Engraving on round objects

[mike klein]

I have engraved lots of text on round objects such as water bottles, stainless steel mugs, etc. and it works great. Today, I had to engrave a graphic which
was in the center of a circle and after it was engraved I noticed that the circle looked egg shaped, meaning it looks longer (or taller) than it is wide. Maybe its just an illusion of being on a round object, I don't know. It is a perfectly round circle in Corel Draw so I know it was created properly.

Is there some sort of adjustments that can be made or is that just the way a round image turns out when engraved with a rotary attachment. I know there is nothing wrong with the rotary attachment, I just put a new motor on it and have engraved several items with no issues.

[Braden Todd]

Im sure there's a formula out there somewhere, but I just "squish" the circle down and print it on paper to hold on the piece to check it. If it looks good then I run the modified design.

Hope that helps some

[Chuck Stone]

easy enough to check with a ruler, then you'll know if it is an illusion or not.
Also, make sure that your rotary settings aren't conflicting with the page
size in Corel. Not sure which rotary you have, but I know that you can specify
one thing on the page, and something else in the rotary settings and get
some funky results..

[Joe Hillmann]

Was the item you engraved on tapered? If so and you had the large end on the drive rollers of the rotary the design would be squished narrower. If you had the narrow end on the drive rollers it would have been stretched wider.

[Kev Williams]

I believe the reason the circle LOOKS narrow is an illusion caused by the curvature of part, simply because the sides of the circle are farther from your eyes; anything farther from our eyes looks smaller.

However, the opposite is actually true! Because you're engraving in a flat plane on a curved part, more surface area on the curved plane is engraved. Even though the sides appear shorter, they're actually longer--

Evidence of this phenomenon is in these pics. The effect IS subtle, but definitely apparent!

I wrapped a piece of notepad paper around a spray can cap, and engraved a 1.25" diameter circle on it. Now, to your eyes, even in a photo, the circle appears slightly taller than it is wide--





Same exact pic, rotated 90° counter-clockwise, the circle appears wider than it is tall








Now, watch what happens when I unwrap the paper and view it flat- Now, the oval appearance has totally reversed! Instead of too tall, now it's too short!






and rotated, the effect is even more pronounced!




For the record, with the paper flat, the circle measures exactly 1.25" top to bottom, and 1.282" side to side. Even though the actual image is .030 longer, it appears an equal distance too short as a cylindrical part... It's all just an illusion! You really can't trust what you see...

by the way, I did not rotate the part, just engraved it "flat". Mike, when you rotated the part to engrave it, you made a perfect circle as measured "flat", meaning your illusion of short sides would be even more pronounced!

[mike klein]

Interesting explanation Kev, makes sense
I am engraving the round metal sport bottles that JDS sells on a Epilog Mini with the Epilog rotary attachment.

I've engraved round images on these before and I guess I 've never noticed this before....had to be doing it all along, just didn't see it.

[Mike Null]

Mike

I don't do mugs any more but I had experienced the illusion that you describe. You can compensate quite easily. Assume you have a perfect circle you are trying to engrave or transfer to a cylinder. The circle is 3" in diameter. In the horizontal plane the image not only appears to be narrower, it actually is, if viewed in a flat plane. My method for adjusting that is to take my calipers and measure the width of the image on the cylinder as the eye sees it then adjust the drawing to produce the 3" width as the eye sees it.

illusion.jpg

[Glen Monaghan]

So, if I understand correctly, the 3.0" actual width of original circular art should be the arc of the cup's side between the red marks for 2.75". That is, a 3" circle wrapped around the side of the cup would appear narrower (you used 2.75" as representative number), so you note where 3 linear inches (measured with your calipers) are across the cup (green marks), measure the actual arc around the cup between those points (you used 3.25" as representative number), and then stretch the artwork to that width prior to actually engraving. Result is the too-wide artwork appears correctly circular when wrapped around and viewed on the cup.

[Mike Null]

Glen

I think that's right. It's a little hard to bring yourself to do it and it may not be worthwhile but it does give the illusion of a perfect circle. It works when dealing with circles but I didn't try it with ovals, although it should be similar.

[Dave Sheldrake]

The Gore method from Cartography can be used for both Spheres and Cylinders if you need point to point visual accuracy in a picture viewed in 3d produced by a 2d machine.

Quite complex but calculable for the nth degree of accuracy although different eyes will often see things differently depending on the person viewing and their eye dominance. (none of us for example, see a rainbow the same as each other) it's due to us having stereoscopic vision (depth perception) an object further away is seen as smaller when it's physical size is the same.

cheers

Dave