A Plastic Screw for an N Scale Con-Cor PA

This week I’d hoped to show you the 3D printed replacement Minitrix Crosshead from last week’s post however due to the heavier than normal snows last night I’ve had no delivery today. So this week I’ll share with you something else which should also be arriving soon.

The venerable Con-Cor Alco PA has been around since 1967 and has been improved over the years but the original design, made by Kato under the name Sekisui, can still be found going strong on many layouts; mine included.  One of the things which made this design different from the later is how the chassis sections are fixed together.  The original design had a solid metal section on the top and two metal sections below, one making contact with each rail through the trucks. Between the top and bottom sections is a strip of plastic for electronic isolation.  It’s all clamped together with screws, metal ones on one side to conduct power from the lower section to the top, and plastic screws on the other to isolate that section.  This works well, but the plastic screws, if removed a lot, can easily be rounded off.  Plus if you drop one into the bottomless depths of an exhibition hall they are very hard to replace.

But thankfully I have a solution.

These have been designed to be 3D printed in Shapeways’ Frosted Ultra Detail material to give the required accuracy for the thread and hardness for the actual screw.

Hopefully these will be delivered soon, along with the other parts, and I will be able to share some photos with you next week.  Plus it will be nice to have my Alco PA back on the tracks.

3D Printed Minitrix Cross Heads

Trix produced a variety of locomotives including a range in N scale, dating back to the 1960s, under the name Minitrix.  Many of these shared common parts and it’s one of these for which I’m creating a replacement; a Minitrix valve gear cross head.

This particular cross head will be for the A4 steam locomotive model shown below.

The cross head is the gray slider which connects the piston and the main connecting rod.  This plastic part slides up and down the metal runner as the piston goes in and out, keeping it level and it also connects the valve gear linkage.

As with a lot of the early plastic parts these can become very brittle and start to break up.  Almost all the other parts of the locomotive’s motion are made from metal, the only exception is the crank pin which drives the eccentric rod.  This is also made from plastic and I’ve previously made this as a replacement 3D printed part; you can read more about that here.   You can identify the crank pin in the first image as it’s gray and not silver, just like the cross head.

The original cross head is a very small part and very difficult to photograph up close so the image below is my 3D model of the part, without any modifications.

The cross head is symmetrical so it can be used on each side of the locomotive.  The box section on top has slots in the sides to allow the slider to pass through. Below the box is a pair of rings, the first connects the cross head to the connecting rod and piston.  As these are joined with a pin the connecting rod is able to rotate as the cross head slides back and forth. The second ring connects to the valve gear linkage, again with a pin allowing it to rotate.

The weak spot on these parts is where the rings connect to the box.  If the valve gear becomes jammed and the wheels keep turning a twisting force is applied at this point. If the plastic has started to break up it will simply snap.

In the image below you can see three cross heads with the original on the left.  The middle one has had the weak area under the box strengthened by adding a larger amount of plastic.  The right hand side one has also had the area between the loops strengthened in the same way and it’s this version which I’m test printing.

This cross head fits most of the British outline steam locomotives including the Gresley A3, A4, Standard 9F, Ivatt 2-6-2 Tank and the Ivatt 2-6-0.  Only the Britannia 7P had a different valve gear with a simplified cross head consisting of a folded metal plate.  Minitrix also made two steam locomotive for the American market, a 4-6-2 K4 and a 2-10-0 Decapod.  As the 2-10-0 shared the same chassis as the 9F this also has the same cross head.  The K4 shared the simpler Britannia 7P chassis.

The part has now been printed by Shapeways and I’m expecting it later this week, and once tested will be made available to buy. It’s often these small, seemingly insignificant parts that aren’t glamorous or even particularly interesting, that 3D printing really comes into its own. The ability to modify, improve and manufacture replacement parts at a fraction of the cost of replacing the locomotive means we can keep the majority of our stock rolling, and it’s why I continue to produce these parts.