Engraving on Laboratory Glassware

ccschreyer

Well, I've successfully drawn with markers on my new Ostrich Bot, and I'm ready for the real work.

I manage a community college chemistry lab and I bought the Ostrich Bot and diamond engraver so I could automate putting identifying numbers on laboratory glassware. (like Station #1-30) (think Pyrex - this is tough borosilicate glass, a little harder than regular glass)  Some laboratory glassware (like a volumetric flask) is quite expensive and I need to track it for student use.

I have previously done this with a vibrating engraver pen, which is effective but sloppy looking.  I also have tried to etch numbers on, but there is a (very thin) plastic protective coating on lab glass that resists the hydrofluoric acid based etchants.  Sanding or scraping removes the coating but scratches the glass.  I have also tried sandblasting the numbers on but that was messy and sloppy looking also.

So here I am with a new toy, which draws well, but I am having trouble engraving.  I can barely see what marks the diamond point produces.

In the engraver instructions, three ways are mentioned to adjust the engraving.

#1 "vibration strength"   I have cranked the trimpot all the way up and when I just turn on the engraver in a single spot (using manial control) it hops around a bit and draws about a quarter inch line (instead of a single dot).  But it does do it fairly dark.

#2 ""speed of engraving" I have tried a pen-down speed of 5-200.  Slower is significantly better.

#3 "pressure applied by engraving tool" How do I change this?

Perhaps if I just keep going over the same places?  How would you sugget doing that?

Also, how would you recommend filling in the engraved numbers?  I need them 1/2" to 1" high. 

Thank you!

 

Windell Oskay

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.

ccschreyer

Thank you very much for your prompt, thorough response.  I haven't had a chance to implement your suggestions.  I have more questions, though!

You mention increasing the pressure with hinge stiffness.  Where do I get stiffer/softer hinges?  I installed the hinge that came with the engraver.

You mention increasing the pressure by changing pen arm height.  My pen arm is at the lowest.  By the "two position setpoints for the pen-lift servo motor," do you mean moving the whole servo motor assembly lower on the pen arm?  There seems to be two holes there.

Can I also lower the pen rotational motor in the eggbot chassis?

 

 

Windell Oskay

The Ostrich Eggbot kit should have come with a more flexible hinge, and the engraver kit comes with a stiffer hinge. You can get additional stiffness by stacking hinges.

The pen arm height is *very* adjustable, perhaps in some ways that you haven't thought of.  There are two positions to mount the servo motor assembly, as described in Step 33 of the assembly instructions.  If you are nearly bottomed out in position, you should definitely be using the lower pair of holes.

The servo adjustment points that I was referring to are the software settings that you use to adjust the servo motor positions.   

The correct position of the pen motor in the Ostrich Eggbot chassis depends on the shape of object that you are engraving.  For an object with (roughly) spherical geometry, it should be at the top position, and for increasingly elongated objects, it should be lowered.