CNC cutting is rapidly becoming my drug of choice. The shear beauty of the finished pieces as they come off the machine is addictive. But getting here was a long and windy road...
This artice is the combination of a number of much longer articles posted on my blog at Red20RC. To view them in their full glory you can go to:
To CNC or not to CNC? That is the question.
I’d been thinking about purchasing some sort of equipment to produce high quality custom components for a while. Everyone was raving about the 3D printing revolution and the chaps at Flite Test were pumping out kits from their laser cutters at a rate of knots.
A CNC cutter seemed the obvious choice. I could cut a variety of components from a variety of materials. They are fairly quick and relatively economical to run.
A short DHL shipping later and my CNC3040Z was unpacked and sat on the workbench.
The first thing I have to say is that the machine is lovely. The quality is excellent and the fit and finish of the parts are really nice. A little bit of assembly is required to attach the stepper motors to the screw drives but that is about all I needed to do.
I was really excited to get started so it was frustrating to say the least that I then went through anything up to a month of fiddling and head-scratching to get the thing running.
There are a number of modern CNC machines that are USB driven but these tend to be higher end kit. Most machines out there seem to be, like mine, driven by a serial connection. If you are considering going down the same route, here are a few things you need to bear in mind:
- You will need to buy software to create your toolpaths and GCode as well as software to actually “drive” the machine.
- You will need to be running a 32 bit version of Windows (not sure about Macs).
- You will need a proper serial port to plug it into (USB to serial adapters don’t work).
- It won’t run off a laptop, so you’ll need a desktop running all of the above.
Other things you’ll need…
Of course, if you want to do any serious cutting you are going to need a good stock of router bits. The shape of these change depending on what you want to do with your machine but as I was only interested in cutting out patterns I went looking for an array of end mills.
If you are cutting “slots”, that is full thickness cuts to create something like a multirotor plate, than you will need to protect the bed of your machine. A sacrifice board is a level surface, preferably of softer material than what you intend to cut, that can be clamped beneath your workpiece. I’ve tried a number of things but have found either foam board or simple MDF to be the best solution.
- Some sort of design/modelling software to visualize your design on the computer.
- Some way to build your toolpaths and generate the code to drive the CNC machine.
- Software that actually “drives” the CNC machine.
I’m still not entirely sure what some of this software does and I’m pretty sure there will be better ways of doing things but I’ll take you at least through how I do it.
3D Modelling and Blueprints
I do everything in Sketchup. It used to be Google Sketchup but now it is owned by Trimble (I guess the same mob that make the Gatewing UAV). Yes, I can hear some of you groaning! I know the pros will be using Solidworks, Autocad or other more advanced tools but I know Sketchup and I find it easy enough to use. It also has the benefit that when I am working with others to produce a design, Sketchup is accessible and I can send files around easily.
Once I have a workable design I use the “Flattery” plugin to export my drawings as an .svg drawing. The good thing about using .svg is that all dimensions and vectors are saved perfectly ready for the next step…
My toolpath software can only accept certain file formats and I can't go directly from Sketchup to CAM, so I have to convert and clean up the drawing first. Fortunately I run a copy of Adobe Illustrator for work and this is ideal for the job. Opening the .svg file in AI reveals a number of additional hidden vectors so I delete those, increase the line width slightly to 0.1mm. Rotate the shapes how I want to cut them and then save as an .ai format vector image ready for the toolpath creation…
I saw Vectric Cut2d recommended on a YouTube video and took a look at the software. There were more complicated and expensive options (such as their VCarvePro software) but Cut2D seemed to do all of the things I wanted so a short credit card swipe later and it was on my desktop.
The software itself is easy to use. It is actually a simple CAD program in its own right and the interface makes it easy to import and manipulate the .ai files. One thing I did learn was that once the vectors are loaded it is a good idea to select everything and hit ctrl+j to close all the lines and make selecting paths easier.
CNC Motion Control Software
The manual that came with my machine stated that it was designed to work with Mach3 Mill from Newfangled Solutions, so that is what I got. I am pretty sure that I am still not using this software to it’s full potential but it really is a complex bit of kit.
In a nutshell it takes the G-Code from the CAM software and sends it in real-time to the CNC machine through a serial cable. It is this software that tells the motors how many “steps” are in a millimeter and how fast to spin the drill and move the gantry. Everything starts and stops here so it pays to read the manual and at least learn the basics.
The toolpath is where the fun really begins. There is absolutely no way I will be able to tell you how to design toolpaths in one article but I will give you some of the things I have learnt along the way…
The first thing to do when you open your CAM software (I am using Vectric Cut2D remember?) is to tell it the size of the blank plate you will be working with. I generally use either a 250mm x 200mm or 400mm x 200mm blank. You also need to tell the software how thick your material is as well as where “home” is so it has a point to work from.I always set home to the dead center of the plate as this is easy to find and mark on the blank with a ruler before I clamp it into the machine.
The next thing to do is to actually import your AI drawings and arrange them on the blank.TIP: Always leave about 10mm clear around the edge of your blank and make sure you keep the corners free for the clamps. (Or you could simply set your material size to 10mm smaller than the real piece).
Once all your plates are in place I found that in Cut2D I needed to select everything and then hit CTRL+J to close all the vectors. This means that if I then try to select a plate outline for example it will select the whole thing rather than a single segment.
Toolpaths are created by first selecting everything you want to cut and then choosing a path style from the menu. For this type of cutting I use either drill or profile. Drill is usefull for the smaller holes whilst profile is the path to use for everything else.
Never try to cut a whole design in one toolpath. By breaking it up into different parts it is easier to monitor and check progress as well as use different size end mills for different components.
Once you have created your toolpaths you need to export them to G-Code. You can add more than one toolpath to a G-Code file but I like to keep it simple and save each one separately. The output I use for my machine is “G-Code Arcs (mm) (*.tap)” and to look at is nothing more than thousands of lines of numbers and letters.
Let’s get on with it then…
So, we’ve got our design and we’ve got our toolpaths, now we need to get the thing cut. Here is pretty much how I go about every project…
Preparing the blanks
I think I already mentioned that I use one of two blank sizes. The smaller size is 250mm x 200mm and this fits most miniquad size frames. The larger plate is 250mm x 400mm for the larger frames.
The sheets come from the factory in 500mm x 400mm sheets and I cut them down with a hand-held circular saw with a thin 40 tooth blade. I cut the plates down in this way for a couple of reasons:
- My CNC machine isn’t that big so I couldn’t fit a full size sheet on the cutting bed if I wanted to.
- It is easier to work with smaller plates and creates less waste if I cut one frame per blank.
- Using the thinner materials, larger plates can bow and vibrate causing problems with cut depth and deflection.
The next thing I do is stick the blanks to the sacrifice boards. As most jobs use both 1.5mm and 3mm plates I stick one of each either side of the sacrifice board.
Hang on. You “stick” the blanks to the sacrifice board? (I hear you ask)
I spent a long time trying to work out how I could cut components without having to have nasty tabs all the way around the edge. In the end I settled on actually fixing the blank plates to the sacrifice board using spray adhesive. The image shows early tests using foam board blanks but I have since moved to MDF blanks as they are both cheaper and stiffer. Here’s what I have learned:
- Cut a piece of 3mm MDF to the same size as your material blanks
- Leave the protective coating on your material
- Spray one side of both the MDF and the material with spray contact adhesive and press together
- Put cutting material on BOTH sides of the MDF. This reduces waste and also adds a lot of stiffness to the surface to be cut. If you are using two thicknesses of material (e.g. 1.5mm and 3mm) put one of each on your sacrifice board and cut the thinner material first to maximise the stiffness.
The last thing to do once your blanks are stuck is mark the centers by drawing a line from corner to corner over the protective coating.
Clamping & Setting Up
My machine was supplied with four screw type clamps that can be moved anywhere around the cutting table by way of square nuts that lock into the channels which run full length.
I marked a straight line (with a set square) across the bottom of my table so I had a datum from which to align my boards and then use the screws clamps on each corner. Alignment is important if you are trying to maximise cutting areas and minimise waste as you don’t want your tool to slip off the edge of your material during a cut. I find that by having the second thicker sheet on the back of the sacrifice board I can clamp right on the corners and still not see any bowing in the center.
With the correct end mill securely clamped in the chuck I now manually wind the point into place so that it is on my center mark, just touching the surface of the material. I do have an electronic z-axis setting tool that I could use but I find it just as easy to find the material surface by hand.
A Short Word About Safety
CNC Machines are designed to cut hard materials at speed using very sharp tools. Once the tool bit is in motion it will keep trying to reach is pre-programmed coordinates regardless of whether it is fiberglass, the metal clamps or your child’s fingers that are in the way.
ALWAYS make sure cutting is done in a safe environment, secure from prying eyes and fingers. If you are going to be near the machine during cutting then make sure at a minimum you are wearing eye protection. You need your eyes to fly so don’t risk them!
The jury is still out on just how dangerous carbon fiber dust is but I know for sure that I don’t want to be breathing a lot of it. If you are running a small bench-top machine like me then make sure you keep a filtered vacuum handy and regularly suck up the dust. If you are going to be doing a lot of cutting then maybe even a cabinet with dust extraction would be good idea.
Let’s get on with it then (again)!
Turn on the power to your machine and then move to the computer. The first thing I do is hit the software “Reset” button to make the connection between the computer and CNC machine. Then I zero the three cutting axis – this is REALLY important as not doing this means the software could think your center point and material surface is anywhere! Now I load my G-Code file and I’m ready to cut.
If I am cutting a very full sheet I will sometimes load the final cut (the component outlines) and “jog” the tool bit around the table at a safe height to check the extremities and make sure everything fits and the clamps won’t get hit.
Only when I am 100% sure I am ready do I hit the green “Cycle Start” button and cross my fingers.
Note: Most cutting software can control the spindle speed through the interface. If yours can’t then you will need to manually start the drill before starting the cutting cycle.
After each cut I check everything is okay by vacuuming off the dust and checking the tool has returned to my center mark correctly. If I need to change tools then I find it is safe to switch the machine off and manually raise the drill to change the bit so long as you remember two things:
- Only touch the Z-axis. Don’t touch the X or Y otherwise you might knock it out of alignment.
- Reset the Z-axis zero in the software so the machine “learns” the surface point for the new bit.
And that’s it really! Just repeat the process until all your cuts are complete and you have a finished set of components.
Once your cuts are complete you should be able to tell (after vacuuming) if the cuts have gone all the way through. Now is the time to find out as it is easy to make some changes to the toolpath and repeat a portion of the job whilst the material is still in place and the zeros are set. Finding out once you have taken the plate off the machine is disastrous!
To make sure I actually remove the components from the blank whilst it is still clamped. To do this I get a small flat headed screwdriver and pry the parts loose. With a bit of pressure the glue or protective sheet will give way making it easy for you to lift the component out.
I can then peel off the protective sheets, check the finish and move on to the next job.
At the end of the day I always take some time to vacuum around the work area to get up any stray dust. I also check the set screws on the CNC machine, give it a wipe over and lubricate the moving parts.
CNC machining is a bit of an art and once you get the hang of it it is a total addiction. You’ll have some failures and frustrations along the way but they’ll all be forgotten the first time you put a frame you’ve designed and cut together and feel the satisfaction of the components slotting perfectly into place.
Like I said at the start, this is only a brief chunk of the full series of articles so if you are interested in this aspect of the hobby, pour yourself a coffee and have a read over at Red20RC. If nothing else you'll be able to work out if I should have called this "CNC Cutting BY a Dummy"!