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Grob 103 Twin Astir, Scratch build, Vacuum forming


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On 10/16/2018 at 10:38 PM, BritJet said:

 Are you using PET for moulding the canopy?

I haven’t decided on how to mould the canopy yet. I’ve found that the drawback with the female moulds is that all details from the moulds show up. That includes the imperfections of the mould surface. Male moulds are more forgiving as details don’t show up as clearly. I also think they are easier to sand and polish which makes them more suitable for canopies.

I could try to estimate how thick the canopy plastic will end up and make a correspondingly undersised male mould.

Edited by martenare
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12 hours ago, martenare said:

I haven’t decided on how to mound the canopy yet. I’ve found that the drawback with the female moulds is that all details from the moulds show up. That includes the imperfections of the mould surface. Male moulds are more forgiving as details don’t show up as clearly. I also think they are easier to sand and polish which makes them more suitable for canopies.

I could try to estimate how thick the canopy plastic will end up and make a correspondingly undersised male mould.

That drawback is also an advantage! Female moulds will give you all the detail on the outside of the mouldings, where you need it, instead of a 'soft' version that you'd get with a male mould. Also the mouldings are the right size rather than oversized by the plastic thickness, the moulded thickness will vary with the shape, so it's difficult to allow for it with a male mould. As good as your CNC milled female moulds are, I don't think they will give a good enough surface for moulding the canopy. I would glue two of your fuselage mouldings together, make the seam invisible and get a perfect surface. If you then follow how I did the Prentice canopy and make a female mould from car body filler, it can be polished before vacforming. If there any imperfections they can be carefully rubbed down with wet & dry, before polishing again.

 

8 hours ago, Bandsaw Steve said:

I’m my view / limited experience, the key is to get the right kind of plastic. Take Britjets advice on this as he seems to know what he’s on about.

That, Sir, will start a very vicious rumour! I just make this stuff up as I go along, some of it actually works now and again......

 

I try different things and if they don't work, try something else. Eventually a solution appears and then it's just a case of tweaking it. Thanks for the praise.

 

PETg is probably the best for canopies as it takes a lot of punishment when vacforming, smells a bit when heated but it's not too bad. I always vacform when the other half is out, to avoid any complaints. It's what Coke bottles, Easter egg packaging etc is made from and these can be used to practise vacforming. I would buy some PETg sheet for the proper mouldings as it will be scratch free and a constant thickness.

 

Steve

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1 hour ago, BritJet said:

That drawback is also an advantage! Female moulds will give you all the detail on the outside of the mouldings, where you need it, instead of a 'soft' version that you'd get with a male mould. Also the mouldings are the right size rather than oversized by the plastic thickness, the moulded thickness will vary with the shape, so it's difficult to allow for it with a male mould. As good as your CNC milled female moulds are, I don't think they will give a good enough surface for moulding the canopy. I would glue two of your fuselage mouldings together, make the seam invisible and get a perfect surface. If you then follow how I did the Prentice canopy and make a female mould from car body filler, it can be polished before vacforming. If there any imperfections they can be carefully rubbed down with wet & dry, before polishing again.

 

I try different things and if they don't work, try something else. Eventually a solution appears and then it's just a case of tweaking it. Thanks for the praise.

 

PETg is probably the best for canopies as it takes a lot of punishment when vacforming, smells a bit when heated but it's not too bad. I always vacform when the other half is out, to avoid any complaints. It's what Coke bottles, Easter egg packaging etc is made from and these can be used to practise vacforming. I would buy some PETg sheet for the proper mouldings as it will be scratch free and a constant thickness.

6

That sounds like a very sound plan. You are right in that my CNC would not produce a good enough surface and the material that I am milling also has microscopic pores that would not be fit for a canopy. A male mould could be primed and sanded to a good surface finish but I would, just like you said, struggle with the correct size. Your idea is much better and I will definitely try it. 

 

PET it is then. If I recall correctly, the canopies of the Twin Astir that I am building had a blue tint to them for sun protection. Are there plastic sheets with a blue tint available or do you think I can spray paint the canopy with a thin layer of blue?
I just ordered a couple of clear PETg sheets 0.5mm. Should I have gone for thicker/thinner sheets?

 

My better half would never allow me anywhere near the ovens with a plastic sheet in my hands. I had to invest in a small and very cheap oven for the garage in order to have somewhere to bake my plastic!

Edited by martenare
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I would build a plastic card box around the canopy area with the top open, spray the inside with release agent and then carefully add the car filler. Once it's cured you can pull the box apart and release the fuselage. You will get a much better result with the female mould.

 

You may be able to get blue tinted PETg but I've never looked so don't know, it would probably be more expensive and have a minimum order quantity as well so might not be worth it unless you are going into production! I think you'd be better off experimenting with clear blue paint airbrushed on the canopy inside, Tamiya do a few clear colours and I'm sure there is a blue one.

 

An oven in the modelling area, that could lead to a fridge, kettle, microwave, bed......

 

Steve

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On 10/15/2018 at 4:47 PM, Marklo said:

The 'Jig' is just heavy card CA's together and is used to set the diehedral, Using Brass gives you a bit of scope to bend it into shape if the adhesive sets a a little off.

3

I decided to follow Marklo's advice and create a jig for my Twin Astir. I fired up Blender and created a little jig kit that my cnc milled from a 1mm styrene sheet.

2018-10-23%2012.16.23_zps1neai4ip.jpg

I then separated and cleaned up the parts before I glued my jig together.

2018-10-23%2013.25.30_zpsif1yi9iz.jpg

The only parts of the aircraft that I have glued together so far are the two stabiliser halves. The rest are still separate but the jig made it possible for me to bring all the parts together and get an idea of what it will look like when it is finished. So here they are just resting against the jig.

2018-10-23%2013.24.56_zpsjkuzxy5v.jpg

The reason why I haven't glued the two fuselage halves together yet is that I am waiting for the cockpit parts. I have created the seats and instrument panels in Blender and sent it off to Shapeways for 3D printing. They should hopefully arrive later this week and I will then be able to get properly started with the build.

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I am trying to plan how to attach the wings securely and correctly. I’ve been given some good advice by bandsaw Steve and marklo and I have bought some brass tube but my big concern is the alignment of the wings.

 

Below is my plan, please let me know if this would work or if there is a better way to do it:

 

1. CA glue 1.2mm Albion Alloys brass tube into the wings. Position it roughly around max camber of the wing. It will function as a spar as well as the attachment to the fuselage.

2. Open up a hole in the fuselage around max camber of the wing. Fuselage halves not joined at this point. Hole a good bit larger than the brass tube.

3. Slide a section of 1.6mm brass tube over the tube that is glued into the wing.

4. Place a ball of fresh miliput on the inside of the fuselage half.

5. Place the fuselage in the jig.

6. Align the wing in the jig and push the brass tube into the miliput.

7. Wait for the miliput to cure and it should now be possible to take off the wing while working on the fuselage.

 

Will the milliput stick to the fuselage and the brass tube or do I have to use CA to glue it in place afterwards?

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Sounds worth a try. Milliputt is amazing stuff. I think it will probably stick if you score some cross-hatching into the relevant mating surfaces, be they plastic or brass. Can’t guarantee anything though. 

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4 hours ago, Bandsaw Steve said:

Sounds worth a try. Milliputt is amazing stuff. I think it will probably stick if you score some cross-hatching into the relevant mating surfaces, be they plastic or brass. Can’t guarantee anything though. 

Thanks. I'll try the scoring!

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2018-10-26%2011.13.04_zpsgoq2ebci.jpg

Two male canopy moulds done in the cnc.

I will follow BritJet's advice. The two canopies from the cnc will be lightly snaded primed and polished to a near perfect finish. They will subsequently be used to produce female moulds using BritJets method that he used for his Prentice.

I thought it would be a good idea to make two as a backup should one of the female moulds fail. It would also be handy to have at least two female moulds on the vacuum forming machine at the same time in order to save time and material.

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Thanks for the nice feeedback. but to quote Oscar Wilde, 'talent borrows, genius steals', the Curtis float plane by Greggles.w Here on Britmodeller is where I borrowed,stole, found my inspiration for the jig, his jig (like yours) is probably at lest as good as the model he's using to make it with, mine really is as rough as a badgers backparts :)

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This is really looking good. 👍

 

Would it not be easier given the small scale, to make the wing joiners from plastic tube with a piano wire insert?.

Carefully mark the wing root position on each fuselage half, and drill two holes to carry a plastic tube. One hole corresponding to the max thickness of the wing, and the other, smaller diameter, near the trailing edge. Once the fuselage halves are joined, fit the plastic tubes through the fuselage and sand flush. 

Once the wing halves are glued together, pack the root end for about 12mm with Milliput.  Fit both the fuselage and one wing into the jig already constructed, and take a small daimeter drill bit through the fuselage tubes to bore into the Milliput in the wing.

Repeat with the other wing and when ready, pass a length of piano wire through the fuselage tube and into each wing.

A 3mm wire at the main max thickness position and a 1mm wire at the trailing edge should be more than adequate to hold the wings securely, and allow them to be removed for any reason.

 

Just a thought. 

Chris.

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On 10/26/2018 at 5:05 PM, Ossington said:

I case you don't know, never try to glue milliput with superglue until it is absolutely solid. If you don't, it will go yukky and never set properly. Not going to do a 1/48 Vigilante oneday? Hint. 

Thanks! I did not know that but now I don't have to do that mistake!

On 10/26/2018 at 11:26 PM, stringbag said:

This is really looking good. 👍

 

Would it not be easier given the small scale, to make the wing joiners from plastic tube with a piano wire insert?.

Carefully mark the wing root position on each fuselage half, and drill two holes to carry a plastic tube. One hole corresponding to the max thickness of the wing, and the other, smaller diameter, near the trailing edge. Once the fuselage halves are joined, fit the plastic tubes through the fuselage and sand flush. 

Once the wing halves are glued together, pack the root end for about 12mm with Milliput.  Fit both the fuselage and one wing into the jig already constructed, and take a small daimeter drill bit through the fuselage tubes to bore into the Milliput in the wing.

Repeat with the other wing and when ready, pass a length of piano wire through the fuselage tube and into each wing.

A 3mm wire at the main max thickness position and a 1mm wire at the trailing edge should be more than adequate to hold the wings securely, and allow them to be removed for any reason.

 

Just a thought. 

Chris.

Interesting idea! I might try that. I haven't tackled the wings yet so the attachment method is not decided yet.

 

The only progress during the last few days is that I took delivery of my cockpit parts (2 panels and 2 seats). I designed them in Blender:

cockpit%20parts%20render_zpsc4i3how9.jpg

 

And this is what they look like after being 3d printed:

2018-10-26%2018.32.55_zpsqpkifns7.jpg

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  • 11 months later...
3 hours ago, OutcastJoel said:

Did this ever finish? Some absolutely stunning work going on in here!

No, I never finished it but I will hopefully get back to it in the near future. Thanks for asking and giving me the reminder! 😁

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3 hours ago, BritJet said:

I would like to see you get back to this and finish it as it would be a shame to have got this far and not see it through to the end.

 

Steve

Same goes for me too.  👍

 

Chris.

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