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Solid cello neck set – how to control your neck projection

May 5, 2015 | Repair & Restoration, Tools and Jigs, Violin Making

When putting a new neck into a cello the mortice is cut precisely so that the fingerboard projects out to a specific height above the cello top ( the neck projection, or pitch is usually about 81mm measured at the center of the bridge).  It is often the case that the neck projection, which was correct just before gluing, comes out higher than intended. What has happened is the the glue applied to the neck root soaks into the end grain and swells that surface causing the neck root to bend backwards.  To compensate for this some makers will set their necks a millimeter or two lower than they actually want, others will insert a wood or fiberglass dowel into the neck root to try and stabilize it.  There is a better way: 3 Tips for controlling cello neck pitch
  1. Glue size the neck root end grain as far ahead of final gluing as possible. Flatten the surface, glue size once or twice, allow the glue size to dry the surface will become curved initially but will more or less flatten out as the glue dries. After a couple of days re-flatten and glue size if necessary, allow to dry as long as possible.  I usually do  this step on the first day of making a new instrument, with a replacement neck I like to wait at least a week before finally gluing the neck into place, the longer it can sit , the drier the size will become and the less moisture it will absorb at final gluing. I have hear of some makers using shellac to size the end grain since it is way less hygroscopic than protein glue, I've experimented with this and  feel that it may compromise the final joint.
  2. Leave the neck root oversized, do all of the shaping when the neck is glued  in place, the extra wood helps resist backward bending when glue is applied
  3. Mis-alignment of the clamp during final gluing is a  second source of neck pitch errors so, after final dry fitting,  use a wedge to record the height of the board end above the top, then the pitch can be double checked when the clamp is in place.
 
The "pitch" or "projection" is measured at the center of he bridge by projecting the center line of the fingerboard

The "pitch" or "projection" is measured at the center of the bridge by projecting the center line of the fingerboard

 
After dry fitting the neck joint but before gluing and clamping the projection at the tip of the fingerboard is recorded on a wedge , it can then be checked after gluing and clamping to ensure that nothing has shifted

After dry fitting the neck joint but before gluing and clamping the projection at the tip of the fingerboard is recorded on a wedge , it can then be checked after gluing and clamping to ensure that nothing has shifted

 
The excess neck root is easily removed after gluing

The excess neck root is easily removed after gluing

 
New neck clamped and glued. The neck root is left oversized to help resist backward bending caused when the hide glue swells the endgrain

New neck clamped and glued. The neck root is left oversized to help resist backward bending caused when the hide glue swells the endgrain

 
Note on other causes of neck movement  The pitch of the neck commonly goes up when a new instrument is varnished and placed in the low humidity environment of a light box, this is the same effect that plagues cellists in climates with wide seasonal humidity variation, it's a temporary problem  and it will go away a week or two after taking the cello out of the light box.  Avoid this problem by using an LED light box