Thanks so much Gordon, I passed along your response to my friend in France. Also passing on to my mentor, as he is starting to do some concave cuts. I'm soon going to be cutting my first concave cut at class. Will let you know how it turns out.
In delving into trying to learn about concave faceting, in my research I ran across a site that has 28 downloadable concave faceting designs,
www. AstonGems.com Thought I'd pass it if you or any of the members in Australia wanted to give them a go.
Shared your reply with my friend, he did have a couple of further questions: You spoke of "1W" & "2W", he wasn't sure what it meant, I suggested to him, that it might refer to the width of the mandrel you are using to cut the concave facet. Not sure if I'm correct, but hope so.
These are his next 2 questions:
1- is the thing to understand is : when you cut concave, you try to replace a flat facet by the same width of a concave one by only playing with the depth of the curve given by the diameter of the mandrel ?
2 - if the centre meet point is deeper in the stone than for a flat facet, does that mean that you might be under critical angle or it doesn't change
Thanks in advance Gordon
Correct the 1W and 2W Are measures of the mandrel diameter compare to the shortest distance across the stone shown on the fasting diagram as W. They Are measures of the mandrel diameter compare to the shortest distance across the stone shown on the fasting diagram as W.
This does not really matter what is more important is the relative size of the facet to the mandrel circumference as this determines the arc or amount of Curvature of the facet.
I usually cut stones about 10 to 12 mm in diameter on my manuals which are half inch in diameter. If I was to use a quarter inch diameter mandrel the facet would cover a greater arc on the circle at the circumference of the mandrel. Meaning this would now be a deeper cut facet relative to its edges. For example if I was using a quarter inch mandrel and cutting a facet that was 6 mm wide at it and then the facet would be almost 3 mm deep at the six the wide end. But if I used a 12 mm mandrel the facet would only cover and arc scribe via 6 mm line across the age of that circle and most likely be significantly less than 3 mm deep.
But if I used a 12 mm mandrel the facet would only cover add arc scribe via 6 mm line across the edge of that circle and most likely be significantly less than 3 mm deep.
Your next question relating to the critical angle, had me thinking a bit more. The issue is that when you cut a concrete facet the entire centreline of the facet starts cutting so that the angle of the facet stays the same. That is if you use a 42° angle as you cut the centreline of the facet (assuming you are cutting a round stone) stays at 42° and cut down into the form flat facet. The curvature of the facet means that it’s entire service is still cut at 42° But because it is curved the edges of the facet are higher than the middle.
I hope this clarifies your friends questions.
Thanks again Gordon, I passed it along.
Not having cut my first concave, I'm learning a lot from what you've been sharing in this dialogue. My mentor, has been learning by 'trial', he has only been doing concave cuts for the past few months. Your experience & suggestions has helped him a lot too!
If you don't mind, Lionel, my friend in France, had two more questions:
1: According to the size of the stone, what is the diameter of the mandrel generally used? ex: for a 8 mm stone, you use a mandrel that is xx mm in size. What size would generally be used for a 10mm or 12 mm size stone?
2: Is there a better depth to try to get the best curvature? ex: if I have a 4mm large facet, which diameter of a mandrel will give the best result? I understand that the smaller the mandrel is, the deeper the cut is, but what about the result?
I'm no expert, just someone who has been thinking about the topic a lot. I have only cut a few concave stones, it takes me ages to cut a stone.
Lionel wouldn't be Lionel Ba by any chance? I've seen some of his work on Facebook and am quite impressed with his experimentation.
I presently have only one size mandrel dictated by the polishing mandrels I bought. They were discontinued stock form Aussie Sapphire, two 1/2 inch darkside mandrels made for an Polmetric concave facetor. The copper ones had a larger shank, too large for my ER11A collet chuck. As a result I had a friend turn three mandrels up for me on his lathe.
Clarry Trevena, who has done a lot of concave work over the years, set me straight on mandrels, they must all be the same size, e.g. accurate to 0.01 or 0.02mm. If they are different sizes the facet won't polish. Mine are all 12.37mm from memory. I can make new ones from 1/2 inch copper pipe and skim them so they are parallel and the same size, once I set up an accurate grinding arrangement.
I have also bought some copper rods 6mm, which when I set up the grinding arrangement I'll size and use.
Polymetric make 1/2" and 3/4" mandrels for their machines and Bob Kelly's Facetron OMF uses 3/4" mandrels.
As for curvature, I can't say, but it stands to reason that the more curvature on the facet, the greater the spread of the light as it strikes the facet. I am not sure if this is good or bad yet. Maybe it depends on the desired effect and type of stone.
You guessed correctly, Lionel Ba is the friend I've been asking the questions for. I've known him a few years via FB and he's been a great help for me in understanding and working with Gem Cut Studio when I've been trying to create or adjust a design for regular faceting. His creativity and innovations in his creations is really amazing, i can't wait to see what he'll be coming up with when he starts to do concave cutting.
I haven't attempted to cut my first concave yet even though my mentor Don Laufer has been encouraging me to. He's also in the process of learning and trying to do various concave designs. He's posted several of them on Instagram under the user name 'headwaterslapidary'. His pear design in citrine is outstanding. He's photographed and shared many of the stones I've cut too, I'm not great at taking photos of my stones. If you have time check out Don's stuff.
I've been sharing all of the discussions on concave cutting with Don too and he's really found it helpful in the process of cutting a concave design.
Thanks again for all your help and being open for questions on the subject.
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