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GMFC 4.0.4

Fall is coming back in France, It’s time to go back to the workshop and cut some foam…

This new version of GMFC mostly correct bugs. Please upgrade to it:

  • [Expert] Bugs in selecting points in alignment mode
  • detection of close points in alignment mode
  • bug save .cncx files
  • bugs in dialog in German PRO and Expert version
  • saving parameters in foam cut dialog
  • remove tip check for orthogonal spar

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Support of TB6600 based motor drivers with the GGC adapter

TB6600 drivers are interesting because they can work up to 40V and 4A. Therefore, they can drive a large variety of bipolar stepper motors. These drivers are also affordable with a price below 10 euros on ebay or aliexpress.

TB6600 drivers are available under several packaging with different maximal current and voltage. However, connections are mostly identical.

On the motor side, there are :

  • VCC (DC+), GND (DC-) that must be connected to the motor power supply.
  • A+,A- and B+,B- the four motor signals.

On the GGC side, there are:

  • PUL+, PUL- that are the step control signal.
  • DIR+, DIR- that control the direction of the move.
  • ENA+, ENA- that control powering of the motor. This signal is optional and avoids heating the motor when not used.

The PUL+, DIR+ et ENA+ should be connected to +5V on the GGC. The pin 20 on  port J4 can be used for this purpose as shown on the following picture.

ENA- should be connected to the DB25  parallel port, either to pin 1 or to pin 17. PUL- should be connected to pin 2, DIR- should be connected to pin 3. Other motor drivers should be connected to pin 4 to 9. This is only an example since the configuration of STEP and DIR signals is fully configurable within the ggc_utility.

Using the ggc_utitity the timer should be set to 50Khz, the duration of the STEP signal and the delay for Dir before Step should be set to 2 us. Here, Motor ON is on pin 1, and should be Active High.

Several DB 25 pins are also available on J4:

  • DB.17 (Motor Enable) -> J4.2
  • DB.2 (step Motor1) -> J4.13
  • DB.3 (data Motor1) -> J4.15
  • DB.4 (step Motor2) -> J4.9
  • DB.5 (data Motor2) -> J4.11
  • DB.6 (step Motor3) -> J4.5
  • DB.7 (data Motor3) -> J4.7
  • DB.8 (step Motor4) -> J4.1
  • DB.9 (data Motor4) -> J4.3

I tested the TB6600 drivers under GMFC using a NEMA23 motor (200 steps) without being attached to an axis. The power supply voltage is 28V. Higher the voltage, faster the movement, but the heat may also be higher. I obtained the best performance with the current set to 2A.

Here is the table configuration. The motor is attached to the YL axis.

The motor is set to half step mode in the driver, so the number of steps per revolution is 2×200. To fully check the speed configuration, you have to performe two ways movements using the Zero axis dialog. The number of steps for performing acceleration is set to 400. The time between two steps for the no acceleration speed is 620us. With acceleration, it goes down to 260us.

These numbers are good. With my Letmathe motor interface, I get sightly better results with my openbuild table, but my power supply has also a higher voltage.

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How to backup and transfer table configuration files

GMFC’s configuration is stored is a set of files that need to be backup regularly in case of a computer crash. If you change your cutting computer, you will also have to transfer these files to the new machine.

Table parameters such as their size and displacement speed are stored in a file named, with name being the name of the table. Foam cutting parameters such as the kerf and the speed are stored in a file named There is also a file named that summarizes all the tables. All these files are text based and can be edited with the notebook.

So where do you find these files?

Start GMFC, and open the dialog “GMFC/Configuration and Files”

The Configuration Directory is shown on top of this dialog.

Now open this directory using the file explorer. After exiting GMFC, you can backup the complete content of the directory or transfer it to a new PC.

When transferring to a new PC, you can copy the configuration files to any directory. Start GMFC, then open “GMFC/Configuration and Files” and set the configuration directory. You should now see your table and foam configurations.

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Using Fusion 360 to design a GMFC project

Fusion 360 is a great CAD program, and quite easy to learn. For now, it is also free for personal usage. I have already used it to design few models.

Here is a tutorial that explains how to design a “simple” shape using Fusion and then exporting it into a GMFC EXPERT (and PRO) project.

First, you need to start Fusion, and create a new design (File/New Design).

Using the view cube in the right, select the front view.

Then, create a sketch (Create/Create Sketch). Select the (front) plane to define the sketch on the front.

Now draw a hexagon using Lines. Then finish the sketch.

Now, we are going to create the second face. First, we need to create a second plane parallel to the first one. Use Construct/offset plane for this. Select the front plane which is the XY plane in the Origin section in the browser, and enter the distance between the two faces (here 60mm).

There is a new plane in the construction section. Right click on the plane and select “Create Sketch”.

Draw a circle on this face, and center it with respect to the hexagon.

Now we need to split the circle so that we can synchronize both faces. Here we want to divide it in 6 parts corresponding to each side of the hexagon. For this, let’s draw lines from between each hexagon opposite points. Change the type of the lines to “Construction Lines”.

The next step is to break the circle at the intersection of the construction lines. Use the break tool in the “Modify” menu.

Cut also the construction lines outside the circle, using again the break tool. Then finish the sketch.

Now, we need to construct the 3D view of the object, so as to check the final result. Select the “Loft” tool in the create menu, then the two profiles.

Here is the result.

Note that we could have split the circle in other ways. The construction lines are there only to ensure that all part are symmetric. What matters is that in the end there are the same number of segments in each profile.

Now we need to create a GMFC project from this shape. The solution relies on exporting both sketches into DXF files. Fusion does that, but it also exports construction lines which would require editing the DXF using a CAD program. My solution is to create a script for fusion that does the exportation while removing the construction lines.

First download the script. It is available here:

Uncompress the zip file in a directory. You need to install the script within Fusion. This should be done only once. Call the script menu using the “S” command. Then click on the green cross and select the installation directory. “ExportDXFSelection” should be now present under “My Scripts”.

Select “ExportDXFSelection”, and click on “Run” to launch the script. A windows is displayed with the number of sketches selected.

Select the sketches you want to export, then click on “OK”. A new pop-up window appears for selecting a directory where the DXF are saved. There is a file for each sketch. The name is constructed by concatenating the design name and the sketch name.

Then for each sketch, the script displays the bounding box, i.e., the limits of the sketch.

Finally, the script displays the delta in X and Y. This information is critical for correctly positioning both profiles when creating the GMFC project.

Now, we can create the GMFC project. Start GMFC, open the panel parameter. Select “Different root/tip”.

Enter the DeltaX value in the “Sweep back” field. Add the DeltaY value to the root “Basic Height” and enter it in the tip “Basic Height”. Enter the distance between the plans into “Panel span”.

Import both profiles using “File/import DXF”. For more details, follow the tutorial at

Enter the alignment mode. We need now to assign the same number of cutting points to each corresponding segment at root and tip.

Double-click on a segment and set the number of points.

Voila, you are nearly done for the design part… now you have to cut the shape.

Note that this project induces a lot of sweep back in X and Y… I used a 100mm wedge for cutting the shape. Fortunately, my Y axes are large. Here is a video of the cutting…

This tutorial is for GMFC EXPERT. If you are using GMFC PRO, you need to transform the DXF files using a CAD tool. Split each segment into the desired number of lines, then finally merge them into a polyline.

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GMFC 4.0.3

It time to release a new version after more than year. I consider version 4.0.3 as a stable one. Please upgrade to this one.

Most of the changes are bug fixes:

  • Improvement in points selection in DXF importation
  • Bug exit spar cutting in separated pass
  • Bug get down surface for closed profile
  • Modification of foam management dialog, move of field C
  • bug postion, horizontal LE in cutting spar & lower surface
  • bug position table+5mm
  • bug pre-heating spar & lower surface
  • Time for stabilizing heat in test cut for foam calibration
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Foam Calibration Tutorial

If you want to get the best out of GMFC, you need to precisely calibrate the cutting speed and the kerf. Foam settings are fully specific to a given foam material. Foam settings are also specific to temperature and humidity. In my experience, foam settings are valid for a range of 5 degrees (centigrade) around the temperature in the workshop during calibration.

Calibration is done using the foam management dialog (GMFC/Foam management). Start by creating a new material. For choosing the name, I specify the type of the foam, the temperature and the type of the wire.

Calibration is achieved by cutting a slice of foam. The dialog allows to cut a stack of slices in a single block. Therefore, you need to enter the block height and the block placement in X0. Enter also the size (width, height) of each slice. The dialog shows how many slices can be cut in the block. Start cutting a slice by clicking on the “Cut” button. Between each slice, cutting stops so that you can adjust speed and heat parameters.

First, we calibrate the high cutting speed and heat. Expanded polystyrene can be cut up to 3-5 mm/s. This depends from the machine speed, and also from the heat (wire and voltage). You need a small piece of foam those edges were cut manually by a knife/cutter since after the foam is burnt, the surface becomes harder to cut in. Remember that GMFC cuts by radiance. So the wire must penetrate the block without dragging it. Specify the high cutting speed and adjust the heat so that the block does not move while cutting the slice. Also be careful to minimize the heat since too much heat increases the kerf and alters the shape.

When speed and heat are fine, you can evaluate the kerf. Use a caliper to measure the slice height after cutting. The kerf is equal to the difference between the measured value and the specified one. Enter the result in the “Root kerf” field. With a 0.3mm Nichrome wire, the kerf should be around 0.6-0.8mm (depending on the speed and heat). Note that in this dialog there is no kerf compensation when cutting a slice.

We now need to evaluate the kerf for a tapered wing those tip is smaller than the root. We already have the kerf at Root, we need to estimate the kerf at Tip. Enter a C ratio (length of Tip/length of Root) that is close to the kind of wings you will cut. C=0.8 works for gliders. C=0.5 is the limit. A smaller ratio induces too much deformation.

Let’s cut now a tapered slice. For this, check the box “Measure kerf at tip“. Enter also the length of the bock in Z and it’s position with respect to the left axis. Be precise since it directly impacts the slice dimensions. Cut the slice by pressing the “Cut” button. Measure the slice height at tip with the caliper and enter the result in the “Tip kerf” field. Note that the root kerf should still be the same.

Finally, we need to determine the low cutting speed and the associated heat value. Note that this does not impact kerf value which should be the same at this low speed. Choose a low speed between 1 and 2 mm/s. GMFC reduces the cutting speed when the wing/shape is high tapered which in turn induces a large displacement (and speed) at the axis level.

Et Voila, you are done. Click on “Apply and exit“…

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GMFC 4.0.2

Here is a new version for the beginning of the year.

There is an important novel feature that improves wire heat. When the wire sits on the table, its temperature is reduced by the table itself. Therefore, the wire temperature may be lower at the beginning of the cut. With this version, the wire raises up to 5mm above the table and waits for a given time before continuing the cut. In the table configuration, there is now two time parameters: one for un-gluing the wire from the table, and the time for temperature stabilization.

Parameters of the wire cleaning are now saved.

A bug in the non-detection of the GGC for Windows 10 has been corrected. Finally, few minor bugs introduced in version 4.0.1 have been corrected.

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Merry Christmas and GMFC 4.0.1

Santa was generous this year. He brought a new version of GMFC in his hood 🙂

Here are the novel features:

  • In variable heat mode, the maximum cutting speed is given by the max calibration speed.
  • The entry to the TE prologation and the round spar cutting speed are now indepedent from the profile cutting speed. This is a major improvement since dihedral in the spar or wing taper may severly impact cutting speed.
  • There is a new Xoffset option in the foam cutting dialog (PRO and EXPERT versions). This allows to quicky shape foam blocks at a given width.

Happy cutting between the turkey and the cake 🙂