Modern Outline Kits GWR Armstrong Class 7 4-4-0 - Part 2
Raymond Walley
In this second instalment Raymond tackles the locomotive’s chassis and inside motion
See also Part 1 and Part 3
Ian Rathbone concludes the series where he describes painting the finished model in Part 4
The Bogie
The bogie frames are a three-layer laminate. The spring hangers fit neatly into holes behind the slots in the outer frame side; a simple and elegant basis on which to build the bogie. As the engine is a 4-4-0 I expected that some form of springing and possibly side control would be necessary, though there is precious little room for any since tolerances are very tight. Peter Roles indicated that the method employed works well so I looked forward to seeing it running in due course (which it did perfectly, but more on this later).
The bearings rest in beams that fit into slots in the main frame and are held in place by 10BA bolts. Each axle has limited vertical movement but enough to compensate the unit. It is attached to the chassis with a pin through the central hole in the cross member. This is a close fit with little vertical movement in action. The wheels take time to fettle and remove all the excess plastic from the spokes but the effort is worth it. The instructions make no mention of the wheel splashers, part B014 on the brass fret.
Bogie frames erected
Assembled bogie, springs to be fitted
Bogies frames, wheels and bearing plates waiting assembly
The Chassis
Erecting the main loco frames
Steam chest and slidebar bracket
Solder reinforced frame joggle
Ashpan weighted with liquid lead,
compensation levers and axle bearings
Axle bearings, keep plates and bolts, compensation lever pivot
The frames look a little flimsy at first sight but once the various spacers are fitted, it makes up into a rigid structure. The compensation is very elegant, except for an enormous 6BA cheese head bolt that holds the bush. I misread the instructions here and merrily opened out the hole in the side to take the bush. Not a bright thing to do; it was a bit of a fiddle correcting the error but it all works fine now. The frames are also handed to allow for the etched lines that make up the joggle. I got mine wrong and it was by now far too late to go back so the joggle had to be done by bending against the etched line instead of into it. Therefore it was not possible to add a fillet of solder to this potentially weak area I got round it by soldering some wire into the etched line and filing it down to match the contour of the frame, as will be visible in a later picture.
At the time the pictures were taken, the frames were not soldered up as I was doing a dry-run.
The ash pan is a separate item with provision to hold the motor in place and adjust the position in relation to the axle. There is also space in the base for some ballast. I used liquid lead with some twopart liquid resin glue from Ripmax to hold it in place (an idea from Bob Alderman). A repeat of the problem I had with the Prairie (See Gazette Vol. 17 No: 7 Page 42) would be a major disaster. The ash pan is clipped in place using the tabs provided.
I decided to modify this arrangement and drilled frames and the sides of the ash pan to clear 12BA, soldered some pieces of brass on the inside of the ash pan and drilled and tapped them 12BA. The tabs were removed and now it is a simple matter to drop the ash pan out to release the motor. I had to modify this later because the bolts need to be inserted through the wheel spokes and the original holes were not in the correct place. At the same time I replaced the 6BA bolt with a short length of studding to which I had soldered a nut, filed to about half its thickness. I am happy now with how it looks.
Part of the rigidity of the frames is imparted by the cylinder head and motion bracket, which need the holes in them tapped 10BA, before bolting in place between the frames. This also allows all of the inside motion to be removed. Once happy that all parts fitted correctly the frames were soldered solid and the etched lines of the joggle filled.
The inside motion
Work progressed on cleaning up the castings for the motion. It is not a huge amount of work because the castings are of high quality but the bearings need careful fettling to obtain a smooth-running set of motion. Never previously having built a set of working inside motion this stage proved interesting.
Slide bars are fitted with piston rods and connecting rods in dry-run mode, hence the rough pins holding the bearings on the cranks. Underneath, the valve chest is fitted with its moving parts in place, rather too soon as became apparent later.
The cast slipper/piston rods were modified. I have never understood why designers incorporate the rod when making a master for casting the piston rod/slipper part. I cut off the rod and drilled the slipper 1.6mm right through, then force fitted and soldered a length of rod into the hole and re-drilled the pivot hole 1mm for the pin. When I suggested this to David Sharp at MOK he said that normally their kits have slippers cast with a hole but that this set had been bought-in from Peter Roles as there was no point in producing new masters when an excellent set was already available.
Next job was to fix the cranks to the axle. Ordinary solder I considered would not be adequate (doubtless some will disagree) and I am not confident with silver soldering so used Loctite 603 as recommended during a discussion on the 7mm E-group This proved a very simple exercise once the parts were physically and chemically clean. To ensure a really secure joint, the webs were drilled 1mm and nickel-silver pins set in place with Loctite 603. I managed to break one expensive stub drill bit doing this.
I initially used pins to hold the straps for the valve gear rods in place on the crank axle but this method proved unsatisfactory, producing loose, inelegant joints making it difficult to determine when the parts fit the crank. I scrapped the first pair and sent for replacement parts. A far better way is to use bolts to hold the parts together thus allowing relatively easy removal I began by turning down some brass rod to 10mm diameter to match the crank on the axle.
The parts were cleaned up, the larger strap drilled 0.8mm and tapped 14BA and the other drilled out 1mm. The parts were bolted together and one pair of ends marked to ensure consistency when reassembling. A rolled tube of 240 grit wet-and-dry was used to open the hole until it fitted the previously turned brass rod. It is then a simple matter to polish them up for a good fit on the crank axle. The parts can be disassembled as required. The forked rod is soldered to the strap and the pins reduced in size. It is important that the straps are fitted to the crank axle with the two oil pots at the top and the forks facing outward from the centre. The manual is far from clear on how the gear goes together but David came to the rescue with some advice.
Fitting the forked ends to the valve slides takes a little effort. There are four lift links provided among the etched parts but, only two are needed. Three of the four ends had some thick scrap etch soldered over the hole, shaped, drilled and tapped 14BA. This gave a basis for most parts to be bolted in place, a poor move as it turned out.
The forks at the top are bolted but those at the bottom are pinned with 0.8mm rod. The eccentric links are also pinned to the valve slides but with 1mm rod. Each lift link is fitted to the outside fork at the top of each eccentric; the excess thickness of the tapped block being filed down to obtain clearance, which is very tight. The bottom ends of the lift links are then bolted through the weigh shaft arm.
Crossheads
Crank axle as supplied
Drilling cranks for their retaining pins
Inside motion components: slide bars, steam chest cover and valve parts at back with cranks, con-rods and crossheads at front
Trial assembly of motion parts
Modifying the valve-rod big ends
Checking the fit of valve-rod big end
The finished crank axle
Fitted valve gear from above
Fitted valve gear from below
It was necessary here to introduce some spacers to keep the lift links vertical so I used a couple of 14BA nuts with the thread stripped out. I had also to unsolder the slide valve box to fit the slides to the eccentrics.
At this point I decided the time had come to drop the whole lot into a bath of Viakal, followed by several minutes in the ultrasonic cleaning tank.
Fitting the wheels and bearings came next and I used the lathe to push the wheels on to the cranked axle. As it needs considerable force to seat them, they are not likely to come loose but the other axle needed Loctite 603 to fix the wheels and one of them wobbled so badly it had to be changed.. Insert photo 21 here Once the axle is fitted in the turned crank unit, it needs a section cut out to leave a slot opposite the cranks for the piston rods to pass through and this looked as though it could prove problematic. How to hold the thing in a vice while sawing? A few minutes thought and I realized that the lathe would again serve; with the axle supported in the chuck and tail stock it was an easy matter to cut using an, oiled, fine piercing saw, there was little to clean up afterward.
The next pictures show the valve gear from top and bottom and installed in the frames. The whole unit, crank axle and valve gear fitted into the frames perfectly; a credit to the designer. Now it was time to make the coupling rods so that it could be tested properly and run in.
Slide bars, motion bracket and steam chest fitted (above and below).
Coupling rods and Crankpins
The outside cranks
Assembling the coupling rods
Assembling the coupling rods
Crank-pin nuts
Rods and cranks ready for fitting to the axles
Beefing-up the valves
Valve gear assembled
Each rod comes as three pieces of etch to fold up and laminate. I used cocktail sticks to line them up in a vice, applied a liberal dose of flux while trying to avoid it reaching the sides, tightened the vice up fully and then used a 75 watt iron with a 179 degree, 2% silver eutectic solder wire. The ends are clamped with aluminium hair grips. The extra bosses are soldered after this operation, despite what the picture may suggest.
It is then simply a matter of cleaning up, filing to shape and polishing. I got the method from Malcolm Mitchell and it is now the only time I use a soldering iron aside from electrical work.
I had first considered using CPL crankpin nuts designed for Harris wheels, which come as a bolt but they were not suitable for the job so instead I fitted a 10BA bolt through a clearance hole in the crank and similar to my method for Slater's wheels generally. The crankpin nuts are CPL for Slater's wheels and need to be opened out and tapped 10BA, fiddly but worth the effort. This shows the completed set, with mirror finish cranks ready to be fitted to the wheels.
Unfortunately, the test run exposed problems with clearances in the motion. My lack of experience and precision, combined with the attempt to make it all disassemble easily was not an ideal combination. In addition the valve crosshead slides had lateral movement of almost half a millimetre, with the result that the gear simply would not run. It had to be taken apart, modified and rebuilt.
The valve crossheads were easy; each crosshead had two pieces of 0.1mm phosphor bronze strip soldered to the outside face, which cured the side slop. I thought about how the motion operates and decided that since the reverser did not operate, there was no need for the eccentric links to be able to move in the vertical plane. The centres had small pieces of nickel soldered in place and the pair were drilled 1mm together in the drill press then assembled to the valve crossheads.
There is now no side-slop in the valve chest and no wobble in the eccentrics. The rest of gear was reassembled using soldered pins. CPL locomotive couplings come with a small etch containing far more small nickel washers than needed and so some spares were used, with slips of paper to then screwed a 10BA bush tightly on to it, prevent soldering the joint solid while assembling all the pivots. A much neater job resulted and finally the gear ran pretty much like a sewing machine but the rods were tight and needed further work.
This then produced a further problem. While reaming out the rods with a broach, the bosses began to de-laminate, rendering them useless. I think it is caused by the burrs raised while using a broach that does the damage. As a result I tried for the first time a set of Precision rods. Ordered via their new website on a Saturday, they landed on my doormat on the following Tuesday; excellent service. They ran first time, though a little tight but a few minutes with a broach sorted that out.
Side view of the completed inside motion
Completed motion from above
Bench testing the chassis
When I fitted the balance weights to the wheels I also used some Milliput between the spokes to represent the molten lead that was poured between the holding plates. It all helps to add authenticity.
The third part of the story will conclude Raymond’s construction of the locomotive and Ian Rathbone has volunteered to write about his experiences of painting and lining the loco and tender which will form the fourth and final instalment
