Scratch build - LNWR Precursor Tank - Scale7
- Thread starter adrian
- Start date
adrian
Flying Squad
My apologies for the delay in updates - I bought a new laser cutter so got slightly distracted with setting it up and running a few special jobs for my better half. Work was ticking along on the Precursor Tank and as alluded to earlier I wanted to sort out the rear radial truck and as usual I had to go and do something different. 
Where to start? I appreciate all the suggestions made previously but none of the solutions particularly appealed to me. The curved channel options provides a compact solution but I struggled with trying to incorporate springing that I was happy with. In 7mm I just have never found coil springs to be satisfactory and think wire springs are the only sensible option. Alternatively just making a rear pony truck with a centre pivot means a pivot point too far forward and interferes with all the brake gear under the cab. If it were to maintain the same radius as the radial truck the pivot point would have to be in the middle of the firebox!
Whilst I was mulling over this dilemma there was a discussion on WT about the swing link bogies fitted to the Ivatt 2-6-2 locos. See the drawings about the swing link bogie got me thinking. The design means that on a curve the outside wheel increases the loading - I started to think what about turning that through 90 degrees and using it for a radial truck? So this is my experimental radial truck progress so far.
The other thing I love as an engineer is pouring over the GA drawings to understand how they built the original locos. So I took a snapshot of the rear bogie arrange and loaded it into QCAD. I created a few construction lines to get the radius of the radial truck and then drew on a couple of 10mm radius circles along the radius to work out intersection points.
The idea being that the if the fixed points on the chassis are narrower than on the radial truck then they will follow the correct radius. Hopefully all will become clear in the following photos.
As per the front bogie everything on the loco is running on miniature ball races. So this is where I started, again nothing special just a few bits of metal out of my raw material drawer.
A couple of brass lengths were drilled for the ball-race bearings and then soldered to a brass strip to make up a little cradle.
So this is my radial truck.
The top strip was drilled and tapped 12BA to make the swing link pivot points. The swing links were cut from a length of steel strip for strength as there isn't a lot of metal around the end bushes. There were drilled out to fit small brass top hat bushes so that they would allow the 12BA screw to clamp tightly but still let the swing link to pivot.
The swing links would provide the side to side rotation required but this still doesn't solve the problem of the vertical movement and springing. In the GA drawing there is a frame spacer just in front of the rear wheels with the brake cylinder mounted to it. Another reason for searching for a compact design. I then looked to use this frame spacer as the mounting point for the other end of the swing links. So I ended turning this little T-bar arrangement.
p.s. never realised this before but now having a set of ER collets for my mini-lathe - chucking and turning square bar stock is so easy with collet chucks it's untrue.
So the T-bar is turned for the droppers and a 10BA thread on the end for a captive nut. The droppers are drilled and reamed to fit on the T-bar with the lower leg threaded 12BA for a captive nut to hold on the swing link.
The T-bar is drilled and tapped 12BA to fit it to the frame spacer.
Hopefully the arrangement now becomes apparent. Note it is NOT a parallelogram - the pivot points on the radial truck are deliberately spaced wider apart than the fixed ones on the T-bar. When everything is central then the radial truck is parallel to the T-bar. When the radial truck moves to the side then the outside swing arm will move further away from the T-bar, the inside swing arm will move closer to the T-bar. If my dimensions are correct then this should match the correct arc for the radial truck.
Initial playing with the arrangement it seems to work as intended. However when I started looking at fitting it the frame spacer revealed a couple of draw backs. i.e. cockup I hadn't thought through properly. As illustrated below the T-bar is too wide and will not fit inside the rear frame, also the swing links on the radial truck are too wide and end up hitting the rear frames before allowing the rear truck to make a full swing. Looking at the GA drawing the rear frame has a double plate reduction under the cab so is narrower than I planned for. So slight modifications are required but it's late so will have to wait for the next instalment.
Where to start? I appreciate all the suggestions made previously but none of the solutions particularly appealed to me. The curved channel options provides a compact solution but I struggled with trying to incorporate springing that I was happy with. In 7mm I just have never found coil springs to be satisfactory and think wire springs are the only sensible option. Alternatively just making a rear pony truck with a centre pivot means a pivot point too far forward and interferes with all the brake gear under the cab. If it were to maintain the same radius as the radial truck the pivot point would have to be in the middle of the firebox!
Whilst I was mulling over this dilemma there was a discussion on WT about the swing link bogies fitted to the Ivatt 2-6-2 locos. See the drawings about the swing link bogie got me thinking. The design means that on a curve the outside wheel increases the loading - I started to think what about turning that through 90 degrees and using it for a radial truck? So this is my experimental radial truck progress so far.
The other thing I love as an engineer is pouring over the GA drawings to understand how they built the original locos. So I took a snapshot of the rear bogie arrange and loaded it into QCAD. I created a few construction lines to get the radius of the radial truck and then drew on a couple of 10mm radius circles along the radius to work out intersection points.
The idea being that the if the fixed points on the chassis are narrower than on the radial truck then they will follow the correct radius. Hopefully all will become clear in the following photos.
As per the front bogie everything on the loco is running on miniature ball races. So this is where I started, again nothing special just a few bits of metal out of my raw material drawer.
A couple of brass lengths were drilled for the ball-race bearings and then soldered to a brass strip to make up a little cradle.
So this is my radial truck.
The top strip was drilled and tapped 12BA to make the swing link pivot points. The swing links were cut from a length of steel strip for strength as there isn't a lot of metal around the end bushes. There were drilled out to fit small brass top hat bushes so that they would allow the 12BA screw to clamp tightly but still let the swing link to pivot.
The swing links would provide the side to side rotation required but this still doesn't solve the problem of the vertical movement and springing. In the GA drawing there is a frame spacer just in front of the rear wheels with the brake cylinder mounted to it. Another reason for searching for a compact design. I then looked to use this frame spacer as the mounting point for the other end of the swing links. So I ended turning this little T-bar arrangement.
p.s. never realised this before but now having a set of ER collets for my mini-lathe - chucking and turning square bar stock is so easy with collet chucks it's untrue.
So the T-bar is turned for the droppers and a 10BA thread on the end for a captive nut. The droppers are drilled and reamed to fit on the T-bar with the lower leg threaded 12BA for a captive nut to hold on the swing link.
The T-bar is drilled and tapped 12BA to fit it to the frame spacer.
Hopefully the arrangement now becomes apparent. Note it is NOT a parallelogram - the pivot points on the radial truck are deliberately spaced wider apart than the fixed ones on the T-bar. When everything is central then the radial truck is parallel to the T-bar. When the radial truck moves to the side then the outside swing arm will move further away from the T-bar, the inside swing arm will move closer to the T-bar. If my dimensions are correct then this should match the correct arc for the radial truck.
Initial playing with the arrangement it seems to work as intended. However when I started looking at fitting it the frame spacer revealed a couple of draw backs. i.e. cockup I hadn't thought through properly. As illustrated below the T-bar is too wide and will not fit inside the rear frame, also the swing links on the radial truck are too wide and end up hitting the rear frames before allowing the rear truck to make a full swing. Looking at the GA drawing the rear frame has a double plate reduction under the cab so is narrower than I planned for. So slight modifications are required but it's late so will have to wait for the next instalment.
Brian McKenzie
Western Thunderer
Super concept Adrian.
adrian
Flying Squad
Intriguing - oil burners are not my forte - do you have any GA's of their solution?
David Waite
Western Thunderer
Hi Adrian
Could you shorten the T bar and put the two swing links closer together?
David.
Could you shorten the T bar and put the two swing links closer together?
David.
adrian
Flying Squad
It was something I have considered and the solution I ended up with is a sort of variation. It's something I want to get sorted as at some point I'd like to build an Aspinall 2-4-2 which had radial trucks front and rear.
Crimson Rambler
Western Thunderer
I think the arrangement @Dave Holt described may have appeared earlier. I think Eric Langridge in Under Ten CMEs vol II illustrated something similar. Unfortunately I can't find my copy to confirm.
@adrian - the Vale of Rheidol comparatively recently modified the geometry of the trailing pony trucks using an approximately similar configuration:-
Crimson Rambler
@adrian - the Vale of Rheidol comparatively recently modified the geometry of the trailing pony trucks using an approximately similar configuration:-
Crimson Rambler
Mike Garwood
Western Thunderer
Adrian
That's a fabulous solution and I think your right about the duplication of your solution in 4mm. However, the photo Crimson Rambler has posted of the Vale of Rheidol, the photo on the right of the two. That looks doable in 4 mm. Fortunately I have another Comet etch to play with (and destroy). First things first I need the centre line of where the radial truck lies in relation to the chassis. Time to fire up AutoCAD.
Mike
That's a fabulous solution and I think your right about the duplication of your solution in 4mm. However, the photo Crimson Rambler has posted of the Vale of Rheidol, the photo on the right of the two. That looks doable in 4 mm. Fortunately I have another Comet etch to play with (and destroy). First things first I need the centre line of where the radial truck lies in relation to the chassis. Time to fire up AutoCAD.
Mike
adrian
Flying Squad
Many thanks for posting those diagrams - very interesting to see the different solutions.
I like the VoR one, the fixed pivot points are much closer together than I'd planned for so may be an option to consider in the next one planned. Interesting the use of the spherical bearings for the up and down suspension movement. There are miniature spherical bearings used in the R/C aircraft world for control rods actuation. I did consider trying them, maybe next time!
The class 40 seems to be vey similar to the solution I have settled on, although as notionally a trailing bogies I have the links working from the opposite side.
I went back to my drawing and found my error on the width. Re-jigging the links to give sufficient side to side motion and not strike the frames actually ended up with only a small modification. It only needed a gnats whisker over 1mm extra, as shown in the photo by moving the links from the rear of the bogie to over the axle centre line allowed me to get the side play needed.
As per the photo's I've now got it mounted to a frame spacer - screwed to a small length of angle brass with two 12BA bolts. I've not decided on the springing arrangements but the 2 redundant holes in the bogie probably means thst there will be length of spring wire located in them to provide vertical and lateral side control.
I like the VoR one, the fixed pivot points are much closer together than I'd planned for so may be an option to consider in the next one planned. Interesting the use of the spherical bearings for the up and down suspension movement. There are miniature spherical bearings used in the R/C aircraft world for control rods actuation. I did consider trying them, maybe next time!
The class 40 seems to be vey similar to the solution I have settled on, although as notionally a trailing bogies I have the links working from the opposite side.
I went back to my drawing and found my error on the width. Re-jigging the links to give sufficient side to side motion and not strike the frames actually ended up with only a small modification. It only needed a gnats whisker over 1mm extra, as shown in the photo by moving the links from the rear of the bogie to over the axle centre line allowed me to get the side play needed.
As per the photo's I've now got it mounted to a frame spacer - screwed to a small length of angle brass with two 12BA bolts. I've not decided on the springing arrangements but the 2 redundant holes in the bogie probably means thst there will be length of spring wire located in them to provide vertical and lateral side control.
adrian
Flying Squad
There is sufficient! It's part of the design I'll probably refine in the next iteration. There is a small top hat bush in the steel links attachment to the truck. Originally I had put a slight radius on the bush to allow a little roll. When the attachment point was at the rear of the truck it worked quite well as a "spherical bearing". Unfortunately moving the attachment point to the centre of the truck has restricted the roll a little. I think there is enough slack in all the joints for a little roll in the truck, but as I say it's one area I'll probably tweak on the next iteration. Every day is a learning day.
Now that was sorted I turned back to the main frames and horn guides. The intention is to fit miniature ball races and rather than castings or etchings I make the horn guides from a bit of brass channel. In this case I've use some U channel out of my raw metal draw and cut one side off to form the L shape.
For those of you with OCD may notice that the rear driver horn guides (on the left) appear to be slighty wider than the leading driver horn guides (on the right). That's because they are!
One thing I love about scratch building is looking at the drawings and trying to understand what the engineers of old designed. From looking at the GA drawings it seems on the Precursor Tank the rear driving axle actually had a 4-coil spring suspension arranged in a square format - the 4 leaf clover in the drawing below. Hence the horn guides were quite deep. My thoughts are to solder on a square plate on at the bottom of the horn guides and then 3D print a 4 coil spring representation to stick on later. There is an additional bracing plate across the frames between the left and right horn guides, that will be covered later on.
Then the leading main axle only has a twin coil spring arrangement. Hence the narrower horn guides.
Against one of the main frames I then clamped a straight edge across the top of the frames parallel to the horizontal datum.
The reason for this is that when I start soldering the brass angle to the frame I can use this with an engineers square to ensure that the angle is truly vertical in the frame.
Once the first horn guide is soldered in then I have a short length of 3/16 brass hex with a small flat filed on one side - it looks not too disimilar to a Monopoly hotel. This allows me to solder the other side of the horn guide in parallel to the first. Hopefully this means everything is straight, square and parallel. However this first one is critical, I soldered this one in and then walked away. I came back later and ran through my checks and it didn't feel right. Something wasn't quite lining up so I dismantled it and started again, this is actually the 3rd attempt with which I was happy.
Once I had the first one done to my satisfaction now is the time where a short stub axle in a horn block is used. With the steel coupling rods it now gives me the spacing to the 2nd axle on the main frames.
Now I had one side completed I can clamp the second side to the first. Using my "Monopoly hotel" stood upright through the two frames meant that I had a reference point to solder the horn guides on. [ note the small step between the horn guides and the main frames is to accomodate the top hat ball race. Hopefully more apparent later on.].
Now I have mainframes with horn guides fitted. Note only the first one was fitted "by eye" the remaining 3 have all been fitted by reference from the first. Hopefully this makes it all square when assembling.
Attention then turned to working out where to put all the frame spacers. Again reference to the GA drawing was useful and within the restrictions of the model I have tried to copy the full size where appropriate. For this build I have decided to try fitting the font and rear buffer beams to the chassis rather than the body, mainly for detailing and painting issues. It'll be interesting to see how it works out. As mentioned they did apply a bracing frame between the horn guides which will be cosmetic additions later on. The rear trailing radius truck is mounted on a frame spacer for fitting just behind the cab. The rest of the bits I've cut out by happy co-incidence seems to match pretty reasonably the etches @mickoo posted a couple of days ago so I must be doing something right.
One thing I did note from the GA is that the rear frames actually have an additional spacer plate behind the rear drivers, so it is 3 plate width.
In Scale7 I'll take anything going so I have an additional spacer so that the rear frames are even narrower and give a bit more swing for the rear radius truck.
Rivet location and frames are marked out for the front of the main frames.
The one remaining spacer now to sort out is the bolster for the front bogie. It's going to need a 3D design to fit in properly. I have a couple of ideas in development but will need to leave that to the next post as I'm still thinking through the details.
adrian
Flying Squad
Slight pause to the modelling activities over the last couple of months, not just Christmas getting in the way but after the loss of our previous pet the house seemed empty so at the beginning of December we had 2 new arrivals! They have kept me rather occupied and busy - believe me they are not always this angelic - much to my wife's dismay they chewed up her reading glasses today!
However some progress has been made on the Precursor Tank - concentrating on the front bogie support. Now anyone who has built a MOK loco may recognise the inspiration for the design from building the ashpans. Just a few bits of nickel-silver sheet with a few slots cut.
It slotted together quite nicely - the angled edges and over length strips are just because they are scrap bits from previous component cut. It will be tidied up later.
I paid particular attention to making the joints as tight as possible as I wanted to silver-solder this component so that later on I could soft solder it to the frames without worrying about it falling apart.
This is after silver-soldering.
Like any soldering joint - seeing the solder flow through to the other side of the joint is a really good indicator that the heat got to the right places and that the solder is through the joint. This is one of the reasons I love silver soldering in the right application, the pale white line along the joints is the silver solder, when it flows correctly you get a really nice fillet along the joint that needs no cleaning up.
Starting to trim the excess metal and file back to surface show that a good solder fillet has been made throughout the joint, although it seems that I cut one of the slots slightly too deep!
I think I'm now at the stage where I have enough of the components cutout, and rivetted, to start soldering everything together for the frame.
For this locomotive I've decided to build it with the front and rear buffer beams attached to the loco frame rather than attach them to the upper body work of the loco. This is for a couple of reasons that may may become apparent later. First I could use them as frame spacers and secondly looking in detail at photographs and drawings the loco footplate on these locomotives actually do not over hang the buffer beam. In fact the buffer beam is slightly proud of the footplate. There is a slight overlap but the footplate only half overlaps the buffer beam.
However some progress has been made on the Precursor Tank - concentrating on the front bogie support. Now anyone who has built a MOK loco may recognise the inspiration for the design from building the ashpans. Just a few bits of nickel-silver sheet with a few slots cut.
It slotted together quite nicely - the angled edges and over length strips are just because they are scrap bits from previous component cut. It will be tidied up later.
I paid particular attention to making the joints as tight as possible as I wanted to silver-solder this component so that later on I could soft solder it to the frames without worrying about it falling apart.
This is after silver-soldering.
Like any soldering joint - seeing the solder flow through to the other side of the joint is a really good indicator that the heat got to the right places and that the solder is through the joint. This is one of the reasons I love silver soldering in the right application, the pale white line along the joints is the silver solder, when it flows correctly you get a really nice fillet along the joint that needs no cleaning up.
Starting to trim the excess metal and file back to surface show that a good solder fillet has been made throughout the joint, although it seems that I cut one of the slots slightly too deep!
I think I'm now at the stage where I have enough of the components cutout, and rivetted, to start soldering everything together for the frame.
For this locomotive I've decided to build it with the front and rear buffer beams attached to the loco frame rather than attach them to the upper body work of the loco. This is for a couple of reasons that may may become apparent later. First I could use them as frame spacers and secondly looking in detail at photographs and drawings the loco footplate on these locomotives actually do not over hang the buffer beam. In fact the buffer beam is slightly proud of the footplate. There is a slight overlap but the footplate only half overlaps the buffer beam.
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cmax
Western Thunderer
Adrian,
Your modelling is fantastic, And thank you for introducing Stan & Flo, Know the feeling losing a pet brings, but look at all the fun you will have in the future, and I'm sure your wife has already forgiven them for chewing up her reading glasses! Enjoy.
Pencarrow
Western Thunderer
Good to see you getting chance to do some modelling Adrian.