New to CNC. Engraving depth

so Im new to CNC. Have a self built 3020. Finally got to the stage of playing about with it. Pocket operations and contour work fine but I have got a dozen v engravers (30degree) so I thought id try my hand at surface engraving.
I made a pocket jig so any table slope would be accounted for. And then tried text engraving.
Media was 0.5mm aluminium dogtags as its relatively soft. spindle speed was max with default feed and speed. If I broke any bits id re-evaluate but I didn’t need to.
My first attempt at 0.1mm engrave depth unsurprisingly blew through the dog tag so I kept reducing the engrave depth until I got down to 0.02mm. but the engraver kept cutting through. Now I know why the ‘better’ engraving method is drag engraving…:slight_smile:
Can it be done? What sort of tolerances realistically can a CNC work to? Would I have better results with a harder or thicker media?
If it can’t be done are v engravers only good for thin contour passes?

Edit. Im using carbide create v6/ UGS and a z-probe measured with micrometer to two decimal places.

What does your fixture look like, and where are you setting Z zero?
Are you double checking Z zero after setting it to ensure it’s at the top of your stock?

If the machine is solid & your axes don’t have any slop, you should be able to get tolerances down to 0.001" (0.025mm)

:smiley: machined out a pocket the same size as the dog tag. Used double tape and ca glue to fix the tag in the slot then ran the probe off the top surface of the dog tag. If thats what you mean?

I was starting the run from the post probe standoff position. Should I be running it from xyz zero at start?
I suppose that would help minimise any extra z axis movement…:smiley:

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You should be able to start from anywhere. I usually raise Z a couple inches to get it away from the part when the spindle starts up.

If you are using some kind of probe or bitzero to set your Z zero, at least until you are comfortable & confident with the process, it’s a good idea to move the tool down to the part & check the Z readout.
If the probing routine (Macro) is off in any way, this will verify it. The readout should read Z0.000 (or very close).

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Thanks. I guess if there’s more slop then the more movements would throw off the tolerance?
The z probe process in UGS is:- medium speed descend, retract, then a slow speed descend then a final retract. I was running from the retracted position.

Would it be worth getting a micrometer for estep calibration.? I did the initial calibration with fairly big movements in each axis so I could see them, but that wouldn’t necessarily be the more accurate…

After you do the Z probe, move the tool down to the part to check it. Or just skip the probe & zero it manually to the top of the part.

Step calibration is best done using the largest motion you can, given the precision measuring equipment you have. I have 8" calipers (resolution 0.001"), so I cut an 8" square & measure it for XY calibration.
I have 6" of clearance below my gantry, so I cut something to exactly 6" tall, then measure it.
(Or I use a 4" gage block, and touch off the table, then the top of the block)

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Ohhh. I was calibrating using axis movements only. Measuring cuts is a much better idea.

Outside contour for nearly maximum bed size rectangle. Then I can workout if I’m calibrated properly.

As further evidence. I ran another engrave test at zero z height. It gouged the media, so I obviously need to rethink my setup for improving tolerances. Or get a drag bit, but then I’ll need to get carbide create v7 to use it and then I’ll lose my save to gcode workflow.

A drag bit doesn’t strictly require Carbide Create v7:

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One other thing to consider.

Drag engraving bits are commonly spring-loaded. Small variations in material surface height are automatically compensated for, within limits.

V-bits are fixed tightly in the spindle collet, and any variation in material surface height will make a big difference in the engraving width and depth. I’ve done a lot of PCB engraving with V-bits. For good consistent results I had to use CAM programs with height-mapping. That map allows the program to alter Z heights in the g-code and compensate for material height fluctuation. Most attempts without height-mapping failed dismally.

You can do a poor-man’s height mapping (but not compensate) by probing multiple locations on the top of the material and comparing the machine coordinates of the Z-zero values. If dead flat, they should all be identical - they rarely are.

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