Warren
To complete the story, there are three other critical factors that need to be considered when drilling blanks.
One is the speed of rotation of the bit - how fast is the lathe, drill press, hand drill, or brace/bit rotating. If the rotational speed is too high, friction between the blank and the bit, and between the blank and swarf in the flutes of the bit, will cause heating, and if there is too much heating, that can lead to problems. The worst case is that too much heating can cause the blank to crack - moisture trapped in the blank flashes to steam which expands to crack the blank. The general rule is that you should drill at the lowest speed possible - my experience is that speeds in excess of around 500 r/min are too fast.
The other issue relates to the rate at which the bit is advanced into the wood. Let's take the 7mm bit you are using as an example. A 7mm bit cuts a hole with a cross sectional area of a tad more than 11 sq mm. So for each mm that the bit advances into the blank, it will cut a hole with a volume of 11 cubic mm, and generate 11 cubic mm of swarf. That swarf has nowhere to go other than into the flutes. I'm sure that there's a defined relationship between the depth of flutes and the diameter of the bit, but I don't know what it is; visually, however, it looks to me like the flutes of my 7mm bit are about half the cross sectional area of the bit. So that means that for each mm that the bit advances into the wood, swarf will fill 2mm of flute. As long as the swarf passes smoothly through the flutes, everything will be fine. But the fact is that flutes become constipated (can't think of a better term), and the swarf stops moving. That means that the additional swarf generated by the cutting edge will be packed into the flutes, resulting in increased friction, and increased heating. So that leads to the second rule of drilling - periodically, you have to stop the forward motion of the bit and back it out of the blank to clear the swarf.
Finally, there is the matter of how fast the bit should advance into the blank. Remember that the cutting edge is at the tip of the bit, and as the bit advances it moves into the hole cut by those leading edges. If the wood has pronounced grain (meaning a differentiation between softer wood that grew faster during warm weather, and harder wood that grew more slowly during cold weather), and if you try to advance the bit faster than it can cut the hole, you will be forcing the bit to follow the softer wood that is more easily drilled. This will result in wandering - and the smaller the bit, the more easily this can happen. This translates into rule that says the slower the bit is rotating, the slower you must advance the bit.
