Twist Break Facets - Crown

[beryl]

. In response to a query from Garry Holloway 2 weeks ago ...
. The attached chart shows the slope (inclination) of crown break facet according to its index (azimuth) from the adjacent main facet if the height of the break facet is to remain unchanged. Here the data are for a crown main slope of 34.5°. The plots are for three heights; others could be added; the .105 value is typical.
. The important thing is the relationship between index and slope changes: for 1° increase in index, the break facet slope increases about 0.7°. Data in the 2nd decimal place have no meaning to a practical faceter - he jst 'fits' the things.

 

[beryl]

... and here is a similar chart for the pavilion - specific for pavilion main slope = 40.75°. Again curves are plotted for only three heights; the middle one, J=.345, is typical for a modern-cut diamond with 80% deep pavilion break facets.
. In this case an index increase of 1° requires an increase of about 0.18° in the slope of the break facet. Again I note that 2nd decimal place is absurd precision to a practical cutter; however, this is the theoretical relationship.
. Some details are not clear, due to the steps I take to present an AutoCAD drawing here. The blurry writing at an angle says 'bend line of break facet', meaning that this facet can be thought of as being bent along the edge joining it to the main facet.
I will bring 'original' art to Moscow for those who want good copies. Iiro, be sure to bring your crayons.

 

[Garry H (Cut Nut)]

thanks Bruce

For reference purposes this should be linked to the GIA discussion several weeks ago http://www.pricescope.com/forum/rockytalky/another-gia-press-release-on-diamond-cut-research-t12787-30.html

 

[Iiro]

Sure Bruce

I take them all

 

[beryl]

. The attached chart converts radial star length dimension (ala GIA and used by most) to vertical break height dimension (preferred by Harding and used for many years). They both define location of the same point on a round brilliant. This will enable you to use the break twist charts I submitted at the start of this thread if you have the typical radial star length definition.
. Using the break height definition it was possible to discuss the effect of twisting the break facets without knowledge of the star and table dimensions; indeed, this shows what table/star combinations can apply to any given break height dimension.
. Note that a different chart is required for each value of main facet slope B, but the formula applies to any value. The more important thing is to observe the trend - that is, that break height decreases about .005 for every 3% increase in star length, or roughly .002 per 1%. It is apparent that the formula can be approximated by straight lines, for those who wish to do so.