Concave Cutter

[Faceting Frank]

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.

[Faceting Frank]

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.

[Faceting Frank]

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.

[MakkyBrown]

Looks good frank. Well done on an operation machine.

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

[FlashGP]

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.

[Faceting Frank]

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.

[MakkyBrown]

Yeah, I do the girdle with the pav as well. Good luck with the concave.

MB

[Rusted]

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.

[steveo]

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.

[FlashGP]

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.

[Faceting Frank]

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.

[Faceting Frank]

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.

[Faceting Frank]

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.

[Rusted]

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.

[Faceting Frank]

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.