Australian Facetors' Guild Limited

Concave Cutting

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  • 29 Jan 2020 7:14 AM
    Reply # 8698867 on 8136454

    Here's something I had not thought about and learned the hard way.

    When cutting facets that are not centered on the line running down the centre of the quill (Centre Line), such as facets on an oval cut, the concave facet cuts deeper on the side nearest the Centre Line.

    I cut a row of step facets each side of the 96, 48, 24 and 72 facets on an oval expecting them to look like step facets.  Instead they were pointy like main facets on an SRB but running along one side of the flat facet they were cut on.  I thought that although I had alligned  the cutter it was out of allignment.  But the concave facets each side of the 4 quadrant facets (96 etc) were symmetrical  with each other.

    That row of 8 concave facets filled in the bow tie on an L/W1.5 oval giving the ends of the stone a deeper colour than the middle of the stone, a bit like a party colour.  Here's A photo of the front and back of the finished zircon

    Happy Faceting

    2 files
    Last modified: 29 Jan 2020 7:18 AM | Anonymous member
  • 02 Feb 2020 1:05 PM
    Reply # 8710740 on 8136454

    Hi Gordon,

    Read through your article about making your own concave cutter Gordon and have already read most of MakkyBrowns thread on Aussie Lapidary Forum.

    The linear bearings he is using for sideway adjustment of the carriage, is the same linear bearing I’m using on my machine mast. I have the height adjustment working in the same manner as Ultra Tec, with a threaded rod. Also have the mast base touching a rail to keep it pointing the same way all the time.

    So getting to the point of this post. As far as I can see the sideways adjustment of the cutter is to maintain alignment with the centre line of the stone as the mast height is changed. Would that be right.



  • 03 Feb 2020 7:22 AM
    Reply # 8711584 on 8136454

    Hi Frank,

    Correct, the sideways adjustment is for allignment.  It also allows a series of parrallel concaves to be cut on step cut rectangular stones.

    If your cutter is pit of allignment, the stone will cut partway around the radius of the cutter and the concave facets won't line up.  If you adjust the mast stop so they meet at the centre of the stone, the next row will be tilted.

    Even the play to allow the mast to slide can make a difference to allignment, but this wouldn't necessarily be noticeable if all facets on that side of the stone are concave cut.  However I am only concave cutting some of the pavillion facets, so it is quite noticeable if the cutter is out of allignment.



  • 03 Feb 2020 11:45 AM
    Reply # 8711896 on 8136454

    Hi Gordon,

    Yes, there is a lot of things that can throw facets out of alignment. Having the horizontal swing parallel with the lap and the vertical swing at 90deg to it is vital. Keeping the mast base pointing the same way as you move it back and forward is as well. As I discovered when I fitted the height gauge.

    I won’t be trying to cut anything fancy, just the pavilion central to the centreline of the stone. Something like the stone you put in the comp, so my machine won’t be anything like yours or MakkyBrowns in the Aussie Faceting Forum. (I go under the name of faceting Frank on that forum). As I see it, I will need fine adjustment in the Y axis and parallel alignment with the centreline of the stone.

    I spent a lot of time adjusting the pivot bearings on the quill base so I could cut the 8-sided step cut I have shown in Facet Talk 228 from 90deg to 20deg with near zero cheater movement. But that only proves the swing is 90deg to the lap and not the machine base, something else to think about.


    I will answer your other post here Gordon so it’s all together.

    If you search on eBay for a C12 ER11M 150L straight shanked collet chuck holder, you will see what I use for a quill. It has a threaded retaining nut and I fit a ¼” collet in one of those. It’s made in one-piece 12mm diameter to such a tolerance that a standard 12mm bearing has to be forced on. You can get them with bigger heads so you can fit a whole range of size collets.

    Because the tolerance of the internal taper on the chuck and the collet is not perfect, there is a very small variation of runout when putting the dop in. I only know it’s there because of the dial gauge and it’s probably only 0.05mm at worst and no problem with cutting tools.

    I understand the problem your having putting the chuck on the motor shaft as I imagine they are not made for each other. That’s the reason I won’t attempt doing it that way, but as I mentioned earlier and  put the 150 long chuck shaft in an ally block, with the motor separate.

    I can’t offer any advice on trying to machine parts to make them fit, as it’s a hit and miss affair. I do notice MakkyBrown has one of those things that allow for misalignment and vibration on his, that’s the thing with the orange rubber. Possibly cut one of those 150 long shafts down to fit in a base and stick the motor on the end.



  • 03 Feb 2020 5:39 PM
    Reply # 8712080 on 8136454

    Thanks Frank.

    The ER11A comes with 5 and 8mm mounting holes that are generally one to 2 thou bigger than the motor shaft and fit with grub screws.  That shaft tolerence is the cause of a lot of vibration if not cancelled by pressure fitting the chuck to the motor shaft, hence the copper shims.

    Andrew solved his fit problems by boreing out the smaller collet to press onto his motor.

    I'm glad I am not the only one to consider that the taper on the collet chucks is not perfect.


    Gordon (AKA, FlashGP62)

  • 29 Mar 2020 8:33 AM
    Reply # 8864525 on 8136454

    Hi all Engineers out there.

    I was thinking of trueing up the outside of the collets by dopping them on the end of a dop then grinding them at the angle used for their outside with a 1200 lap 

    But then thought of dopping them and with the angle at 90 degrees, grinding them with the collet chuck without locking nut using lubricant and 600 grit.

    Would this be better at making them concentric?



  • 31 Mar 2020 8:47 AM
    Reply # 8868566 on 8136454

    Hi Gordon,

    As I mentioned earlier in this thread, I have a slight runout on my ER11 quill and collet, which varied slightly every time I put a dop in.

    So, I repetedly fitted a dop turning the collet a bit each time and recorded the runout with the dial gauge.

    I found there was a point where the error in the collet and ER11 quill cancelled each other out and if I fit the collet the same way every time, the runout is from 0 to 0.01.


    Last modified: 31 Mar 2020 8:48 AM | Anonymous member
  • 01 Apr 2020 7:26 AM
    Reply # 8870887 on 8136454

    Hi Frank,

    I have observed the same thing, but with a greater run out at the cutter, between 0.4 and 0.04mm.

    When I assemble the belt driven version of my cutter, I'll try grinding the inside face of the collet housing and collet exterior as mentioned above to see if I can improve them.

    An  ex army engineer mate of mine who used to teach machining at TAFE thinks it'll make a difference.  My greatest concern is that unlike a lathe, using the faceting head for allignment is not rigid, so a much greater chance of harmonic vibrations throwing evertrhing out.

    Regards Gordon 

    Last modified: 23 Jan 2021 8:12 AM | Anonymous member
  • 22 Jan 2021 7:42 AM
    Reply # 9905571 on 8136454

    A fellow faceter in France is just starting to try to do concave faceting and had a question, if anyone can provide an answer, I'd appreciate it. Thanks.

    "if you have a SRB to cut with 2 angles for the pavilion, lets say 41° and 42° what will be the modified angles in concave mode ? of what i think i have understood, is +2° then it would become 43° and 45°"

  • 23 Jan 2021 8:32 AM
    Reply # 9942990 on 8136454

    Hi Glenn

    If you are trying to match the facet to surrounding facets, such as replacing 4 of the 8 main pavillion facets with concave, the size of the mandrel relative to the size of the stone will influence the change in facet angle.  For example a mandrel that is 2W will give a shallower curve than a mandrel that is 1W and hence require a smaller angle change.  

    This means trial and error unless you cut every stone to the same size and can repeat your first stones settings.  

    It is one of the issues Rej was grappling with in his thought to add a concave function to GCS.

    If cutting the whole main facet row as concave there is no need to change angles if you partly cut the mains and then finish them off on the concave.  All that happens is that the centre meet point is deeper in the stone than for a flat facet.

    As for light return, I postulate that a concave facet is flat along its length and curved across its width.  As a result it should optimise light return at the same angle as a flat facet. All that happens is that the curve reflects light in a radiating pattern instead of a beam.

    Whether I am on the mark or not should show in this year’s comp.


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