Engraving irregular surfaces

[Stepper Monkey]

What are the possible solutions for rotary engraving over curved or irregular surfaces?
I can think of a few ways to accomplish this. First would be to have the bit and collet in a splined holder that mates to a splined reciever chucked into the spindle. The splines would allow for bit rotation but also vertical travel up or down over gentle curves in the workpiece. The weight of the holder and/or springing in the reciever would mean that constant pressure was exerted regardless of the telescoping of the reciever.
The second and probably coolest would be for a microcontroller attached to a scanning touch or laser probe that rode next to and paralleled the engraving bit. This way if the probe could map the surface and send additional g-code interrupts to move Z up or down in real-time during the engraving if it encountered an elevation change.
Third would be for a very light handheld spindle to be attached to a suspension system with an adjustable spring or gas preload to effectively counteract almost all of the weight of the spindle, allowing the spindle to effectively "float" in Z. The workpiece itself would then act to drive the spindle up and down.


How is this problem actually solved in real life? I'm sure someone has thought of it, but I can't find ANYTHING really. Perhaps people just only bother to engrave on flat stuff?

Ideas please!

[svenakela]

DIY spring loaded engraving tool holder



Regards,
Sven

[Robin Hewitt]

apologies for butting in, I'm new here and just having a look around...

Constant pressure works fine on our Borries marker which uses compressed air to push down, but that doesn't spin the tool so it doesn't drill. Is this spring really weak or is there some kind of depth stop ground in to the tool tip? How come it doesn't dig in?

[Stepper Monkey]

Originally Posted by Robin Hewitt apologies for butting in, I'm new here and just having a look around... Constant pressure works fine on our Borries marker which uses compressed air to push down, but that doesn't spin the tool so it doesn't drill. Is this spring really weak or is there some kind of depth stop ground in to the tool tip? How come it doesn't dig in?

The spring is somewhat weak, but there is no depth stop to keep it from digging in. So yes, it can and will dig in if it pauses over an area. The only way to solve that is to keep it moving at a steady IPM and RPM, which incidentally is how to control the depth - slower IPM and/or faster RPM and it engraves deeper. I know it sounds like a pretty fly-by-night solution, and it really is, but it is actually quite amazingly consistent once you figure out the settings.

The big downside is the trial and error to figure out the speed/material/depth settings in the first place, but in my book it still beats the other alternatives for rotary engraving for right now as it costs way less than 1% of the nearest "correct" rotary method to implement. That and much of what I want to do can't be done by non-rotary engraving.

[Robin Hewitt]

Are you using standard carbide burrs for this, what shape? Presume you need a fairly wide angle so it wants to move up?

I've just bought some fine pointed trees to do some engraving with variable depth but I don't think they'd last very long without the z screw.

Why doesn't the side force on the tip prevent the tool retracting? Ball splines? Vibration?

Sure sounds like fun though

Robin

[Stepper Monkey]

My guess is that with a few ounces of side force the bits actually wouldn't be able to retract for the very reason you suggest. With very fine needle tip bits like the 15 degree half-round vee .005 carbide tips I am using, even dragging your fingerprint across the tips when they aren't spinning will damage them or snap the end clean off, yet they hum right through titanium without complaint while spinning - so in practice the side forces have to be way, way less than a few ounces in normal use. They spin at such an insane speed relative to thier lateral motion (feed) that they really don't have any side loading to speak of. The side forces for rotary engraving in general are very, very small as compared to traditional scratch engraving. I haven't done the math, as I don't have the proper equipment to calculate the various vectors on the tip, but since the testing seemed to work out fine so far I doubt I'll bother.

In short, it works because the bits would fail from side loading way before you could ever get the spring piston to sieze in the barrel.

As I work up to testing max feed rate with stronger bits, like 90 degree vees, I am betting that at some point your concerns will come into play though. The required vertical motion allowed by the tool may also allow the bit to induce induce some type of high frequency ocillation or "bounce" (I guess that would be analagous to a form of "chatter") that the spring pressure may not be able to attenuate. So far so good however!

I'll keep everyone here up to date as I try more variations with this thing. Anyone else run into troubles with thiers yet?

[Robin Hewitt]

I've ordered the half round cutters.

The plan is to put a light weight die grinding pencil on the end of a 6" arm. I will pivot it down and rely on the minimal side forces to keep it on course.

Well, it might work

[Stepper Monkey]

Originally Posted by Robin Hewitt I've ordered the half round cutters. The plan is to put a light weight die grinding pencil on the end of a 6" arm. I will pivot it down and rely on the minimal side forces to keep it on course. Well, it might work

Good luck with that! I would strongly suggest using only the strongest bits to practice with 'til you get the hang of the material, say 80 and 90 degree bits. I have a sizeable stock of used 15 degree bits I can afford to break, but they are pretty fragile, and the tips twist off if you aren't careful. I busted a number of them learning, more than I would have been comfortable with if I was actually paying for them! If I was buying bits new I would make them at least 45-60 degrees. That would be my best guess for practical general use.