Rotary Engraved Image Questions

[Frank Defert]

I have been trying my hand at rotary engraving on stainless water bottles using cermark. The strength of the black engraving and placement on the bottle both look good. Where I am having a problem is with the image changing from the screen on the computer to the stainless bottle.
The image from top to bottom is the right size, but from left to right it narrows and becomes elongated as opposed to it's actual shape round. All the details of the image are there crisp and clean with no blemish or fault. I have used this image on flat surfaces with great results.

As always thank you .... Frank

[James Stokes]

It sounds like it is slipping in the rotary, That will cause that to happen.

[Frank Defert]

If the rotary was slipping wouldn't the laser run over the already lasered image?

[Carl Sewell]

Do you have the rotary fixture setup correctly? The rotary requires that you specify the diameter of your object so that it can calculate an incremental value for each rotary step. For example, if you are using a 3" diameter bottle and you tell the rotary it's a 6" diameter, you're not going to end up with what you expect.

I don't have an Epilog so I cannot tell you the setup steps or parameters, but I would look there first.

It appears that the Helix uses the page size in CorelDraw to tell the rotary the diameter and length of the object. You specify the diameter by using the circumference of your object as the page HEIGHT. The length of the object is the page width.

See Page 156 of this PDF: http://www.epilogfiles.com/mini_helix_manual.pdf

[Martin Boekers]

Frank, I run two Epilog EXT's and once in a while they both do that. Haven't been able to figure it out.

I end up "stretching" the image sometimes to compensate for it.

If you come up with a good answer let me know!


Marty

[Michael Doyle]

I do a lot of pens, and have to change the aspect ratio depending upon pen diameter. I haven't done any larger diameter items.
What I would do is set up a square the size of your drawing, with a line throught the center on your item's long axis. Draw a box the same size on a piece of tape an attach it to your bottle. Be sure to include the center line. Use your red dot to align the center line of your on screen pattern with your tape pattern. Run in vector with power off to see where the outside of the boxes line up. Change the size of your on screen box until you get it to match the size of your taped box.
Once this is done, you can calculate the ratio of the change needed for that diameter by comparing the new box size to the original box size. It shouldn't take more than a couple of minutes to do it.

[David Fairfield]

It may not be due to slippage. The shape of the cylinder can distort the graphic, in much the same way that a fun house mirror changes a person's reflection.

Sometimes widening the graphic is all you need to correct its appearance on the finished product, but if the object is significantly tapered or puckered you need to compensate by warping the graphic in various ways to counteract the distortion. I'm sure there is a mathmatical formula for it, but I use simple trial and error on a sacrificial object.

HTH
Dave

[Tom Delaney]

If the cylinder you are trying to engrave is not parallel to the laser at all points the engraving will appear to be distorted from the left to right (or up and down if it is really wierd in shape). Several months ago somebody posted (I think it was Frank but my mind is not where it used to be) a formula to calculate the distortion that you had to place into your .cdr file in order to compensate for the out of parallel condition (think of a vase where it curves from a small to large circumferance but not in a straight line (wish I could draw!). It will look wierd on your screen but will engrave correctly (IF you calculate it right). Otherwise, trial and merror (painters tape is a wonderful thing but it does pull ceremark off - so give it a test before you spray!). Anyway - a search of distorted images might help.

[Dan Hintz]

I posted in that thread... I offered how to calculate the exact dimension changes, but I also suggested for compound curves to just scan in the objects outline and distort to the outline.

[Frank Defert]

Thanks for all the replies. After reading it seems that the distortion of the image is somewhat " normal " and I need to compensate for this in advance of lasering. It just doesn't seem that you should have to do that?

I have searched the forum and found lots of good reading but not the thread that explains the formula for calculating the adjustments to compensate for the distortion, anybody?

As always thanks .... Frank