Barleycorn Lignum Vitae Box

1 August 2013 

Cutting the barleycorn pattern on a curved surface requires a little more care than on a flat one.

I used the set-up recently developed for cutting lattices on the side of bowls and chose Lignum Vitae as my box material.  It is very stable, takes fine detail very well without crumbling and if well sharpened tools are used will cut to a polished finish. 

To form a barleycorn pattern, basic cuts of suitable diameter (more about that in a bit) are made so that the circles just touch.  If there were say twelve basic cuts around the surface they would be 30 degrees apart.  More circles are then cut in the space between these circles at what would be fifteen, seven and a half and smaller angles if required.  A well cut barleycorn should have the whole surface patterned and each of the cuts should intersect at a sharp edge.

Accurate repeatability and angular setting are everything in this form of turning and the odd tenth of a millimetre or a few arc minutes will be visible in the final pattern.  

Where to start?

There is obviously a trade-off between the diameter of the circles and the cut depth needed to make the pattern on the whole curved surface.  Hence large diameter circles on small diameter cylinders will not work and because the rotation axis of the tool is a 90 degrees to the work axis then the points at which the circles touch will be beneath the surface of the cylinder.  

So what diameter of cuts is required, how many should there be and what should the cut depth be?   I get into the right area using a combination of simple trigonometry and trial and error.

It is quite easy to show that if t is the maximum cut depth (remember the surface is curved so it varies), D is the diameter of the cylinder and N is the base number of touching circles then:

d (cut diameter)=(D-2t)sin(180/N)

For the piece I made the diameter (D) was 69.95mm, depth of cut (t) 1.7mm, N was 12 and so the cut diameter was 17.22mm.

I found the cut depth and base number of touching circles by trial and error on a similar diameter piece of scrap.

The cut depth required also depends on the cutting angle of the tool, which in my case was about 110 degrees. The whole surface needs to be patterned and the more acute the angle the more circles need to be cut. Too many and the result can just look like knurling. If the angle is too obtuse then the pattern has insufficient depth to be distinct. I don’t know if there is a magic number but 110 degrees is in the right ball-park although I shall probably increase the angle a little for my next project.

The job should be started by taking trial cuts on two of the base circles (30 degrees apart) and at a little less than the full cut depth.  Then slowly increase the depth and check with an eye glass until the just touching condition is achieved.

For this pattern I required 96 circles (3.75 degrees apart) to make a properly formed barleycorn.

Making the Box

I made tenons at both ends of the work to chuck mount in compression mode, and then I cut the barleycorns for the box portion.  

After parting off the lid section using a thin parting tool this was mounted in a second chuck and the barleycorn on the lid was cut.  A second chuck is useful so that the box does not have to be removed and then re-mounted and centred for the final operations. 

The lid was then hollowed (bowl gauge, scraper, and skew chisel) and finished and a recess turned on the box to suit. I mounted a slide rest on the lathe so that I could get these two mating surfaces as parallel as possible achieving a really nice fit.

Then the box was hollowed out and finished (Forstner bit, scraper and skew chisel), and the lid re-united with the box.  Then with support from the tailstock the cylinder body and lid were finished.  A vee cut was made at the lid to box join to disguise any differential effects that may result from changes in humidity or release of stresses.

The lid was then mounted on a tightly fitting jam chuck to cut the pattern, which is 24 circles (15 degrees apart), 1.1mm deep.  Here the circle radius is equal to the offset (eccentricity) so that all circles pass through the centre.

The box was then mounted on a jam chuck and the base and box were finished.

Finishing

I find that shear scraping, then 360 and 1500 grit Hermes Webrax will produce a good finish. Lignum Vitae does not respond well to sanding (for me anyway).  Use waxes very sparingly.  The merest hint of Carnauba Wax on a buffing wheel works well.