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Old Jul 15, 2014, 08:34 AM
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Northamptonshire, UK
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Royal Marutaka B-25 Mitchell Scratch Build

Having looked around for a B-25 kit for a reasonable price and not finding one, I have taken to building one from the freely available Royal plan.

After posting a request for help for some of the formers that are missing on the plan, and for the original building instructions, I am now ready to start building.

I am not a speedy builder, and have more than one project on the go (although this one is the only scratch build on the bench)., so please don't expect it to be complete any time soon...

I plan to trace the formers and the like using CAD, and generate DXF files that I can use to create nc files for cutting on my home CNC router that I built last year.

I had the chance of an Aerotech kit for 230, but declined as I saw too much plastic and foam. I hope I do not regret that decision

My first hurdle is to re-trace the components with balsa sizes in mind as the plans are a bit hit-and-miss in this respect, and I don't want to have to be filing slots etc when building.

First up is building the inner wings/Nacelles along with modifications to allow the use of "normal" retracts, and to allow the wheels to retract further into the nacelles...

Edit: Apologies for the word "scratch" in the title, as it's obviously not a scratch build, but a build from plans.
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Last edited by geoff.sim; Jul 18, 2014 at 06:20 AM. Reason: Wrong title
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Old Jul 15, 2014, 08:54 AM
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Moab, Utah, USA
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Looking forward to following this build. The B-25 has always been one of my favorite WWII planes. Which version do you plan to build?

Larry
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Old Jul 15, 2014, 09:16 AM
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Northamptonshire, UK
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Plan

Hi Larry.

Yes, I agree. This is a model that I have wanted for some time. However, I kept away from twins when I used i.c engines.

I have attached the plan and original build instructions (thanks to glen_j). These plans are stand-off scale I believe.
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Old Jul 16, 2014, 02:08 AM
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kent, uk
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Glad to see you're making a start Geoff, i look forward to seeing the build
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Old Jul 16, 2014, 03:24 AM
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I have ordered some balsa and ply stock. In the meantime, I am busy tracing (and modifying) the formers using Photoshop and TurboCad.
Photoshop is used to create images of the parts I am interested in, and TurboCad is used to trace the parts and create .dxf files.

I import the .dxf files into a program called CamBam, which allows me to create .nc files that are used by my CNC router software, Mach3. This in turn controls my CNC router, that cuts out the parts on a cutting table. My cutting table has a cutting area of 560mm x 320mm, and can cut materials up to about 1/2" thick, albeit in many passes.

So, the process for each part is...

PDF Plan -> Photoshop -> TurboCad -> CamBam -> Mach3 -> CNC Router.

What a palaver. However, the results are worth it. Also, as this model is a twin, cutting multiples of the same former is a breeze. For example, to create the two nacelles, I have to cut multiple complex plywood formers (2xW16, 2xW17, 4xW18, 2xW19, 2xW21) accurately, in lite-ply and ply. I expect to be able to cut these 12 formers on my router in about an hour and a bit (ok, it took me days to create the .nc files ).
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Old Jul 16, 2014, 07:21 PM
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San Jose, CA
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I take it Turbocad can't import .pdf files directly? Autocad can, which is what I use..

SteveT.
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Old Jul 17, 2014, 02:24 AM
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TurboCAD was cheap on eBay! TBH it is fine for what I am using it for. The main issue is in keeping the scale across software components. I find I have to use 72dpi for TurboCAD to import without me having to re-scale.
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Old Jul 17, 2014, 02:59 AM
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Nacelles

While working out what W18 looks like, I found that this former hangs the engines on the wings with a 2 degree down thrust angle - something to bear in mind when constructing the center sections of the wing, to make sure both left and right are the same.

I used Google Sketchup 7 to import the dxfs and test fit before cutting out. Lucky I did, because the formers seem to need adjusting to get them to fit together properly.
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Old Jul 17, 2014, 08:07 AM
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Hey Geoff,

I'm following with great interest, as I've always been fascinated by the B-25. Thanks for posting the plans and manual in your other thread; they're now in my extensive collection of "one of these days" future builds. In the meantime, I'll enjoy watching yours!

Looking at what you've drawn up in Sketchup, I'm going to have to learn how to use that package. I'm designing in CorelDraw, but every now and then a little 3-D visualization would go a long way...

Steve
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Old Jul 17, 2014, 12:11 PM
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I have done some more work on the nacelle area.
It is getting there, but there are still issues with the formers F19 and F17.
I added the ribs to check that they fit ok, especially the F5 pair.

Not sure why I am getting so many problems in this area. Either the plans are not very accurate, or I am tracing them incorrectly. I have took to tracing on the centreline of the drawn line on the plans. Maybe this is incorrect and I should be assuming that the author has traced these formers onto the plan, and that I should be tracing on the inside line of the drawn line? Even then, I would not expect to have so many alignment issues. Maybe someone who has built one of these kits could comment?

Once I can get them all to line-up/fit, I intend to look at modifications on the retracts area...
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Old Jul 18, 2014, 01:42 AM
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Here's W18 Geoff
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Old Jul 18, 2014, 03:16 AM
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Cheers for that, Glen. It's pretty much as I guessed with the exception of that little "bump" in the slot. They are also on W19. I wonder what they are for?

Edit: A load of balsa/ply just turned up. If I can sort out these formers, I may be able to cut them out this weekend...
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Old Jul 18, 2014, 05:54 AM
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The bump is for the landing gear leg if you are using retracts you can omit the bump
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Old Jul 18, 2014, 06:18 AM
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Quote:
Originally Posted by glen_j View Post
The bump is for the landing gear leg if you are using retracts you can omit the bump
So they are! I will not include them as I intend to redesign this section to allow "standard" retracts.
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Old Jul 18, 2014, 06:49 AM
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Ilkley, West Yorkshire, UK
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I'm along for the ride too! Always liked the B25 - I grew up calling it a Mitchell, because my late father (Blink Snr) told me that he had always hoped to get posted to a Mitchell squadron in WW2 - at one point he was presented with paperwork posting him to a Marauder squadron which concerned him a little!!!

Quote:
Originally Posted by geoff.sim View Post
I used Google Sketchup 7 to import the dxfs and test fit before cutting out. Lucky I did, because the formers seem to need adjusting to get them to fit together properly.
By gum, chum! Didn't know you could do that!! I use DraftSight which is pretty good (ie free) and I am now reasonably comfortable with it (and it's free). Not only is it free, but seems to be able to save stuff in a number of Binary and ASCII .dxf formats (no idea what all this means, mind you!). I need to look into this, because Sketchup is free.....

EDIT: What's the wingspan?
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Last edited by Colonel Blink; Jul 18, 2014 at 06:56 AM.
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