Laser Etched and Cut Salisbury & Dorset Road Bridge in 4mm scale for Verwood, Dorset.

[Steve Smith]

Verwood Station was somewhat inconveniently sited on the Cranborne road, quite a distance from the village of Verwood.  When the railway was built the road, that became the B3081, was diverted over a standard design Salisbury and Dorset Junction Railway brick arch bridge.  It seems that the Albion Inn was built fronting the old road with the knowledge that a railway station was planned at that location, but ended up in the station yard with the diverted road running behind it.  After the closure of the railway, the bridge was eventually by-passed and the road once more runs in front of the pub.   Incidentally, the sainted B3081 winds all the way from Ringwood to Prestleigh (near Evercreech), close to where I now live!   Happily, the bridge (and pub) survive, and have been the subject of many useful photographs.  Last year I was able to survey it as it is now in the garden of the Albion Inn, see Disused Stations - Verwood.

 

My first excursion into designing for laser cutting and engraving was the LSWR Type 1 signal box, and that was definitely problematic, and I ended up producing a much more accurate model using 3D printing, now in test build.  The problem with laser cutting and engraving is that actual material thicknesses vary greatly, sometimes in the same sheet, this is particularly true of MDF.  There is also no standard for the colours and line thicknesses in the drawings used.  I wished to be able to cut and raster engrave, and that proved beyond most laser cutting software, that manage to corrupt and change the files that you import.   

 

Anyway, my signal box had been cut by DCC Trains Automation using a Trotec laser, and that had proved that these things could perhaps be done, so after my survey visit, I embarked on a design for the bridge, and these are scans of two of the resulting etches.  There is one other etch for the underneath of the arch, that also has filler pieces for what I'm going to call the string course below the parapet.  The arch is correctly mainly in stretcher bond with some rows of headers to bond the arch.   As a bit of an enthusiast for traditional building techniques, I was keen to reproduce the brickwork as accurately as possible.

The brickwork sheet is in 1.5mm MDF, but MDF of this nominal thickness is quite often 1.6mm.  The carcass is 2mm MDF, and this is quite often a bit less at 1.9mm!   The arch liner is in 0.5mm PresspahnK, and I really wish this brilliant material was available smooth faced in the sort of thicknesses that MDF comes in!

 

And this is the carcass built up - it's only actually glued (Rocket Card Glue) at the ends, the slots and tabs were all a lovely fit.  The frame reproduces the crown of the road across the top, but not the camber without extra work.  Next up, assembling the brickwork etches.

[Steve Smith]

Below is the assembled bridge with the first coat of shellac in the form of some white knotting. 

All the arch faces and abutments were dry fitted using spring clamps, so that the joints were all nicely aligned and the abutments tight up against the bottom of the arch.  I then glued the pieces in one at a time, using the neighbouring clamped or glued pieces to locate them accurately.  The parapet was epoxied into place using a jig to keep everything straight, otherwise only Roket Card Glue was used.  The deeply etched bricks had an inclination to spall off with heavy-handed treatment, and some repairs were needed using narrow strips of card.  The coat of shellac should fix that.   The suction on the MDF is prodigious, and a second coat of shellac will be applied later today.    Once that is dry, the plan is to paint it brick coloured, and once that's fully dry then I'll apply the mortar paint, wiping off the surplus.  After that dry brushing to restore the colour, and picking out of individual bricks, bearing in mind that the headers in East Dorset are often 'blue'.  

 

Brickwork, to mitre or tooth join, that is the question.  Mitres are all very well, but laser cutters like the Trotec can't do them.  They can be done in a jig, and this is OK for simple box like objects.   For this more complex model, I opted for comb joints.  Results for comb joints can be mixed to say the least, partly because we never know exactly how thick the next piece of MDF will be, and also due to the kerf (gap) left by the cut.  The kerf for something like this can be reckoned to be 0.1mm, and we can adjust for that, but having done that, why did my joints still look wrong?  My take was that the engraved joints hadn't been carried across the face of the brick and then down the ends, so that's what I've tried to do here.

 

[Blandford1969]

Looks incredible

[DGO]

13 hours ago, Steve Smith said:

Brickwork, to mitre or tooth join, that is the question.  Mitres are all very well, but laser cutters like the Trotec can't do them.  They can be done in a jig, and this is OK for simple box like objects.   For this more complex model, I opted for comb joints.  Results for comb joints can be mixed to say the least, partly because we never know exactly how thick the next piece of MDF will be, and also due to the kerf (gap) left by the cut.  The kerf for something like this can be reckoned to be 0.1mm, and we can adjust for that, but having done that, why did my joints still look wrong?  My take was that the engraved joints hadn't been carried across the face of the brick and then down the ends, so that's what I've tried to do here.

 

 

Your assertion that mitres cannot be cut with the likes of Trotec lasers is tecnically not accurate, it is possible to laser cut at an angle if the laser cutter has the added hardware uually used for working on cylindrical objects, however I am only aware of one company currently using such techniques and they don't use it for mitres but instead use it for adding brick detailing, that would be intentio model railroads https://www.intentio.shop/index.php?route=common/home