Linear rail concave cutter

[Faceting Frank]

CONCAVE CUTTER.
This cutter only cuts facets that revolve around the centreline of the stone, same as all the flat facets. Have included a picture of it showing the entire unit.
I built the whole unit for about $400.

Below is the components I bought online from overseas. From Banggood online shopping website.
On their front page select. Tool industrial & scientific  - Machine tools & accessories – Motor.
That page will show all the electrical components you need. 12v dc motor, 12v worm gearbox motor, 240-12v transformer and variable speed controller.
I selected the following. Right angled worm gearbox 220 rpm, 775 12v dc motor, 2 hanpose ac110/220 to 12v dc transformers, 2 machifit variable speed controllers.
 12v on/off switches and 12v cable connectors from supercheap
Current total advertised cost for the two motors, two transformers and two speed controllers is less than $60 US plus freight. Wiring the whole thing up is very simple.

From the front page again. Tool industrial & scientific  - Machine tools & accessories – Linear motion.
You need a Linear slide rail with two bearings, 300mm long is long enough. These are the square rails with the bearings that have red and green plastic on them. Not the round rails which are no good for this application.
The rail and bearing I looked at was a Machifit HGR20 300mm long rail with one HGH20CA bearing for $34.25 US, you will need an additional bearing to suit. On eBay I found a 500mm long rail with two  HGH20CA bearings on it for $94.50 AU.
One bearing supports the quill and motor assembly, the other the stone dop stop.
The quill from eBay is a C8ER11M 150mm long straight shanked collet chuck and wrench, the one I thought was ideal for the job was $42.20 AU. It has to be 150mm long and C8, which is 8mm diameter shaft C12 and C16 stand for 12 and 16mm. I use one as my faceting machine quill and their great.
The chuck that goes in it is a ER11 ¼” collet chuck and they cost about $10. The aluminium,  brass, copper bar came from eBay Australia.
I will go into detail on how to make the individual components as we go along.

[Faceting Frank]

Concave Cutter rail, rail bearings, drive motor, collet chuck and shaft assembly.

Have included a drawing of these components but avoided putting too much detailed information on it, as they can be difficult to follow. I’ve described the aluminium components as I would have purchased them. But because I’ve got a lot of old cutter components, I used them where practical. You will notice that in some of the pictures.

The collet chuck shaft support is bolted to a 50x6 piece of flat bar 120 long. The 25x25 shaft support is 50mm long, it doesn’t want to be any shorter. The shaft bearings are plain 8mm ball bearings, 8x16x5 wide with rubber seals, they will be a press on fit onto the collet shaft. The 25x25 aluminium bar will be bored through 9mm in the centre and the ends bored 16mm, deep enough to suit the bearings. The bearings are fixed to the 25x25 bar with grub screws.
I made the collet shaft and the 50mm long support out of an old faceting machine, so its 12mm diameter shaft and 50x25 bar, but 8mm diameter shaft is much easier.

The 50x6 flat bar fitting position to the bearing is shown on the drawing. You will need ¼ inch brass rod, (not from the hardware as it won’t be straight) to make the shafts for your own cutting tools. You can use a length of this (shown as green on the drawing) to accurately align the collet up with the bearing rail. Then tighten all the fixing bolts up.
I’ve made the motor support out of 10mm thick aluminium flat bar as it makes for a nice square edge to fit to the 50x6 flat bar. The motor fixing to the 10mm bar includes rubber washers, these are to provide a soft support for the motor. I included two bent flat bars (shown in the picture) to stop the bolts vibrating loose. But they turned out to not be needed.
I got sick of waiting for my flexible coupling from China and made up a coupling myself from plastic rod from eBay (see picture). I would recommend doing the same, because they don’t touch each other except during rotation and they don’t need to be accurately aligned.
I also made some polishing mandrels for cerium oxide out of it. I made the rest of my mandrels out of 12mm copper rod 50mm long bored out ¼ “ and turned in the lathe after fitting the ¼” with 35mm protruding. The collet chuck needs 20mm and I added an additional 15mm for clearance.

[Faceting Frank]

Concave Cutter dop stop.
The dop stop is fitted to the top of one of the linear rail bearings and moves along the rail to suit the angle of the faceting machine stone and dop. I already had a rail and bearing from the mast of an old faceting machine and wasn’t game to buy another bearing for it. Because I have a total of four 16mm rails and bearings and their all different. So, made up something from a lump of 50x25mm ally bar. I suggested 20mm linear rail and bearings for this as it will have a bit more body to it.

I haven’t put any dimensions on the stop drawing apart from the 70mm high back bar, because I don’t know how big the bearing is. I’ve used 10x10 bar for the lock underneath, it could be anything. One of the 6mm bars is bolted to the bearing with countersunk screws, so the top one can fit flush on top of it. Everything else is bolted to the top one.
The 70mm high bar must be perfectly square to it, those plastic right-angle squares you get from the stationaries will do to set this up. If after its all bolted together it’s not square it will have to be shimmed till it is.

 The top 6mm bar is bolted to the bottom one with a single 6mm cap screw “see picture”. This will have a clearance hole on it so it can be moved to set up the ¼’ rod to the back stop and tightened.
Once this is accurately aligned with the cutter quill and chuck it is permanent and never needs doing again.

Got the idea from someone on You Tube and improved on it by making the curved stop longer so the dop is against it before getting down to the mandrel. It works really well because the rotation of the mandrel keeps the stone in place.

[Faceting Frank]

SETTING UP THE DOP STOP
The initial setup of the dop stop is by putting brass rod in the concave cutter chuck and leveling the 6mm thick flat bar to it, see picture “Dop stop alignment”.
 Hold everything together then tightening the cap screw to lock the top plate in position. That way when the stone dop is against its stop, it will be exactly in line with the cutting tool. This must be done before initial setup on the faceting machine.
I use cap screws as much as possible, they have a compact round head and allen keys are more easily used in confined spaces.


INITIAL SETUP OF CONCAVE CUTTER
The timber base is easily fixed to the machine base with 25x3 ally angle, screwed into the timber and bolted to the table by tapping 6mm threads into the table. The size and location of the timber base will be determined on initial setup.
The position of the concave cutter on the machine table will be at an angle because of the machine spindle. (see drawing). Also, there will be a limit to how close the linear rail can get to the faceting head base (see part A in drawing) when the quill is at 90deg.
During the use of the concave cutter the faceting head base will not move, during angle changes everything will be moved along the linear rail to suit. That way the centreline of the stone is always directly above the cutting tool.

So, with the faceting machine quill at 90deg and setup like part A in the drawing. The quill against the stop as in the picture “Dop stop setup”. Slide the faceting head base back in its slot along with the concave cutter, till you find the most convenient spot to locate the concave cutter. Chances are the faceting head will be right back in the slot.
This will be the same place the faceting head is located every time the concave cutter is used.
With the dop accurately aligned against the stop, tighten the mast base and bolt the concave cutter to the table, two 6mm bolts will be adequate.

[Faceting Frank]

ONGOING SETUP OF THE CONCAVE CUTTER
Bolt the concave cutter to the table and with the mast base in its position, quill at the required angle. Slide the dop stop into position and setup as per drawing “Stone and dop alignment”. Any fine adjustment can be done with the adjusting screws on the cutter base, then fit the curved dop stop.

Slide the rest along the rail into position, which once you know where everything goes will help with a couple of ink marks on the rail. The oscillating pushrod on mine looks a dodgy looking thing, as I expected to have a bit of trial and error on the pitch of the holes and radius of the pin on the geared motor to suit 50mm long cutters. Turned out to be spot on, radius of the pin on the geared motor is 15mm giving a stroke of 30mm and the hole pitch is 75mm. So, didn’t bother making another one.

Not saying 75mm is right for this, you will have to work it out so the linear rail bearing can’t come back and hit anything to do with the geared motor setup.
The pushrod wants to be lift off, mines on a 10mm round bar because originally, I was operating it by hand. Long 6mm bolt with the head cut off, tapped into the 25 square quill bases will do, with some tube on it to bring the height up.

The geared motor cable connectors were tiny, so I made up standard 12v connectors to connect to. Hence the plastic top.

Splash pan is something I stole off the wife, doesn’t have a slot in it yet on the righthand side to allow to cut girdles. I only use baby oil as a lubricant and that works fine.

[Faceting Frank]

Rest of pictures.

[Faceting Frank]

Tried to make the info as easy to follow as possible. easy to get down on paper after having already made it. But not so easy to follow when you know nothing about it. I did spend some time in working out the stone-dop stop setup and it will cut concaves on the pavilion to a perfect point.

More than happy to help if someone wants to give it a go.

[Faceting Frank]

Haven’t bothered to make up a light box to take stone pictures in, so still a bit primitive in that regard.
This is a concave stone I finished a couple of days ago on this machine.