Dealing with odd shapes in a laser

[David Somers]

Has anyone taken the time to make accommodations for surfaces that are sloped when you engrave or cut? See what you think.


I do a lot of wood turning. I do all sorts of things from hollow forms to bowls to platters, upwards of 30 to 40inch diameter. And I do small things like Christmas ornaments that have a globe measuring 1.5 inches diameter. A lot of these pieces have sloping surfaces where I envision engraving or cutting happening.


For example.


A the rim of a platter might drop 15mm from the outer edge to the inner edge. A distance of 20 to 30 mm. My initial thought on deal with that would be to burn the pattern in bands. Assuming that a band with a width of 5 or 10mm stayed within the acceptable depth of field of the lens then I would try to burn the patter in a series of bands of the width. It would be time consuming I assume, but these are one off pieces and not part of a production line.


Another example might be a vase. The shoulders of a vase would likely have the same problem of exceeding the depth of field of even a 4 inch/100mm lens. The height of the neck and top of the vase are likely to be in the way of allowing the laser head to travel across the entire piece. The idea I came up with was to modify chuck style rotary device so that the chuck held a plate to which the base of the vase was attached, and the entire rotary device then had 1 side lifted up until the surface you were engraving or cutting was parallel with the focal plane of the lens. Then you could rotate it as the laser cut.


Has anyone tried solutions like that? Or had other thoughts for similar shapes that might not otherwise be run through a laser?


Obviously your laser needs to have enough height available to accommodate this.


Any thoughts? Am I nuts or do you see some possibilities for this type of thing?


Thanks for your thoughts everyone. As I said earlier, I am exploring the use of a laser to be an add on to my wood turning, among other things, and have been trying to learn as much as I can about how these things operate and what their firm limitations are.


Dave

[Mike Null]

There are a number of posts on this topic. Most have the subject "cutting a bevel" so that's where I'd suggest searching.

What happens in your scenario is that the engraving will be elongated as the lens is de-focused. A longer lens like a 4", for example, will produce better results than a 2" lens but still has a limited focal range and that will result in a distorted engraving.

[Dan Hintz]

David,

You're pretty much spot on how most of us deal with these types of engravings. For example, the ULS rotary is mounted via a pin hinge at the far side of the platen... for sloped objects, the near side of the rotary is propped up to level out the engraving field. Think of beer tumblers and the like.

For objects with a large change in focal area, doing it in stages is pretty much the only way to do it beyond using a longer focal length lens (with the requisite increased depth of field). You have to be careful on high-resolution substrates, however, as the junction between two bands can be more noticeable without proper care.

Once in a while I'll stick a bowl/vase in my machine, but I'm typically selecting pieces that I know will be manageable. Sometimes that means tweaking the design before it leaves the lathe, and sometimes it means a piece just doesn't get engraved.

[Martin Boekers]

You may want to consider doing a sand etch for some of these pieces.

[Mike Null]

Marty

I think he's interested in doing wood.

[David Somers]

Mike,


When I get home this evening I will do a search on "cutting a bevel." Thanks for the reference!


Dan,


Nice to see another woodturner on the forum!!! <grin>
I had seen a number of the wheel based rotary tools that use a scissor type jack at one end to lift the end of a sloped piece up. For some pieces that would work. For others I would lose contact between the bottom of the piece and the drive wheels at the other end. That was why I was looking at some mechanism that would support the bottom of the piece at an angle, while still driving the rotation, with the whole rotary lifted at one end. When I get home tonight I will try to sketch out what I mean. I haven't seen one of these paired wheel style (or even chuck style) rotary attachments in person, only photos of them. So I am not sure if they can be adapted that way. it looks like they could without too much fuss though.


And obviously, one of my other questions from last night on Depth of Field related to this question about odd shapes. I was trying to get a handle on how much wiggle room you really had on a sloped surface while still getting a decent cut.


One of the uses for this was for inlay work on the sloping portions of the pieces. I have a feeling there might be issues if the inlay work used veneer inlaid into the engraving....at least you might need to play a bunch to find the right approach to make it look snug. But the other inlay technique I was looking at his with is a metal powder inlay technique that uses fine metal powders, packed into the engraved areas, saturated with super thin CA glue, and then turned/sanded down to match the pieces surface and buffed up to a shine. It can look quite nice. It would be more flexible a technique than if you were trying to match up cut veneers to the engraving with the slope involved. Or at least that is my impression from pouring over this forum.


Martin,


I have just started looking at other approaches to this as well. Sand blasting was one. Not sure what kind of detail I can achieve with that but will dig into it. And I was also started to look at the CNC approach, though I am not sure that won't involve trading one frying pan for another...solve some problems, introduce others in other words. This has been interesting.


Dave

[Mike Null]

David

My earlier response in this thread was more a response to a question you posted in another thread. (my bad)

To answer your question about how much wiggle room you have in engraving an irregular surface my experience is that it is slightly under +/- 0.100" with a 2.0" lens.

[Martin Boekers]

Marty

I think he's interested in doing wood. I used to order plaques from a company that did sand etching, most came out nice, the only issue is that most plaques are glued up. The glue was harder than the wood so there was ridges there.

[Dan Hintz]

I had seen a number of the wheel based rotary tools that use a scissor type jack at one end to lift the end of a sloped piece up. For some pieces that would work. For others I would lose contact between the bottom of the piece and the drive wheels at the other end. That was why I was looking at some mechanism that would support the bottom of the piece at an angle, while still driving the rotation, with the whole rotary lifted at one end. When I get home tonight I will try to sketch out what I mean. I haven't seen one of these paired wheel style (or even chuck style) rotary attachments in person, only photos of them. So I am not sure if they can be adapted that way. it looks like they could without too much fuss though.


And obviously, one of my other questions from last night on Depth of Field related to this question about odd shapes. I was trying to get a handle on how much wiggle room you really had on a sloped surface while still getting a decent cut.

Here's a pic of the ULS rotary:
273511

The closest end sits on pins and can pitch up/down along that axis. The far end slides forward to cradle both ends of the object. The far end can also be propped up by whatever you have sitting nearby (useless vendor catalogs, failed engraving attempts, etc.). By doing so, any conic object can have its surface brought into focus. Of course, compound curves are another story and you have to do some serious jiggering with the file to do things in bands, refocusing with each band (a good system will allow for automatic Z-height adjustment with each band, based upon what's in the file).

For DoF, we covered a lot of that in your other thread, but be aware... engraving and cutting are two different things. The DoF for cutting tends to be quite a bit less than engraving... you may get an acceptable engraving at +/- .25", but only +/- 0.10" for acceptable cutting. It depends upon your laser power, which focal length lens you select, etc.

[David Somers]

Thanks Mike! I did catch that. It was a good answer in both contexts! That was more wiggle room than I actually expected. When I get home tonight I will pull out a piece that I was using as an example and see what those numbers look like when applied to the piece, and how much other types of compensation might be needed.

Dave

[David Somers]

Dan,

It looks like the drive motor is on the near side of the rig?

Thanks for the info on cutting versus engraving and how the DOF affects those two operations. That was on my list to look for info on this evening. For some of the cutting applications I was thinking of that may not be very critical. Aside from engraving for the sake of engraving, or as one step in different inlay techniques, I was looking at using this for some of the piercing techniques that get done on thin wall turnings. The work is typically done with a dental drill, or less ideally, a Foredom or Dremel tool. If you turn you are likely familiar with the technique? I figure the laser would have to be pretty sloppy indeed to fail to equal the cutting quality from a dental drill or dremel. Depending on the wood, the wall thickness would be an 1/8 inch. No more than that.

Thanks!!

Dave

[Chuck Stone]

I think that if your piercing is anywhere except the middle band, you'll also need to watch out
for the angle. An Razertip or Paragrave tool is usually held perpendicular to the surface when you
are finishing the pierces area, but doing that on the laser presents some difficulties. You might
even need to skip the rotary and place the piece on sandbags for EACH cut. Still .. not as labor
intensive as the turbine tool

[Joe Hillmann]

When using a rotary on a tapered item the image get stretched and/or compressed to make up for the differences in diameters.

With a chuck type rotary you tell the laser the diameter of your part. Where the part diameter is the same as what you told the laser it is, 1 inch will equal 1 inch on the x and y axis. Where the actual diameter is smaller, the image will get squeezed together in the x axis and the y axis will be the same. Where the diameter is larger, the image will get stretched in the x axis and the y will stay the same.

With a roller type rotary 1 inch equals 1 inch where the part contacts the roller. Where the part is smaller it will be squeezed. Where the part is larger it will be stretched.

What all that means is if you plan to use it for doing inlay your parts probably wont fit without a lot of hand work. If you plan on using it for piercing where it is squeezed or stretched you may wind up with ovals instead of circles, and then you may run into the issue Chuck Stone brought up.

I would suggest finding someone near by you who has a laser and see if they would be willing to do some experimenting with you to see if a laser fits your need.

[Dan Hintz]

Dan,

It looks like the drive motor is on the near side of the rig?

Thanks for the info on cutting versus engraving and how the DOF affects those two operations. That was on my list to look for info on this evening. For some of the cutting applications I was thinking of that may not be very critical. Aside from engraving for the sake of engraving, or as one step in different inlay techniques, I was looking at using this for some of the piercing techniques that get done on thin wall turnings. The work is typically done with a dental drill, or less ideally, a Foredom or Dremel tool. If you turn you are likely familiar with the technique? I figure the laser would have to be pretty sloppy indeed to fail to equal the cutting quality from a dental drill or dremel. Depending on the wood, the wall thickness would be an 1/8 inch. No more than that.


Correct, drive motor on the near side.

A word of warning... if your wall is not equal thickness all over (and doing that while turning is an art unto itself), you'll have to cut with the maximum amount of power needed to get through the thickest section. In a nutshell, you'll have to set things to a higher power than if you were cutting a consistent 1/8" piece of veneer or plywood. Cutting with more power than you need can cause undue burning in some cases, so you'll have to experiment.


When using a rotary on a tapered item the image get stretched and/or compressed to make up for the differences in diameters.

What all that means is if you plan to use it for doing inlay your parts probably wont fit without a lot of hand work. If you plan on using it for piercing where it is squeezed or stretched you may wind up with ovals instead of circles, and then you may run into the issue Chuck Stone brought up.

Joe brings up a good point. For piercing and infill it won't be an issue, but if you're wanting to do inlays, you'll have to do some more tweaking to your drawing. It's not insurmountable, and I've posted directions on how to handle it similar situations in years past, but it will take a different mindset in setting up the file. We can go into more detail when you get to that point.