Aussie Lapidary Forum
LAPIDARY => Gemstone Faceting => Topic started by: Faceting Frank on May 04, 2020, 08:46:19 PM
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R B Operated Oscillating Concave Cutting Attachment
Firstly I would like to thank MakkyBrown and FlashGP for their contribution on the forum on this subject. They have helped me a lot in designing and building my concave cutter attachment.
This concave cutter attachment is designed to cut facets that are in line with the centreline of the stone only. It doesn’t have a base that can be set at different angles, the cutter always points the same way, which is in line with the faceting machine quill.
This makes it way more easier to make. The only fine adjustment needed to be built into it is to keep the cutter perfectly in line with the centreline of the stone.
I don’t like the idea of locking the quill direction so it can’t be turned. I want to look at the stone with a loupe, very difficult if you can’t point it straight at you.
So I’ve made a stop for the dop to rest against while cutting the facet, it can be adjusted to suit the angle of the quill.
The machine mast base is locked in position and the concave cutter assembly moves along a linear bearing rail as the stone angle changes. A fine adjustment is built into the linear rail base to align with the stone if required.
I’m going to buy a 12v geared motor to operate a pushrod assembly to provide automatic oscillation. But at the moment it doesn’t have automatic oscillation, the movement of the cutter is manually operated.
This consists of moving the cutter body against two stops, with a rubber band providing an auto return in one direction.
Used it for the first time today and apart from the adjustments I can see need making with some bits, am very happy with the results.
So will add a lot more info and pics over the next few days.(http://aussielapidaryforum.com/forum/gallery/9/medium_120565-040520203615.jpeg) (http://aussielapidaryforum.com/forum/index.php?action=gallery;sa=view;id=9208)
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First time I’ve ever used a concave cutter, so bit of a learning curve to go through.
Very happy with the test facets on an old 12mm round quartz stone I had dopped for the job. Surprised on how quick the 12mm copper mandrel charged with 1200 grit cut into it at around about 500rpm.
Made two other 12mm mandrels from HDPE plastic, one for 3000 grit pre-polish and one for Cerium oxide. Only issue was with the 3000 grit putting grooves in the facet, can only see them with a 10x loupe. So may try finishing off at a slower speed and get the oscillating hand working faster.
Just skimmed the top layer of half of the copper mandrel to see if there was any difference in performance.
(http://aussielapidaryforum.com/forum/gallery/9/medium_120565-050520181805.jpeg) (http://aussielapidaryforum.com/forum/index.php?action=gallery;sa=view;id=9209)
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A couple of more pictures of my Micky Mouse version of a Concave cutting Attachment.
So far so good, started cutting this years comp. stone Turtle Shell 3. Not sure how the pavilion will turn out, but I know the crown is difficult.
So gave myself plenty of depth, so I could do a trial run on the crown to see how it goes.
(http://aussielapidaryforum.com/forum/gallery/9/medium_120565-050520215851.jpeg) (http://aussielapidaryforum.com/forum/index.php?action=gallery;sa=view;id=9211)
(http://aussielapidaryforum.com/forum/gallery/9/medium_120565-050520215652.jpeg) (http://aussielapidaryforum.com/forum/index.php?action=gallery;sa=view;id=9210)
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Looks good frank. Well done on an operation machine. beers
Your not wrong with that crown looking very difficult. 1 degree angle change using my dodgy 45 degree dop is going to be interesting. I have started trying to cut anything yet, I will be in last place :) I have no doubt about that. I'm half tempted to cut crown first as it concerns me more than cutting concave which I've never done.
CheersMB
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Hi Guys,
The turtle's crown is easier than it looks. Its a modified Bar Round 1.
The trick to the crown is the relative speed the different facet meets move as you cut.
Regards
Flash.
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Got all the pavilion mains cut to pre polish and the girdle polished. Don’t know if you guys like polishing the girdle before transfer dopping but I do.
So its into the unknown with the concave facets tomorrow.
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Yeah, I do the girdle with the pav as well. Good luck with the concave.
MB
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I have started to leave the girdle polishing to after transferring, I think it works better.
Before transfer take it to pre polish, after transfer the first step is to polish the girdle facets. It's the best way to make sure your transfer went well. With just a light touch on the polish lap you can see immediately if things are lined up and can correct them if they aren't.
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I like Rusted idea of making sure your transfer went well. I usually get clue or wax on some of my girdle facets so thats why I pre polish girdle facets when doing pavilion and dont usually polish them any way.
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Hi,
I polish the girdle before transferring to cut the crown, that way I can make sure I don't overcut the pavillion girdle meets.
But I also cut a row of Witness facets just before pre-polishing the Girdle and use these to allign the crown facets in much the way Rusted uses when polishing the girdle facets.
Technically you only need one witness facet, but a row of them speeds up pre-polishing and polishing the girdle facets and gives me a couple of extra witness facets to confirm allignment.
Happy faceting.
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Had to do a few mods this morning, so didn’t get started till after lunch.
Bit of fiddling around with the cheater and mast height, to keep the facets over the girdle meets and meet at the centre.
The dop stop works well (one of my pics), something I saw on You Tube. Just needed some height to it so your up against the stop before getting down to the mandrel. Not only does it allow you to look at the stone through a loupe in the normal manner. But the stop and mandrel are very close, making fine adjustments much more precise.
Started the 3000 grit pre polish, but picked up a grain of 1200 making scratches. So will have to clean everything and start again.
All in all going along ok.
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Polished the concave facets first and discovered a problem as far as a competition stone is concerned.
Started polishing the flat facets and I’m not going to get the meets at the girdle with the concave facets because the concave facets are too deep.
You can’t cut the concave facets at the pre polish stage to meet the girdle, they have to be short.
The stop for the dop instead of locking the entire quill up works really well. Because the mandrel rotates clockwise it keeps the dop up against the stop, also once the stop is setup accurately it’s permanent. All you need to do is slide it along linear bearing rail to suit the angle of the cut.
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Not having a lot of luck with this thing. Decided to pre-polish the concaves again to get them all the same depth into the girdle as well as meet at the pavilion. Also decided to polish them with cerium oxide while setup, now the sods have started scratching as I get near the end.
Probably something to do with the soft HDPE plastic I’m using, because cerium oxide on a hard plastic lap is perfect for quartz. 60k diamond doesn’t work either, so will try hard wood as a last resort.
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Thinking out loud here Frank
How would you go tinning a brass mandrel with a high tin solder.
A brass Mandrel, that's a type of monkey isn't it, lucky it wasn't here this morning, quite frosty.
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Don’t see how you could Rusted; you need to end up with a perfectly smooth finish.
I’ve never done it before but I’m assuming you would need to melt the tin, heat the mandrel up so its hot enough for the tin to stick to it.
Flux the mandrel and dip it into the molten tin, lift it out and let the tin run off like paint.
You could try heating the mandrel up with a blow torch, hold it vertically and rub the tin on and let it run off.
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What is the exact diameter of your mandrels, business end as well as where it goes in whatever you are using as a chuck or collet? Also length?
I will have a play and see what I can come up with. I have some Phenolic and some acrylic and will try the solder thing. If they work they are yours.
I'm always looking for a challenge.
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The picture includes the collet chuck, collet, linear bearing and rail I have put together to make this thing. Like everything cheap they come from China via eBay. I use a collet chuck for a quill on my faceting machine and their dead accurate, however get one less than 12mm diameter for this job, so you can use smaller bearings.
The mandrels are 12mm diameter 50mm long, the shafts are of course ¼” to fit in the chuck. I have found the polishing mandrels need to be .5mm smaller so they don’t round the edges of the facet.
Anyway, this manually operated 2 bob contraption works and works well, I was worried about having a job for each hand and having to think all the time.
But the stop for the stone dop works and works well, just have to remember to have the quill against the stop before bringing it down to the mandrel. The rotation of the mandrel keeps it there.
Happy concaving.
(http://aussielapidaryforum.com/forum/gallery/9/medium_120565-110520194321.jpeg)[/url]
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Is that 50mm of working surface at 12mm dia? how long does the 1/4" shaft need to be to go in the collet? Total length?
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I have successfully made a tin alloy concave faceting mandrel by tinning a brass mandrel.
If you want to have a go make your Brass base mandrel only a couple of thou under your finished size.
It is difficult to keep a thick even layer of liquid solder on the brass, as you apply more solder gravity makes it run and drip off.
Spin the mandrel in the lathe chuck, (cordless drill would do), Clean the brass thoroughly any contamination and you will have trouble, heat the brass, apply flux, heat to solder melting temperature and apply solder as it is spinning, allow to cool, turn down to desired size.
If you don't have a lathe you could try tinning by applying solder to the brass mandrel and while it is still hot wiping the excess solder off with a cloth, this will give you a microscopic layer of tin but it might be enough.
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The dop goes in the collet 20mm, I added an extra 15mm to the mandrel. The mandrel is 50mm total length, picked that length by guessing the length of Ultratec’s.
I do have a lathe, so will have a go at your tinning idea.
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That is a great idea just tinning the mandrel. I'll do that if I can't polish on aluminum or brass. I've been busy and not had a chance to try my machine. And with only a couple of week till entries close hopefully I'll have some time soon.MB
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The pressure of the comp entry date is getting to me as well.
When I said earlier that the concaves cut faster than the flats, so leave them short at the pre polish, is probably wrong. I now find myself with the concaves 1mm short of the girdle.
Anyway don’t have time to muck about with it so have it on the transfer jig. If I have time will retransfer and do some more work on the pavilion.
As far as a competition stone is concerned it’s a joke, but I want the concave section to remain part of the annual competition. For that to happen it needs entries.
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Did a trial run on the crown of Turtle Shell 3 with the girdle still at 1mm.
It is easier than it looks, the only issue I had with it was putting the dop in the 45deg dop the right way round.
With mine the stone dop can spin round anyway, so getting it set the right way was an eyeball and then cheater job to get the E facets meeting the A facets. I cut the E facets at 2deg, 1deg is too fast. The table has no meets anyway so you can’t tell.
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Finished Turtle shell 3 this morning but will not be entering into the comp. If I had more time I would re-1200 grit it all and start again.
Had to recut the mandrels to stop the cerium oxide rounding the corners of the facets. Now have them 12mm 1200 grit, 11.6mm 3000 grit, 11.2mm cerium, that works well. Also the flat facet v concave depth. First started with the concaves to deep, then pre-polished everything to find after polish the concaves short.
If it wasn’t a competition stone it wouldn’t matter, but re cutting has thrown out all my number 1 facet lengths. Their all over the place and I have to fix that without altering the girdle, as the crown is finished, stone redopped and girdle thickness set.
I enjoyed making the concave cutter, it cost me about $350 to make. That includes the value of the parts I already had. But need some more practice with it.
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Well done Frank but you should enter it, pretty sure it's free to enter and it will be better than mine I haven't started ??? . Was only one entry last I remember Flash saying. Going to try and find time to dop up a bit of quartz tomorrow and cut the pav an see if my machine actually works.MB
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Hi Frank,
It wont hurt to enter it, itll probably cost nothing extra if mailed with your other entries.
Regards
Gordon
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Ps. No entry is a bad entry. Some are just better than others. The feedback should be worth the effort of submitting it.
Interesting comment about the rounding of the facet boundaries. My nandrels are all started out of equal size, 12.32 +- 0.01mm. Havent measured them lately. I use diamond on a darkside polisher, or Zirconia batstick on the darkside, both instances, very little polish. Most rounding for me comes from letting the stone wander off the centreline through slipups.slip ups
Regards
GP
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I can’t enter it now because I’ve mailed my other five stones. The mail these days seems to take longer and there is only 9 work days left in the month. Don’t want to leave it any longer, if they hadn’t knocked a whole month off the entry date, I would have had heaps of time.
Definitely have one in next years comp. and the next I.F.C., got to give Barry Chapman a run for his money. I only had the minimum three stones in this year’s I.F.C.
Having decided to scrap it last night have been experimenting on it with different polishing agents and mandrel materials and speeds. Only have hardwood and HDPE plastic, Zirconia and Cerium oxide for polishing Quartz. Found the best to be HDPE plastic and cerium at slow speed and lots of pressure. Also well scored with a bit of hacksaw blade, wet and not a lot of polish, slow oscillation.
The edges were only slightly rounded, but I wanted them to be sharp like the flat facets, reducing the diameter did that. The stop keeps the dop in place regardless of the pressure you put on the stone.
I think that would be an advantage over a locked quill because applying pressure would want the stone to move off the side of the mandrel.
One tip on the crown of that stone, don’t use big difference cheater settings on the E facets to meet the A facets. Because if you do the table won’t be square and you can’t do anything about it.
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I was thinking if i posted Monday it would be fine, but perhaps not. Given how slow the rest on my mail is atm.
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We find getting birthday cards interstate lately is taking two days longer.
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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
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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
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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
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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.
(http://aussielapidaryforum.com/forum/gallery/9/medium_120565-220520093909.jpeg) (http://aussielapidaryforum.com/forum/index.php?action=gallery;sa=view;id=9232)
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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
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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.
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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.
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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
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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.
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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. :(
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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
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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.
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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
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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
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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.
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Hi Guys,
If you are interested I started a Facebook group called Concave and Fantasy Faceting Design.
My intent is to foster discussion among a wider group than is available on ALF. Members of the half a dozen facebook groups I am a member of are granted automatic membership if they apply to join.
Regards
Flash
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I would be interested in joining when I get into it. Got my geared motor attached the pushrod assembly will operate off, but stopped work at the moment because of the stone I have on the machine.
It’s “Barion Heart – Transplant” for the US comp. They have made the stone a specified size and I’m finding you cant cut the stone accurately with the current cutting sequence. Because nearly every facet on the pavilion is cut including all the breaks, before the girdle is started. A typical meet point cutting sequence, which if you want the stone to be within 0.1mm and the meets flawless under a 10x loupe at the girdle doesn’t work.
So well into the pavilion with my own cutting sequence which is going well, will concentrate on finishing it before getting back to the concave cutter.
Going to enter stones in the Gemboree 2021, there is a concave stone in that comp.
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Hi Frank,
Great to hear. I am also planning on entering the 2021 Gemboree. It'll be good to have a concave section with more than 2 entries.
Would ypu mind if I posted your photo above on the new website. A few members of Faceting Technical have mentioned they are interested in getting a machine so I thought it might inspire them to see 2 or 3 different ideas. I am also going to ask Makky because we each have approached the design from a different angle.
Regards
Gordon
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No problem, when I get it finished and the bugs out of it, I’m going to post detailed info on how to make one.
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I remember posting that I got my concave stone away in time for this years comp. but cant find where I posted it.
Anyway much to my surprise I came second out of three entries with a score of 93.05. So done some more work on it lately to get the auto oscillation bit working.
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Did you make photo's of the stone you send in?
I am very curious what you have cut!
I have been wanting to make a concave cutter for years, but I have been struggling with my new faceting machine for a long time.
A machinist changed a lot of things about it, and I have just started cutting stones.
So for now I just want to cut some stones.....finally
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No I didn't Giel.
You can find the cutting sheet on the Australian Facetors Guild website.
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Hi Frank and Giel,
Hans is photographing place winning stones along with the medals and trophies for the AFG publication Facet Talk. You'll be able to see the cut there.
You cut a nice stone Frank.
Regards
Gordon
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I checked the AFG website but could not find the photo's, maybe they are not uploaded yet.
Or maybe you have to be a member to view the photo's.
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Hi Giel, the results are on the website. But the photos are going to be published in Facet Talk.
Hans takes delivery of the trophies this sunday at our local guild meeting to photograph the trophy winning stones with the trophy, then sll the photos will be finished.
Regards Flash
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Hi Giel and Frank,
John has already sent the next edition of Facet Talk to the printer so we are aiming to get the results and photos into the following edition.
In the meantime we are seeing if we can post the lhotos on the Guild's website. I'll let you know if we do.
Regards Gordon.
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Let's hope that non members can also admire the photo's.
I am very curious to see the cut stones in the concave section!
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Hi Guys,
The photos are up under the Competition results tab on the AFG website.
https://facetorsguild.com.au/AFG-Competition-results (https://facetorsguild.com.au/AFG-Competition-results)
Non members should be able to see them, but space restrictions mean they are a bit grainy.
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Got my name mentioned a couple of times there, but nothing compared to the US competitor John Lee. His consistency was amazing.
I'm a member of the US faceters guild and compete in their comps, mentioned on their forum about John's success in our comp.
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Hi Frank,
Yes, he had an impressive total and won two of the trophies. But your results were pretty good also
Regards
Gordon