Fast Tracks

Cutting Those Pesky Gaps!

By Tim Warris (This newsletter was originally published on February 1, 2004)

Every hand built turnout needs isolation gaps to electrically isolate the frog from the rest of the turnout. Without them there would be a dead short in the turnout.

Commercial ready-to-run turnouts provide isolation by using an all-plastic frog, or by using the switch points to route power to the frog. This not only leaves a very unsightly gap in the switch points, but also relies on the points to route power to the closure rails and the frog, which can be a constant source of trouble.

Hand constructed turnouts or crossings on the other hand need to have the isolation gaps manually cut into the rail. There are a couple of ways to do this.

Why I Hate Dentists

The simplest and most common method of cutting isolation gaps is to use a cut-off disk with a Dremel Moto-Tool. (Photo 1)

There are two types of cut-off disks that you can use. The first is a ceramic disk; a very thin cutting wheel that will cut a clean, discrete gap. The downside to using this type of wheel is that they are VERY fragile and will break very easily. Twist the wheel even slightly while cutting the gap and BOOM! It will explode! So safety glasses and great care are essential.

Personally I prefer to use ceramic wheels, as I like the thin gap and the smaller diameter of the wheels are less likely to nick the stock rails. Fortunately they are sold in packages of 25, so blowing up a few wheels is not hugely expensive! A good description of ceramic wheels and places where you can buy them can be found here.

The second cutting wheel option is a fibreglass reinforced cut- off disk. These wheels are very durable and long lasting, and short of taking a hammer to them, very resistant to breaking.

The downside to fibreglass wheels is that they are a bit thicker than the ceramic disks and are usually only available in larger diameters.

Fibreglass wheels do work well for larger scales (HO and up - Photo 2), but in scales where there isn't much space between the rails, like N or narrow gauges, cutting gaps on a finished turnout can pose a bit of a challenge. Dual gauge turnouts are even more of a problem as the rails are very close together, which can prevent your from using a cut-off wheel at all. I have several times "nicked" the guardrails while trying to cut the gaps for the frog with fibreglass wheels.

Why I Like Jewellers

So for these types of turnouts there is a third method ? the Jewellers Saw. (Photo 3) There are a number of places you can purchase Jewellers Saws and blades on the web. Here is one.

This nifty little tool is very handy for all aspects of model work, but are particularly useful for cutting isolation gaps. By opening one end of the saw the blade can be passed through the ties to cut the rails at the frog, even in the tightest of spots!

After threading the blade through the ties, start cutting with a long even stroke until the blade bites into the rail a bit, and then short back and forth strokes. Rail being very soft will cut quickly and cleanly. (Photo 4)

This is one of the handiest methods I have found yet to cut the gaps in tight spots. Of course this will only work with a "bench built" turnout, like the turnouts and crossings produced in a Fast Tracks fixture (shameless promotion!)

Isolation Insulation

To keep the gaps from closing up due to expansion of the rails or the bench work shifting (which can happen!), you may want to fill in the gaps with small pieces of styrene.

The gap cut with my jewellers saw is about .01" thick, which just so happens to be one of the thicknesses that styrene is available in. I hold the styrene in place with a small drop of CA adhesive (crazy glue or Zap!) applied with a toothpick.

Once the glue has cured I shape the styrene to match the rail and after the rails have been painted the gaps will be virtually invisible.

- Tim

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Have a question or comment about this issue of the Fast Tracks newsletter? Then post a reply! I will be happy to respond to any posts. - Tim

Hey, go easy on the dentists. Someone has to do the work. Dentists are always "down in the mouth" because people hate them. Tim Warris wrote: