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Author Topic: The Tora Bora Line  (Read 493 times)

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Offline Ollie3440

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Re: The Tora Bora Line
« Reply #15 on: January 11, 2018, 05:02:47 pm »
Afternoon.

Will be watching this layout progress with great interest. That bridge is mightily impressive and I'm keen to see how the rest progresses. Nice one!

Cheers

Ollie

 :NGaugersRule:
Hi I'm Ollie, and I'm a addicted to buying MK1s......

My Previous Exhibition Layout - The Sheaf Valley Railway
http://www.ngaugeforum.co.uk/SMFN/index.php?topic=24.0

My Current Exhibition Layout - Wenlock
http://www.ngaugeforum.co.uk/SMFN/index.php?topic=23228.0

Offline Dizz

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The Tora Bora Line - Royal Albert Bridge - 1
« Reply #16 on: January 14, 2018, 06:46:31 pm »
Since 2008 I had been designing radio controlled model aircraft using AutoCAD and making the parts on a friend’s CNC router and laser cutter.  In August 2015 when I “retired” I treated myself to a Chinese laser engraver/cutter in anticipation of being able to spend more time building models.  I finally got it installed and at least functioning in the man cave in February 2016.  Then followed a frustrating 3 months setting it to work properly and gaining operating experience.



The first issue to overcome was that the control panel was in Chinese!



With some lateral thinking I managed to change language first to Danish and then English so I was then able to start setting parameters.  The included instructions were next to useless, but as I already had a good knowledge of operating the other laser I at least knew what questions to ask on Google.  Every step was dogged by some frustrating minor issue that took several days to diagnose and rectify.  Anyway, by the end of May the laser just about did “what it said on the tin”, but still left much room for tuning to truly get the best out of the machine.  This coincided with me returning to work and also with requiring a solution as to how the model railway project would tackle the door gap.

Online port perran

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Re: The Tora Bora Line
« Reply #17 on: January 14, 2018, 06:59:46 pm »
Wow. That’s some machine and way beyond my levels of comprehension.
I thought it was some kind of modern tumble dtyer with that big blue pipe comng out of the bottom!
By the way, I notice that you are just up the road from me.
It's you railway so build it as you want and run whatever you like. The only rule is - ENJOY :
My Layouts -
Port Perran:- Trepol Bay:-

Offline Dizz

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The Tora Bora Line - Royal Albert Bridge – 2 Inspiration
« Reply #18 on: January 14, 2018, 07:00:24 pm »
I drive over the Tamar road bridge at least 4 times a month when traveling to/from Torbay and every time I am impressed with Brunel’s Royal Albert Bridge masterpiece.  One trip in July 2016 it occurred to me that I might now be able to laser cut my own version of the Albert Bridge to carry the railway across the door gap.  I looked at some photographs when I got home and came up with a way I could construct it.  I then played with scaling a side elevation drawing in ACAD to see how it would fit/look.  It was clear that if I used 2 spans as per the real one the bridge would not get anywhere near 1:148.  A single span scaled to the width of the door would be better, still not 1:148 but close enough that it could “look right”.  There would obviously need to be a certain amount of artistic licence required anyway (not least in that it would have to carry a double track) so I continued tweaking the scale of the side elevation in ACAD until I was happy.     



My laser has a 700x500mm bed, but with allowances for carriage overrun the actual maximum cutting dimensions are approx. 650x470mm.  Having finalised the scale of the bridge side elevation it was a case of deciding how I could make it.  I buy sheet material in 600x300 and 600x400mm sizes and the Balsa is 915x100mm or 915x75mm.  The next stage was to manually tracing over the side elevation to generate the framework, design the cross braces, design the bridge deck, project and draw the layers for the balsa support tube and design the end towers.



Offline Dizz

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Re: The Tora Bora Line
« Reply #19 on: January 14, 2018, 07:05:28 pm »
Wow. That’s some machine and way beyond my levels of comprehension.
I thought it was some kind of modern tumble dtyer with that big blue pipe comng out of the bottom!
By the way, I notice that you are just up the road from me.

Think of it as a special sort of printer, because effectively that's how the computer treats it.  The blue pipe is for smoke extraction.
Yes....saw you at the Hayle Christmas show..........not that you would have known who I was  :beers:
(I was with Mike and James in the afternoon)

Offline Dizz

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Re: The Tora Bora Line - Royal Albert Bridge – 3 Laser cutting
« Reply #20 on: January 14, 2018, 07:34:20 pm »
Once all the drawing is complete I group the various parts by material and lay them out to minimise wastage. 



Although there is a add-in for AutoCAD that allows me to send the drawing direct to the cutter I prefer to run through a further step of importing the vector drawings (*.dxf), applying laser settings and saving as a dedicated cutter file for the machine.  That way I have more control to be able to change laser settings on the day and select individual parts for cutting if required.



As the ply and wood are natural products there is always a slight variation in density from sheet to sheet and even across the same sheet.  In addition the laser wavelength is attenuated by moisture in the atmosphere and as the light path from tube to material can vary by up to ~1100mm the laser power has to be slightly adjusted from job to job to get the best cutting results.  The machine is set for the laser to cut along the line and I always aim to use the minimum power required at a cutting speed fast enough to just produce a clean cut all the way through the material with a nice golden colour edge to the cut.



As an aside and keeping it simple, there are 2 parameters under my control which will determine the quality of the cut: laser power and cut speed.  Ramping up the power increases the width of the cut, but higher powers greatly reduce laser tube life: it is not a straight line relationship either, there is an exponential factor. My laser has a nominal 60W tube (£480 to replace) and the anode current limit I apply is 22mA which equates to 73% power control setting.  At that power the tube should last around 1000 hours actual laser firing time.  If I ramp it up to 100% and 30mA I would be lucky to get an hour out of it.  Most of the work I do is between 25% and 38% (7-12mA) and hopefully that will equate to around a 10,000 hour life. 
Slowing the head speed allows more time for the beam to cut through, but it also means an overall wider cut, allows more time for the edges of the cut to be charred and on thicker material gives a slight coning angle to the edge (wider cut at the top).   
I have built up a table of settings for different thicknesses of different materials to use as a starter, but I always do a test cut on the day and adjust as required before starting a job.
« Last Edit: January 14, 2018, 08:14:28 pm by Dizz »

Offline Dizz

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Re: The Tora Bora Line - Royal Albert Bridge – 4 Construction
« Reply #21 on: January 14, 2018, 07:51:59 pm »
I admit that the way I decided to replicate the side suspension lattice work is not the most efficient use of the plywood with a lot of waste, but it is the easiest way of constructing the structure.  I also admit that having drawn it I knew how it was intended to go together so was able to crack on building without having to identify parts or get my head around any instructions.









If I remember correctly I think I was able to glue all the main components together in under 2 hours.  The glue was left to set overnight.  The next day I shaped the suspension beam to shape with a razor plane and assorted sanding tools.  I gave the balsa a couple of coats of sanding sealer and rubbed down then glued the main structure together.  I attached the deck with 1M x5mm screws so it is possible to disassemble if access to the track is needed.



I sprayed my bridge with rattle-can grey primer, which I think is actually a little darker than the paint on the real one at the moment.  The “I K BRUNEL Engineer 1859” on the towers is etched by the laser and about 1mm deep.  I left the paint a few days to harden off properly then filled the lettering with light weight white model filler.  I left it 10 mins to start hardening then wiped over with a damp cloth to remove excess and get a nice crisp edge to the letters. 

Offline Dizz

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Re: The Tora Bora Line - Royal Albert Bridge – 5 Wiring
« Reply #22 on: January 14, 2018, 08:09:25 pm »
The bridge is located in place with brass furniture dowels as used for base board alignment. 



I realised that they were not suitable to also act as electrical connections and my first intention was to connect the bridge track to the DCC bus with a plug and socket hanging down below the deck.  However this did not seem to be a very neat solution and I came up with the idea of using copper pcb pads and phosphor-bronze spring clips.  I canned this idea because I couldn’t find any suitable metal and also the clips would stop the bridge from seating down properly on the supports.  Luckily though in my box of model aircraft spares I came across a canopy latch and it occurred to me that it would be ideal as a sprung electrical contact.  The canopy latches had dropper wires soldered to them and were epoxied into holes in the bridge supports at a suitable depth.



I heavily tinned the pcb before I glued it to the underside of the bridge deck so there was a little “give” for the sprung pin to push into. 



When the bridge is pushed into place it sits down correctly on the supports and my multimeter shows zero resistance between the main tracks on either side.



My track is pinned place, but I suppose it could have been glued.  One day I will get around to weathering the bridge and track.




Online weave

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Re: The Tora Bora Line
« Reply #23 on: January 14, 2018, 10:13:02 pm »
Hi,

All looks amazing. I will have to go back tomorrow when less Sundayed and can concentrate properly on what you've done.

Great stuff.

Cheers weave  :beers:

Online Mito

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Re: The Tora Bora Line
« Reply #24 on: Yesterday at 10:23:55 pm »
Fascinating! Something I would love to do but I have more than enough on my plate now. Can we have a photo of the bridge in position to be able to appreciate it in its entirety please?
You know you're getting older when your mind makes commitments your body can't meet.

Offline Dizz

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Re: The Tora Bora Line
« Reply #25 on: Yesterday at 10:59:40 pm »
I will get some photos tomorrow evening, in the mean time here is the only one I have on my phone.


 

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