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View Full Version : Question about creating acrylic jigs - using a trotec laser



Andy Jones
04-15-2014, 1:58 AM
Hi everyone,

Long time lurker, firs time poster.

I recently purchased two trotec machines. Speedy 300-30 watt, and Galvo fiber laser with all the bells and whistles-xyz axis. We sell engraved products such as business card holders, keychains, etc in bulk. Average order is around 500 pcs. Hence the reason we wanted to buy a setup where we could run jobs at a higher level of production vs. engraving 1 at a time. The co2 machine was purchased for engraving certain materials, but to also help cut jigs for the Galvo fiber laser.The machines and reps from trotec have been great so far but we are running into an issue that we just can't overcome. We are currently cutting jigs out of 1/8" acrylic. A typical jig would be 16x16 with the same item template cut evenly throughout. Example 9 across, 4 high utilizing illustrator to evenly space them out, say 10 mm apart. The issue is the result of the entire jig is not consistent. Majority of the engravings are not centered. Majority of them are off center to the right.




There are a few variables I have determined are at play.
1. The items may not always be 100% exact. (made in China...go figure)


2. XY table is not exact. Using an L bracket to push to jig up onto the table.


3. The acrylic jig I am cutting is not being cut to an exact specification.




1. I don't think there is anything I can do to overcome this.


2. My first few trial runs we're way off until it was determined that the L bracket was off slightly skewing the x/y placement. This has since been fixed so I am able to eliminate this possibility.


3. I have cut jigs on multiple settings, different speeds, power, etc. But I am noticing an inconsistent spacing of the templates. I am using a Caliper to measure each cut/ item space and they typically are off by an average of +/- .05 mm. By the time I get to the 5th or 6th item on the row, that .05 adds up and the mark is really off. This issue tends to be consistent throughout the 4 rows.


Is there anyway I can overcome this? Using any other materials to make a more accurate jig, or way of Fine tuning the settings to get a more accurate cut with the acrylic?




Any help would be greatly appreciated as I am dead in the water searching for a gust of hope..


Thanks!
Andy

Dave Sheldrake
04-15-2014, 5:41 AM
I'll run off a multihole jig on my Mitsubishi later today, I have a feeling the combination of all the errors may be where the 0.05 is coming from.

Acrylic isn't good for lasered jigs for high levels of accuracy, it tends to creep when cut and that can lead to small errors.

Try adding a single point to the left hand side of the jig that is spotted each time a jig hole is cut (to get all the movement in one direction relative to a datum rather than incrementally from the last position)

cheers

Dave

Scott Shepherd
04-15-2014, 8:17 AM
.05mm? That's .002" of an inch to most of us here. Even stacking that up 5 times isn't going to get you "way off". Sounds like something else at play here. For our fixtures, we cut the outside to shape, then put it on whatever table we'll be using to engrave (not the vector table) to cut the openings. I want the openings to be cut when the actual fixture is in it's proper place. I don't like cutting the fixture all in one piece and using the vector table and then trying to make that work without the vector table.

Richard Rumancik
04-15-2014, 9:40 AM
Andy, please confirm that you didn't miss a decimal here - as Scott noted +/-.05mm x 5 = .25 mm which is .010" max. off of desired position on the last item. For most decorative applications you probably can't get (or don't need) better than that. If it was a small jewelry item maybe that error would be too much but for most applications it would be fine.

If you truly need positional accuracy of +/- .10 mm (+/- .004") or better for all the items then you need some pretty serious tooling and procedures. Acrylic fixtures are fine for most promotional product marking but for high accuracy you probably need metal fixturing which will be very expensive. This isn't consistent, though, as you admmit the parts themselves are not very accurate.

When you make the fixture on one machine and mark on another it adds a couple errors.

1) On the co2 laser, if you mark two vertical lines 24" apart, what is the measured pitch? There may be an error on positioning. (Same issue with other axis.) On my laser, if I mark 24.00" lines it measures 23.96". So for accurate work I have to compensate for that. Since you are using two lasers, you possibly have an additional calibration error on the second machine. That machine needs to be checked for calibration too.

2) How do you ensure that the "0,0" of both machines is identical? In my case I make a fixture on one machine and mark the part on the same machine. Even then, between runs, I find that my 0,0 origin will drift and when starting a new run I may have to shift my entire graphic a few throusandths in the x or y to correct it. (It is easier for me to move the graphic than re-set the origin in the machine.) In any event you need to find a way to establish/correct the origin on the second (marking) machine.

3) When making fixtures that need high accuracy try to pick up on important features and edges. For crude alignment a "pocket" in the acylic sheet works fine but if there is .2 mm slop in the pocket you can't get .1 mm accuracy. Sometimes it may be necessary to spring load the pocket to push the part into a "corner" so it positions consistently. I can't see how this level of complexity should be necessary for keytags and promo prouducts, however.

4) When you mark the parts, make sure the fixture is screwed or clamped down on the table. It can't be "loose" on the table.

Could you provide a few pics of your fixture?

Dan Hintz
04-15-2014, 10:00 AM
Majority of the engravings are not centered. Majority of them are off center to the right.

Are all engravings in a batch off by the same amount and in the same direction? If so, the template itself is out of position.

Does the error grow outwards from the center of the template? If so, the location of the actual template holes does not match what the galvo is using.

Is the error zero in one spot (not the center), but grows as you move towards an edge of the template? If so, the galvo may need alignment to the center of the working area.