Double sided engraving

[Josh Jackson]

Ladies and Gentlemen of the Creek,

I have yet again, stuck my proverbial foot in my mouth. A customer has ordered 500 of a custom alder wood coin. Sounds marvelous, except...the border is a rope design. (see image) AND it is to be engraved front and back. This has had a 30/70 success rate of completely aligning the sides up. I am looking for suggestions and recommendations.

The first side engraves well, cuts out well (aside from the burning, which I would like to clear up too) but when I flip it and fit it snuggly back into the place it was cut from there seems to be consistent misalignment. I've tweaked it over and over, but can't find a good solution. So I turn to the wisdom of the Creek.

Thank you.
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[Mike Troncalli]

Two potential issues that I can see which might be your problem.

1. Angled cut.. Assuming that you cut these out yourself.... If so then are the sides a perfect 90 degree cut? if they are angled at all once you flip them over it can shift them a bit..

2. Software control - Have you checked ever single setting in your software to be sure both files are exactly the same... Postion, engraving size, etc, etc...

[Scott Shepherd]

It's not going to line up perfectly with that design by flipping it over. Your rope pattern isn't symmetrical. The pattern is leaning to the left, and if you flip that, to place it back in the cutout, it's going to be facing the wrong direction (to the right) on the side that is down. Only way that would work is if it was symmetrical. If you are in CorelDraw, take one side, mirror it, and then take the other side and overlay it and you should see the misalignment problem. You need to fix that. There are a number of ways. You could reverse the rope direction on one side, which would mean that they line up.

[Gary Hair]

I would laser one side, then the other, then cut. If you superimpose the patterns in corel then you can rotate one or the other to get them properly aligned.

[Dan Hintz]

It's not going to line up perfectly with that design by flipping it over. Your rope pattern isn't symmetrical.

This^^^. Flip one side over in CorelDRAW and overlay it... you'll see. There's no hope with that specific design, so if you want it to match, you need to add some form of symmetric border around the rope.

[Mike Troncalli]

Could you get away with using wood that is 1/2 thickness than you are using now? Engrave 500 with each side and then glue them together to make one of the regular thickness..

[Bill George]

What does the customer say? You might be fixing a problem that is really not that important to him/her.

[Samuel Espy]

I agree with the comments regarding the purely geometric considerations of aligning the flipped designs, and those considerations will have to be sorted first. I would also add to the discussion that you might consider making a fixture that holds numerous coins. Each coin would reside snugly in a circular hole in the fixture that is cut ALL the way through the fixture. After engraving one side, you would flip the entire fixture (not extract a coin, flip it and re-insert in hole). Thus, after solving the geometric flip/alignment issues discussed, there should not be any ambiguity in the rotational orientation of each coin as there is if you remove coin, flip, and reinsert. You may already be doing it that way, but that was not my impression from your description.

[Keith Colson]

Hi Josh

I typically get around 50 microns of error when doing flipped alignment. Assuming your artwork has the symmetry as noted above this is the process I would do if I wanted a precision result. I do my circuit boards on the laser like this. they need to be precisely aligned. As suggested you need to make sure your laser beam is perpendicular to the table too.

1) Engrave a complete panel with two dowel pin holes cut into it, say at the top two corners.
2) Double side tape some acrylic to your engraving table and cut some dowel holes in it, Mount some precision steel dowel pins in the holes. I set them up for a push fit.
3) Flip and press your panel onto the dowel pins and do the side two engrave.

This avoids the misalignment you get from placing individual coins. I have this process working really well. You just need to put the dowel pin circles in both sides of the artwork, and they must match when flipped.

edit. I forgot to mention, you must leave your laser cutter on (do not re-home) and do not move the bed "Z" or you will not make these numbers I mentioned.

Cheers
Keith

[Ryan Daily]

Hey Keith, could you elaborate more on this process? Do you have any photos or links? I have been having similar 2-sided issues.
Thanks!

[Dave Gates]

Josh, I did a double sided engraving job where the front and back were not symmetrical. For the sheet of wood I used, half the setup was Side 1 and the other half was Side 2. After engraving and cutting, I took the pieces for Side 1 and flipped them into the cut out holes of Side 2 before proceeding with the final engraving pass (and put Side 2 pieces into the Side 1 cut out holes as well). The rope design looks like it will have to be modified a little like others have said. Couldn't you just flip/mirror the rope design on one side?

[David Somers]

Josh,

What Keith Colson is describing is the same technique used on CNC routers to do two sided router engraving. If you go to Vectric.com or any of the other major CNC software companies you will find some nice videos that show the process. Dont worry about the parts of the video showing the development of the graphic. Just focus on how they do the alignment. There was a nice one in the tutorials for Vectric that showed both the software side of this and then showed the process on the table of the router actually doing the cut. It was nice and clear. Keith is suggesting the same thing, but adding in the use of pins to keep your media above the table for a cleaner cut. Otherwise the process is the same.

When Keith mentioned the laser beam being perpendicular to the table I believe he meant that the table itself has to be on a plane parallel to the X/Y axis of the gantry. And also that your guides on the table being used to place your workpiece are aligned with the gantry as well. (Correct me if I am not interpreting you correctly Keith! <grin>)

[Michael Hunter]

David -

If the laser beam hits the final lens off-centre, then the beam will slope away to one side : not perpendicular to the table.

[David Somers]

True Michael! Thanks for showing that. Would it be fair to say all 3 apply in this case then? The beam itself, the table being aligned to the gantry plane, and the guides being aligned to the X and Y axis?
Dave

[Michael Hunter]

Yes - everything needs to be aligned properly!

Big problems doing anything if the table is tipped relative to the gantry movement, as the focus will be lost as the head moves.