My current experiment (I'm always experimenting!) is making a bow drill set suitable for kids to use - my two daughters (6.75! and 5) like playing with my sets and want to be able to use the bow drill but due to their size and age it's proving difficult. So I'm now experimenting with sets suitable for use by kids and I'm using my daughters as test subjects! One issue is with applying enough downward pressure and co-ordination of holding the bearing block and the bow so for now, it needs assistance from me to hold the bearing block to provide the required amount of downward pressure and to hold the base board firm so they can concentrate on the bow action.
In my experiments, you need about 12-15lbs of downward pressure with a 1.5cm hazel spindle on lime. My daughters can't physically apply that amount of pressure. However, this is less pressure than I usually apply so it's slightly easier for kids to rotate the drill as opposed to under heavier pressure. If the drill starts squeaking I apply slightly more pressure but not too much as to make it harder to rotate the drill. The other consideration is using easy wood combinations, such as Hazel on Lime (or Ivy) and having a spindle which isn't too wide so there is less surface area (so not as much effort is required to rotate the drill as opposed to a wider drill.) My current spindle is 1.5cm wide but that may need to be made a bit thinner. I'm also using a longer spindle so it doesn't matter as much if they don't keep the bow straight as there is more spindle for the cord to travel up and down.
I'm also using multiple twists of the cord (at least 8.) The advantage of this is that the cord doesn't need to be tight and there is less chance of slippage.It also doesn't matter as much if the bow isn't kept straight. Another advantage of this method is that you can use thinner cords that are more prone to breaking with a taut cord.
And finally, I made a smaller sized lighter weight bow with enough space for two hands to grip - my daughter is scared of touching the rope and it's easier if she uses 2 hands. The only thing now is getting my daughter's attention span for long enough - the first attempt was promising but it was cold and they soon disappeared indoors! .... watch this space..... I have successfully welcomed an ember with this setup, applying less pressure than usual, but it needs at least 5 minutes of a daughter to really test it out!