People often ask me why the edges of their laser cuts are not square. The laser beam is being focused at an angle to a spot, so no cut can be perfectly square, but there are things that can make it worse. Note: All of the images are exaggerated to show the affects.
You first need to understand how the lens works. Laser cutters use a collimating lens. This means it takes parallel rays from the beam and focuses them to a single spot. For a couple of complicated physics reasons, it can never do this perfectly, but it should do it close enough not to be a factor in this discussion. Below is a picture of a collimating lens. A typical beam width is usually about 5mm-8mm and a typical lens is about 20mm-25mm wide.
You can see that the beam forms an hour glass shape. This can cause a little angle. With a 6mm wide lens and a 50mm focal length, this angle is typically 3-4 degrees.
To get the least affect on your part, you might want to center the focus in the middle of your material.
If you are getting a bigger angle than a few degrees, it is more likely because the beam is not in the center of the lens. The lens will still focus to that same point, but the hour glass is at quite an angle to your work piece.
This type of angle is offset in one direction, so you may see it more in certain directions of travel. If the beam is moving from right to left in the above image, you might not notice the problem at all.
Does a longer focal length help? It can, but due to the complicated physics issues I referred to earlier, a longer focal length creates a larger spot size, which reduces power density. See this calculator.
3D Systems recently acquired Geomagic. Several years ago they acquired Alibre. Alibre is a parametric CAD program. It appears that they are simply re-branding the existing version of Alibre as Geomagic, but will incorporate feature of the Geomagic product line over time. Here is the basic product line.
I have used Alibre and Cubify Invent and find them to be quite capable. Some day I may loose my easy check out access to Pro/Engineer and these products are on the radar as possible solutions. I found the even Cubify invent could do a few little tricks that are a pain in Pro/E.
In a couple weeks we are going to do a CNC V Carving night at Pumping Station One. We hope to fabricate a number of designs on the CNC router. This blog post will serve as a basic introduction to the concept and will help people get artwork ready.
What Can V Carving Do?
V Carving uses a V shaped bit to to “carve” a design into the material. Because the bit has a v shape, you can cut narrow shapes with the tip or wider shapes near the bottom.
You can even cut shapes wider than the widest part of the bit by doing multiple passes of the bit. The depth of the cut is proportional to width of the cut, so you need to make sure your material is thick enough. If you have very wide areas, you can set a depth limit and you can make that area have a flat bottom with a second flat bit.
Normally routers cannot cut square inside corners because you are cutting with a round bit. V Carving can get around this limitation because the bit can rise up into the corners until it gets to the zero radius tip.
Finishing Tricks
V Carvings can look great simply cut in the natural material, but they can really pop when you put a contrasting finish in the carved areas. This is a time consuming process and can be difficult to do well.
You can often shortcut this process by using masking materials. You start by applying a background finish to the work piece. This is then masked with tape or specialized masking material. The router then cuts through that as it is cutting the design. Now, only the cut area is exposed and you can simply spray paint the exposed area. If the design has multiple colors you can cut one color, paint, remask and repeat the process.
The files should in in DXf, DWG, AI, EPS or PDF format. Many programs like InkScape and CorelDRAW can output these formats. If you have hi resolution bitmap, some of these programs can convert to a vector format. Feel free to try that, but help with that will be beyond the scope of the session.
The quality of the masking material comes into play with very fine details. If your design will leave tiny isolated dots of masking, some materials may not stick well enough and break free during cutting. If that happens, you can manually touch up those areas later. I like to use Avery Paint Mask, but plain masking tape, adhesive shelving paper and materials for vinyl cutters also work.
What is good artwork to start with
Avoid very thin lines lines. The material needs to be needs to be perfectly flat and consistently thick for this.
Very large areas to cut will take a long time, so avoid them for this session.
Multiple colors. Multiple colors is OK, be each color needs to be separated from the other color so there is masked uncut area between them.
This logo would work. The red would be the base material and all other colors would be cut and colored.
This one would be very hard to do because of the adjacent colors.
How to bring the artwork to the session.
To save time, the artwork should be as ready to import as possible. Problem artworks will be pushed to the back of the queue and might not get cut.
The artwork needs to be in a digital, vector format. By digital I mean you can sent the file electronically. Vector files are created with actual geometry. Lines are lines and not a string of pixels. Scans and photographs are not usable. If you zoom in and the image gets pixelated, it is not ready to use. The shapes also need to be closed. If you have a square, for example, all the corners need to meet or the software cannot determine inside from outside. Tiny gaps can be closed in the CAM software, but if you can see them, close them.
Try not to be too complex or have large cut areas. These will take a long time and will limit how many people we can accommodate in one session. We can still create G-Code, but you many need to cut it at a later date.
If there is time I my cut small PS:One snowflakes for the people who did not bring any artwork.
Material
The idea material is a smooth, pre-finished piece of wood. It needs to be as least as deep as 1/2 the width of your widest feature if you are not planning a flat bottom. The material should be as flat and consistently thick as possible or the results can be distorted, because the depth of the cut is so critical. Avoid oily or wet finishes because the mask material may not stick well. Plywood does not look well and often interior layers have voids.
I met programmer and maker, Joe Walnes, through a few local Chicago maker groups. He showed me a really cool web based G-code viewer he wrote to preview his 3D printer G-code. It used WebGL for super smooth motion of the model. It also allowed you to drag and drop your own files right into the page. It worked great, but really only worked with 3D printer G-code. He posted the code on GitHub.
I have a couple programming projects in the works that need a G-code viewer, so I decided to update his program to handle more types of programs. Joe had a really nice UI and design pattern for the code, so I left that alone. He also helped me out with a few issues as I worked.
G-code Parser
A parser is a bit of code that breaks down text into tokens, or the basic grammer of the G-code. He was working with very well formatted G-code so his parser was pretty simple.
G1 X5 Y5 Z6 E0.124
I was dealing with really Fugly lines of G-Code like this, so I needed to totally rewrite the parser.
N100G17G91G1X5Y5Z6M03S1000(comment)G1X5;comment
Graphics Generation
Reprap 3D printers basically use G1 (straight moves) for everything. I needed to add the code to handle G2 and G3 (arc moves). This was a little tricky because there are no arcs in WebGL. I had to break them into small line segments. Joe also treated each Z level as a separate layer. That is nice for printers, but not for general G-code. I changed that and the way the color of the lines worked.
A Work in Progress.
It works on all my CAM generated 3D printer and CNC router G-code, but I want to add code to deal with more advanced features that are often hand coded like incremental moves, machine offsets, parameters, math functions and subroutines.
I will post the source code soon.
Usage
You need a WebGL capable browser like Chrome, Opera or Firefox. I hard to turn on WebGL in my Firefox. I got it to run on my Android phone in Opera, but could not spin/zoom the model with the screen controls.
To view your own files, just drag and drop the G-code into the browser. It will use the zoom settings for the previous model, so if you drop something that is a different size or offset to the side you may need to zoom around to find it.
The tool is attached to a hand held frame. Actuators within the frame can move the tool to compensate for errors you would make when trying to cut a complex shape.
A high contrast pattern is placed on the work piece. They are the horizontal bands of shapes in the image above. The tool creates an internal map of the work piece using a camera. It can then accurately determine it’s location anywhere on the work piece without the drift that might occur using incremental positioning sensors. The outline of the cut to be made is shown on the screen. The location of the tool center is also shown on the screen. You only need to follow the line within the correction limits of the tool. The tool digitally corrects to produce remarkably good results.
This same idea could be applied to a lot of different 2D fabrication tools. There is a very comprehensive paper here showing all the details.
Thermal friction drilling very cool (hot actually) drilling technique that is especially useful for creating tapped holes in thin wall materials. The pressure and friction of a specially designed conical bit heats the material to a plastic state and forms it into a hole that has 3-4 times the depth of the wall thickness. It looks like it is best for hollow tubing because the back side of the hole is a little rough. The bit forms the top of the hole into a flat bushing which is perfect for fastening to round tube.
The bit is made from carbide and is held in a special holder/collet assembly that isolates the heat generated on the tool from getting to the machine. Lubrication is required to prevent the material from welding to the bit. Any machine that meet the speed and power requirments, including drill presses can use this technique. According to the web site a 6mm hole would require about 2500-31100 RPM at 1.2kW (1.6 hp).
Most of the thermal drilling companies recommend thread forming taps. These taps form the threads by displacing the metal rather than cutting it. The resulting threads are smoother and stronger than cut threads. I have used them on conventionally drilled holes. The pilot hole needs to be slightly larger than a standard pilot hole. The taps are a lot stronger because they do not have flutes. The can be run faster and don’t bind up with chips. It looks like the combination of thermal drilling and thread form taps creates a very clean operation.
Inventables is now taking pre-orders for Hadron ORD Bots. I will not be shipping anymore kits and Inventables is now the official, authorized distributor of the kits. Go here to get on the list for one of the units. The Inventables kits will also have the following improvements.
Bright Dipped Finish
The previous version looked great, but the matt finish quickly gets dirty and scuffs often looked like sceatches. Bright dipping is a chemical polishing that is done before the anodizing process. The goal is to get the finish quality of a Mag Light.
Open Ended Wiring Holes.
A well wired ORD Bot is a thing of beauty, but once wired some of the parts are trapped by the wiring.
By adding open ended wiring holes, the wires can escape out the sides. This will allow you to completely side off thing like the gantry without removing any connectors.
Wider Extruder Platform.
Deep extruders like some of the Wade’s family of extruders previously had to overhang the front a bit. This adds more depth and an extra hole pattern to give you more options.
Loop Belts
With loop belts you will not have to measure and cut from open ended stock. They will be a lot easier to install and will have twice the grab in the belt clamps.
Build Platform Changes.
Added holes for the ORD Bot heated build platform.
Increased spacing between the wheels. This improves rigidity, reduces the sensitivity and cleans up the busy center area where the clamp and switches. fit.
Electronics Plate Changes
The Electronics plate will now have hole patterns for the RAMPS, ORDuino and Azteez controllers.
There has been a lot of interest in an ORD Bot with a bigger build area. This is the Quantum’s larger brother. This uses the MK1/MK2 heated built platform which is about 214mm x 214mm. You can go larger, but you are on your own.
The name is not yet determined. I wanted something similar to Quantum that conveyed a larger volume. A search of units of volume reminded of one of my favorite units “The Firkin”. Not only does Firkin sound cool, it is part of the FFF (Furlong/Firkin/Fortnight) unit system and it is a measure of beer. The other suggestions was Hadron, which is very similar to Quantum but larger and means thick or stout in greek. Firkin is cool, but Hadron is more family friendly.
They share all parts except the MakerSlide length, the handle and the build platform.
The ORD Bot made its debut at the Chicago HackerSpace, Pumping Station One. They host a monthly DIY CNC night that is open to the general public. I have been going for about 6 months and it is always a lot of fun and you meet some great people. Last night’s event was very well attended with about 30-40 people. Many of the people were there for the first time and found out about it via the Hackaday blog post. They usually have a 15-20 minute presentation and then everyone starts breaks away to work on projects or discuss ideas.
I was drafted into the presentation role to present the ORD Bot. It was received very well. Everyone liked the simplicity and clean look to it. Just about everyone picked it up to hold it in their hands. You don’t see people just spontaneously pick up someone else’s 3D printer very often. I guess when it is small, cute and has a handle, it is just asking to be picked up.
Up until then, the ORD Bot had never actually tried to do a full print. I had a lot of trouble getting the PLA filament. I had some get lost in shipping and two orders canceled due to lack of stock. I had some scraps of ABS that I used to test the extruder and I had run all the axis motors. I had done all my testing on a desktop PC at home and brought my laptop to the HackerSpace without all the software. I hooked into the PS1 Wifi and got the nessesary software.
This is the software “stack” I ran. Sprinter was already loaded on the Arduino. I used Slic3r to create the G-Code. I used Printrun (Pronterface) to interface with the printrer. Jeremy from tinymachineshop had a similar setup on his Makerbot and had Slic3r setup in about 3 minutes with the settings I needed. He also donated a length of black PLA
I decide to run the 20mm calibration cube STL that comes with RelicatorG. Slic3r generated the code in about 1 second. We spend about 5 minutes running PLA through to clear out the old ABS. The first attempt immediatly had a problem with the Y axis. I releazed that the set screw was never tighten on the pulley and it was just running on shaft friction. I tightened the set screw and the print ran great. I probably could play with the retract a little.
We were amazed how quiet the machine ran. You can compare it to the background voices in the video. There is even a Makerbot in the background about 10 feet away you can hear. I am not sure why it is so quiet. I think it might be because there is really nothing to resonate. All the flat parts are pretty small and pretty well bolted down.
The completed part measure about 20.015mm square. The layering looked quite even. There were several 3D printer owners in the room and they all commented that it took many hours of trial and error to get the quality I got on the first try. I am sure the tools have evolved a lot since they started and they helped by giving me the good Slic3r settings.
I took a bunch of pictures, but I left my camera at home. I will post more soon. Here is a picture of the part I took later. If anyone else has some pictures, please send me a link.
I have developed an inexpensive control system (less than $70) that can be used to both get more cutting power out of a DC discharge laser and significantly improve cutting accuracy for home built laser systems. The control system implements a control technique known as Pulse-Per-Inch (PPI) control. PPI control involves pulsing the laser every time the head travels a certain distance. PPI control allows a CNC laser to produce consistent cuts at the same power level setting over over a wide range of speeds. In effect, pulsing the laser as a function of distance along a cut decouples the power input to cut from the speed that the head travels. Therefore, the speed and acceleration of the CNC system have minimal bearing on the cut characteristics. Furthermore, the unique transient rise response of a DC discharge laser allow PPI to deliver more power to a cut in comparison to the same laser system with just on/off control.
Background and Motivation
A while back, I was active on a forum in which we were discussing the time it takes to turn a laser on and off and how that relates to engraving control. One of the forum members from Full Spectrum Engineering posted a high-speed intensity spectra for the cheap DC discharge lasers that we use for DIY laser cutters. I was quite surprised by the spectra. I expected to see a nice exponential rise to set power level, but what we saw was a rapid rise to a very high power level (nearly double the set value) followed by an exponential decay to a set value. The Spectrum in question is shown below (credit for the spectrum rests with Full Spectrum Engineering). The yellow square wave is a 5ms pulse sent to the laser power supply. The green spectra is the intensity spectra of the laser. For whatever reason, the magnitude of the spectra is upside down (I think the ground and signal leads were reversed), so on for the digital signal and higher intensity for the laser power spectra are down rather than up. Anyhow, the spike in intensity is caused by the necessity voltage to start a plasma in a DC Laser. The laser power supply generates a very high voltage to start the plasma which is stored in a capacitor. When the signal comes to turn on the power, the power supply dumps this charge into the system and then supplies a nominal (still very high) voltage to sustain the plasma once it is on.
