October 25, 08 by Andy
There are a couple of features that I’ve come to the conclusion are essential, when generating g-code. Sure, there are a lot of features that are necessary, but these are features that perhaps a newcomer to CNC might overlook.
Tab Generation:
When cutting out a part from a piece of wood, there is a moment when the bit will cut through the last piece of wood holding the part to the rest of the wood. At this point the forces may cause the part to simply be forced out instead of precisely cut out. If the part is complex it is possible that this may happen even earlier in the last cutting pass, causing the wood to be snapped and the part to be damaged.
The solution is holding tabs. These are small pieces of wood that are left in place to hold the part to the rest of the wood. After cutting has finished the tabs can be broken, perhaps with a Dremel, then sanded away. This creates control over the process of removing the part, and will avoid any damage.
It is possible to manually design in the tabs when creating the part in a CAD program, but then some manipulation is required in the g-code generator to obtain the correct toolpaths. A quicker and easier solution is to use a g-code generate that can automatically generate the tabs. The usefulness of this should not be overlooked.
Lead In Moves:
Plunges, where the bit is moved vertically into the wood, put a lot of strain on the CNC machine and the spindle. For example on my CNC machine I can sometimes see the Z-axis assembly move slightly up and back when it encounters the resistance of the wood.
The solution is to use lead in moves. These are moves that gradually move the bit vertically into the wood at an angle or from the side. This reduces the stress on the mechanical components, which should last longer. As an example for my current projects I use a lead in move of seven degrees.
September 23, 08 by Andy
The best introduction I have found is from the Enco catalog. Fortunately you can view the catalog online. The introduction covers materials, types, number of flutes (including the trade offs), end cut type, shank type and surface treatments.
View the introduction at:
http://www.use-enco.com/CGI/INPDFF?PMPAGE=114
Note that this gives you page 114. Enco’s next release of the catalog may change the location of this introduction to another page. If you see this link is out of date please let me know the new page number.
September 15, 08 by Andy
Recently I wrote about the steps I took to reduce the cutting time of a particular part from 67 minutes down to 20 minutes. I have now purchased a couple of 1/8″ four flute endmills from Enco.
I can now cut the 1/8″ wide groove in the design, plus the outer profile with holding tabs much more quickly. In addition I can use the 1/8″ bit to perform a quick roughing pass on the pocket operation. Once complete I switch to the 0.0571″ endmill and make a finishing pass around the edges of the pocket, increasing the detail.
The new time, including the manual tool change, is about 10 minutes. Not bad. At the end of the cutting process the motors, stepper driver chips, transformer and dremel are all a lot cooler.
One small downside is that the 1/8″ bit make a lot of noise.
September 15, 08 by Andy
Previously I wrote about a DXF exporter for Inkscape. Bob has updated his Better DXF Output exporter for Inkscape 0.46. All the files are now in one place and it is under the GPL v2 license. This is great news for CNC users and I hope it leads to more use for this handy Inkscape extension!
September 11, 08 by Andy
I am working on a design that basically involves cutting out a 4″ diameter disk from 1/4″ thick wood, with some shapes pocketed into the surface of the disk. I have been using 10 inches per minute (IPM) to cut through the Poplar and I had no clue if this is average, fast or slow. What I do know is that it is painful to watch.
At 10 IPM cutting out the design took 67 minutes. That is probably split 60%/40% with the majority on the pocketing. Too long for my liking. I decided to try and reduce the cutting time. First I increased the speed to 30IPM, which is nearly the maximum that my PC can go. Next I reduced the depth of the pockets from 0.15″ to 0.10″. This saves an extra pass.
The new time to cut? 20 minutes. Less than 1/3 of the original time. But that’s no good if the result is a ragged mess. Tonight I found out - there are more burrs on the wood, but the vertical edges look just as good as the ones cut at 10 IPM. So I will likely continue to use 30 IPM in the future.
My next aim is to split the cutting into two parts. The pocketing with a 0.0571″ end mill and the rest with a 1/8″ end mill. This should reduce the time even further.
September 07, 08 by Andy
If you search around for ideas on how to import DXF files into Inkscape or convert DXF files to SVG files, there are a lot of results. But they mostly seem to be shareware or orphaned applications that haven’t been updated for years. However there is a simple, obvious (once you see it) and free solution to converting DXF files to SVG files (which Inkscape can load).
It’s called Open Office Draw.
Yes, that’s right. Open Office Draw can load a DXF file and save an SVG file. I’ve tried it and it worked - I was able to take a DXF file, convert to SVG, load into Inkscape, edit, save as DXF (see my other posting from a couple of days ago on this) then load into CamBam for CAM processing.
September 04, 08 by Andy
Update: see this later post before following the instructions below.
I don’t care for most of the DXF editors available. They seem a bit clunky and not too friendly. But I do like Inkscape. Unfortunately it doesn’t export DXF files.
Here is a way of getting Inkscape 0.46 to export DXF files which can then be processed in a CAM program to generate g-code for a CNC machine.
Firstly install Inkscape 0.46. It must be this version.
Next go to this post on BobandEileen.com, right click on the link to “dxf_templates.py” and save it in C:\Program Files\Inkscape\share\extensions.
Next step is to go to another post on BobandEileen.com, right click on the two .py files (”simpletransform.py” and “better_dxf_outlines.py”) and save in the same place. Then do the same for “better_dxf_outlines.inx”.
Restart Inkscape.
Create a drawing and then move it to the bottom left corner of the page. This corner ends up being the origin. If you want your drawing centered on the origin then center it on the corner of the page.
Go to File -> Save As…
From the list of file types in the save dialog window choose “Better DXF Output (*.dxf)” and save the file.
Now open the DXF file in your favourite CAM program, such as CamBam.
Note that you may need to scale the drawing in your CAM program. Even though I had my drawings correctly sized in Inkscape, they seemed to be quite a bit bigger. If anyone knows how to solve that please post a comment.
July 18, 08 by Andy
CamBam Plus has the option to automatically generate tabs. This is pretty nice and might make me change how I use fixtures to hold workpieces down. I’m starting to have concerns about the double-sided carpet tape, as I’ve seen the wood moving slightly during cutting.
I decided to create a simple test - cut a 0.5″ x 0.5″ square with tabs, in 1/4″ poplar. Also I cut the entire piece out so I could examine it and show it off to people (total size is 1″ x 1″).
CamBam Plus automatically generated the position of the tabs, but I needed to decide on the height and thickness. My first test using tabs with gears failed as the tabs were so thin they were non-existent. In this test they are 0.1″ wide and 0.07″ high. In order to saw the tabs later the outer profile cut has to be wide enough to get a tool in there. In this test it is 0.125″ wide.
The feedrate was 10IPM.
Here is the video - the final piece is shown at the end. In the EMC2 image you can clearly see the tabs at the bottom of the cube on each side. Sorry about the poor quality.
July 13, 08 by Andy
Today it was time to give some new gear generation code a try. Using CamBam I created gcode for a gear with a 2″ pitch diameter, 18 teeth, 20 degree pressure angle and a diametral pitch of nine. I then cut a couple of them out.
The original plan was to work on precise cutting of two sides of an object. That idea failed so I had to eyeball it. Here is a video of my Fireball CNC V90 and EMC2 (running on Ubuntu Gutsy) in action:
The gears fit together nicely.
June 23, 08 by Andy
The next step in testing the CNC machine is to try cutting a hole. For this I needed thinner wood than the scrap pine I’ve been using. Home Depot sells small boards of oak and poplar so I picked up a piece of poplar. It’s 1/4″ thick.
I also needed to raise the sacrificial platform so the tip of the end mill can reach the bottom of the wood. To do that I just cut some more 16″ x 9″ pieces of 1/4″ MDF and stacked them (see the post on fixtures for more information and pictures on what I am talking about).
I used CamBam to draw a 0.5″ x 0.5″ square and then created a profile on the inside using my 1.45mm (0.0571″) end mill. CamBam showed me that I would have slightly rounded corners, but that’s ok. I decided to cut the profile in passes, increasing the depth by 0.05″ each time. This results in five passes to get to the bottom of the wood. Tedious, but better than stressing the end mill and Dremel.
I was afraid that the tip of the end mill might bind in the sticky double-sided carpet tape so I held on to the poplar with one hand and kept my finger on the power button with the other hand just in case. I was also afraid that the cube being cut might fly out as it came free.
It turned out pretty good. No sticky residue on the end mill and no gouging of the sacrifical platform. The cube in the center held in place during cutting and while I lifted the poplar. It came out when I removed the carpet tape.
The cut is nice and clean with no burrs. I guess the carpet tape is a good method to continue using.
