I have used Tom's setup with great success, but every alignment took 5-10 minutes, mainly adjusting (playing) with both the mirror and the laser location.
Once the setup was in order it took only 30 seconds to slide the mast so that the laser beam hit the polished girdle facet at 90 degrees,
and adjusting the radial adjustment (aka. cheater, index splitter) & fine angle knob (i have Graves 5XL faceting machine) so that the laser spot reflected back to where it was from.
Later i realized my 6 meter light path was superfluous and maybe about 2 meters of total light path would give reasonable accuracy to get the job done. I was hoping to find something more compact with less moving parts. I tested various setups until i decided to build the cheap laser in to the small IKEA shelf top- lamp. The lamp has been fixed on top of my shelf so that i have clear view to it's hair grid during the alignment. I also made additional semi-transparent mirror unit at 45° on the front of the laser to turn the laser beam to the wall, giving 6 meters light path. It was not needed - it is just so much easier to look up when adjusting the machine.
You don't want to find the most powerful laser for this tool, mine is only Class 1M (1mW) so i don't have to use protective eyewear.
Remember, it's your responsibility to be aware of the risks of laser radiaton - not mine!
The most tedious task is to find suitable lamp for the tool. You want to be able to "calibrate" the laser head, meaning it has to be adjustable but still solid enough so it does not drift the laser spot once fixed. You want to hit the laser beam perpendicular to you cutting/polishing lap. When you select the spot place carefully, you get additional bonus - it gives you a nice reference mark for lifting the quill always at the same location while cutting or polishing.
The whole process of radial adjustment takes now anything between 15 and 45 seconds. I only need to polish one girdle facet for reference. Usually 3k diamond prepolish done with BATT- lap is reflective enough. Since one direction of laser spot is adjusted by fine angle micrometer, i can easily look at the digital angle readout of my machine and see 0.10 degrees change makes about 6 mm (1/4 inch) spot movement. The spot can be adjusted precisely to perfect retroreflection (back to the hole it came from). It is safe to claim i'm able to adjust the beam within 1 mm giving resolution of 0.02 degrees. I'm not going to say anything numerical about accuracy, but so far i have not had any need to touch the "cheater" while cutting. (I use term cheater when it is needed during cutting, the very same knob is radial adjuster in my terminology when used after transfer).
How to build it
As you can see from the image, there is no high-tech involved here. If you have any experience with electronics and you know how to bore you can easily do it yourself:
1. Lamp housing - small adjustable shelf-top halogen lamp (do not remove halogen lamp holder)
2. Halogen lamp holder with wiring to power source (2 AA-batteries and switch at the base of the lamp)
3. Pinned wires from laser diode to the lamp holder (make sure about the polarity)
4. Red laser diode (disassembled from $10 leveling laser)
5. Plastic holder for laser diode, glued to the housing
6. Laser beam
7. White or light gray thin plastic screen with 2mm hole at the center. Top side inked black. Black "hair" cross at the bottom side.
1. Black hair grid
2. Exit for the laser beam
Girdle alignment procedure
When you have done your first side of the gem together with the girdle, use your favorite method to transfer the gem. Make sure you don't bury all your girdle to the dopping medium. If you have keying system you can use it for quick pre-alignment.
1. Insert a small mirror on the top of your true cutting or polishing lap. Best mirror type would be first surface mirror - i don't have one. I have used my faceting machine to cut this mirror and glued small dowel for better handling. Make sure the laser beam hits to the mirror.
2. Look up to see if the laser spot has been mirrored back to it's origin. In this case i have purposely twisted the lamp arm to incorrect angle.
3. Twist the lamp arm so that laser spot aligns right in the middle of cross hair. How do you know it's there? What you can't see from the image is that laser light has diffracted when it came through the hole. In addition to spot itself one can see co-centric red rays caused by diffraction.
4. Select the index opposite to the girdle facet you are aligning. If your design does not use base indexes, you need extra care here. Remember, when you transfer you have mirror indexes. For example, with 96 index gear your pavilion index 3 becomes to index number 93 when cutting crown. Just remember this mirroring before selecting the opposite index.
Adjust your faceting head to ~90 degrees.
Insert your dop in to the quill, but don't tighten it yet.
Slide your mast so that the laser beam hits the girdle.
Rotate the dop until you see the girdle facet is mirroring the laser beam up.
Tighten the dop to the quill.
5. Again, look up and find your mirrored laser spot.
6. Adjust your faceting machine head so that laser spot is aligned to the grid. There is two adjustments you need to make:
You can move the laser spot in east-west direction by adjusting your faceting head angle.
You can move the laser spot in north-east direction by adjusting your radial adjuster (cheater).
It's done! Now you just select the actual index and start cutting. It is good habit to make some reality checks. I always leave quite thick girdle first, so i still have enough material to make minor radial adjustment if something went wrong with the laser.
Note! It's not critical to always adjust the spot exactly back to the hole. Instead you need to adjust it to the same location as it was with the mirror. I keep the mirror on the lap while adjusting. If you need to recheck the reference spot, you just swing the faceting head away for a while.
While playing with my laser i figured one could insert a small mirror (it's easy to make with your machine) on the top of the adapter.
I bored a small (diam. 6mm) bed for mirror and glued the mirror with easy melting Graves shellac. I took the task of careful adapter adjustment with UltraTec's DAD- calibration block (Fig. 1 and 2).
When everything was in order i gently heated the mirror so that shellac softened enough to make the mirror adjustment possible. After 10 minutes of tapping the heated mirror with the tweezers i was happily adjusted the laser beam.
Now it takes less than minute to insert the table adapter together with the dop and adjust the table level parallel to the lap (actually parallel to the mirror on the lap). Of course there is always little cheating involved in table cutting - this modification just makes it easier to bother to use the adapter.
I'm writing this new rule down here mainly for myself - never ever again use the table adapter as a heating jig to get the gem off the dop!
1. Select some base index, for example 96 with 96 index gear. Set your radial adjustment to zero to have maximum adjustment room in either direction:
Insert the dop in the quill but do not tighten the quill locking nut yet.
Using hard stop of your machine, lift the quill so that the facet is about in horizontal orientation. Lock the dop screw now.
Slide the mast so that the laser beam hits the facet (Fig. 1 - left small image)..
Find the laser spot on the screen. Adjust the spot to the center of the grid using the same controls as previously (Fig. 1 - right small image).
Read the angle from the digital angle dial (138.27° in this case).
Make simple math: 138.27 - 180 = -41.73. Forget the minus sign. The facet angle is 41.73°.
Adjust your faceting head to the calculated angle using hard stop. (Fig. 2) Rotate the index wheel 180° (to the opposite index).
Ink your facet with your favorite color. Lower the facet head to the stationary lap until it touches. "Polish" the ink by rubbing the facet to the lap and see how it is removed from the facet surface. Usually it's darn close.
Remember! You have not measured the absolute angle of the facet to the girdle - you may have tilted dopping or tilted gem - but it is the right angle for this particular facet for your current lap.