VLS6.60 laser does not cut perpendicular

[Keith Colson]

I have been doing some reasonably precision things on the machine lately as I have been making a 1926 style mechanical TV. I thought I would simply cut some little belt pulleys out of acrylic. I made them, and fit one on a motor and it wobbles like crazy

I did some measurement and found when I cut a "tight" 5mm hole and stick a shaft in it. Then measure the shaft angle I get 3 degrees left and 1 degree back. 3 degrees is huge!

I looked around and cannot see anyway to adjust this other than make a custom mirror holder for the mirror in the focusing head. Has anyone solved this problem on their machine or have a solution? Cutting a perpendicular hole would actually be quite useful to me.

Cheers
Keith

[Matt McCoy]

How thick is the acrylic? Could it be the hourglass shape of the beam?

[Kev Williams]

Light bends and plastic melts, and they and other factors affect how a laser cuts acrylic...

For some plex parts I make, I have e-plastics cut 3/8" thick white plex into 3/8" square rods, which I cut to 4" long with my LS900. I do them in one pass, which is quite slow, takes like 3 seconds to travel the 3/8" across-- and the cuts are always angled. I grabbed 2 of them from their box, and found a piece of 3/8 thick pine or whatever it is. I set the wood in the laser, and cut it thru, same power, speed and same direction to match how I cut the plex. I put the cut edges of the plex above the wood, and tooks some pics...

Note the plex cut is very angled and not consistent, the cut appears to curve even...
--but the wood cut is near dead to rights straight up & down....

I've always assumed if I adjusted for the angle, the angle would just change.. "precision cut acrylic with a laser" IMO is an oxymoron ;)

FWIW the 2 plex pieces are random, not the exact cut, but definitely typical-- and, I haven't adjusted my beam or mirrors in any way for maybe 5 years....
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[Brian Lamb]

Cut the hole slightly undersize and ream it to size on the drill press if you want accurate holes at 90º to the surface.

[Rich Harman]

First thing is to be sure that the beam is perpendicular to the work piece. Aside from that I don't think it can ever be that precise.

I laser the holes undersized then use a reamer. Even so, that reamed hole ends up being a couple thou off center usually, but it is exactly 90 deg to the surface.

[Joe Pelonio]

Back when I first got my laser I learned this when trying to put together an acrylic box with laser cut 1/4" acrylic. The hour glass beam shape was the explanation, and there is no cure other than machining. The thicker the material, the more off it is. Many people doing a lot of 1/16 never notice it. For precise 90 degree angles the CNC is a better tool.

[Joseph Shawa]

Late on my answer but if you are not getting a perpendicular cut then it is because beam from mirror 1 is not at dead center of mirror 3.
You might get a vertical beam and can test it without your lens in but as soon as you put your lens in it will be off slightly because of diffraction.
You If you are dead center of your cutter head and hit the same spot at extreme Z positions without your lens then you should get the same result with your lens in. If not then you have an uneven lens seat.

[Rich Harman]

Late on my answer but if you are not getting a perpendicular cut then it is because beam from mirror 1 is not at dead center of mirror 3...

I mostly agree but you can be dead center and still have the beam going at an angle. For example, if your third mirror has three adjustment screws you can align that mirror with all the screws turned nearly all the way in, turned nearly all the way out - or somewhere in between. If they are all turned in then the mirror will be closer to the second mirror than is optimal. To hit the center of the lens the beam will have to be angled slightly away from perpendicular. If you align with the screws turned out then you will need to angle the beam back towards the lens slightly.

Conversely, you may be able to account for the beam not being perfectly centered by using those same screws to move the mirror in or out slightly.

You also can have the case where the beam is centered but the laser head is not mounted perfectly perpendicular.

To test for being dead center I use thermal paper. Fire a spot at both extremes of Z axis movement to check - with the lens in place. The thermal paper allows the spot to be visible even though the beam is spread to a large diameter.