I recently acquired a vintage FSM enginehouse kit based on John Allen’s award winning structure that was a centerpiece on the original G&D. While it appears to be a fairly faithful reproduction, I did notice one clear difference. Whereas the original had round arched doorways for the engines to enter through, pictures on the FSM box show a diagonal arch using straight cuts and trim. While I could construct it as is and have an outstanding structure, It would be nice to customize it to look like the original. Unfortunately, I have almost no scratchbuilding experience and cannot figure out how to go about making the round arched doorways. The problem is twofold. I must figure out how to cut a perfectly round semi-circle of the arch with a diameter the exact width of the doorway. And if I can figure out how to do that, I would need matching trim pieces with the same radius as that of the round arch. I’m guessing I would need some special tool but don’t know what that might be. Any suggestions would be appreciated.
Whether the front wall stock is wood or plastic, you could easily use a larger piece of wood as a base, and construct a jjg with small blocks and glue. That will allow you to use a compass to scribe the arch you want onto the door tops. A bar scribe with a sharp point might do your cutting for you. A much larger jig could be made so you can use a router to cut the arches.
A wooden jig, in long use, will probably allow the center hole to wear larger, allowing play and eventually slop into your scribing and cutting. A small piece of sheet metal flashing would extend the life of the center hole for a good while, where-as brass, being softer, will wear a little quicker.
An arch that’s just slightly smaller than the door width will be easier to sand smooth and look good, while an arch that’s too large will require opening the door width to match, and at some point, will be visible in comparison with other doors.
How you cut the trim will depend on how the arch and the wall containing it are to be finished, to look like wood or stone or brick etc. At some point, you are going to cut two arches, with coaxial centers, which differ in radius by how wide you want the trim piece to be. You might be able to use something soft and easy to cut like cardboard, stiffened with shellac or plaster, or balsa, and you might have to use something harder like styrene, depending on finish treatment.
The softer the trim material, and the narrower the finished arc, the more likely that cutting will crack or distort the piece. The sharper the cutting tool and the lighter the pressure on each pass, the less likely distortion will be.
Let’s assume the FSM model you want to rework is cuttable with an Xacto knife. Styrene plastic or wood counts. To cut the round arch doortops, I’d make a cutting jig, a piece of plywood or masonite with a half round opening of just the right size cut into it. Use this to guide your Xacto knife to score the material deep enough to permit the round cutoff to snap off. In fact I’d rig up something to secure the jig to the work, C-clamps or something like that. Have some spare #11 blades and replace the blade as soon as it dulls. That oughta get the doortops cut half round.
Next trick is finding or making a brick arch or maybe a cast concrete arch to dress up the newly cut opening. I’d look thru the Walthers catalog and visit Grandt Line’s web site and maybe find something the right size, perhaps a round top window frame that could be adapted. Or, try making the required round trim piece out of tiny brick sized bits of strip styrene. Perhaps make one good part and make an RTV mold and cast the rest of them out of casting epoxy. Or, cut the entire semi circular trim piece out of sheet styrene and score it with your Xacto knife to simulate brick work.
The problem with using a stencil for cutting is that cutting alters the stencil, worse with each successive cut. You could use a stencil for marking the piece, then cut freehand, but I think a better way woud be a fixed wooden jig, with a solid steel center screw or bolt, and a square box beam or rectangular bar of brass from the hobby shop to guide your exacto knife.
I’d practice with balsa or cardboard to dial in the radius and procedures before cutting actual model stock.
I like the idea of individual bricks or stones for the arch trim though. You expect a little irregularity in masonry work, and that would cover any less than perfect curved cuts. It’s a lot of work though. The same brass and wopod jig that cuts your front wall stock should be adjustable or alterable, with a second guide bar, to cut the trim perfectly, even from pretty thick styrene.
Get one of these from MicroMark.
As, in your original post, you clearly indicate that you have limited scratchbuilding experience, I’d be very cautious about dramatically altering the construction design of the FSM’s John Allen engine house. While the methods suggested by other here certainly can work for someone with experience, the can of worms that opens up in regard to precisely modifying other kit parts to obtain a correct fit could prove more complex and challenging than you may initially anticipate.
Under the circumstances, I’d definitely try to fabricate a duplicate of the entrance out of strip wood first, to see how well I can make the necessary modifications, rather then jumping in and possibly ruining a fine, expensive, FSM kit.
CNJ831
Thanks to all for the excellent advice. I probably should have mentioned that this engine house is a wood structure with clapboard siding, not brick or stone. Probably the best advice is that whatever method I choose, I should try it on some scrap pieces first until I’m confident I can do it right. I still haven’t decided whether to go to the trouble of rounding out the doorways or not. Even without that detail, it looks like an outstanding structure. I hope to have it become a showpiece on the layout.