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Old Jun 19, 2013, 03:39 AM
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Build Log
F5B Glider Build Log, Part 1.

I'm currently in the process of cutting some moulds for a F5B glider from MDF, and intend to see if I can do a build log whilst moving through the various stages.

Background.

The Machine: This is a commercial item, purchased from Germany. It has a cut size of 1250mm by 650mm by 125mm. It is a hybrid of the two machines availible in this AL-Series. it is the length of their big version and the width of the smaller one. The reason for this is the physical size, at the time of purchase I had nowhere to fit the bigger one! See link below - it's in German, but Google does a good job of translating!

http://www.team-haase.de/index.php?o...id=4&Itemid=30

Control Software: This was upgraded at the time of buying the machine from WinPC-NC light to CNCGraf v4 using a USB interface. Why the upgrade? I was assured that it was far better at handling 3D work. I have no regrets and the ability to control the machine using USB means that I could use my laptop, if required.

http://www.team-haase.de/software/cncgraf

Design Software: I have genuine versions of Cut2D, Cut3D and Aspire v3 from Vectric. Aspire is rather expensive, but it does so much! I have yet to use anywhere near all of its capacity. What I have used is enough to make me seriously consider upgrading to the current version 4, despite a cost of 300.
http://www.vectric.com/
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Old Jun 19, 2013, 03:39 AM
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Model Design.

Unfortunately I am not competent to design a model suitable for F5B, or any other discipline for that matter. I am indebted to Dominik Diefenbach (aka plastikmann) for his Teaser F5B designs. He has drawn up this model and offered it to the forum as an open source design.

http://www.rcgroups.com/forums/showthread.php?t=1549405

http://translate.google.de/translate...ger%25C3%25A4t

Dominik has drawn up the Teaser design using Catia, a very expensive program, and saved them for the web community as .stp files, both positives and mould negatives. These are easily imported into Aspire and modified, if required. I needed to change the wing moulds into two pieces as my machine will not cut a 2m length and I do not have the space around the machine to move the part by some method of indexing. I also used Aspire to mostly fill in the alignment holes around the edge of the moulds. (My cutters will not cut to a depth of 50mm and it also speeds up the cutting time. Alignment holes will be drilled through later on a drill press).
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Old Jun 19, 2013, 03:40 AM
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The original Teaser fuselage design was of a classic F5B shape - T-tailed and boxy to take a 6S lipo pack.

His more recent design is for a slightly longer, slimmer V-tail fuselage. This will easily take 6S in a 2x3S configuration. I also think it should make a very nice high wind slope soarer/DS glider, with the addition of a nose cone.

I decided to start with this fuselage mould. Once again I used Aspire to mostly fill in the mould alignment holes, before generating the cut files using the relevant Post Processor.
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Old Jun 19, 2013, 03:41 AM
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Mould material: I use MDF for a number of reasons but the main one is cost! I would love to use aluminium for my moulds, but it is expensive and the need to use cutting fluid would mean my small workshop would potetially be sprayed in the stuff. Not ideal. The next option would be Corian, again it is expensive and I cannot find a supply of cheap offcuts. It may be an option if I ever need to make replacement moulds. So that leaves MDF. I don't even use the water-resistant stuff that the Australians like.

For the wing moulds I have stuck two pieces of 25mm MDF together to get the required thickness, using white PVA glue.
The fuselage mould uses two pieces of 18mm MDF. This is all aquired from my local B&Q, have a look in their scrap bin at the timber cutting area, you might get a suitable piece for a fraction of the normal cost.
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Old Jun 19, 2013, 04:07 AM
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V-tail fuselage mould.

As previously stated, this is from two laminations of 18mm MDF. The block is held onto the machine bed by simple clamps which engage into slots in the sides. These slots are cut using a biscuit jointer.

Since I like to do all the cutting on the pieces before removing them from the machine, I do not have to have any alignment pins or other devices to accurately position the mould blank. This further saves time.
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Old Jun 19, 2013, 04:16 AM
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The MDF block for the V-tail fuselage mould is wide enough for both fuselage sides to be machined from it.

My control software, CNCGraf, gives the option of setting up multiple, named, zero points. This allows me to have a zero point/start point for the left hand half of the mould, as well as for the right hand half. Once again this is a time saving issue for me. I only have to set the tool up once and I can cut multiple different parts in various areas of the work-piece.

All I have to do is make sure I select the correct zero point (hence the advantage of naming them) and the right cut file. It's not pretty if I get either of those choices wrong!
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Old Jun 19, 2013, 04:47 AM
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Okay, so the work-piece is now clamped to the CNC machine and it's XYZ zero point is set in the control software.

Note: there are two options here. I can set it to be in the centre of the material, or on the lower left corner. I choose to use the second option, both in the control and design software.

The chosen tool, as set up in the design software/cut file, is fitted to into the spindle and, at this point I use a tool height sensor to measure the tool length.

http://www.team-haase.de/zubehoer/werkzeuglaengensensor

Since the material being cut has not yet had its height above the machine bed measured, I don't need to measure the tool length at this stage, but, in my opinion, it is a good habit to get into doing this every time the tool is changed.

The tool needs to protrude far enough out of the collet to allow the spindle collar to be above the work-piece at maximum cutting depth. I've been there - again, it was not pretty!
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Old Jun 19, 2013, 05:01 AM
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Workpiece Thickness

Material Height. This is set quite simply with the tool height sensor, once calibrated. Basically, to calibrate this one, in CNCGraf, a tool is fitted to the spindle and the Z-axis is lowered until the tool just touches the machine bed. Then it is lowered onto the sensor, also on the machine bed, under software control, which then calculates the height of the sensor. This measurement is then saved.

To calculate material height, on the machine, the sensor is placed on top of it and the tool lowered slowly, under computer control, until it is detected. An automatic tool measurement is then carried out. The software is able to work out the material height from this.

Time to start cutting!!
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Old Jun 19, 2013, 05:16 AM
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My software, from Vectric, generates two sets of cut files. The first, the "Roughing" cut, is designed to remove most of the excess material, using fast speeds and large diameter cutters (fast and large are relative terms here and are dependent on material being cut and type/size of cutterbeing used). The second set of files the "Finishing" cut.

My CNC machine will traverse at upto 6 m/min, but will not cut at that speed. From trial and error testing I have found that it will happily cut MDF at 2.4m/min (approx 100"/min), so this is what I use. It is slower than some, but I don't break tools at this speed!
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Old Jun 19, 2013, 05:25 AM
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First Cuts

Enough chat, onto the cutting. This is the result of the "Roughing" cut for both V-tail fuselage mould halves.

You can easily see the steps in the moulds where the 6mm tool has removed the excess material. At it's closest point it is within 0.5mm of the "Finishing" cut surface.

All parameters were set as per previous post.

Total machining time: 1 hour.
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Last edited by heli_bee; Jun 19, 2013 at 05:57 AM.
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Old Jun 19, 2013, 05:55 AM
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Second Cuts.

Now for the "Finishing" cuts. This is done to a finer tolerance, using a smaller tool bit.
Somehow I screwed-up on the first finish cut. I managed to set the Z-axis to start 6mm lower than it should have! The result of this is a 6mm deep groove in the bottom of one mould halve.
Not too much of an issue, other than being frustrating, as I didn't break the tool bit, and I have to soak the mould halves with epoxy resin later.

Total machining time: 6 hours.
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Last edited by heli_bee; Jun 19, 2013 at 05:59 AM. Reason: Added tool details.
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Old Jun 19, 2013, 06:19 AM
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Sealing the mould

Next up is sealing the mould. This is done by brushing the MDF with epoxy resin and using a heat gun to "sink" it into the MDF, (thanks for this tip Erazz). Three, or four, applications of resin will without time to cure between coats will usually be enough to fill the MDF. It's amazing how much resin the MDF will absorb!

I have tried this with polyester resin, but it cures far too fast with the application of heat. I currently use Sicomin Composites SR8200 epoxy and SD7204 hardener. This combination gives a normal pot life of around 2.5 hours, this is great for bagging wings with as well.
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Old Jun 19, 2013, 06:23 AM
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The "Screw-up" mentioned in post #11 was filled with polyester gelcoat.

I used this because it is thicker than normal resin and I had some that needs to be used up. Oh yes, it is also much cheaper than epoxy! Does anyone else see a pattern forming here?
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Old Jun 19, 2013, 06:32 AM
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Cut Paths.

Whilst I am waiting for the CNC machine to finish it's cutting I thought I would compare the design software cut path and the actual cut path.

They seem like a pretty good match to me.
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Old Jun 19, 2013, 09:26 AM
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heli_bee, those successive coats of epoxy along with the "fix" you had to make...

If you were to leave the moulds on your CNC table between each epoxy coat, and also after the fix, why not just run the same cut file again which would smooth out all the fuzzies and fixes brought up by the epoxy coatings and the fix?
The epoxy that saturated into the mould would remain at and below the surface, and you would end up with a very smooth finish that is ready to go to production of the part by virtue of the freshly machined surface.
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