Undercutting Tone Holes

You may have heard the term "undercut tone holes" or have read about "undercutting tone holes," but what exactly does that mean?  We spoke with flute finisher Karl Kornfeld and repair technician Rachel Baker about undercutting.  Rachel shared with us that undercutting tone holes is something that is fairly recent.  Both soldered and drawn tone holes can be undercut, although soldered tone holes would tend to have more undercutting.  Why is this?  Well, essentially, with soldered tone holes, a hole is cut into the flute body, and then then "tone hole" is soldered onto the body with the inside edges of the hole flush with the actual hole in the body.  Because the tone hole is soldered straight down onto the body, there could be sharp edges where the tone hole meets the body.  These sharp edges can cause turbulence inside the body and need to be rounded.  The rounding of these edges is what is known as "undercutting."  Less undercutting is needed on drawn tone hole flutes because the tone hole is formed by pulling metal up from the body.  Because the metal is not pulled exactly in a straight line, there is already a bit of rounding on drawn tone holes.

Undercutting is an extremely delicate step that takes care and precision.  If there is not enough undercutting, the player might feel more resistance when playing.  If there is too much undercutting, you can lose focus in the sound and definition between notes.  As Rachel reminded us, as air travels through the flute, there still need to be nodal points for the air as it travels.  Too much undercutting can disturb the air flow.  We've included some photos of two tone holes on a drawn tone hole flute.  One hole has been undercut, and the other has not yet been undercut.  It's difficult to see, but the undercut tone hole has a bit more of a reflection on the edge of the hole in the body.  The other tone hole is a bit more "'dull" around the edge.  Coincidentally, we caught up with a former Powell flute finisher who is now working on saxophones with soldered tone holes.  The tone holes are soldered with the same process that Powell uses on its soldered tone hole flutes.  Because the saxophone tone holes are so much bigger, it's much easier to see from that photo the difference between the undercut tone hole (smooth around the edge) and the one that has not yet been undercut (rough around the edge).

Many older flutes do not have undercut tone holes, so if yours does not, it's okay.  There are many types of flutes for many types of flute players!  If you have additional questions, always remember contact your authorized repair technician.

Red arrow points to undercut tone hole, orange to tone hole that has not been undercut yet.

Close-up of tone holes (top, undercut and bottom, not undercut)

You can really see the difference with these soldered saxophone tone holes.  Red arrow points to undercut.  Orange arrow point tone hole yet to be undercut (you can really see the rough edge).