Bow drill enhancement

[Jeff Wagner]

I have stumbled across something concerning bow drill spindles that seems very interesting. I have never heard of this before and have not yet tried it to confirm, but on an intellectual level it seems to make perfect sense.

The premise is, the working end of your spindle starts out flat and becomes spherical or pointed because the friction with the heath board is highest at its outer edge and is reduced as you move inward toward the center. Therefore, the spindle material at the outer edge is worn away more rapidly than the material at the center causing the spindle to become pointed. The fire making instructional video I am now reviewing suggests that the center of the spindle that wears more slowing now causes a non friction generating bearing point that impedes friction and the generation of heat at the out edge. In other words, as your spindle end changes from a flat profile to a rounded one, the amount of work required to gernerate heat at the outer edge increases.

Now here comes the good part....

It was suggested that carving a hollow in the center of your spindle eliminates the interference aspect and allows maximum friction and heat to be generated per turn of the spindle. They simply took a knife point and simple dug out a concave at the spindle tip.

This concept is making very good sense to me because a concave tip is exactly what I get on mullein hand drill shafts. The soft pithy core wears away very quickly leaving the harder outer skin.

How about somebody with a bowdrill set up give this a go and report back?

 

[Alberto Hernandez]

Hola Jeff

I have had problems with hand drill spindles that become hollow while I am using them to get a coal. Because some times they produce a ant hill affect on the fire board that causes a the spindle and fire board to bind!

I prefer my hand drill spindles and especially my bow drill spindles to have some kind of a blunt point when I use them to get a coal! By having some kind of a point they burrow in to fire board mush better for me. I find that a flat spindle has a tendancy fly out of the hand hold when I start spinning!

Tu amigo
Alberto

 

[rich59]

I like to think that there are two parts to the tip of a drill. The centre and the outer ring. In reality of course there may be no absolute distinction.

The centre is travelling slowly whilst the outer ring is travelling fast. If there is equal pressure in both areas then the outer ring will produce the most heat.

Another factor in this is the curve of the drill tip. If the outer edge is curving away from a central point then it presents a much larger surface area that dissipates the heat making it harder to get a fire.

With the bow drill much of this is academic as you should have ample friction from even light pressure due to the high speed of the bow drill.

With hand drilling it is highly relevant as you have much less speed and rely much more on pressure.

Different woods produce diffferent shapes of hollow in the hearth. A truely hollow drill with a thin out rim may produce a thin rim with a proud centre hollow in the hearthboard. But if the outer rim of the drill is thick then the central hill regularly breaks off so the depression wears down fairly square.

If I bow drill I find a key challenge is keeping the drill in the hollow while burning it in at the start of a new drill depression. This is much more of a problem for bow drillers than hand drillers because the bow puts a lot of sideways pressure on the drill tending to force it out of the hollow. Having a drill that can be sharpenned to a point (having a hard centre) is the best solution to this. Insert said sharpenned drill point into a small depression formed with your knife point and it will usually form a stable drill and hearthboard mating. You often can't achieve this with a hollow or pith filled drill.

So, my preference is a pointy, hard core drill for the bow, and a pith filled drill for the hand drill.

 

[running bare]

this is all well and good but in reality is the heartwood of the spindle not harder than the outer sap wood?