Sunday, January 29, 2012

Spline box how-to

Mount a blank on the lathe between centers in face grain (side grain) orientation.  Turn a tenon on each end.  I leave the blank square so the next operation is both safer and easier.
The blank is taken to the band saw and a cut is made that follows the contour of the grain. Blank selection is important as you want the grain has some contour that runs side to side, not diagonally. The end grain should also run as perpendicular as possible to the side grain.
The blank can then be mounted between centers again and turned to a cylinder.  Alternatively each half can be mounted in a chuck and turned to a cylinder independently.  When demonstrating this project I opt for the chuck method as it is safer than turning a blank between centers that has been cut in half.  Can you see the joint in this photo?
There it is.

The 2 halves turned to a cylinder and set aside.  The contour is what makes this box interesting and necessitates the spline technique as it's not possible to turn a tenon on the irregular surface and still have a good grain match.

The first half is then mounted in the chuck and the inside is turned.  The body is partially hollowed and a recess is turned around the inside, this will be used to accept the spline that will act as the faux tenon.

The blank for the spline is mounted between centers and turned round.  I usually opt to use the same wood and turned in the same orientation as the body of the box.  In this case the only matching wood I had was in end grain orientation but I used it anyway.  A spline in a contrasting wood is another good look.
The spline is turned to the correct diameter.  It can normally be remounted and carefully turned to get a good fit with minimal runout.  I actually don't mount this between centers.  I jam the blank against the flat face of the spindle and drive it with friction and pressure from the tailstock.  I use the face of the spindle as it will ensure that the blank is perpendicular to the ways.

Once a good fit is attained the spline is glued in place.  The round mark visible here is from the end of the spindle.

The chuck is then returned to the lathe and the center of the spline removed to that only the solid wooden ring is left.  The rest of the body is then hollowed and the spline is blended into the rest of the body.  I opted to reduce the length and soften the edge of the tenon slightly.

The other half is then mounted in the chuck and a recess is turned to match the diameter of the spline tenon.  From here on out the process is mostly the same as a standard box.  We now have 1 half with a tenon (spline) and 1 half with a recess.

The first half is mounted onto the 2nd half and the live center is brought up to support it as the outside is turned.  I round it off and take it down to a little nub.  The top half can be removed to check the wall thickness as work progresses.  The blank centers very well as the contoured joint can only fit in one position.  The dimple from the live center also helps keep it centered.

If the fit is tight enough the nub can be removed without the need for tape.  I rushed through this box so the fit was looser that it should have been so I needed some painter's tape to hold it together while I cleaned up the nub.

The rest of the end half is hollowed and then reversed for for final turning.  I opted to use my vacuum chuck but a jam chuck should also work.  The outside is turned to match the first half,

The 2nd half secured on my vacuum chuck.  My primary vacuum pump pulls 24" of mercury which translates to 12 PSI.  The PVC is 2" diameter which is roughly 3 square inches so there is roughly 36 pounds of air pressure holding this in place.

Turning complete, next up is the texturing that is used to help disguise the joint.

The outside is textured with a round brass bristle brush.  It's held in a jacobs chuck and the lathe rotates it while I hold the box against it.  The softer wood is worn away while the harder wood is left behind.

The finished box.  Another reason I like doing this style of box is that I don't have to sand the outside at all.  The entire surface is textured, more grain is worn away where it's softer but even the harder grain gets some texture.  Any time spent sanding on the outside is completely wasted.

Another view.

Open view.  You can clearly see the end grain orientation of the spline, the grain runs vertical rather than side to side.  If it ran side to side it would blend better with the body and be much less visible.

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