minnie scale 1:12

[Lindo]

I am in the process of building "minnie"
From the book Chapter 1,the parts for the boiler call for a 12" sq 16 g plate that can be cut into to pieces.to avoid waste.I did this when purchased the material,but the main wrapper plate was cut 4" x 10 1/2".but further into the book LC Mason is stating 3 - 9/16" x 11" long.The width is no problem but what
is the real length before bending.I calculate it to be 10.755".
Thanks
John

 

[Jasonb]

Depends where you take the neutral axis or median radius of the bent sheet, but plus 0.44 x thickness is a common one.

So internal diameter is 2.5" plus twice the median (2*.44*0.062) = 2.5 + 0.054" = 2.554" median diameter

2.554 x 3.142 = 8.025" but we want half a bend so 4.013

Two sides @ 3.250" + 4.013" = 10.513" total length of wrapper.

But in practice I would cut it a bit longer as its hard to get the bend dead in the middle of the sheet and then trim off the excess.

J

 

[Lindo]

Thanks Jason
this helps alot.
I will machine 2 4 mm dowels holes in the wrapper sheet,5 .25" in from the long end and turn up a bar to the outer diameter with 2 x corresponding dowels inserted.
After annealing I can slip the strip onto locating on the dowels of the forming bar,bend and it should bring the sides down the same length.The plate should not slip out of position.
while I was waiting for the boiler material,I started on the wheels,all done off the lathe.the rears see photo started out of steel tube,quite a chunk of material removed.
Also made the spoke/hub building fixtures out of MDF.The front slips into the rear fixture,and all machined bolted to a faceplate.I will leave on so I can
align the spokes by putting the faceplate/fixture to the lathe,accurately maintaining concentricity.
I am making the rear hubs in 3 pieces see photo of the blanks now waiting the mill time for tear drop profiling and holes and spoke slots.
forward and onward.

John
Spain

 

[Lindo]

Hello Jason and other minnie builders.
As i am a novice at this project and an ex Formula 1 engineer,maybe I am
overthinking this boiler making construction phase.
The manufacture of the mechanical side,example gears,crankshafts etc etc is no problem.
Dumb question,reading LC masons book,I see no mention of drilling the
stay holes for example in the throat plate prior to brazing to the barrel.
I would of thought all the fixing holes,steam fitting holes etc in the various boiler pieces be done prior to bolting/riveting and final brazing together.these boiler pieces can all be milled and drilled accurately on a machine.
Then the pieces be put together like a meccano set to ensure everything goes together,including the horn plates.
For example,if the throat plate is brazed to the barrel,it would be difficult to follow up with drilling the stay holes,and again the smoke box end,why not drill the 20 rivet holes on a tilt circular table (horizontal) whilst holding the barrel between centers with the wooden plugs still in place.
I need some advice on this issue please.
I was recently told,if I construct this model like a precision race car,the final tractor will not run,it needs plenty of slop.I just think,don't work hard,work smart,but maybe I am over enginnering this.
All help appreciated.
John

 

[xpylonracer]

Hi John, nice looking work so far. I haven't built a steam engine but would think it's true some parts will need more clearance than you are used to, even allowing for scale, but surely there will be a drop in performance if clearances are too great at the important parts. Perhaps someone with experience can enlighten us !!!
Rgds, Emgee

 

[RonGinger]

The biggest problem I had with my Minnie was getting the stay bolts in the boiler to align with the side plates. I really did not like this assembly- trying to get the whole array of stay bolts to align with the plate, then getting both sides to align well enough for all the shaft bearings was really hard.

Id like to see a structure where the horn plates are joined by some cross bracing, then the boiler mostly sits into the structure, much like a locomotive boiler just sits on the frame with a single point attachment at the smokebox.

 

[Jasonb]

You need to remember that at the time Mason wrote the Minnie series most model engineers did not have a milling machine so everything was done on the lathe with manual marking out and only using the handwheels, certainly no DROs then. I made mine without a mill, few pics in this album page 5

The reason the stays are drilled after some of the assembly is that things are likely to move about a bit during soldering so not much point in drilling the sets of holes to super accuracy only for the two plates to be a few thou out when you come to put the stays into place.

You will want a bit of running clearance in the gears and shafts due to the fact that the whole engine uses the boiler as it's chassis and when that warms up it will expand and you could find things going tight on you. Same goes for the valve gear, what works when cold will change as the boiler will get longer but the reversing rod, valve rod etc will stay the same length.

J

 

[Jasonb]

Should also say that the book and drawings show a somewhat old school way of doing the stays in the sides of the firebox, current practice would be to have 4 hollow stays each side where the hornplates bolt on and the remaining 5 would just be 1/8" copper rivits pushed through from the inside, lightly bent so they don't fall out while soldered and then the excess cleaned off

 

[Lindo]

Jason
Looked at all your photo's excellent work and craftsmanship,well done.

Yes lining up the numerous holes in the horn plates will be challenging.a fag
packet sketch of your ideas would be useful.

The boiler side plate stays I'm thinking of initially putting in a known set of datum holes for dowel pins and move off from there.

Regards
John

 

[Jasonb]

This is a boiler for a 2" engine but shows the hollow stays where the hornplates bolt on and the remaining stays as rivits. You can either leave teh stays solid and spot teh outside from the hornplates or solder them in hollow and spot the hornplates from the inside out.

On teh minnie the hollow stays are left over length and you can then set the boiler up on the mill and machine tham back to 1/16" projection to give the gap that the 1/16" stay nuts would have done.

Don't know what part of the world you are in but run any alterations to the published design past your boiler inspector first.

J