Friday, November 5, 2021

A Screw On Lead Extension

The switching lead on my mini test layout would be pathetically short without an extension.  Since I expect to carry the module up and down stairs and round corners, the extension has to be removable.  I wanted it to lock nicely into place and be easy to install and remove.  I ended up using a threaded insert and a screw with a knob.

Here's a side view of the extension installed.

And here's what it looks like when removed, with the layout flipped over to show the threaded insert.

There's enough of the 1x3 going under the layout so the extension can't tip down.  The top piece the cork roadbed is glue to is also a 1x3.  In between a spacer of a scrap of 1/2" ply and a scrap of paneling made up just the right thickness to match the door.  When I tested clamping it on to see how much the extension would flex, I was surprised to see the whole door bow up along it's length as the inside end of the extension pressed up on the bottom of the door.  That's why I added the 1x2 stiffeners along the front and back edges of the door.  The extension itself flexes a little, but not enough to worry about with only the weight of an engine and cars on it.

The brass threaded insert has a 3/8-16 machine screw thread on the inside, and coarse wood screw threads on the outside.  You drill a 1/2" hole to screw the insert into.  I picked up both the insert and the screw with a knob for a head at a local hardware store.

Getting the insert screwed in right proved to be tricky.  My first attempt I used a washer engaged in the two slots at the top of the insert to screw it in, and it dug itself it several degrees off from vertical - enough that it affected how the board seated against the door.  Some experiments in scrap wood showed that it's very difficult to get it to follow the hole straight in.  What I ended up doing is getting some 3/8-16 threaded rod, which I threaded a regular nut onto and then the insert.  I wedged the top of the rod against a handy prop (my drill press) to keep it exactly vertical over the hole and held there firmly enough to resist the tendency to wobble when screwing in the insert.  I used a wrench to turn the whole thing to drive the insert straight in.  Not too hard once you know what needs to be done.  Here's the setup, doing a trial run on a scrap of 2x4.

The end result is easy to remove, easy to install, and seems to align itself quite nicely.

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