touch up) Sprinkle some abrasive powder (Valve lapping compound work fine), turn the bit in a drill in and out a couple time, done. Look at the brighter edges, still all in same planes. Don't forget to do it always to the full length of the bit, not just a portion of the tip.

All that to say, that AT BEST; Your edges could be scraping, never cutting.
Just make sure that there are no nicks etc that could scratches the inside of the hole. Rounded edges are fine, just no scratches...
So unless you have good reasons to, leave the edges alone, concentrate on the nose. Which you could treat just like a turning gouge or a carving gouge.
Work on both sides (inside/outside) leave the edge stout to be long lasting.

In case you were wondering how to best roll an edge on the stone, there is actually a jig to help you with that operation, it's called the Oar sharpening jig

The only jig I know that handle easily curved edge like on a carving gouge.
I never used one, I sharpen all my gouges by hand.
But this would be great to re-established a proper constant bevel on a mangled tool due to careless sharpening.

Next part, chucking it in a brace, which ones, when?

Bob, spinning tales, with or against the grain?


  1. Nice write up and thanx for the Oar jig link. I'll definitely look into that as I would rather use a jig then freehand.

  2. I have often thought it would be nice, but carving tools are very small and go fast.
    If you do get it, Ill be curious to hear what you think of it, maybe ill buy one after I read you :-)

  3. It is very possible to fit a Stage 1 tuning kit to your automatic Mini in the same way type because they are more visible and make the engine sound a bit throatier. Should I buy another Mini with a sounder shell and use that or should I pay thanks
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