Concave Cutter

[MakkyBrown]

Hmm, thinking now posting on Monday will probably be too late as only 4 days available after posting. I suppose my only chance is if I have time to cut it tomorrow and have it in tomorrows mail.
MB

[FlashGP]

Thanks Makky, good luck all.  Hopefully the express post will get there on time. 

COVID 19 is something we didn't anticipate when setting up the schedule.

Regards
Gordon

[MakkyBrown]

I'm sorry Flash for not having tried to cut the stone sooner. I was going to earlier in the week but I got a flue shot, and I've never had such a sore arm from one before.
MB

[Faceting Frank]

I want to fit a geared motor to my concave cutter, to provide auto oscillation. Using a pushrod, just like a motor crankshaft and piston setup.
Looked at You Tube vids of cutters and got a general idea of the strokes per minute, but just wondering if any of you guys have found faster or slower strokes better.
Thanks in advance.
Frank.

[MakkyBrown]

How long do you and others put into cutting a competition stone ?
I've cut/polished the pav/girdle(without concave facets), one meet I might have to push a bit. But it is all a case of how much magnification do the stones get inspected under. And what counts as rounded facets. I know it's 10x but 10x seem to be a bit variable. I use a 20x lens out of a camera but no idea what it really is.
Did you take the sharp edge off to help the concave facets start? I'm a bit concerned as I have no stop to cut against and am just going to lock the head pivot up.
Bit frustrating cutting with no time as I'll have to enter it with faults if I somehow manage to finish it. Like discovering a veil in the stone when polishing and having to polish on my go to tin lap that is well over due to be machined. Tomorrow I'll give the concave a go.
MB

[Giel]

I am very curious to how your finished stones will look. You guys are getting me enthousiastic again to finish my concave cutter.
I had so many problems with my new faceting machine that the concave project has been on hold for a while.
Had to fix the machine first, and I have to facet some stones and get back into it before I can cut concave facets.

[Faceting Frank]

I started keeping a track of how much time I was spending on my competition stones a couple of years back, as it was seeming to take forever to cut one.
Mainly because of the points I was losing on various things, such as meet points I couldn’t spot. I remember losing all three points allocated to the girdle thickness because I was told one side of the stone was 0.05mm too thick.
So I bought myself one of those loops with a measuring scale on it and a good quality 10x loupe. Fitted a digital height gauge to the mast a few years back, that turned out to be brilliant. I could keep a record of where all the facets were and go back to one to fix something.
So answering your question I average 50 hours, in 3-4 hour sessions. Have spent as much as 80 hours on a stone.
The required standards are unbeleavable.

[MakkyBrown]

Farout Frank, that is a serious amount of time. So it really is a quest for perfection. I'd imaging you'd then keep notes on each facet as you cut the stone.

I wish you lots of success after investing so much time.
MB

[Faceting Frank]

It can get to the stage of keeping records of mast height and cheater settings of individual facets. On a complex stone with a few layers of facets it starts out nicely with them all neatly against the girdle. Then the next row goes on and some of them are not quite right and need a bit of adjustment. So what happens you have this cumulative error building up, which your stuck with.
But being retired and having heaps of time on my hands I love it.

[MakkyBrown]

Well that didn't go to well, I chipped the cutlet on the collet chuck. Encoder is skipping steps and I lifted the handpiece up when i shouldn't have. 
Takeaways.Using 1/4" shaft as the mandrel.
-3k on brass cuts really nicely
-100k on aluminum looks to polish, thought my lighting is not the best but doesn't look bad.
-100k on brass doesn't polish and leaves circular marks
-1/4 in not forgiving and easy to notice if it has walked very slightly when starting. I suspect this is the reason 1/4" is not commonly used. I may also need to upgrade to a hard stop. 
-Need to start really gently, cutting a flat first would have helped.
The machine needs modifying and upgrading to a more powerful processor so i can make full use of the encoder and so it doesn't miss steps.

Stone wise, a bit of a disaster. But the machine has potential with modification. To be able to cut on brass and polish on standard aluminum, I'll be happy with that.The cut with 3k on brass is fairly aggressive, I'm not sure if going straight to 100k on aluminum will pollish harder materials. The 1/4" aluminum is also a bit flexible.  When i make mandrels in larger diameters I'll use a brass center.
Sorry Flash I won't be able to enter, even if i had time to recut I need to do machine mods first.

[Aussie Sapphire]

The cut with 3k on brass is fairly aggressive, I'm not sure if going straight to 100k on aluminum will pollish harder materials.

We have sold a zinc lap from Gearloose that tended to be quite aggressive with #3000 as a pre-polish.  He used to recommend using #8000 diamond instead for pre-polishing on that more aggressive lap.  Maybe that might be worth considering on your brass mandrel.

cheers
Leah

[Faceting Frank]

I had no idea on mandrel sizes as well MB, picked 12mm because it was about the average size Ultratec sold. Looked massive when I made them, but turned out to be ok.
Also looked at the speed they were being run at in You Tube vids which looked incredibly slow. Thought they were running them slow just for the vid. I was thinking of the mm per second a lap rotated at and run my mandrels to suit. That was wrong the slow speeds are the way to go, I was surprised on how fast a 1200 grit cut.
Having had a bit of practice with mine I would suggest ditching any electronic or mechanical means of controlling the facet depth and just cut by eye, its way easier.
So have some more bits to buy for mine and cut a few stones with it, so I know what I’m doing.
When it’s all working ok will post some dimensioned CAD drawings and name all the parts.

[MakkyBrown]

Once I have it setup properly, I should be able to cut to depth on the encoder. Can't atm as the carriage is not running true and I need to shin the horizontal rails a bit. Once that is done it will be like cutting a facet.Also having the faceting head locked up makes it harder to inspect facets, I had to stand up and look down on the stone.MB

[FlashGP]

Hi Frank,

Regarding your oscillator speed question.  I have a 30rpm worm drive on a small motor driving a pushrod arrangement you describe.  It is powered via a pulse width modularor speed comtroller.

 I drive it at about 2/3rds max speed most of the time, except when positioning the cutter to set up the mast distance so I don't run off the end of the cutter or changing mandrels.  Being able to slow it right down helps here.

Hi Makky,

I spend 20 to 40 hours on a comp stone.  Some cutters for the IFC spend up to 200 hrs per stone.

The judges use 10x alplanic loups (I think that is the term).  In camera terms they are aspherically ground so a grid has straight lines when viewed through them. 

My loupe is a Belmo 10x triplet loupe.  Judges use them, they are s bit like a 10x microscope.  I have a 30x cheap loupe that isnt as powerful as the Belmo.

  Before the Belmo I used a focussing lense from a Juke Box slide projector and looked through it in the reverse direction it projected in.  It was nearly as good as the Belmo.

The judges we have selected are considered to give pretty good feedback.  The marks on the judging sheet will tell you where you did well and where you need to improve.

Meetpoints take a while to learn.  The judge looks at the shape of the facets next to the meetpoint.  If the meetpoint is in, all facets touching that meetpoint will have a point where they meet.if one or more have a straight line instead of a point, then the adjacent facets are not meeting.

When cutting I generally prepolish in the order shown on the sheet and polish from the centre to the girdle.  This is because the higher angles produce less shift in edge length as you cut compared to the shallower angles.  On the crown it also compensates for thr crown not being exactly parallel to the plane running through the girdle.

Regards Gordon

[FlashGP]

Just finished changing the screw thread used to adjust the allignment of cutter to the centre of the mast to a piece of 6mm threaded stainless from Bunnings.  At 1mm per turn I get 3.6 degrees between 0.01mm. 

Made the anti backlash device by bolting a piece of flat to a piece of angle then threaded through both pieces.  Loosened the retaining bolts, inserted the rod through both pieces with a 2 turn gap then tightened the retaining bolts enough to eliminate the backlash but still let the rod turn.  I found the previous 8mm per turn acme thread for positioning CNC cutters combined with dial indicator too fiddly.

Also fitted a digital linear measurer for a lathe so I can offest the position of the cutter and return to the centre and rebuilt my quill stop so it can be repositioned in the direction of travel of the mast as angles change.

Am planning on testing it out on the Lest We Forget design to  create radiating rays in the background.