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From FLM Br 50.849 to NMBS 25.001

Additional tools allow a different approach for conversion of RTR models, demonstrated with a NMBS/SNCB class 25 loco.
See also part 1 with introduction on these loco's here


The NMBS had 2 machines of the Br 50 ÜK series with the old form of Wagner deflectors numbered 25.001 and 25.013. The latter was always coupled to a 32 m³ 'Wannentender', where 25.001 can be coupled with the 26 m³ 'Kastentender' after 1955. The 32 m³ tender requires far more work for conversion thus the easy approach is for 25.001 after 1955. An excellent photograph and published in a host of literature was taken by J.H. Renaud in 1956 showing it with a 26 m³ tender.

Photo of SNCB 25.001 by J.H. Renaud.

A model of this loco can be build using the older FLM Br 50.849 model with Wagner deflectors, but it needs a boiler with the frostprotection, 1 dome and 2 sandboxes and removal of the preheater and numerous tubes and handrails and a cab with single window and ÜK type of roofvent. Of course it is possible to scrape the existing boiler bare of all superfluous detail and fill up the holes again but this means a lot of careful work with a high risk that something goes wrong and leads to havoc. You also have to convert the cab by replacing the vents and close front window or use a Br 52 cab with 'only' a frontal window change, but this cab won't fit on your Br 50 boiler because of different arrangements in the r-t-r models. Building an entire new boiler from metal is feasible, such as shown in part 1 on the type 25/26 elsewhere, but adding all the fittings from scratch to a bare boiler is also lot of work. An intermediate way looks to be using a 3D printer as this allows the major parts to be directly printed only needing the thin free standing parts, such as the hand rails and sanding tubes, to be added by hand. There is no longer any need for castings or additional models as supplier of parts. Another advantage is of course that with a good base drawing other variants with slightly different appendages and even different boilers can be easily developed by changing those specific parts in the drawing and put the printer at work again. To produce additional drawings access to sufficiently detailed information of the original is of course necessary, as well as some experience in 3D drafting and knowledge of what your 3D printer is capable to print. The latter here is still a learning process as the printer seems capable of finer detail than thought. For instance if the resin would be more rigid than even free standing handrails and tubes would be feasible too. I decided to make these of metal wire, because I expect that the printed ones will not survive the handling of the model in practise. Thus this still leaves some craft work in order to finish this loco.

Drawing and design

All dimensions for the drawing were already available because of the earlier exercise of making a new boiler in brass. This allowed a quick approach in 3D of a first sketch of the boiler. The cab however had to be drawn up from zero. The drawings were made in Cinema 4D (C4D-R21) a professional package for visualisations.
In order to make it a replacement job all holes and screw fixings for fitting it to the frame were measured and copied from the original FLM Br 50 boiler. Of course this involved a test run with a redo for further adaptions but actually the first print was pretty much spot on and useable as oneoff. Of course some things come to light such as the differences between the FLM Br 50 and Br 52 frame, the earlier made metal example was finally combined with a FLM Br 52 frame.

new boilernew boiler drawn in C4D.

The new boiler as seen on screen, below chimney adapted with a large opening for the Br 50 frame and one of those horrible tubes for fixing it.

ÜK Cab with the 'old' rounded front windows but with a 'new' roofvent. Through the ring for fitting it to the boiler you can see a 'click' space for fixing it to the existing FLM frame.

The new cab to go with this loco complete with lifting hooks on the roof and wind deflectors at its window and a set of 'click' spacings on the inside of the cab. The rivets are left off on purpose as this allows refining the cab sides with wet and dry for a smooth surface.

printed results
class 25 boiler with frost protection, 2 cast sandboxes and 1 dome and various tubes and appendages.

This is what the printer delivers, a new boiler and cab still on their printed ladder network. Several amendments (some adding, but mostly deleting) in the posts of this ladder network were done for later prints.

The new class 25 boiler showing its belly, these ladder remains can be quite quickly smoothed

There is some cleaning up to do after cutting off of the ladder network. The ladder remains can be easily removed and the remaining tips first flattened with a stick with #180 coarse sanding paper and then polished smooth with #600 wet and dry. The big rectangular hole still shows a small piece of the ladder network going through, this can be pulled out from the front with tweezers. I found it necessary to include a structural reinforcement in the boiler to keep it circular during the printing and hardening. The original lead weighting piece fits between this reinforcement and the front of the cab, but, when necessary, this part can be cut out with a jewellers saw, for instance when you want to fit a loudspeaker below the chimney.

Test fit on the loco of the first design
first impression

The printed boiler and cab fitted to the loco, the white stuff collected in corners is dust from sanding that still has to be washed off with a soft brush under running tapwater.

To be followed up with further updates, but having already too many projects to finish on the desk this will be a low profile project as older items will get preference. Particularly this outcome leads to a review for the class 64 (P8) and trials to finish this model with a 3D printed boiler complete with ACFI and all of its tubing.
Click here to visit that page.

tubes and appendages added left hand side
left hand side with handrail, tubes and steps added

steps and tubes added right hand side
right hand side with tubes, handrail and whistle

Here are 2 pictures where most of the free standing items are added, the hand rails from 0.25 mm gitar string on split pin posts formed from 0.2 mm brass wire filed to half round. The steps are hand filed from 0.2 mm NS and the Walschaerts reach rod is constructed from 0.5 mm brass filed flat and various pieces of 0.2 mm NS soldered to it. The steam whistle and 2 safety valves were turned on the lathe.

complete with new cylinders
narrowed but still loosely fitted.

Getting closer, a set of new cylinders with the frost protection box on top was printed, original design by Moritz L., but I heavily modified his design to fit on the original FLM parts and added that box on top. However these will get another update as it turns out there is a difference in those boxes between class 25 and 26, this one was based on a class 26, those on a class 25 are a bit smaller, compare with the top photo. More important is that the gross overwidth at the cylinders has been taken out, the screwed metal part that hold the Walschaerts was cut and reassembled again with 2 pieces of NS to fit the narrower design. The thing still to do is to set these red plastic link hangers below the footplate further inwards with scalpel and cyano glue.


tender design
Schematics of tender #3, essentially same as #2 but adapted for on board electronics.

A new tender is necessary as the FLM tender is far too high and has fantasy wheel spacing. The watertanks are too high by some 0.5 mm, the coal bunker over a full mm and the coal load adds another 1 mm. The tender design #2 was slightly modified to get all the electronics into the tender instead of using the boiler for this. This involved dropping the flywheel and shifting the motor backwards. Basically it was made by profiling a strip of brass bar, 8 mm high x 5 mm thick and 50 mm long by drilling some 1.5 mm holes for axles and gears and cutting some parts away. The bogie sides are identical to #2 and 3D printed on the Anycubic.

new tender parts
Some CNC milled parts to start a new 26 m³ tender.

new body top for tender
The above parts soldered together making up a new tender body.

Preview: all parts fitted together. Next thing to do is to add weight and electronics and testing the combination.

The internals complete with electronics.

The mechanical design is identical to tender 2, but the motor is shifted towards the back and the flywheel is dropped in favour of a more compact fit with all electronics in the tender. The motor is now mounted in a squarish block of 10 mm brass that also supports a piece of thin pcb that holds the wiper pickups on six of its wheels. The last axle has 2 wheels with traction tyres. The decoder is mounted on top of that block and the speaker and stay-alive are mounted in front of the motor. An extra capicatator can be mounted behind the worm when necessary. This all fits the available space below the coal bunker, the space beside it is reserved for weight. There are 2 screws, front and aft, that hold the lot together

blocks of lead
The remaining space in the water tank is filled with 2 dedicated blocks of lead.

top view
Top view of assembled construction.

Trial runs, click to open.
class25 on Splitveld

brick wall
current state. Note the different boxes on the cylinders and that the tender is now much closer coupled, lamps and lighting need further effort as well as the colour/weathering.

to be followed up ...


My conclusion is that this 3D printing route saves a considerable amount of work compared to constructing a full metal boiler with all appendages from scratch. It is certainly the best route for a ÜK boiler with the full frost protection intact, adding the free standing items is 'only' about a days modelling work. With the experience of the P8 boiler added, a host of this craft work can be saved by adding further items that can be printed too into the 3D design (i.e. whistle, safety valves, sanding tubes, reach rod, air pump connection and 66% of the steps), but that is rolling insight, generally called experience.

For a standard BR 50 boiler (i.e. without frost protection) scraping off the closed cast hand rails and replace these with free standing wires is probably a faster method as this only concerns replacing 3 to 7 wires/tubes depending on what one finds acceptable. However the 3D route allows to produce all slightly different variations of the BR50/52, e.g. also those with only one rectangular sandbox and single dome but with old BR50 cab and all those rebuilt DDR versions with their front skirts and the various types of wind deflectors of course.

copyright: Henk Oversloot
date: 27 May 2020
update: 24 June 2021