Curved diamond crossings for the M&N RR - Part 6

Preventing short circuits for DCC

To make the diamond compatible with DCC operation, two of the frogs need to be isolated. Here is a schematic diagram of part of the M&N RR. The ‘north’ rail is marked in red and the ‘south’ rail is marked in black. The first crossing I am building is on the right. At places where red rail meets red rail or black meets black, there will be no conflict (short circuit). However, where red and black meet, the frog needs to be isolated with gaps in the rails so its polarity can be flipped for the running direction to eliminate the short.

Auto-reversing the frog polarity

The two frogs where red and black meet will need to be set to ‘N’ or ‘S’ depending on which route is in use. An auto-reversing module will take care of this polarity alignment automatically once the crossing is installed.

The whole scheme relies on the fact that only one route through the crossing is fully aligned for polarity at any given time. When the metal locomotive wheel passes onto a frog that is not aligned for polarity it creates a short, the auto-reverser flips polarity of both switchable frogs. The toggle in polarity happens faster than the DCC booster can respond to the short and the DCC locomotive continues as if nothing happened. The flipping of polarity only happens when the first metal wheel in a train (usually the locomotive, but not always) hits the first misaligned frog. Once the auto-reverser acts, the entire running route through the crossing will be aligned for polarity. The route not in use always has conflicting polarity on the switchable frogs, but this is moot since the crossing is occupied and the empty route can’t be used till the crossing is clear.

Gap locations

There are two types of gaps to be cut; gaps that cut both running rails and guard rails and gaps that only cut running rails. Where possible, I prefer to set the gaps in places where only the running rail needs to be cut. That way the cut is simpler and there is only one rail to be filled and smoothed. Referring to the diagram below, the four gaps on the diamond itself will all cut a guard rail. No other choices available.

With the non-diamond gaps, we have some flexibility in their exact location. Ideally, each gap should fall just outside the guard rail yielding a simpler gap and minimizing cosmetic impact.

Countering this line of reasoning is an operational consideration that would keep all gaps close to the frog as possible. You don’t want the possibility of two wheelsets shorting across two gaps simultaneously. When this happens, the auto-reverser will not be able to align polarity to clear both shorts and the booster will step in and put a temporary end to the DCC action.

So the deal has to be that only one route at a time can be used, all waiting trains must be outside of the outer gaps for correct operation. And since those outer gaps will be just outside the ends of the guard rails, you can see that for this crossing, the rule will be to not enter the crossing until all guard rails are fully visible.

(Note: I made this photo after I had cut four of the gaps so you can see the gapped rails in the diamond near the frogs. Only one set of gaps has been trimmed and sanded in this photo.)

I marked the cut locations on the crossing rails with a blue Sharpie permanent marker, (near the tip of the orange toothpick end in the photo below). I wanted to keep the cuts as non-descript as possible so elected to put them close to the frog since that is a visually busier part of the assembly.

Sawing gaps

Quite frankly, I do not relish sawing the crossing assembly I have just worked so hard to put together! Cutting the frog gaps requires a steady hand and good quality jewellers saw with lots of teeth per inch. I used my new jewelers saw from the Music Medic which I got for about $17 and it is sooo much nicer than the Xacto one I tossed on first use.

For DCC, we just need to create an open circuit to isolate the frog. The smaller the gap, the better. The jewelers saw blades are so fine that I oriented the blade by feeling rather than sight. The blades cut a 0.015” gap which is getting to be pretty good visually and should not be a source of bumps or wobbles for rolling stock, especially after sanding wheel path smooth.

Carefully, and gently, cut the frog gaps in the locations shown on the earlier diagram. I started with the diamond gaps. I’ll determine the locations if the outer gaps later when I’m closer to installing the crossing on the layout. I made a small gap-cutting jig from a piece of scrap peg board. Any small piece of hardwood with a few holes in it would do. The jig minimizes the stress on the crossing by providing a flat surface for the crossing to lay against as it is being cut.

To stop the crossing from sliding around, I taped down a patch of the sticky film to the jig. This stuff is quite handy.

The best thing you can do once cutting is to take your time. Rushing will likely end up with the saw on the floor and the crossing embedded in the wall! Code 55 rail is pretty soft, and code 40 is even flimsier, so the idea is to sever the rail with lots of tiny cuts using a sharp saw blade while the rail that is being gapped is well supported. I turned the crossing upside down so the rail was not being pulled down between two ties as it was being cut.

In this orientation, the whole rail supports the downward pull of the blade. Even then, if you pull too hard on the saw, the assembly wants to bow slightly. You can feel it lightly flexing as you hold it down with your fingers during the cutting. In other words, the span across the 1/4” holes in the peg board are enough to initiate folding of the rail so be careful and take your time! If you rally pull hard and the blade jams, it is possible to introduce a local hump in the rail being cut.

Another potential gotcha comes from the fact that one rail is becoming two during the cut. Downward force from the saw during cutting can do nasty things to the new pieces of rail that were previously attached as a continuous piece. This is the point where you find out that while the rail was solid and secure as a continuous piece, one or both pieces is now not secure enough on its own.

With the gap location sitting over a convenient hole, the blade is threaded through the board and between the ties in the crossing and then the clamps are tightened to secure the blade. This saw has a three sturdy clamps, two for the ends of the blade and one to adjust the tension on the blade. They are forged rather than cast steel with well fitting thumb screws. The holding pads are nice and hard so the blade is held firmly in place.

The jig is about ten inches long so I hold the peg-board down against the table top with my left forearm and steady the crossing by pressing down on the ties close to the gap with the fingers on my left hand. The set up is a bit fiddly but once ready, the whole thing is plenty stable. I moved the green cutting mat to the edge of the bench to protect it and made triple sure I was not slicing into the Corian counter top! I gently cut the gap on one side of the diamond then cut the other side. Each rail took about 20 strokes of the blade. I used the edge of the pcb tie as a guide for the blade to keep the cut straight.

Cutting off more than I expected

This is a good place to emphasize the importance of soldering the point of the diamond to the pcb tie under it. It’s kind of hard to do when installing the diamond due to access for the soldering iron tip. The trick is to put flux in place on the tie supporting the point before the diamond rails are in place, solder the diamond rails to the other ties then, sweat solder between tie and the point of the diamond, aided by the flux. If you don’t do that, well, this is what happens…

Yup, that’s the point of the diamond by the tweezers alright! The one I so carefully sanded into perfect alignment! Not to worry. Notice that the gaps at the left hand end of the diamond are already cut and filled. The exact same thing happened when I cut those gaps too - the point stayed on the sticky film when I lifted the crossing off the gap cutting jig. The point can be carefully glued back in to place as part of the gap filling process. But it would have been much better if it had stayed soldered in place.

Filling the gaps

With the cuts made, I filled the gaps with small sticks of 0.015” styrene strip.

I glued the 0.015” styrene strip in each gap using CA (super glue). The tip can be glued back in at this point too! 0.010 strip was a little small for the job. The styrene needs to be snug in the gap but not so tight that it puts stress on the rails. The styrene stick should extend over the full cross-section of the rail and can extend below the base by a few thousandths of an inch.

Here’s another good thing to know, a drop of Super glue can drip off the underside of a crossing that is being glued onto the paper towel under it. The paper towel will glue quite nicely to the pcb ties, and, the cutting mat below it. Who knew?!!! The paper towel comes off both with a little persuasion from hobby knife if caught quickly! These are the breaks when you start at 11:30pm!

Filler materials

Styrene is okay as a filler but it doesn’t shine or clean the same as nickel silver.  This is going to be true for most non-metallic materials used to fill gaps in rails. As a result, gaps tend to stay visible, the styrene (or ABS or wood or whatever) gets dirty while the the rails shine. Styrene does the job, but I think I might try ABS which is closer to rail color next time.

The other problem with styrene, or similar candidate materials for gap filling, is that it is a lot softer than the adjoining metal rail. So you can end up with the styrene surface sitting lower than the rail head. This can happen if the styrene gets abraded or heated or compressed by something pressing on it. The net result is a rough spot on the rail. Something to be avoided at all costs as nothing destroys the realism of N Scale faster and more effectively than shaky rolling stock. Keep it all looking great by keeping it running smooth!

Keeping it running smooth

Once set, I cut the gap fillers flush with the top of the rail using a single sided razor blade. Flush cutting rail nippers could also be used but they leave some material above the rails. I cut down in to the strip along each side of each rail, then cut in to the material being removed from the rail path at angles to remove it.

The next step is to smooth the wheel paths. I carefully filed the excess filler material to leave a smooth rail path. I first used a thin metal nail file and a disposable nail file to smooth the wheel paths out at the newly filled gaps.

The tops of the rail heads were smoothed using a disposable nail file. I like these as they wear out after a dozen or so strokes which lessens the possibility of over-filing the rail. I cleaned out the flange ways with an stiff-bristled artists paintbrush and tested each route with a MicroTrains truck with the deep ‘pizza cutter’ flanges. Everything ran smoothly as did the four-axle locomotive trucks I slide through, a Life Like SW1200 and an Atlas Dash8-40B.

These filled gaps will be slightly less obvious once the assembly is painted and ballasted, as will the gaps in the pcb ties.

Checking for open circuits

As each pair of diamond rails adjacent to each frog are cut, the two pairs of rails on the routes entering the crossing at that end are now electrically isolated from each other. I checked this with my mini-meter. Initially it showed a short till I realized that the keeper ties on the ends of each route were not gapped. A few cuts with a razor blade on the keeper ties and each route showed a nice open circuit between its rails.

The Ideal Gap

The gaps have to be filled for operational and cosmetic reasons. Operationally, we just need to ensure that the gaps won’t be easily bridged or that the rails can’t expand to eliminate the open circuit. We also want the gap to be silky smooth so the wheel path is uncompromised. From a cosmetic standpoint, gaps look unprototypic and, even worse, they are eye-catching as they break the flow of the rails, the source of the beauty of track. To lessen this negative effect, gaps should be as small as possible and located in as unobtrusive setting as possible. Here’s the before and after shots:

Next steps

Now the surgery is completed, it’s time to get this puppy on the roadbed. I will be adding the wood ties to the diamond and its approaches and matching the diamond crossing to the height of the existing track. Then it’s time to get it wired up and fully functional.

Thanks for reading and stay tuned for more!

- Coxy

Part 5: Installing the diamond.

Part 7: A quick reality check.