Fitting and Fixing the Rosebud Grate in the Polly V

Fire Grate Past Frustrations

Owners of Polly V locomotives suffer the frustration of juggling the original bar grate into the firebox opening and then getting the ash pan to seat correctly before inserting the retaining pin.   With the old bar grate I had mounted some reference blocks on the ash pan floor to keep the fire grate roughly, but still lose, while the ash pan was juggled into the correct position and the retaining rod inserted.   On a cold frosty morning at the track, when keen to get steam up, this was always a pain to do.   I resolved to fix or at least ease this problem on the new Rosebud Grate.

Resolution

I have a ball bearing rolling road that fits on my B&D Workmate stand and this allows easier access to the underside of the engine.

Polly V Engine on Rolling Road
Polly V mounted on the workmate rolling road with finished grate at the front

I mounted the new Rosebud Grate in place in the bottom of the firebox and threaded some 16 SWG wire through the firebox door and into a pair of the holes in the centre of the new Rosebud Grate.  I could now pull the grate upwards and hold it in place in the firebox floor opening.   I did a dry fit of the ash pan and inserted the retaining rod.   Looking back down into the ash pan from the rear of the engine I could see that with the grate pulled up tightly into the firebox opening (using the wire) there was about 3mm of clearance between the ends of the new grate supporting pillars and the ash pan floor.

Leaving the grate securely in place, held with the wire, I dropped the ash pan out of the engine and cut a piece of card to exactly fit inside the ash pan bottom surface.   I painted Engineers Blue on the ends of the Rosebud Grate mounting pillars.  Very carefully I offered and fitted the ash pan in place and inserted the retaining rod.   I then released the retaining wire on the grate and let the grate drop so that the grate pillar ends contacted the card and left blue witness marks.  I tightened the restraining wire again to lift the grate and then dropped the ash pan.   I had four usable blue marks on the card showing where the pillars were located relative to the ash pan base.

I drilled out just the two best marked diagonals pairs with 5.5mm holes and then dropped the grate out of the engine and mounted it on the ash pan.  I cut the length of the two M5 mounting screws such as to be tight into the pillar holes while not quite gripping the ash pan floor.  This gives a degree of movement (aka slop) when offering the assembly into place on the locomotive.

Magic !

Now only a single integrated lump (grate + ash pan) needs to be juggled in place on those frosty mornings.

rosebud grate fitting to ash pan
Items used for the Rosebud Grate fitting and mounting.

Update :-

This afternoon I ran the Polly up on the rolling road with the new Rosebud grate fitted.   After lighting with white spirit soaked kindling, the safety valves were blowing after 17 minutes.   The fire looked bright and even on the blower and on no-load running.   It is not a true test but certainly looks promising.   Following the rain over the weekend, we have been given permission to run steam once again at the Club so maybe an outing is called for on Sunday to get a real feel for the changes.

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    Microscope Addition to Assets

    I often have a look on Banggood for tooling items for the workshop but the other day a low cost microscope caught my eye.   I regularly get thin slivers of brass and steel in my hands and fingers and they are a real pain to find never mind remove.   I thought for the price being asked this microscope might make a low risk purchase to help my failing eyesight.

    It arrived today, looked really cheap and nasty out of the box yet I am staggered by what it does.   The screen is HD and there is a card slot for local storage.   You can record stills or video.   Have a look at the following link : –

    https://us.banggood.com/Wholesale-Warehouse-Mustool-G600-Digital-Portable-1-600X-3_6MP-Microscope-Continuous-Magnifier-with-4_3inch-HD-OLED-Display-wp-Usa-1119592.html?rmmds=myorder

    I will be using it for media shots on my blog that’s for sure.

    Microscope set up out of the box showing part of a US 1 cent piece

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      Rosebud Grate CAM and manufacture on Tormach PCNC440

      While doing the drawings for the Rosebud grate on Fusion 360 I cheated slightly.   From my measurements, I made a best estimate sketch of the needed grate size to fit the firebox floor and having drawn this up, I did a 3D print of an equivalent size thin piece of PLA.  Having trial fitted this printed plate I did some trimming on the Fusion drawing ready to create the CAM.

      I had bought in some 150mm square black mild steel plate and cut it roughly to width but left the length at 150mm this being longer than needed.  This allowed me to clamp the ends of the stock to my tooling plate on a piece of MDF.   I had one clean cut edge on the cut stock to use as a reference.   When mounted I checked this with the Haimer to make sure it was running parallel in the X plane.   Note I cut the MDF to roughly the same size as the plate so as not to interfere with the clamping.

      I did a PathPilot width and length measure using the Haimer and found the centre of the plate and set this as G54.  My Fusion drawing and CAM were referenced to centre. I was now ready to go.

      First operation was to spot the matrix of holes and the second op was to drill them out to 4.1mm.   Third op was to countersink the holes to 3mm depth.  This was a bit interactive.   I just worked on one hole only to start with and did repeated cuts using a BS3 countersink until the depth was correct.   I then did a ‘chose similar size’ selection in Fusion CAM and then ran the full op.

      This now left machining of the profile of the plate to the size of the fire box floor dimensions as per the CAM and my dummy PLA plate.

      rosebud grate first operations
      First Ops on the Grate, spot drilling and then hole drilling. Note the clamps on the outside of the plate area.

      Clearly the clamps were now a problem as the end areas were excess material on the length.  To get round this I removed the drilled plate from the MDF (the MDF had already started to degrade and swell with the cutting fluid) and mounted a new piece of MDF on the tooling plate with M8 fixings.  As you will see below I went a bit OTT with these …. there is even a hidden countersink one under the plate to stop the MDF bowing upwards …

      I remounted the grate on the new MDF with a single woodscrew in one of the grate holes and checked and adjusted the angle of the plate so the good edge was running true in X as before.  I then added a ‘sprinkling’ of more wood screws so the plate was firmly in position and running true.   I then re-referenced G54 to the centre of the grate as before.

      Plate remounted on a new MDF backing plate and held in place with randomly placed wood screws using the grate hole matrix.

      Now I hate making swarf (chips) of material if it is not necessary … so having got the plate securely in place on the MDF I then took it off again and cut off the excess material on each end of the length.   Sad really but you never know when you might need a couple of small pieces of steel …

      The grate could now be mounted back on the MDF with the plethora of screws positioning it back as before.  I did re-check with the Haimer and also rechecked the Z height once again.

      The CAM adaptive profiling was with an 8mm cutter.  Obviously I was cutting air at each end of the grate where the stock was now missing but not a problem.

      Profiling almost complete. The MDF is starting to degrade and swell.

      I could have used a super glue and masking tape holding method but the black mild steel does not have a smooth surface like BMS and I was doubtful how well it would hold.   With hindsight the method I adopted did give me some flexibility in the process method.

      The final process on the Myford Super 7 was to make four posts to sit the grate at the correct height spacing above the ash pan to match the old bar grate position.  These posts were fixed onto the grate by sacrificing four of the holes.  This of course reduces the hole count and therefore the hole area percentage occupancy from 15.17% to 14.62% – but not worth worrying about.

      Finished grate standing in the ash pan and also the 3D printed test shape

      So now I have to prove that all this effort was worthwhile and the grate will make a difference to the Polly V steaming.  More to follow on this in due course.   We have had some rain over the past couple of days so the Club track will no doubt be open for steaming in the near future.

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        Golf Scoring Aid using Thumbwheel Switches

        I seem to have created a small demand at the local golf club for the simple golf scoring aid which I had designed and fitted to our electric trolleys.   The design makes use of old thumbwheel switches which manufacturers don’t seem to use these days with the advent of electronically created equivalents.

        Thumbwheels are cheap to buy on EBay and with the addition of a simple Fusion 360 designed and 3D printed block to mount them, they fit nicely on the side of the trolley scorecard holder accessory.   The design uses two thumbwheels so the user can score their own and their partners score.   Simple but very useful especially since I can’t remember what day it is never mind how many shots I have fluffed.

        golf, scoring aid, 3D printed mounting, thumbwheels
        Simple Golf Scoring Aid using Thumbwheel Switches

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          Rosebud Grates for Live Steam Locomotives

          The Rosebud Phenomenon

          I recently had a discussion with a fellow model club member about fitting a Rosebud grate to a Polly V locomotive.   There is quite a lot of discussion on various forums of this style of grate so I won’t repeat what has already appeared in the likes of Model Engineer.   The gist of the design is to replace conventional live steam bar type grates with a plate having a matrix of holes with back countersinks such as to occupy around 15% of the grate area.   The effect of this change is to get better combustion of the fuel and better efficiency.   Most users report only a fine powder residue after steaming and have observed that the fire appears to ‘float’ on the plate surface.  The back side countersinks appear to create a sort of Venturi effect to boost the draught to the fire.

          How to Calculate the 15% matrix

          So all this got me thinking.  This would be an easy job to run on the Tormach and all I needed was the design entering on Fusion 360.   Which brought me round to the calculation of the 15% surface area for the holes on the new rosebud plate.  Those who know me will bear witness to my weakness for doing spreadsheets and this little problem suggested a spreadsheet was needed.

          Below is a simple sketch of a rosebud fire grate with dimensional attributes.  In the calculation I have allowed for a border around the holes in case there are any no-go areas for the hole matrix.  I have now updated the spreadsheet to allow holes to be ignored such as where used for mounting pillars.   I also give the XY coordinate of the corner holes relative to material centre to help the machining layout.

          Below is a screen shot of the resulting rosebud grate spreadsheet and you can download it as a ZIP file via the following link – rosebud_grate_calculator 2

          Here is the finished Fusion 360 drawing ready to run on the Tormach.  This shows the bottom surface with the 4mm through holes having been half depth countersunk.   Clearly four of these holes will need to be sacrificed for mounting legs onto the locomotive ash pan and these are removed from the above calculation.

           

          So all is in place ready to cut metal and I will keep you posted on the progress and steaming results in due course.  (There is a slight problem at the moment in that we have a steaming ban in place because of all the dry vegetation at the club track).

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