Messages

4881

Show"N"Tell ( Your Machines) / Re: The Laser Project.

« on: February 22, 2012, 05:19:16 AM »

And to be more correct, I think that the laser never really does several pulses on top of one another (well if it's focused fine enough that is), as the axis is in a constant move. I think the correct way of thinking about it is the dots intensity. And the other thing to consider while at it is that the exposure time is determined by the axis feed rate.

Hi Dan,

You can do the maths – when the laser is pulsing at twenty thousand dots per second with a line scan speed of just four hundred pixels per second, there can indeed be many dots on top of one another. If it was not for this fact, the plugin would not, I think, be able to produce any shades of grey. 

Tweakie.

4882

Show"N"Tell ( Your Machines) / Re: The Laser Project.

« on: February 22, 2012, 04:29:43 AM »

Well, I think the plugin doesn't control the laser power whatsoever. All it does is trigger it on and off.

Dan

Hi Dan,

In a way you are right – the plugin does indeed just switch the laser on and off but it does in a cunning way called modulation. As laser ‘burn’ is related to both laser power and exposure time when the plugin switches the laser on it does it in pulses or dots per pixel – a few pulses on top of one another produces a light mark and many pulses on top of one another produces a dark mark. You have seen from Art’s description that these pulses or dots are pretty quick and providing the step-over has been set correctly they will blend and this in turn produces the shades of grey. I have tried to describe the process in my PDF http://www.cooperman.talktalk.net/Engraving.pdf which I am sure you have probably read already.

Overall, the function is not dissimilar to a PWM spindle speed controller – it only turns the spindle on and off at full power but by varying the duty cycle it controls the speed. There are a few more refinements in a speed controller such as measuring the back EMF and adjusting the pulse width so that a constant speed can be maintained under varying tool loading but basically all it does is turn the spindle on and off.

Tweakie.

4883

Show"N"Tell ( Your Machines) / Re: The Laser Project.

« on: February 20, 2012, 12:22:00 PM »

Hi Dan,

I think you have got me there, because I just don't know the answer.

On it's own, the plugin will not always smooth a gradient of tone without producing 'steps' as can be seen in the background of this image which is a smooth transition from light to dark. I have spoken with Art about this and he feels that the cause is mathematical in nature.



Once an image has been 'halftoned' this does not happen, the gradient is much smoother.

As for why ? hopefully I will eventually discover the reason but for the moment it illudes me.

Tweakie.

4884

Show"N"Tell ( Your Machines) / Re: The Laser Project.

« on: February 20, 2012, 11:14:18 AM »

Hi Dan,

A very good question (you always ask good questions) – as far as I can tell, yes the dots are all the same size so this would indicate that the laser has been fired at exactly the same power level throughout the whole image. The thing that varies is the dot density and this is the clever part which gives the illusion of shade.
In some areas of the image the dots actually merge because the burn marks on the wood are larger in diameter than the laser spot that created them. So, in theory, if I could increase my axis speed and reduce the burn time of each dot they would be smaller in diameter and I could increase my resolution. But, as said before, everything is a compromise. 

Tweakie.

4885

G-Code, CAD, and CAM discussions / Re: I am missing a simple g-code for an engraving program.

« on: February 20, 2012, 07:56:18 AM »

doomcuroe,

Please don't make multiple postings of the same question.

Thank you.

Tweakie.

4886

Show"N"Tell ( Your Machines) / Re: The Laser Project.

« on: February 20, 2012, 07:25:53 AM »

Exactly the same technique as previously used but this time with a photographic image.

I really don't know if all CO2 lasers behave in the same way but, as mentioned in way back postings, when using the Mach plugin directly I am only able to achieve a maximum of 5 shades of grey which, I suspect, may be a function of my high voltage power supply operating in a digital rather than an analogue way. Converting an image to half-tone seems to overcome this limitation by introducing the visual effect of many more shades.

My congratulations on an excellent program John.

Tweakie.

4887

Show"N"Tell ( Your Machines) / Re: The Laser Project.

« on: February 20, 2012, 04:59:21 AM »

The recent period of cold weather has rather slowed things up a bit but spring is now on the way.

Another forum member ‘PicEngraver’ has sent me an advance beta copy of his new program ‘PicDither’ which will convert a bmp, jpg, etc image to halftone. This is capable of applying a number of different industry standard algorithms which in their own way produce differing results but the overall effect is extremely similar to the, rather overpriced, commercial program ‘PhotoGrav’.

These are my first tests made from a standard .jpg image which was then laser engraved onto MDF  using the Mach3 Impact / Laser plugin. It is early days and I have not yet tried a photographic image but that is next on my list.

Further details of ‘PicDither’ can be obtained by contacting John through his website  http://picengrave.com/

Tweakie.

4888

G-Code, CAD, and CAM discussions / Re: Milling the table

« on: February 19, 2012, 11:59:04 AM »

Hi Michel,

As all my work is done with a vacuum table or double sided sticky to hold the work I fitted a sacrificial plastic sheet onto the machine's aluminium table then milled that flat and parallel. Made a lot of mess but was easier than milling the alloy.

Tweakie.

4889

Show"N"Tell ( What you have made with your CNC machine.) / Re: Mechanical engineer and craftsman

« on: February 14, 2012, 05:56:35 AM »

Hi Ed,

That is just incredible, such detail and precision. Thanks for sharing.

Mind you, I think he would have a fit if he saw the inside of my workshop 

Tweakie.

4890

Show"N"Tell ( Your Machines) / Re: The Laser Project.

« on: February 14, 2012, 05:32:18 AM »

Hi Greg,

Yes, I think that is about the size of it - always follow the commands with an axis movement (which would normally always be the case anyway).

There may be some occasions when it is necessary to ‘fire’ the laser for a short period when an axis is stationary, as would be the case when using DotG in ‘dot’ mode but as it’s post processor if fully configurable a ‘dummy’ axis movement can easily be included. This was tested yesterday for the attached image and it works well.

As mentioned in the earlier post we do need the ‘turn off all outputs’ checked for safety reasons this way the laser will be turned off with ‘feed hold’, ‘limit switches triggered’, ‘estop’, M30, etc.

Tweakie.