We have had good luck engraving pyrex with the Ostrich Eggbot engraving tool– a good example might be the Klein bottles that we wrote about recently. ( http://www.evilmadscientist.com/2013/eggbot-klein-bottle/ )
For each of those three adjustments, there are tradeoffs between intensity, speed, and precision:
1. With higher vibration strength (as you have seen) you make a bold mark more quickly, but the tip can bounce around more, making a wider line.
2. With slower engraving (as you have seen), you make a bolder mark. With faster engraving, you cover more ground quickly, but weakly.
3. Higher pressure will help to make a bold mark more quickly, but can lead to uneven depth, and a loss of precision in that way. (For thin glass, it can also lead to breakage more readily.) On the other hand, increased pressure can decrease the line width when the vibration strength is turned up all the way.
The pressure can be adjusted in a couple of different ways. The primary variables are the type of hinge that you use and the “rest position” of the engraving tip. The latter can be controlled by moving the pen arm up and down (using the big brass thumbscrew on the proximal pen arm), and by the two position setpoints for the pen-lift servo motor.
Using the thin (more flexible) hinge, and aligning the engraver tip such that it barely touches the surface, you can make it so that the engraver exerts nearly zero pressure on the glass, which will give the finest resolution and very good precision (but very light lines). Lowering the pen arm so that the “pen down” position would naturally rest below the level of the glass will begin to exert more pressure, and you can increase that further by using the stiffer hinge.
Making multiple passes is another fine way to make the lines darker. To run the same job twice, either just start again after it finishes– if everything is setup well, it should go *exactly* over the old lines again –or (select all and then) duplicate all of your objects in Inkscape before plotting.
The best way to fill the numbers will probably be to use the “Hatch Fill” extension, which you can find in the “Eggbot Contributed” menu under Extensions in Inkscape. Be sure and check the “crosshatch” button to get a solid-looking fill with the engraver. We used this with a very coarse fill on the Klein bottles, and it looked quite good. For your smaller numbers, you’ll want to use a finer fill, perhaps spacing of 5 steps to begin with, with cross hatching at 45 degrees.
