More on viaducts

Things which I'd think about:

1) Vehicle overhangs means that they might strike the viaduct walls. On a fairly tight corner the overhangs on vehicles can be considerable. I would measure this carefully (both inside and outside overhang), and then allow a bit more because models sometimes come out over-scale(*), and stock will "hunt" on the track and within sloppy journals to some extent. 4ft radius is fairly generous, but you'll still have some overhang.

2) Front face. I dislike hardboard. Its cheap, but that's about it. You can get very thin plywood (1mm!) which curves easily. I might use foamboard (a bit thick for the above trackbed walls), which can be bought from art shops (expensive) and sometimes scrounged from old shop window displays (free).

3) Stone/brickwork. If planning to scribe it, then usual technique is to apply DAS modelling clay in very thin layer to front face (often with aid of PVA to help sticking). Then scribe with dental probe, old shaped nail in block of wood, or similar. Some of the very very best models are made this way. Alternative might be large acres of plasticard embossed brick, or even printed brick papers if the viewing distance is appropriate (could always write software to generate brick patterns and fill colours in Corel Draw (or similar) and produce any pattern required on home printer).

(* I have a friend who built a beautiful small bridge who now has to cut it down the middle and widen by about a milimetre to clear the stock he built for the model. )

- Nigel

On Sun, 15 Jun 2008 19:36:52 +0100, "Nigel Cliffe" said in :

Yup, the curves are being double-checked as we speak. This is one reason I decided to stick with double and not quadruple track round some of the layout, the overhangs are a pestilential nuisance. Luckily 4' is the smallest radius on the layout, the old one had 3' curves which were much more of a problem.

I'll see if I can find that at my woodyard, but the HB is OK in my experience, stable as long as you seal it properly. This is in an outbuilding, though, so may get damper than I'd like.

Looking around, I can find 3mm flexible ply in 8 x 4 sheets for a reasonable price, that's the way to go I think. I kind of wish I'd built the main curves with that, I used 6mm because it's what I had (and what the local timber yard sells) and it does work, but it was hard work curving it.

I'd forgotten this trick, which is perfect for the job. Thanks. I remember now that you can roll the clay really thin, onto board, and then roll it for the inside of the arches. I've found some instructions for that. A massive improvement over what I was thinking of doing!

Great tips, thanks.

Guy

posting.http://www.chapmancentral.co.uk>

I built a bridge which needed a curved exit ( it was removable for access to other parts). I quickly discovered the need for retaining walls to prevent major damage due to accidents and the clearance problem ( 1M radius). The thing which really caused grief was the entry/exit to the curved end . This eventually came good when I banked the track in a more prototypical manner. This eventually cured problems in other places too. The main lesson was not to "finish" the job until thoroughly tested ( it is still not decorated!)

Regards

Generally on prototype viaducts the arches were the same radius front and back, and the curve was taken up by the pillars being slightly wedge-shaped. I hope this helps your calculations.

Sorry. missed the last post...

You could score the hardboard with one on those old-fashioned line markers - with a slightly blunter point... should keep you occupied for a day or two! _ __|_|_

On Mon, 16 Jun 2008 02:29:13 -0700 (PDT), Sailor said in :

Finished? Small danger of that. I live in uk.rec.sheds.

Guy

On Mon, 16 Jun 2008 12:44:16 +0000, beamendsltd said in :

Is a marking gauge, innit?

Guy

Dunno - but it's the first thing we had to make in woodwork at school!

Shortly before the dinosaurs died.

Cheers Richard

To make an arch that is a different radius on one side as compared to the other, one would need fractionally tapered bricks. Alternatively one could make a series of narrow parallel arches, each with a brick less than the one on it's larger side. Neither method is really practical. Of course if the span of each arch differs then the height of the arch will also differ, which introduces another geometrical difficulty. As the required radius decreases, the difference in length of the pillar sides becomes greater if one's arch is the same radius on both sides, until one reaches the point where the pillar ceases to exit on one side. I built a brick arched viaduct on 18-24" radius ("J" shaped) and effectively foreshortened the inside as against the outside, so that the lengths differed but the heights remained the same. The arch underside brickpaper was a very odd shape, (edges cut in-situae) but by making that in two halves with the odd shaped bricks at the top of the arch it looked ok. Luckily it was in from the edge of the baseboard so one couldn't get one's eyelevel to a position to look up under the tighter curved arches.

The structure of the viaduct was MDF deck and pillar faces with the outer surfaces made of old floor lino/vinyl tacked on.

Regards, Greg.P.

On Tue, 17 Jun 2008 08:53:56 +1200, Greg Procter said in :

Yes, I get this.

My plan is to build the arches parallel and bodge the far side of the pillars which will not be visible to the operator.

Guy

The main problem with that construction is that you will always know! =8^))))

Now that I think about it, why not do the under-arch brickwork on CAD generated paper on a home printer? The taper per row would be very small, but of course lots added together creates the noticable total error.

Greg.P.