Method to accurately measure engraving depth to achieve AS478-15B compliance standard

[John Kleiber]

All,

Today I ran a series of tests to achieve a mark on 7075 aluminum at a depth required by AS478-15B standards.

Height of the engraved font is a mere 2mm. I believe I have achieved proper depth, however how do I make sure.

What methods are available to measure engraving depth of very fine engravings?

Thanks
John

[Kev Williams]

by fiber or mill?

One way to find out is locate someone with an expensive optical comparator to measure the depth--

But what I do is copy the engraving sequence (do exactly what you did to make the small letter) and make a engraving big enough to get the tip of a small depth micrometer into. For the most part, whatever you did for the fine engraving will repeat with say a 1.5mm box...

Another novel idea that I use, is Silly Putty. I have a series of engravings at different depths, from .001 to .010 in .001 increments, and to .020 in .002 increments.. I press a piece of Silly Putty into my new engraving, then another piece into the test piece- Now take a 10x or so loupe and check them out- At 10 power the difference between .002 and .003" is visible...

If engraving by fiber, beware of slag, as it can give false depth readings as it's standing above the base metal..

[John Kleiber]

by fiber or mill?

Kev,

I am using Fiber.

I think the easiest thing to do at this point is the 1.5 mm box idea. Its 7075 aluminum so yes there is a rough bottom to the engraving as well as rough edges.

Generally speaking any aluminum engraving I do is = to or > than customers requirements. However in this specific instance where strict guidance is reference in the drawing, I was concerned to exceed AS478-15B min/max requirements for fear of being out of compliance.

-John

[Kev Williams]

Laser milling to a certain depth creates a few questions that don't apply to with tool milling: How do you create a consistent depth of cut using a beam of light, and because a laser beam creates microscopic peaks and valley 'mountains', which depth applies, the peak or the valley? I had a customer require a .004" minimum depth on some aluminum parts, and they called me out because some places measured .0038"... reeeaallly?? I'm going to get NCR'd over 2/10,000 of an inch, on engraving that serves ONLY a visual purpose? I asked them, did you measure the surface peaks, or the bottom valleys? A depth micrometer will measure the distance from the surface to the peaks- in my case, .0038". But once you mill down the engraving to the exact point all engraving has been removed, and measure THAT depth, you're likely going to find it's around .0043"...

Running a very tight hatch routine will combat the difference in peak and valley depths, but nothing I can think of can or will serve as a cutting depth gauge for laser machining. And many engineers who come up with these machining rules seem to be clueless about stuff like this...

[Gary Hair]

Its 7075 aluminum so yes there is a rough bottom to the engraving as well as rough edges.

If you lower the power and increase the frequency you can reduce or avoid these two problems. Running multiple hatches will smooth the bottom of the engraving as well as speed up the process. I usually run a final pass at higher speed, lower power and higher frequency to clean the bottom of the engraving if necessary.