Rosebud Fire Grate on a Silvercrest BR Class 4

I received a blog enquiry asking if I could make a fire grate for a Silvercrest 5″ BR Class 4. The owner had inherited the locomotive but it was missing the fire grate. Given the age of the model Silvercrest were unable to help. I gamely said I would ‘have a go’ but on the basis of making a rosebud rather than a conventional bar grate.

What I didn’t realise was that the grate on this model is made up of 3 separate long sections which are shuffled into place via the firebox door …

I asked the owner to first of all make a template of the grate size based on three equally wide sections of hardboard. This resulted in an overall grate size of 57mm x 209mm. From this I used my Excel spreadsheet to derive a grate with 15% hole area occupancy from a matrix of 25 x 6 holes each of 4mm diameter counterbored with a BS4 centre drill.

I thought it might be a good idea to have a draft angle on the inner edges of the grates. Don’t ask my why – it just seemed like it would make it easier when dropping the middle section into place and perhaps reduce air leakage through the joint .. and of course this makes the design more complicated than necessary and added to my machining woes. This can be seen on the image above.

The design was created as a single grate and then extrude cut into the three sections so the middle and outside grates could have their own CAM program. Here is the Fusion model view looking from the under side.

Having grasped the basis of the design I sent the owner a set of 3D printed grate sections which he cross checked and confirmed would fit correctly. (Could these be the ultimate chocolate fireguard I wonder ?)

The next step was to think about the CNC CAM operations and the stock holding. I find holding the stock and the order of doing things to be the most challenging part of the machining process. As the three sections of grate were all under 25mm I opted to use 6mm x 25mm BMS as the base stock material. I had the Fusion 360 model reference the stock at the centre point. This is easy to probe using the Tormach PathPilot inbuilt routines. I cut three pieces of the BMS to 215 mm lengths and mounted each in turn on parallels in the machine vice. I had a vice stop set up to make the process repeatable. Each section was then drilled for the 4mm through holes (50 off in each) and then counter bored with the BS4 centre drill such that the taper on the drill finished flush with the stock top. This just left the outer profile of the model to be cut on the residual stock.

This was accomplished by using a piece of 30mm wide BMS as the sacrificial backing jig mounted on parallels. This was centre referenced and a number of the 4mm hole locations were drilled at 3.2mm and then tapped 4mm. These holes were then used to bolt down the three grates in turn so they could be profiled to size. The profiled width had to be the maximum width ignoring the intended draft profile (i.e. 19.10mm + 20.916mm). Here is the end view (upside down) showing the three grates with their draft angle .

The machining process was then transferred to my Myford VMB manual mill. This was set up with a tilting vice set to 10 degrees. The two bright edges of the centre grate and one edge of each of the outside grates were then blackened with a Sharpie before each being placed in the tilted vice and skimmed at 10 degrees such as to just remove the blackened surface.

The resulting grates all butted up together nicely and the job was complete. Here is the bottom side view.

These were shipped off to the owner for trials and he later confirmed that they fitted nicely into the firebox. I am not sure if anyone has made a rosebud for a Silvercrest with split grates so it should be interesting to see how well this configuration steams out on the track.

I think the above should give you enough to think about but there is one other spin off that resulted. While drawing up the grate in Fusion 360 I decided it would be much more convenient if the rosebud grate design could be automated using Fusion’s parametric functionality.

This resulted in the Excel spreadsheet being modified and the Fusion model also being updated. The result is the ability to get all the factors needed for the Fusion parameters from the Excel sheet (but you have to manually copy them between Excel and Fusion).

Here is a ZIP file with both these files included.

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Simple Water Level Sensor for Live Steam Locos

While coming to the end of a running session of my 5″ loco at the club raised level track I suddenly realised the axle pump was not making its normal rattling noise. Water level was likely very low and there appeared very little in the glass sight gauge. My state of panic endured until I reached the filler hose.

On returning home to the workshop I resolved to try to make this alarm situation more readily visible. The common solution is to add a second sight glass feeding from the water tank and visible in the ‘cab’. My engine did not have a great deal of room for this addition.

Sacrilegiously I began to consider an electronic solution which probably wouldn’t make me popular with the mechanical diehards but the challenge appealed.

The electronic solution turned out to be a multi faceted activity. An initial bird’s nest lash up was followed by a PCB designed in Fusion 360 Electrical. The PCB was milled on my vacuum table fitted to the Tormach 440 with the Gerber and Epsilon files converted to GCode using FlatCAM. Having completed the PCB board this led to a customised enclosure designed in Fusion 360 and 3D printed in the Qidi X Smart 3.

The circuit is a NPN transistor that is turned on by water conduction between two sensor probes which in turn drives a second NPN to illuminate a flashing LED. The module is powered by a CR2025 button cell.

The enclosure consists of three parts, the base, a snap on lid and a round boss that matches the diameter of the water filling hole on the engine side tank. The two water sensing probes are made from 16 swg wire protruding from the boss.

The round boss is held in place with a M2.5 countersink screw and could be dimensioned to suit different sized filler holes. Having it as a separately printed item is also useful in that it allows the enclosure base to be 3D printed without support.

Here is are some views of the completed assembly.

It seems to work quite well and is surprising sensitive down to the last few millimetres of the probe rods. Time will tell.

If you want more details then please send me a message.

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French Model Steam Engine Gathering

5″ Gauge Activity at Montcuq

We were very lucky to be invited to this event near Cahors. A significant number of the engines were Polly designs and the owner of the track has had a long standing relationship with Polly Model Engineering.

The track has two loops and a very significant incline which tested not just the engines but also the drivers. One very encouraging aspect of the weekend was the number of young people, both male and female who were enjoying driving the track.

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Replacement Whistle on Polly V Steam Engine

Spluttering Banished to be replaced by a Banshee

It had become a standing family joke on steaming outings that my Polly V whistle left a lot to be desired.   Quite often it would do nothing more than a feeble splutter.   The Polly V kit supplied whistle is fitted under the running board on the left hand side of the cab.   The pipe run is long and somewhat tortuous.   I had insulated the pipe to reduce feed loss but this made little difference.   The whistle valve also had a gentle leak and was very stiff to activate.   All in all not a good setup.

A recent article in Engineering In Miniature (EIM) by Richard Wightman (September 2022) went into detail about a whistle and valve combination he had created.   The whistle was fairly conventional but very compact.   The steam control valve was unusual in that he used a standard tyre Schrader valve.   This tweaked my interest and I set about upgrading my Polly V locomotive using this technique. Here are some Fusion 360 images and shots of the new valve and whistle mounted in place on my Polly V.

Here is a blow by blow description of the process as a PDF download.

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Bad day steaming with my 5″ Polly V live steam locomotive

Last Sunday was a members steaming day at the model club.   I told all the family I would be there and to bring the grandchildren along for a ride.

I arrived early with a view to getting steam up and be ready to go … in theory.

First problem was that the connecting rods had stopped in a position that was blocking fitting the fire tray holding pin.  This immediately got worse in that the cylinders had seized since my last run.  The more experienced members told me not to worry and if I could move the wheels enough then fit the fire tray and get things warmed up.   After some rocking I managed this slight movement …. but forgot I hadn’t filled the boiler and the fire was burning very brightly.   Mad panic to get some water in quickly.

Steam pressure up to 90psi but no joy on the cylinders.   Gentle tapping on the con rods lead to heavy tapping on the con rods and lots of slide dragging back and forth on the steaming bay.   Still no joy.  Cylinders locked.

Now running out of water in the boiler.   Grandchildren arrived.  “Why can’t we have a ride grandddad ?”.   Grabbed the handpump leaver and rapidly pumped away to get water into the boiler …. too rapidly in fact and broke my hand pump leaver.   Still had a few PSI of pressure and thankfully the injector kicked in … for a short while until the pressure dropped.

How bad can this get ?

Some slight movement on the cylinders now.   Grabbed some blocks of wood and got the engine airborne so the axle pump could be used … if I could just get the wheels to fully turn.   More tapping/thrashing at the con rods and suddenly the wheels jumped into motion.   Phew.   Water now flowing into the boiler, fire good, pressure good.   Get out on the track quick and keep the grandchildren quiet.

Back home in the workshop  I made a new hand pump handle and fitted new oil port plugs on the top of each cylinder each with a M3 feed hole.   This means I can syringe oil into the valve and cylinders a lot more easily after future steamings.

It is a hobby after all but this was one of the most stressful steamings I have ever experienced – all because I was in a rush last time out and did not do the end of day drain down processes correctly because it was tipping down with rain.  Lesson learned.  Thankfully no long term damage to the cylinders.

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