Trotec Rotary Engraving Issue

[Greg W Watson]

I am having a problem that I am trying to overcome with my trotec rotary attachment. When I do beer glasses like the attached image, or really any glass that has any kind of taper I am running into an issue where the closer it etches to the bottom of the glass the image starts moving inward. The red arrows in the other attached image show where the image is being squeezed inward.

Is there a setting to overcome this?

Thanks

Greg

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[Mike Null]

My first thought would be to adjust the image in Corel to compensate.

When I was doing mugs, even though they were straight sided, the round image I used would appear to be an oval longer vertically than horizontally. To solve this I increased the width of the circle in the drawing. To arrive at this dimension I used calipers to measure the drawing as it appeared on a finished mug then increased it accordingly. That produced an image that looked like a circle though in fact it was a horizontal oval.

[Greg W Watson]

uggghhh I was hoping to NOT have to do that. It will be different for every design.

Greg

[Dan Hintz]

I discussed this very topic at length a while back, so a search might turn up some useful old threads. Essentially, you need to reverse the shape of the glass... luckily, a simple taper is easy enough to do. If the top of the graphic is going around the part of the glass that's 10" in diameter, and the lower portion is going around a potion that's 8" in diameter, you need to widen the lower portion of the image by 10/8 = 25%. Instead of having straight edges, it will look like this:

.._
./ \
/-----\

Ignore the periods, just for spacing

[Greg W Watson]

That is actually a very easy way to do this thanks Dan, used the perspective tool and it worked

[Mayo Pardo]

Just because I'm curious even though I don't have a rotary attachment yet...

Does this distortion happen because the diameter is smaller at the lower part of the glass or does it happen because that part of the glass is positioned lower than the top part of the glass?
Could it be eliminated by putting a shim under the lower side of the rotary holder?
If the rotary holder allows you to adjust the surface of the glass so it's parallel to the laser, then the shim idea is already taken care of so never mind on that.

[Mike Null]

Mayo

It's because the shape of the object is conical--smaller diameter at the bottom than the top. The image to be engraved is rectangular when it should be adjusted to be trapezoidal.

[Dan Hintz]

Just because I'm curious even though I don't have a rotary attachment yet...

Does this distortion happen because the diameter is smaller at the lower part of the glass or does it happen because that part of the glass is positioned lower than the top part of the glass?
Could it be eliminated by putting a shim under the lower side of the rotary holder?
If the rotary holder allows you to adjust the surface of the glass so it's parallel to the laser, then the shim idea is already taken care of so never mind on that.

You always shim to level the face of the glass, so that's not the issue. Imagine a glass 360" in diameter at the large end and 180" in diameter at the small. For every 1 degree of rotation, the large end shifts 1" but the small end only shifts 1/2". Pictures start to look funny real fast.

[Kristina Jones]

There is a great 5 part post about using a Trotec rotary here - http://www.engrave.ca/ This answers a lot of questions about this and how to get your material set up correctly in the rotary attachment.

[Ross Moshinsky]

The cheater way is to play the with diameter. If the glass is 3" at the top and 2" at the bottom and where you're engraving is about 2.75-2.25", using a 2.5" diameter can often make the problem negligible.

[Greg W Watson]

The cheater way is to play the with diameter. If the glass is 3" at the top and 2" at the bottom and where you're engraving is about 2.75-2.25", using a 2.5" diameter can often make the problem negligible.



As luck would have it I have a customer who is way too fond of his caliper set.... sigh :(