Australian Facetors' Guild Limited

Faceting the crown first.

  • 06 Jan 2019 7:06 PM
    Message # 6985463

    Faceting has been an occasional hobby of mine for over 40 years. During that time I’ve always cut my stones in the traditional way, pavilion first. It allows you to accurately determine the maximum size stone you can get from the piece of rough. But since getting involved in competition cutting I’m becoming more convinced that’s the wrong way to go.

    Because the girdle has to be perfectly even all round, the first row of facets on the crown adjacent to the girdle have to be adjusted very slightly to create that. This in turn creates issues with meets on the next row, and so on.

    I was just wondering if anyone on the forum cut the crown first as standard and what issues if any it created cutting the pavilion.


  • 07 Jan 2019 8:40 AM
    Reply # 6986216 on 6985463
    Anonymous member (Administrator)

    I am not sure why you think that would reduce the error - it would simply transfer it to the pavilion side of the girdle. Much better to work out where the error is coming from (e.g. machine set-up, transfer jig, incorrect lighting, under-powered optics, technique, etc) and work to reduce it.

  • 07 Jan 2019 9:57 AM
    Reply # 6986275 on 6985463

    Hi Frank,

    I think you would have a lot more trouble getting the shape and meets right if you started with the crown.  This is because most diagrams are written the way they are displayed, pavillion first, AND there is usually a larger variation in crown angles than in pavillion angles.

    I have found that at the end of the prepolishing stage, if there is error in the star facets, polish the table then prepolish the meets in again starting with the row next to the table and progressing to the girdle.

    The change in angle means that as you progress through the tiers the amount of error reduces with each row.  That is an 0.1 mm error at the star facet becomes an 0.02mm error at the girdle.

    I also polish from crown to girdle so I can polish in the meets.

    If you look at a 15 degree triangle, a small change in hight translates to a large hypotenuse change, but at 50 degrees the change is much smaller.

    The effect is much less on the pavillion so if you cut the crown first, any error at the girdle will be harder to eliminate at the pavillion.

    Anyway, that's my theory and it works for me.

    Last modified: 07 Jan 2019 10:01 AM | Anonymous member
  • 08 Jan 2019 11:22 AM
    Reply # 6988278 on 6985463

    Its not a transfer jig issue in the sense that I can detect any problem with it, or any other mechanical issue that I can see. The only measurable defect there is on my machine is what I can see on the dial gauge. The runout at the dop is 0.015mm, allowing for the extra distance the dop is away from the pivot arm that will be more like 0.025mm. Doesn’t sound a lot, but as Gordon says it will make a difference to the facet lengths from one side of the stone to the other when levelling the girdle.

     

    What made me think more about finishing the crown first is the cut for this year’s US masters.

    It has a very complex crown and the stone must be cut to a precise size. The pavilion is very simple and they recommend cutting the crown first.

    So, I’m thinking the next time I have to cut a comp stone with a relatively simple pavilion I will cut the pavilion to the 1200 stage, setting the size etc. Transfer dop and finish the crown. Transfer dop again and finish the pavilion, that way the minor adjustments to level the girdle are not going to have a knock on effect with a lot of other facets.


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