In this review I will show you how to setup and operate the CNC Foam Cutting Machine that we built in Part 1 HERE. A CNC Foam Cutter will allow you to quickly build wings, fuselages and other shapes from various weights and types of foam. The machine uses a temperature controlled hot wire to cut (by melting) the foam to the correct shape. The machine is a 4-axis machine and has two independent sides, so different shapes can be cut on each side. This machine will cut tapered shapes as well as straight shapes.
For this review Hobby CNC provided a FOAM Pro Major Electronics Combo while Precision Tech Machining provided the lead screw, lead screw nuts, bearings and bearing blocks. Gilles Muller provided the GFMC software and A Main Hobbies provided a ProTek 1010b+ iCharger to use as a hot wire power supply.
Here is where we left on on Part 1:
Degree of Difficulty
Skill Level: Medium
Time Required to Build: 5-10 Hours
Frustration Level: Low
Width (wing span):
Length (wing chord):
Motors: 130 oz-in Torque Stepper Motors
Foams this machine can cut:
- White Expanded polystyrene “breadboard foam”
- Pink, Blue or Green Extruded Polystyrene Foam Insulation
You will need some basic tools:
- Side Cutters
- 000 Steel Wool (for cleaning the wire between cuts)
A really handy tool to have when cutting foam is a metric tape measure. I found this one at Ace Hardware for under $10 bucks. The GMFC software uses all metric, so this tape will help you measure your machine and position the foam before cutting.
Hot Wire Setup
When it comes time to selecting the cutting wire, there are lots of options. The perfect cutting wire would be readily available, cheap, very small in diameter, would not stretch when it got hot and be very strong.
|Stainless Steel Wire (Also called Stainless Steel Fishing Leader)||Carried by most fishing places, cheap||Stretches, hard to find very small diameters|
|Nichrome Wire (Most hobby shops carry this)||Carried by most hobby shops, small diameter, cheap||Stretches, brittle|
|RENE-40 Wire||Very Little Stretch, Strong, Small Diameter||Special Order, Cost|
I’ve tried all these and prefer the RENE-40 wire for several reasons. It does not stretch as much as stainless steel or Nichrome and it is very strong and comes in a small diameter.
SkyKing Products LLC has been selling small amounts of RENE-40 wire (50 feet) to the hobbyist for many years. There RENE Cutting Wire is 0.014 inches in diameter and at the time of this writing was $36 including shipping.
To install the spring you will need a spring, crimps and the RENE cutting wire. I found my spring at Ace Hardware, C-141 a 5/8 x 2 1/2 x 0.072 Utility Extension Spring. Use the spring on one side and the other side will attach to the screw going through the block. Use the crimps to secure the wire into a loop. It helps if you can loop the wire through the crimp at least 1 extra time. The crimps I used were Du-Bro Catalog number 896. The RENE wire is strong, but very stiff and does not bend well so take care not to make sharp bends with the wire.
You need to install the wire so the spring is a little stretched. Exactly how much you will need is hard to describe with out some way to measure it. You can study the spring in the picture above to get an idea. When you cut a tapered wing this spring will have to stretch to make up the extra distance as the two axes move out of plane.
There is a better way to tension the wire, but you have to build a bow and support the bow. The bow hangs under the machine so you will need to add legs to the machine. With this method, the wire tension stays constant as the axes move out of plane. Searching the web will yield more than a few examples of how to do this. If you cut a lot of highly tapered shapes, this is the best method for tensioning the wire.
There are 3 kinds of foam that I’ll look at in this review. Each has their place, and I will explain the common uses of each.
The most common is white or “breadboard” foam, also called Expanded Polystyrene. This is the foam that most ARF’s (if they use foam) will use. White foam comes in various densities including 1lb/cubic foot, 1.5 and 2.0 lb/cubic ft. Generally 1.5 or 2.0 lb foam will produce the best combination of strength and weight. The higher density foam will require more heat to cut as well. Typically the white foam sold at the big-box home improvement stores is the lighter 1lb foam and has “regrind” or recycled foam pieces. This foam will not cut as nice and the wire may drag or hang in spots. For the best cuts, try and find “virgin” foam that has no “regrind” in the 1.5 or 2.0 lb densities.
Extruded Polystyrene is typically denser than than the white foam and it generally does not come in various densities. I have not worked with the green foam, but from my experience the pink foam cuts just a little nicer than the blue foam. I’ve used this foam with great success in my Combat airplanes. Depron is also an extruded polystyrene foam that comes in many thickness that are popular with electric foamies.
EPP foam is cool stuff. It is by far the most “crash resistant” of the foams. This foam has large beads and wings cut from this material absorb impacts very well. This for is also much less stiff than with the other kinds of foam. The disadvantage of EPP is you will probably have to order it, and shipping can be expensive. If your going to make a plane to bump around, this is the foam for you.
|Advantages||Disadvantages||Where to buy|
|White Expanded polystyrene “breadboard” foam||Cheap, available, lightweight||Strength||Home improvement stores, insulation supplier|
|Pink, Blue or Green Extruded Polystyrene Foam Insulation also can be purchased as “Depron”||Strength and available||Heavier, more brittle||Home improvement stores, insulation supplier, Mail Order or Hobby Shop (Depron)|
|EPP Foam||Bends without breaking||Most Expensive||Mail order|
Cutting By Radiance
So how does this thing cut the foam anyway? The theory is simple. A tensioned wire is heated by an electrical current source, this causes the wire to heat up. The heat that radiates off the wire melts the foam. Enough heat is generated by the wire that the foam will melt in front of the wire as it moves through the foam.
The amount of power that runs through the wire and the speed that the wire travels is important. Set the power too low, and the wire will cut by contact. Too high, and the amount of foam that is melted will be excessive. Also with too much heat, the surface of the foam is distorted. Cut too fast or slow and you will either cut by contact or melt too much foam.
Remember that Power = Volts x Amps, you can read more HERE. For a typical power supply (not a special constant current power supply) you hold the voltage at some level, and the amperage will fall out depending on the resistance in the wire. To make the wire hotter with a typical constant voltage power supply, you dial in more voltage. This will cause the amperage to go up and thus the wire will be dissipating more power.
I know what your thinking, but how do you get an accurate size shape? The answer is with the software. Kerf is the distance that the wire needs to be offset. The GFMC software allows you to insert two different kerf values. The GMFC will cut tapered wings, and on the tip (normally the side with the shorter chord), the Kerf will be bigger than the root because the wire is moving slower. To compensate for that, GFMC has two Kerf values (S and S/2).
When the software cuts any shape, the left and right axis must move to the same percentage of the line that you are cutting. For example, if you are cutting a tapered wing with a 10″ root chord, and a 8″ tip chord – the wire must move faster on the side of the larger chord. So when the wire is at 10% on the tip – the wire is at 10% at the root. Also the wire will enter and exit on at the same time from the leading or trailing edge.
Cutting Wire Power Supplies
To cut the foam you have to provide power to the wire to heat it up. You can do that with AC (Alternating Current) or DC (Direct Current) electricity. A search on the web produces results for hot-wire power supplies of both types that you can buy or build yourself. The best is the Direct Current variety because of heat control and heat stability.
Don’t confuse the cutting wire power supply with the one that will supply DC to the HobbyCNC controller board! These MUST be two separate power supplies! For the HobbyCNC board I used a converted ATX 400W computer power supply. The HobbyCNC site has these instructions on how to modify the power supply for use when it’s not in a computer.
ProTek 1010b+ Charger (with Foam Cutting Software)
For this review A Main Hobbies provided a ProTek 1010b+ Charger. ProTek has introduced their 1010b+ advanced battery charger that has some unique software that will allow it to be a foam cutting power supply. The best part is you can power it with a 12V power source and get up to 40V output! It also has an easy to read LCD that will let you dial in the amount of voltage and readout the amperage. A stable power supply is very important to getting consistent cuts with the machine.
The ProTek charger comes well packaged in a full color box. The ProTek 1010b+ can charge up to 10 Amps. It comes with a board to connect to 2S through 6S LiPo batteries. It also comes with a temperature probe for safe charging and alligator clips. The manual comes on CD and can also be downloaded HERE. The ProTek 1010b+ will charge or discharge several different chemistry batteries including Lilo, LiPo, LiFe, NiMh, NiCd, and Lead Acid. In addition to being a charger, it has software to drive a brushless DC motor and be a foam cutting power supply. The 1010b+ charger also supports the open-source software Logview (http://www.logview.info) that can display, plot and analyze the charge and discharge data. You can also update the firmware via the included USB cable from the Internet HERE.
HobbyCNC Heat Control (Included with the HobbyCNC board)
While not a power supply, the HobbyCNC board comes with a wire heat circuit. This circuit has two positions, Auto and Manual. On the Auto position, it will interface with the GFMC software to vary the heat as the wire moves around the shape. One example where this works well is going around a square corner (like in a spar slot), the software will reduce the heat to for a sharper edge. There is a switch to change the mode to manual, and in this mode you can press the two buttons (up and down) to vary the heat of the wire. The HobbyCNC board can handle up to 35 Volts DC of voltage at on the bow power supply terminals. To power the bow you can build your own power supply from parts with instructions from the HobbyCNC site.
HobbyCNC or ProTek?
When the HobbyCNC and the GMFC software work together, the quality of the cut is definitely better. The software knows when to dial the heat down. It does this around corners and when the wire speed slows down. The HobbyCNC board acts like a valve on the current, so you need to supply the board the maximum voltage that you are going to cut. The HobbyCNC board will then supply less voltage out when the GMFC software demands it.
The ProTek will simply supply a constant voltage current to the cutting wire. It will maintain a constant voltage to the wire. The ProTek does a great job of maintaining a constant power to the bow. As a bonus with the ProTek, you can also charge up to a 6S LiPo battery. This charger has also been recommended on other R/C Sites for it’s precision while charging. The ProTek has an internal DC-DC converter that will let you output up to 40 volts on the wire. This is handy when you want to clean the goo off the wire.
With either setup you need to provide a power supply. With the HobbyCNC board, you will need at least a 24V 2 Amp supply. The HobbyCNC site has instructions on building one of these for about $75 bucks. You cannot use a converted computer ATX power supply with the HobbyCNC setup. The ProTek charger isn’t so picky and I was able to power the ProTek with a 400W ATX power supply.
Which cutting power supply you choose will depend on your needs. If you need the most accurate cuts, then use the HobbyCNC setup. The ProTek has a nice LCD panel that gives you great information when cutting foam. Having this data will help you set up the machine the same way each time you use it. This is a feature that the HobbyCNC board is lacking. The ProTek also does double duty as a kick butt 10 Amp 6s LiPo charger.
The GFMC software is used to drive the HobbyCNC board and make cutting files. It has several special functions, particularly for cutting wings. It can cut just about any profile, so you can cut pieces for fuselages. I’ve even used it to cut foam blocks to help me transport my planes to the field.
Inside GFMC you will tell the software how big your CNC machine is and what kind of foams you will be cutting with. The program will allow you to create and name various types of foam where you can preset the cutting speed and the kerf. You can also have multiple machines with different settings.
Because GFMC has so many features and functions, watch the video below for a quick introduction.
The GMFC software was primarily designed to cut wings, although other shapes can be cut. For cutting wings, the program will allow you import an airfoil profile and add features to it like spars and other cutouts without having to draw them in a CAD program.
And you don’t need a CAD (Computer Aided Design) Program Either…
You can import airfoil coordinates from a .dat and .cor files. A .dat file is a standard way to store the airfoil coordinates in a text file. A .cor file is a CompuFoil format that also stores the coordinates in a text file. The .dat/.cor files are common files for storing airfoil coordinates (without spar slots and other cutouts) and you can download thousands of airfoils HERE. They both can be opened in a text editor like Microsoft’s Notepad program. Both .dat and .cor files are text files with airfoil coordinates in “unitless” dimensions. GMFC will export a profile to .dxf so you can open it with a CAD program and create more complex shapes such as hollow wing cores. You can also import a .dxf file of an airfoil or any shape from a 2D CAD program.
Example: NACA 2414 Airfoil .dat file
To cut shapes other than wings, you can draw that shape in a 2D CAD program and save that file in the .dxf format and import it to GMFC. Once you import the file, you will need to edit the points of the shape by telling GMFC the zero/starting point and the direction of the cut.
One feature that I really like about the GMFC software, is that you only need to purchase a license for the computer that is going to run your cutting machine. You can download and install GMFC Pro on your other computers, and the “cutting” feature will be disabled after 30 days. You will still be able to use the software to design your cutting profiles. I normally design the cutting profiles in my office (in the big comfy office chair of course!) and then transfer them to the computer in the shop via a USB stick.
Software Setup and Calibration
First you will need to set up the GMFC software to work with the HobbyCNC controller board. Goto GMFC>Table configuration…
I decided to run the motors in half-step mode, so I removed the jumpers from the HobbyCNC board.
Set the steps per revolution, screw thread and the table size.
Jogging the Machine
Once you have the table setup you can power on the HobbyCNC board and jog the machine around manually. This way you can check the operation of all the motors. If you need to troubleshoot – DO NOT DISCONNECT ANY OF THE MOTORS WITH THE POWER ON!!! This will blow the stepper motor driver chips.
Once you are done moving the machine to the extremes and checking for binding, move the machine to the home position and zero the machine.
Move the machine a distance and measure the actual distance it traveled. Start with something small like 20 mm, then do it again for something bigger like 500 mm Do this a few times in each direction. If you are loosing steps, you have too much friction in the mechanical assembly or have the table settings wrong.
The Kerf is the amount that the machine needs to offset the wire to account for the amount of foam that is melted by radiance. To determine the Kerf, a test cut must be made with the same type foam, the same speed and the same power to the wire. For example, if you were cutting a 100 mm X 100 mm square, and the test cut was only 98 mm tall, you would put the kerf as 2 mm. To make a text cut, GFMC has a cutting file that is used where the large side is 80 x 40 mm and the small side is 40 x 20 mm.
Before cutting the foam, I like to set the kerf in the foam management setting to zero for S and S/2. This step isn’t necessary because you could add or subtract to the kerf that was set when the test cut was made, but it does cut down on any potential math errors!
To keep from wasting foam on test cuts, I prefer to use the machine to slice 10 mm off the end of the foam block. Then I cut those pieces into 150 mm long lengths. Using a few pins, stand one up on either side of the machine. Set the machine up as if you were cutting one continuous block of foam.
After the test cut, measure the block height (not width) and subtract the difference from the size the block was supposed to be and the size it is and put that value in S for the large block and S/2 for the smaller block. This is your Kerf. For this task I prefer a metric machinist rule. You can also use calipers, if you are careful to not squeeze the foam. The calibration takes 5-10 minutes and uses very little foam.
How often should you calibrate the machine? First you will need to calibrate the machine for each type of foam. Then you will need to calibrate when the temperature changes. I can’t give you a good idea on how often to calibrate for temperature, but if you want the most accurate cuts, you will need to calibrate often. Next any time you change cutting speed, you will need to calibrate.
Designing Your Cut
There are two ways to design a cutting profile. You can use coordinates from a .dat or .cor file or import a .dxf file from a 2D CAD program. With the .dxf method, the only extra step is to define the zero point coordinate and the cutting direction. Other than that, the steps are identical. GMFC will preview both the root and tip profiles as well as the wing planform (top-down view). To see the top down view, goto View > Wing.
To set up a cutting profile goto Project > Current Panel Configuration….
For a basic introduction of aerodynamics, NASA has a good site: HERE
|Foam Thickness (mm)||This is the height of the foam block. In the image above it would be the distance between the two horizontal blue lines.|
|Panel Span (mm)||This is the distance between the root and the tip.|
|Sweep Back (mm)||The amount the tip profile is set back from the root profile (horizontally)|
|Dihedral in degree||The amount the tip profile is higher than the tip (vertically). Check this box input the dihedral.|
|Wing Panel||Check this box if you are cutting a wing panel. This will allow you to add spars to the wing. On more complex profiles (even airfoils that you have drawn the spars in) you may want to make sure this is unchecked.|
|Skin||Checking this will allow you to put in a thickness to offset the cutting in to account for a wing skin.|
|X Centered||Checking this grays out the sweep back option and centers the root and tip so the wing has the same sweep on the leading edge and trailing edge.|
|Different Root/Tip||Check this box to select a different profile from the root and the tip.|
|Airfoil File||Click the button to select a profile|
|Chord (mm)||This is the chord in millimeters. See the image above.|
|Leading Edge Margin (mm)||This is the amount the wing is set back from the front of the foam block|
|Trailing Edge Margin (mm)||This is the amount the wing is set back from the rear of the foam block|
|Washout (deg)||This is the amount of degrees difference between the root and the tip.|
|Basic Height (mm)||This is the distance between the profile and the bottom of the foam block|
|Camber (%)||This changes the camber of the wing.|
|Thickness (%)||This changes the thickness of the wing.|
|Skin (mm)||Thickness that the cutting line is offset to account for wing skins.|
Ok, now for that extra step when working with .dxf files. Goto File > dxf/Hpgl Import…then select a file.
Now press the button. and you will get something similar to the image below.
Next, hover over the very last point in the trailing edge and right-click then select “Define Zero Point Here”. Then press the button.
Now you need to check and see where the cutting line is. It should be on the outside of the profile. You can check by going to Cut > Panel Cut…. and selecting cancel. The cutting lines will be shown on the profile of the airfoil.
If the lines are on the inside of the airfoil, the numbers are going the wrong way. Press the button and right click over the zero point and select “Reverse Point Numbering”.
When you are ready to cut a profile goto Cut > Panel Cut…..
|Cut (Right Wing / Top first)||Selects which wing panel to cut and if to cut the top or bottom first.|
|Wire Kerf Compensation||If the Profile box is check then the software will compensate for kerf, otherwise it will cut directly on the line. If the Ref. Lower Surface box is checked, then all the wing cores in a multi-panel wing will be at the same height so you can vacuum bag these cores easier.|
|Block Trimming||Allows you to select trimming the foam block before, after or while cutting.|
|Block Position||This option allows you to position the block. For wings that have taper and dihedral you may have to play around with the block positioning until you don’t get an error in the bottom left.|
|Cut to File||This allows you to “simulate” the cut to view it in an external program.|
|Intermediate Section||This allows you to cut a panel that’s bigger than your machine and allows you to select where to section it.|
|Cut Parameters||Allows you to select the foam type and speed|
|Repeat||Allows you to make more than one cut at a time. Great for production work. This program stacks the wings the height of the foam block that you entered and will cut the profile one right after another.|
When you have adjusted all the parameters, power up the HobbyCNC board and turn on the wire heat. Then press Cut.
Building the CNC machine is the first step in an adventure. Once you start using your machine, you will discover new ways to make foam cut shapes. Like a good airplane or helicopter, you will learn to “tweak” it the more you use it. In this article, I described how to run some of the basic features of the GMFC software. As you get familiar with the software, you will see that I have hardly scratched the surface.
This CNC machine and the GMFC software can be used to make many different shapes. I’ve cut wings, fuselage pieces and even repair parts! This machine will let you make single or multiple shapes quickly.
I really enjoyed writing these two articles. Building the machine was fun and now that it’s complete there is almost no limit to what I can make!
RCU Forums: batchelc
8502 Mulberry Rd.
Chesterland, Ohio 44026
GMFC (Foam Cutting Software):
Precision Tech Machining (Lead Screws):
Sky King RC Products, LLC (Cutting Wire):
2110 13th Ave. NE
Watertown, SD 57201
GMFC (Foam Cutting Software):
A Main Hobbies (ProTek 1010B+ Charger):
424 Otterson Dr. Suite 160
Chico, CA 95928 USA
Toll Free: 1-800-705-2215
Int’l & Local: (530) 894-0797
Special Thanks To: