Designing a CNC Gem Stone Faceter

[dsadams]

I am looking for information to design and build a CNC Gem Stone Faceting Machine, I have Raytech Shaw Machine that I will either convert or use parts of to build a new machine. Are there any fellow Rockhound / Lapidaries out there that are interested in this type of machine?
I was watching my buddy running his CNC Taig Mill and I became inspired to take a shot at this. I will be looking for deals on small steppers or servos if the prices are right and I will also be looking for all of the other stuff that this machine will require. as for making some of the parts, I bet my buddy with the taig mill would like to start to recover some of the clams that he shelled out for it. Any and all info will be helpful. Thanks
Dave

[Evodyne]

Dave,

I don't know a thing about gems, and I'm asking out of curiosity: how are facets made? Is it through cleaving, or grinding/lapping? If you already have a machine that does what you want (with you at the helm), I would think automating it would be the place to start.

I'm guessing that it is no so much a milling machine with cartesian (X,Y, & Z) motions, as a machine with polar motions (rotating your stone) in two planes. And then some way to advance your cutting/grinding head. Am I even close or am I way off base?

I can't imagine that speed is your biggest concern-if not, then PC control from Windows becomes a reality (parallel port contol under Windows has a limited maximum pulse rate).

Assuming that this is good for you, you can interface directly from the PC port to step+direction stepper drivers: I like the Geckodrives.

Let's say you need three steppers and the little NEMA 23's are OK. If you were to opt for the Geckodrives, you would want bipolar steppers. I have some that are rated at 5V, 1A per phase. Let's use these ratings. What does this mean in terms of other hardware?

Geckodrives require a power supply voltage between 24 and 80 VDC. In addition, good high speed performance requires that the power supply be be 4 to 20 times the rating of the stepper. Some typical power supply outputs are 24, 36, and 48 VDC-any of these would work.

Our reference motors have two windings (phases), so worst case (both windings on at the same time) we are talking 2 amps (1 per winding x 2). NOTE: this really isn't a normal condition-both phases energized at once-but if we design for worse case nothing will go up in smoke.

You could find either one supply capable of supplying 6 amps (3 steppers * 2 amps) or three separate 2 amp supplies. Again, a little overkill, but you are erring on the safe side.

The basic G201 stepper drives are $114.00 each. Do a little eBaying and factor in your power supply costs.

What else? While not strictly necessary, end of travel limit switches (set up to kill your drives if triggered) will keep you safe. These can be mechanical, optical, magnetic, etc.

Also, some sort of "home position" switches to tell the PC that the machine is at a know position: all other motion is relative to that place.

Oh, the G201s are microstepping drives (with a fixed factor/multiplier of 10). So, the reference steppers are rated at 1.8 degrees per step or 200 steps per revolution. Using the G201 you would see 0.18 degrees per step/2000 steps per revolution. Figure out what this means in terms of movement on your machine.

Not precise enough? Use toothed wheels and belts with a reduction ratio designed to get the resolution you need. Example 3:1 reduction would mean 6000 steps per revolution.

Software? Hmmmm. I don't know if any of the typical CNC stuff would be what you would need. I suppose a good description of your machine would be needed to determine that.

BTW, did you know that there was a "Glass, Plastic and Stone" forum? It may be the better place to post, if for no other reason than it is for people with interests similar to yours.

I don't know if any of this helped, hopefully it did.

[dsadams]

Thanks Evodyne,
The facets are made by grinding on 6" diameter laps, I usually start with about 325 grit for roughing out the stone then move to 1200 grit to cut the facets then if the stone is one of the harder ones I skip to one of the polishing laps, for sapphire it would be 100,000 grit diamond on a ceramic lap. The gem stones that I cut are from 4 mm to 3/4" diameter and the speed that the laps need to run are 1000 rpm for roughing out and then slower for the faceting and then really slow for polishing on ceramic lap with 100k grit or faster for polishing on phenolic or tin laps.
The machine will need at least 3 axes and possibly a 4th.
Thanks again,
Dave

[Evodyne]

Dave,

Interesting! The Raytech Shaw Machine you mention, does it lend itself to the addition of steppers, or do you really think you will need a new machine?

I'm still wondering about the CNC software more typically used for routers/mills being exactly what you woyld need.

Good luck!

[dsadams]

Well the machine doesn't lend itself to just adding steppers. I will try to post a picture with a discription of how it works and also with my idea of how I invision the robotic arm to work on this piece of equipment.
Dave

[alipavsky]

Dave,
I can offer a bit of insight into your issue. Although I don't personaly cut stones, I have been designing and manufacturing jewelery for 15 years. I have also built and/or redesigned most of my equipment so I'm familiar with both sides of your question. I know you're not going to like this but you may need a 6-axis CNC machine to automate the functionality of a manual faceting machine. Here's my logic.

A manual faciting machine has 2 or 3 axes, depending on machine, that are set with angle adjustments. However, the last, and most important, axis is the hand presure you place on the stone as you grind the facet. The hand presure greatly influences the angle of the cut. It is precisely because of this hand-presure variable that you will need up to 6 axes. This would work by axes 4,5, and 6 being rotary tables perpendicularly attached to each other, creating a 3-axis rotary platform capable of presenting the stone within a spherical space. This whole platform would ride on your x-y cross slide and your z-axis would have a small, high-speed grinding wheel.

There's my two cents worth, I hope it helps.

Andrey

[sdjdave]

DeBeers has marketed an automated cutting system for Diamonds for years now, a lot of them are used in the Israeli cutting industry. They don't sell them, you lease them, and I'll bet they cost a bundle. There is a Korean company making a cnc stone cutting machine that is very simple, but VERY expensive (I think $23,000). It has a small lap, which is just run with a small motor, there is an axis (X) that moves the head across the machine, the head pivots to set the angle to the lap (Rotary motion there, Y? or A?), the dop rotates to index each facet (Rotary motion again, B?) and the whole head assembly moves up and down to control the depth of cut/size of facet (Z). The motors were all very small Vexta's, none of them was as big as a 23. The software they use is called GemCAD (I think). I saw this machine in Tucson two years ago at the GLDA show. I have literature on it at home somewhere. If I find it I can give you a better description, but based on what I remember about the machine, I think it was a 4 axis. Since then I have seen pictures from cutters ads showing the machines in their factories. I think the key to all of this is the GemCAD software. I haven't seen it offered anywhere, but I didn't get the impression that it was written by the company that made the machine.

You could use a 2 axis machine, but use an index plate with the plate latch pulled up momentarily to start a motor rotating the dop (slowly), then dropping the latch. When the latch drops into the next index notch, it trips a micro switch that stops the motor, leaving you locked at the next index point. That may be the best method to automate your Raytech, since you don't need to do anything except automate the angle, the indexing, and the up/down of the column. It may limit you to standard type cuts though. The GemCAD software looked like it would allow you to do all kinds of bizarre cuts, but how many of those have you cut and sold lately?

I realize this is being posted 9 months after your initial post, but maybe you have been stopped by a lack of information. Hope this helps, or gives you ideas to improve what you've gotten done so far.

If you've done something, post some pics. I hope to do a preformer sometime in the near future. Use a micrometer head to dial in your largest dimension, load your dopped rough, turn it on, and walk away. When it's done, it'll tell you and you'll have a perfectly proportioned oval/pear/marquise ready to finish.

Dave

[ViperTX]

Dave, dig up whatever information you have. This has always been a fascinating field.

[lwalker]

...and a month later on an old thread :-)

I"ve started work on a rock slabbing saw, possibly with power feed depending on how "into it" I get. So any more discussion on this topic is definitely interesting.

[ViperTX]

I have a Lortone rock slabbing saw in my garage...it's pretty simple.

[jhwatts]

Please see my post below. I have a few smaller steppers I would donate to the cause and I would be glad to join forces and build one.

http://www.cnczone.com/forums/showthread.php?t=13228

[epineh]

Hey Dave, any progress? I am a faceter as well as a newbie cnc builder.

I had been trying to think of ways to automate the process as well, I was leaning towards building and indexing head that you enter index no's (manually) and it moves to that position, and also the same method for the angle setting, getting this working then automating it.

Getting usable software might be difficult, but I also thought about making a five axis machine, entering a tool value similar to a slitting saw in dimensions, but of course this would be a small lap, then trying to set up standard software to grind away.

Dunno, haven't thought too much on it lately, I think it would be pretty sweet to put a dopped stone into the machine and watch it take shape, then come out finished.

I would think re-dopping would have to be manual, and knowing when the facets are polished properly would be the hardest to set up.

Russell.