Monthly Archives: December 2017

3D Printed Minitrix Cross Heads Part 2

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Two weeks ago I shared with you my designs for a 3D printed replacement N Scale Minitrix cross head; you can read the post here.  This week I have the actual 3D printed cross head to show you.

The design, as pictured below, followed the original closely with the exception of the weaker areas which were strengthened by adding a bit more material.

The actual parts look like this, next to a broken original.

The parts have been cleaned by submerging in Goo Gone for 24 hours, rinsed in warm water and left for a further 24 hours in open air; this is my normal cleaning process for all parts 3D printed in Shapeways’ FUD and FXD materials.

In the image below you can see a new and an old cross head on the slide bar, the slots in the sides are just right to allow the crosshead to slide without being too tight or too loose.

The upper hole on the cross head is for the piston and connecting rod joint and as you can see below this fits together well.  The piston is actually a bar with a ninety degree bend in.  The bar passes through the cross head then the connecting rod.  When it comes to getting the bend through the two parts it needs a bit of a push.  This also stops the bar from falling out.

One thing I did learn at this point is the piston bar needs to be fitted after the lower hole is connected otherwise it’s very hard to assemble!  So it came out again.

Also, to connect the lower hole, make sure the crosshead is on the slider bar first. Don’t ask me how I know that!

The lower hole connects to part of the valve gear. This time it uses a metal pin.  The pin is actually a tube and the end is flared over to prevent it falling out.  I carefully un-flared the pin by squeezing it together with a pair of tweezers.  Then the remaining part of the old crosshead fell away and I was able to push the pin through the new one.  This is probably the stage when the new cross head is most at risk of breaking, so try to make the end of the pin as close to the diameter of the pin as you can.  Once it has passed through the pin can be re-flared by pushing in a needle file or something similar.  It doesn’t need to be flared much, just enough to stop the pin from passing back through the hole.

The piston bar can now be inserted, completing this part of the assembly.

The wheels and motion can now be inserted into the chassis; this is a really tricky job!

And here we have a repaired Minitrix cross head.  The chassis rolls up and down freely and is ready to have the motor and other parts refitted.

The cross head is available on my Shapeways shop in sets of two and four here via the links.

I will also try to keep a few in stock, although not until after Christmas, so you can also contact me directly for a set.  I will also offer the cross heads in a set, along with my replacement Minitrix Eccentric Rod Crank Pins.

Next week I’ll have another how-to to share with you about coal loads.

A Plastic Screw for an N Scale Con-Cor PA

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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

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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.