I left this until last simply because the posts in this series have been long enough, without adding text about the problems you can encounter as you go along.
1. Leaks. If the bore is not completely airtight, the tuning will go wonky, the flute won’t play well, and things will be pretty awful in general. So: if your tuning doesn’t make sense, or if you can’t get a clear note out of the flute no matter what you try, you could do worse than check the seams for leaks. Block the TSH with a blob of blu-tack or similar, if you have holes cover them all with more blu-tack; and blow into the South end of the bore. Dampen your fingers and feel along the seams; you’ll quickly identify if any air is escaping from the flute where it shouldn’t. If you have leaks, they should only be small; only under the most severe of circumstances will you have to re-split the flute and re-glue it. Inspect where the leak is, and put some glue along the place where it is. Your normal wood-glue should do the trick; but sometimes if the holes are very small, some CA glue may run into the holes more easily. If you have a long split, run a bead of glue along the seam, then use a knife blade to part the split slightly, so that the glue is drawn into the split. A final way to draw the glue into the leak is to suck gently on the South end, with the holes and TSH still blocked. NOT heavy suction; better to suck gently for a long time than to pull hard and pull all the glue out into the inside of the bore.
2. Gaspy or poor quality sound. No leaks, and everything seems right? Check the bore for splinters and roughness – these destroy sound and are easy to get rid of. Next double-check the important particulars: Flue 0.7-0.9mm deep; cutting edge at exactly half that; TSH exactly the same width as the flue; TSH less than 7mm North-South; ramp/flue smooth and not lumpy or sloped from side to side; smooth, finely sanded base on the bird; straight cutting edge; straight flue cliff and bird exit edge; cutting edge and under-ramp all the same thickness… all these details will affect sound quality. Better always to start with a narrow TSH (North-South) and widen until sound is perfect than to make it too big and end up with a rough-sounding flute.
3. One hole gives too high a note. We all make mistakes; and it’s all too easy to be a little over-enthusiastic with a burning iron and end up with a note slightly higher than needed. In this case, you can use a CA glue (superglue etc) and an accelerator spray to apply to the North edge of the hole; turn the flute blow-hole down, put a small drop of the glue on the North edge of the hole and spray the activator – the glue should set in a couple of seconds. Check the tuning of the hole; it should have come down a little. Repeat until it’s a little low, then use your tuning file to bring it back in.
4. Early Wet-Out. “Wet-Out” is when moisture gathers in the flue and obstructs sound production; it becomes more difficult to blow, and notes are slower to start (in extreme cases, if they start at all). If this happens early (after only a few minutes of playing), it’s possible your flue is too long. Make sure it’s about 20mm from cliff-edge back to the start of the ramp; you can reduce this section by file or chisel and file; make sure you leave it absolutely flat left to right throughout the new transition.
5. No back pressure. This makes it very difficult to play, as all your breath is gone very quickly. The flute may sound great, but you need to breathe in, on every third beat! This could be a case of too deep or too wide a flue. Check your measurements again. If your flue’s too deep, work on sanding the nest until the depth is reduced; if it’s too wide, use a bead of CA glue (use your activator spray to set it) on either side to make it narrower; then use your dremel Router attachment once the glue is set, to make the sides of the flue vertical again. Sand the nest to reduce any bumps of glue.
6. TSH or Flue splits or splinters. Sometimes a little eagerness or indiscretion when cutting the under-ramp to the cutting edge will lead to a chunk of wood splitting off where the cutting edge, the flue, or both should be. DO NOT PANIC. The size of wood we chose to start with has room to make repairs even for such a drastic and seemingly disastrous event. Use either an electric planer or a jack plane to steadily bring down the nest until it’s perfectly flat; the flue and cutting edge flush with the nest surface. Then use your original method to re-cut the flue and cutting edge, making sure you go slowly and making sure the TSH ends up between 3 and 6 mm North-South. This process will reduce the thickness of the front of the flute; if you’re shaping with a lathe, be sure to plane the other three surfaces until the flute is square again. If you’ve split off too much and your front wall ends up only 3 or 4mm thick after this repair, you’re either going to have to shape by hand and finish on a non-circular shape, or you might have to call this “a stick” and start again. Don’t give up is my advice; unique shapes after repair work make for interested buyers!