As promised in my last post I’ve now received the first test print for my new 3D printed axles for this locomotive, and in this post I’ll share with you how well they worked out.

This was the first test print for this design and although the design has been well thought out there’s always a chance that something’s not right, so I printed three sets.

Each set contains one drive gear and three axles; below you can see an original axle and gear for comparison.

With the new parts removed from the sprue, a closer comparison can be made. And for me, the first thing that stands out is the size of the square hole. It appears much smaller on the new part than the original, which in fact it is, but by design.

The reason for the smaller size is based on the actual axle size on the wheels. I need the replacement parts to be a tight fit but not so tight that the new parts split. Sadly they do. The reason for this is due to variances in the wheel axles. The wheels are not machined, they are cast. Then they’re plated which gives them their shiny finish and conductive coating. However, this leads to a large variance in the thickness of the axles. It’s not enough to be really visible, but it’s enough to make a difference in splitting an axle or not. So I’ill need to adjust my 3D model to allow for this.

In order to complete this test, I still wanted to see if all the other aspects of the gear and axles worked, so I filed the square hole with a square needle file to enlarge them slightly.

Now the main drive gear is fitted. Quartering the wheels, or making sure the wheels are at 90° to each other, was very easy with the square holes, and the two wheels pushed into the axle without splitting it. Although the axles have been designed to correctly space the wheels it’s good to check the distance between the back of the wheels, which should be 14.4mm for HO. I use a laser-cut ‘back-to-back’ checker as you can see below. This should be a snug fit between the wheels.

All the axles have now been fitted and spaced. At this point, I check to make sure all the wheel assemblies can move freely and none of the valve gear is misaligned.

The last part is refitting the base plate to hold everything in. For this locomotive, the base plate also holds the pilot and trailing truck and clips onto the chassis. There are two screws in the base to hold it on.

And then comes the test, does it work under power? At this stage, I’m looking for three things; does it run freely, does it run at slow speeds without binding, and most importantly, how does it perform under load? If you followed my previous design post about the first generation of this locomotive, the original gear was very thin and the first design for the replacement gear cracked or stripped its teeth under load. Luckily the second generation already has a much thicker gear. In the short video below you can see I tested all of these.

And I’m pleased to say it passed all three tests. Given that I had to enlarge the square hole in all the parts I’ve now updated my 3D model to reflect this and I’ve ordered a second test print. I also made a slight change to the angle of the teeth on the main gear. I don’t see a problem but it brings the angle more in line with the teeth on the contacting gear.

Hopefully, the new test sets will be here soon and I’ll replace the gears in this chassis to see how well they fit. If everything is okay, with no splitting, I’ll release the Bachmann HO 4-8-4 Gen 2 replacement axles and gears for sale.

Back in 2018 I shared with you my design for replacement axles and gears for a HO Bachmann 4-8-4 Northern.  You can find the post here. That turned out to be quite the adventure, as simply creating replacement parts didn’t work out so well!  Due to the thinness of the original components I had to engineer a much thicker gear which could handle the force applied from the motor.  This turned out to be a success.  But this gear and axle set only worked for the first generation of the Bachmann HO 4-8-4.   As it turns out, although Bachmann redesigned and improved the second generation, it too has problems with split axles and gears, and I’ve regularly been asked if I could produce a set for this locomotive.  Well, I finally have and in this post I’ll share the design with you.

To start, I want to explain the difference between the three generations that are commonly available.  I’m not going to go into any of the body or cosmetic differences, just the drive train so you can tell which generation you have.

The first generation, as I’ve already covered in other posts, has a pancake motor which drives the rear axle.  Below is an exploded schematic drawing, courtesy of John on the Tyco Depot Train Collectors & Model Railroading Forum. You can just about see the gear on the rear axle.

Looking at the underside of the locomotive there’s a bump over the rear axle to make way for the gear. 

This is the main clue that it’s a first generation.  This locomotive also has round wheel stubs and holes in the axles.  That’s what makes quartering the valve gear tricky on this locomotive.  ‘Quartering’ means setting the piston rods and side rods on each side of the locomotive at 90° to each other; as the wheel stubs are round it’s easy to get it in the wrong place meaning the loco won’t run smoothly.

The second generation had a lot of improvements such as a can motor and thicker gears and axles.  But most importantly the axles and wheel stubs also have square stubs and holes, making quartering far easier to do. You can just about make out the cracks in the axle and gear.

Below is an exploded schematic drawing, also courtesy of John.

The motor this time drives the third axle and there’s no bump in the base plate; this along with the lack of wires running to the tender tells you it’s a second generation.

The third generation, which I believe is the version currently available, had a total redesign.  It still has a can motor which drives the third axle, but everything else has changed.  Looking at the exploded schematic drawing below, again courtesy of John, you can see the axles are a different shape. 

To my knowledge, these don’t fail.  These are normally marked as DCC ready and will have wires connecting the tender to the locomotive; the DCC socket and speakers for sound are located in the tender.

Looking inside the second generation you can see the axles and drive gear on the third axle. The second axle is clearly split and once I removed the others I discovered they had split too. As the wheel stubs are square they don’t need to be such a tight fit as the round ones on the first generation.

Interestingly the axles are almost identical to the axles used on the UK Bachmann (former Mainline) designed locomotives that I have 3D printed replacements, for as shown below.  These were for a GWR Hall class and can be found here.

However, upon measuring the US 4-8-4 axles and gears I found them slightly different, so a new design was needed. These have been designed to be 3D printed in Shapeways Smooth Fine Detail Plastic.

The kit I’ll supply will contain three axles and a main drive gear. The idler gear between the worm and axle gear to my knowledge never breaks so I haven’t supplied that; if yours has, please let me know and I will add it to the kit.

These have now been sent off for a test print and I’ll see how well they work.  As with the first generation, and the UK Bachmann locos, I’m restricted by the size of the axle box and the size of the wheel stub.  This means the axle itself nearest the wheel is still not as thick as I would like it to be, and it’s possible to crack the new axles if not cleaned properly before use.  Once I’ve test fitted a few, if it appears too easy to crack the new ones I will include a few spares in each kit.

So now I’m waiting for the 3D printed parts to arrive, and I’ll share them with you when they do.

This week I have the final stages of testing to share for the HO Baldwin RT-624 before it’s released next week.

The big news is the etched brass has arrived. Thanks to all of you who pre ordered, your etch will be in the post this week.

The etch fret contains handrails, sun visors, grab irons, windscreen wipers, train phone antennas and MU hoses.

All the parts for the HO RT-624 kit look like this.

You may also need the truck conversions to rotate the Bowser trucks in the donor chassis as shown below. These are the same as for the HO DT6-6-2000 model and when I release the RT-624 next week I’ll include a link for these.

Lots of the details on the RT-624 etch are the same as the DT6-6-2000 but I still wanted to test fit as much as I could. Please note that in the photos the shell and 3D printed parts haven’t been fully cleaned yet and the brass is not glued in, just resting in place.

The end handrails are half etch and half 3D print. For strength the etch parts pass through the 3D print.

The sun visors fit nicely into the angled slots in the shell.

The side handrails all have holes or locating lugs to fit into and it fits well, but as you can see its a little bent. This is simply because I got a little over enthusiastic cutting it out. As the etch is thick, 0.5mm, it takes some force to cut. I cut mine out on a cutting mat with a large craft knife (Box Cutter) and the force bent the handrails. The trick is to put something hard and flat on top of the etch to prevent the part you are cutting out from bending up.

The grab irons are basically the same as the DT6-6-2000 but I’ve made a slight change to the holes which made them a perfect fit. I fitted them without bending. Once pushed all the way in they easily bent down. Each grab iron has a small lug preventing them from being pushed in too far. When fitted with glue these will be secure and strong.

All four on the side are the same.

The top grab iron doesn’t bend.

The last check I wanted to make from the etch was for the MU hoses. Each end has eight and they push into the square holes in the 3D printed section of the end handrail.

It looks a little out of line as nothing is glued in place but once it’s all painted and securely fitted I think they’ll look very good.

I now have a few small details I want to tidy up on the 3D printed shell but it’s almost finished. Next Monday I’ll release the model in two versions. The PPR early version with the Trainphone and the later without. There will also be one extra RT-624 for the single Minneapolis Northfield & Southern locomotive numbered Twenty Five.

My apologies for the lack of posts over the last month but this week I have an update to share with you on the Baldwin RT-624 project; the test print has arrived.

The photo above shows the shell and parts after they’ve spent twenty-four hours in white spirit which was used to remove the waxy residue left over from the print process. Some of this residue has turned to powder and can still be seen on the shell. To find out how I remove this see my ‘How To’ post here. I will be doing that next.

The 3D printed parts included with the shell are the crew in the cabs, horns, pilot blocks, Trainphone receiver, and end handrails centers with walkways. As you can see below the receiver and walkways have broken away. This is not an issue but I will update the 3D model to ensure this doesn’t happen again.

The receiver is designed to fit into a ‘n’ shaped slot at the top right of the nose; there’s only one on the loco, the other end doesn’t have one. You can just about see the slot in the loco shell below.

The receiver fits perfectly and once cleaned up can be permanently fixed in place with a touch of superglue.

On the pilot beam in front of the nose you can see four holes; these are for the end handrails centers.

These also fitted perfectly and have the holes to receive the brass handrail section which will complete the nose handrails. But both handrail center posts should have a walkway between them; this is the part that broke away and which I will ensure is 3D printed as one part in future prints.

The reason they came apart was an incorrect joint in the computer model which has been fixed. For this model the parts have simply been glued back together with superglue. Superglue is ideal for this because it’s made from acrylic, as is the 3D printed model. Other glues may have an effect on the 3D printed material and plastic glues may melt it.

The floor tread pattern actually faces towards the nose of the locomotive; this is because when it folds down to allow loco engineers to walk from one loco to another, the walkway pivots at the lower connection so the pattern will then be facing up. The face you will see looking at the nose will be the underside of the walkway. The square holes are for the MU (Multiple Unit) hoses which will be on the brass Additions fret.

With the handrail centre section repaired it again fits nicely into the holes in the pilot beam.

Looking from along side the locomotive you can see the walkway patten. You can also see the radiators in the side of the shell and I’m very pleased with the finish on those. You can also see I’ve added a little material under each handrail location along the side which will ensure each fits in the right spot.

Before I went any further I was keen to test fit the shell onto the chassis and did hit one design flaw. A different feature from the DT6-6-2000 is the RT-624 has a longer walkway on one side for extra batteries and a hollowed-out area. I copied the modelling for this from my N Scale model not thinking about the HO chassis and as you can see below the chassis hits it.

But that’s why I do test prints. The problem is easy to fix in the computer model and I can make a correction to this shell so that it’ll fit the chassis. I think apart from that, the shell is looking good.

I have ordered the first batch of etched brass Additions for this locomotive, and thank you to those who pre-ordered these. Once they arrive I’ll be able to do the final checks on the shell and, if all goes well, the HO Baldwin RT-624 will be made available to buy.

Hopefully next week I will have a cleaned up test print to share with you fitted to the chassis.