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Australian NR in G - Scratchbuilding. - More Practical Help - Your Model Railway Club
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 Posted: Sun Nov 27th, 2011 09:10 am
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Petermac
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It gets drawn into the joint by capilliary action DD.  That's where punching the holes in the rear lamination comes in - you put a drop of glue in each "hole" so you get a bond over the full sheet area.

Warping is however, always a problem during the drying process and it must be held flat until thoroughly dry.



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 Posted: Sun Nov 27th, 2011 11:39 am
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Petermac wrote: It gets drawn into the joint by capilliary action DD.  That's where punching the holes in the rear lamination comes in - you put a drop of glue in each "hole" so you get a bond over the full sheet area.


That's the first time I have heard of that one Peter, sounds like a really good tip.



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 Posted: Sun Nov 27th, 2011 11:45 am
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Petermac
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It seems you're never too old to learn Bob.

Actually, I'm pretty sure I learnt that tip on here.  I wonder if it was one of Perry's "masterbuilds" .............:roll::roll:  Or maybe it was from one of my boat building friends ...............:hmm



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 Posted: Sun Nov 27th, 2011 12:06 pm
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Wayne Williams
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Petermac wrote:
Actually, I'm pretty sure I learnt that tip on here. 

I can't remember exactly which build I used those holes, but I can tell you they worked great. It does take some time to drill the holes but it's worth it.

One thing to remember about the liquid solvent for styrene. It causes [one side] of the styrene to melt and the other side stays dry. So the inherit physical properties of the sheet are changing. That causes the warping. By NOT applying the solvent in every spot along the edge that you can get to will help reduce the warping.

When I apply the solvent on say a three inch square of styrene, I will only apply it on the ends and a spot in the middle on one edge, and on the opposite edge, I apply it on four equally spaced places. That way I don't have a straight line between spots on the 3" square, that the piece can curl around. One more thing, it is very easy to apply too much solvent, if you do that, it's just like applying it along the entire edge. That capillary action works way better than you realize.

Does it totally stop the warping, NO, but it does dramatically reduce it. As Petermac said, I always lay the part being bonded down flat and lay a heavy object on top for at least 30 minutes. On really large parts, I leave it over night.

Wayne



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 Posted: Sun Nov 27th, 2011 12:58 pm
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Petermac
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I just knew I'd read it on here somewhere Wayne - sorry I forgot it was your idea. :oops::oops:

I was going to have a look tonight to see if I could find it - now I'll have time for another glass of Bordeaux instead.



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 Posted: Mon Nov 28th, 2011 08:02 am
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Max, this is awful, when I saw 'NR' I thought 'Goodo...some more!'

Re cutting plastic sheet, please consider this, I have used it successfully in 1/4" perspex for boat windows etc, even cutting round curved aluminium templates. An ordinary 'all hard' hacksaw blade does for the smaller stuff, but if you can scrounge a worn-out machine hacksaw blade you'll be much better off. The side clearance is the important part.

When cutting large pieces from stock sheets of 6ftx4ft 1/4" perspex the piece was clamped down with the straight edge and a respectably deep groove cut, then moved over the edge of the bench and 'snapped' by two people, the bulk of the material being clamped right on the edge of the bench.

Good luck, chin up!

Doug



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 Posted: Mon Nov 28th, 2011 08:03 am
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Sorry, forgot link!

http://yourmodelrailway.net/view_topic.php?id=5042&forum_id=11&highlight=scrutter



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 Posted: Mon Nov 28th, 2011 08:12 am
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Thanks, Doug.  I used a proprietary brand cutter or draw knife similar to the one you made.

I found that when I snapped the piece off, it left a lip along the edge which needed to be finished off.  I used a sanding block, but finishing by hand left an uneven edge.  I tried cuting right through with the draw knife, but I found that it jammed in the cut.

The problem is that the model is 850 mm long.  This makes very long panels and that's where the challenge arises.

It's nice that others are thinking about it.  :thumbs



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 Posted: Mon Nov 28th, 2011 08:58 am
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Petermac
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As an alternative Max, have you thought about using high grade ply for the large side panels ?

Plasticard is great stuff but it does have problems when it's glued.  As Wayne pointed out, the glue tends to introduce stresses on one side only.  Even when not laminated, you can often see a slight "kink" where inner strengtheners have been glued on.

John Flann seems to have had plenty of success glueing plasticard to timber so that's perhaps not a problem.

Also, didn't RJR build a large scale loco on here some time ago ?  I think he used thick plasticard rather than laminations - it might be worth a search to see if that thread is still here.



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 Posted: Mon Nov 28th, 2011 09:03 am
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Yes, Peter.  John used styrene for his G scale models.  They were a bit smaller than mine.  It's the panel size that's the killer.



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 Posted: Mon Nov 28th, 2011 09:19 am
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Petermac
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That's why I suggested ply Max - just for those huge side panels.

The rest could be plasticard - a superb product for the finer detail but, IMHO, a PITA to keep flat and straight in large sections.

I'll be watching developments with interest.  A problem like this is simply a hurdle to conquer.  I, on the other hand, am no conqueror so, as with things "athletic", I'm prepared to watch others exhibit their skills from my armchair !!! :cheers



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 Posted: Mon Nov 28th, 2011 09:31 am
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MaxSouthOz
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Two things worry me about the ply, Peter.  Does it come in 2 mm?  And how will it go when I have to scribe the detail into the surface?



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 Posted: Mon Nov 28th, 2011 09:58 am
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Petermac
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It comes in as low as 1mm here Max.  You do have to buy it from a model shop or somewhere like that and not a builders merchant.

The quality of "Marine" modelling ply is extremely high with super-smooth surfaces and resin bonded plies.  I had imagined you'd use the ply as a sub base and glue on the detail rather than scribe it in.  I'm sure if you just "roughed up" the areas necessary to take the various grills etc., that would give you an excellent key for the glue.

It has always worried me a little when it comes to glueing one product to a different one but John Flann reckons, with modern glues, he has had no problems at all.

My mind suggests that the ply might need some kind of primer/sealer because, being a "natural" product, it will always "move" slightly whereas the plasticard is totally stable, but it may not in fact,  be necessary.

It might be worth some experimentation.............:hmm



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 Posted: Mon Nov 28th, 2011 10:55 am
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Food for thought, Pierre. :hmm



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 Posted: Mon Nov 28th, 2011 11:01 am
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I don't use these sizes or thicknesses of material (I max out on 2mm) so I'm talking out of my hat.
Is it possible to find appropriate thicknesses of styrene sheet and shape them afterwards?
From experience, styrene accepts glass paper and fine files quite well.
Just blow a raspberry if this is a silly idea.



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 Posted: Mon Nov 28th, 2011 11:41 am
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Petermac wrote:
whereas the plasticard is totally stable

I don't think plasticard is as stable as people believe it is. It is made from styrene and styrene has a coefficient of expansion via temperature. As the material heats up and cools down it expands and contracts. The amount it moves is directly related to it's thickness and overall size. The thicker it is and the longer it is, the more it moves.

Try taking a piece of styrene, measure it's length carefully, note the ambient temperature, then place it in the refrigerator, leaves for an hour or so, then remove it and immediately measure it again. It should shrink.

Conversely, set it outside in the sun on a warm day, heat it up for an hour or so, then do the measuring, bring it inside and place it in the refrigerator, this process will give you the maximum movement on the styrene sheet.

It is an interesting test to do, and does get the brain to thinking.

Wayne



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 Posted: Mon Nov 28th, 2011 11:47 am
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Petermac
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I would do DD but they're out of season just now ....................:lol::lol:

I have no idea what thickness of plasticard is available but I'd imagine, judging by the prices for "our" type of card,  the cost of stuff thick enough for Max's job would be frightening.

Also, I wonder if there's a technical reason it's only normally seen in relatively thin sheets.  That "warp factor" may creep into the manufacturing process if it got too thick.

Doug's perspex might be an alternative - that does come in thick sheets as does normal "plastic".  It might be the styrene that's the problem.  Whilst I'm no chemist, I'm sure that carbon atom combines to take on many different forms ..............



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 Posted: Mon Nov 28th, 2011 12:10 pm
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Petermac
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You're right Wayne - I hadn't thought of temperature changes.  I should have said "relatively" stable vs timber.

I was thinking more along the lines of humidity.

I'd guess that, where Max lives, temperatures would fluctuate relatively slowly whereas humidity change is normally very rapid - one minute it's "wet" because it's raining, the next, it drops like a stone when the sun comes out again.  In my experience with model boats, timber is as good as any hydrometer for giving an idea of being either wet or dry.



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 Posted: Mon Nov 28th, 2011 06:58 pm
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Hi Pete.  I have a sheet of 2mm and a sheet of 3mm - both 1.2m x 1.2m, so I have to persevere with them if I can.

I think Wayne is right about the coefficient of expansion and contraction.  My feeling is that I need to design in a way of taking that into account.  In the same way that railways deal with the expansion and contraction of rails.

It's turning out to be an interesting discussion.



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