Crab Comet conversion

[billbedford]

Fascinating Bill! Beyond my powers of comprehension.

Then I suggest that model railways is not really for you. Perhaps you should take up something less challenging, maybe like knitting?

[Le Corbusier]

Fascinating Bill! Beyond my powers of comprehension.

Then I suggest that model railways is not really for you. Perhaps you should take up something less challenging, maybe like knitting?

Harsh perhaps

[Julian Roberts]

Can't be bothered with that sort of stuff

[Julian Roberts]

I'd have thought Bill you would want to encourage modellers, assuming you make a living from selling us stuff?

If this was my first attempt I wouldn't be writing it up here but comments like your last would be... off putting. I don't subscribe to the view that to make a model you have to fully understand every minute detail of the prototype, nor have a degree in engineering as a minimum requirement. Or the idea that every last invisible detail should be modelled to perfection. My gurus that I have never met are Iain Rice and Mike Sharman. They write in simple terms and above all encouragingly. My guru inspirer, unfailingly positive and encouraging, who I have met is Allan Goodwillie who makes it his mission to explode any P4 snooty mythology that may exist in some people's eyes, to get in there and HAVE A GO. His loco building thread is well known here on this Forum.

Having said which, on with the KNITTING haha. I will do this in several posts as it's easier when using a phone to do it like that. Nothing very profound coming up here...

Several tasks remain before the chassis can be up and running and I said I would detail where I go off piste from the instructions. Having abandoned the kit's springing for the rear two axles for reasons already expained the compensation twin beams had to be filed to give an exactly level middle axle with the rear one at its desired ride height (as also explained previously). After this the little rod on the hornblocks that was to give ride height for springs was removed.

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The front and middle wheel sets are going to be coming on and off the chassis frequently during construction. It is a total pain getting both hornblocks of a wheelset vertical and slotting into the chassis at the same time so I'm trying on this kit soldering them to a tube to surround the axle to make them into a pair. Obviously done in situ in the chassis with plenty of felt tip marker pen to prevent accidental fixing of blocks into the chassis. A slot is cut for oil.

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Some gears of the gearbox may foul the rear spacer and be damaged, not in ordinary running, but when taking the the box/wheelset assembly in and out of the chassis. So a slot was cut and filed, and a small bit of brass to prevent it lifting too far. The plastic gears are quite vulnerable to being bent.

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The box 2nd stage was Araldited and then excess filed or cut away as the beams may rest against the box. There is enough space for them but if they rub against it they must be able to move smoothly so that area was checked for free movement in the range of possible motor swing. The beams shouldn't go quite as high as the 2nd stage in normal orientation.

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The Araldite was also used to fix on the worm and an outside tube to make the beams able to slide into each other as a pair to make (dis)assembly easier.

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[Julian Roberts]

The wheelsets were now assembled on the GW Models quartering jig. I coloured the jig with the same colour code as the hornblocks (blue on the right like the Tories, no value implied in the the word 'right'). It's done in one shot with no messing about with BB or quartering in the future. I won't go into BB arguments here. I boobed a bit on the front wheelset and set it in the low 17.60's, fortunately without locking it up solid against the washers. I'm perfectly confident this will have no adverse effect whatsoever on running, and the mistake gives me a minute bit of extra room for the crosshead. The middle wheelset came out with the desired 0.5mm play each way.

To lock the wheels on the axles I cut a slot and fill it with a piece of piano wire and Araldite.

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Now I had hoped to finalise how to get 50g Exactoscale springs into the place where the supplied ones go. They have the behaviour I want, as a 0.5mm movement of decompression has little effect on the force they exert, so they would be pushing nearly 50g through the wheel onto the track even if there is a dip in the rail level. But I found it impossible to use them without their deforming and conclude the only way to use them is to mount them in an enclosed cylinder as happens with buffers. And that seems a step beyond me for now in that the way it would be mounted isn't possible... or rather, just too complicated to think out at this stage in comparison with the easy option, to use the springs provided.

These may not provide enough downward force on a track dip without too much upward force at the desired ride height. So the steering of the loco may be iffy unless the pony truck can provide effective directional force. This will all have to be worked out by trial and error later on.

Next the brakes, as it will be them that will prevent the wheelsets dropping out of place. So they will be removable. A 1.6mm tube was soldered in the holes for the brakeshafts and filed back to put the brakes the right distance from the chassis. The 0.7 rod that will hold the brakes will be removable.

The front brake assembly is separate from the others. The instructions were confusing here and I found the bracket had to be fixed on in the reverse direction to that shown there, but as per the numbering on the fret. Anyway this worked out to give the right clearance of brake to front wheels.

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The brakes themselves were numbered on the fret slightly confusingly. There is a full depth etch and a thinner one for each brake. The main thing is that the front ones are a different length to the others. I found myself making each brake into one that could be used either way round though that may not have been the design intent.

I couldn't see how the real thing would work if I followed what I thought he instructions say as far as the linking if front brake set to rear/middle one is concerned. But as it's all pretty unnoticeable I didn't get too headbanging about it being absolutely correct - it was invisible on all the photos I could find.

The rear fixing bracket had been attached some time ago. The hole in it for the brake pull rod shaft brought the brakes a little too close for model comfort. Shorts would be likely. So I arranged the pull rod to be about 1mm further forward by tiresome opening out and fixing on a bit of strip with hole in desired location.

On the front brakeset pull rod I soldered the cosmetic springs to give a travel limit to the hornblocks and prevent the real springs falling out. They are a long way inboard of where they should be but I can't get steamed up haha about this almost invisible kind of stuff.

The time taken to make all the brakegear removable makes the whole process much more time consuming than on previous locos. The brake assemblies will have pickups attached so need to be also fairly rigid. The pull rods prevent the wheelsets falling out. They have a removable shaft at each end.

The rear brake shaft is also the compensation beams' fulcrum so here just small tubes were soldered on the chassis sides for the brakes which will just spring on and off the beams' rod, which itself is removable too.

A bit of spacer was made that would fit into the chassis, and screwed onto the body rear fixing hole. Then with great care the chassis was aligned correctly in every way (with the front end screwed down into its place) and tack soldered to it. Now it's a matter of not losing the tiny screw...

The front end had a long piece of doubled up EM spacers ( the ones I have fit between the Bachmann cosmetic frames) attached with similar care.

I will post photos separately

[Julian Roberts]

Some pix to illustrate previous post. Apologies to the squeamish for liberal use of solder on invisible parts.

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[billbedford]

I'd have thought Bill you would want to encourage modellers, assuming you make a living from selling us stuff?

If this was my first attempt I wouldn't be writing it up here but comments like your last would be... off putting. I don't subscribe to the view that to make a model you have to fully understand every minute detail of the prototype, nor have a degree in engineering as a minimum requirement. Or the idea that every last invisible detail should be modelled to perfection.

Engineers communicate with drawings, so if you admit that, after all the time you have been attempting to model, you find some relatively simple engineering drawings 'incomprehensible' then I would suggest that you haven't understood the basics of what you are trying to do.

It's a bit like someone wanting to become a musician by joining the Portsmouth Sinfonia.

[RobM]

Engineers communicate with drawings, so if you admit that, after all the time you have been attempting to model, you find some relatively simple engineering drawings 'incomprehensible' then I would suggest that you haven't understood the basics of what you are trying to do.

It's a bit like someone wanting to become a musician by joining the Portsmouth Sinfonia.

Bill, having been a model engineer since the 1960's with a live steam 2" scale compound traction engine, a 3.5" gauge live steam loco and a number of stationary engines under my belt I am fully conversant with engineering drawings and would say that the drawings on your drop box are to me comprehensible but not exactly relatively simple with having to scroll all over the place and not being able to see the drawings in their entirety. From my 4mm modelling experience in latter years I have used little of my model engineering skills (including reading drawings from the full size works) but it has not precluded me from producing the models I make.
From what I see, Julian is addressing various problems and 'sorting' them with belt and braces but eventually the visual effect will be convincing and will hopefully conclude with a good running loco.
As an aside and which Julian will not concur... , in the '60's I was a 'musician', well a drummer in a number of bands......couldn't read music but I knew a duff note......and I play keyboard and guitar by ear.......

Julian, keep up this thread which is educational to both noobies and the more experienced............
Rob

[Julian Roberts]

Rob
On the contrary I would say that defines you much better as a real musician than the ability to read a lot of notes in a short time in the note factory as we sometimes call our worser weeks. By some definitions I am just a note operative. The ability to see the meaning of the notes on the page is unfortunately not a prerequisite to earning a living in the "industry" nor is any "meaningfulness" a sine qua non of contemporary music...rather the inverse may sometimes seem to apply.

Anyway to business...thanks for your words!

I am surprised no one has hounded me out that the brake pull rods are actually just a single one on the chassis centreline. ..according to the instructions: which say the assembly is more robust if done as two nearer each side. My reason is that I also intend to put pickups on them so have a greater amount of fixing points for pcb strip. One's eye is not drawn to this error....and it's all a matter of trompe d'oeil I think, Rob?! - given appropriate use of colour.

Bill - I make kits I don't design them. Surely it's the designer who needs to understand the drawing?

In any case I was being a bit flippant as I was running out of time and should have been more grateful for your posting them. I did see that there is a difference between the two types, just did not understand the rather complicated Crab mechanism. What would the reasons be for such differences, what do they intend to achieve....indeed is it correct the say there are differences ? - however much, or not, it is necessary to model them.

[Philip Hall]

Glad you haven’t been discouraged, Julian. Your posts on here have been the source of many questions and subsequent answers, and whilst your way of doing things might not be mine you seem to be enjoying working your way around problems which is really what this modelling lark is all about. When it came to knitting, I always looked at my mother when she was doing it with complete bewilderment! I feel the same way about some engineering drawings, despite having been taught at school how to produce some. But that was fifty years ago...despite this handicap I seems to be have been able to produce some acceptable models. I often have to phone friends to ask what a particular chunk of metal does on a steam engine.

I seem to be spending most of my time these days hacking RTR mechanisms around, and that has strangely taught me a lot about what is good to have and what isn’t needed if you don’t fancy it. Things like absolutely true and round wheels, and how much you can get away with leaving alone to produce something that has a bit of life to it, without spending half a lifetime building one engine. I wish I could match Allan’s production rate!

To get back to your engine, pickups hanging from the brake gear are a good thing so long as you don’t want to take them off again (which I try not to want to do) and I used exactly this dodge on a big GWR 72XX built from a PDK kit. I set the brake rodding either side to try and keep the juice on each side apart. It gives a lot more flexibility when deciding where to place the collectors. I have some incredibly thin PCB which I attached to the cross shafts without making them too thick.

Keep up the good work and we will be interested to see the finished engine.

Philip

[Julian Roberts]

Thanks for that Phil!! A quickie re the incredibly thin pcb....any suggestions how I could get some? Was going to use double sided.

I realise Bill I've completely misunderstood the drawings in my previous post. From your comments with those drawings I really don't see what happens. Do the wheels stay parallel to the frames in both cases?

In any event the model will go round a far sharper bend than the real thing so I fail to see how the prototype is more than a guide, and relevant only to the ideal but impractical model.

[Philip Hall]

Julian, please PM me with an address and I’ll look some out for you. Might be a few days as the contents of my workshop is still strewn around its new building, and Christmas is coming...

I should say that thin stuff like that is best Araldited (proper full strength Araldite, not the fast setting variety) to the cross beams as I like to minimise the amount of heat applied by soldering the pickups to just the one side. I have had the copper on some PCB strip off as the glue failed with heat, although I think the new fibreglass stuff is better.

Philip

[grovenor-2685]

If its any help here is the pic of the NRM Crab that they gave out as wallpaper when the repaint was done.

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You can blow it up a lot if you download it.
Regards

[PeteT]

Another 'here here' for keeping up with a useful thread. We all have our ways of doing things (both assessing the prototype, and recreating it, and making it move) so its all good food for thought!

Eileens Emporium do 0.25mm thick double sided PCBs - hidden in the miscellaneous products section:

https://eileensemporium.com/index.php?option=com_hikashop&ctrl=product&task=show&cid=4184&name=thin-pcb-approx-2-5-x-2-x-0-25mm&Itemid=189&category_pathway=1108

[billbedford]

Bill - I make kits I don't design them. Surely it's the designer who needs to understand the drawing?

That is only true if you built the kit as intended. As soon as you start making 'improvements' then you need the information the drawings provide to understand where the kit is deficient.

[Julian Roberts]

Many thanks for several replies, and the picture, Keith.

Regarding using Araldite to fix the pcb, then soldering pick ups to it Philip...I use 24 hr stuff having found the 5 minute version is less good than Devcon for example. But even 24 hour Araldite, will that retain its adhesion even when heated up in soldering? Perhaps it's just the usual thing of being quick with the iron...?

Yes fair enough Bill and I may have to post sometime my even more boring thoughts that I edited out earlier, on the model design of the pony suspension.

I have realised the beams have a design fault. The weight of the loco will be borne (bourne?...born?) by a soldered join, nothing inherently mechanically strong. I made them like this thinking the weight would rest on top of the beams, but it rests underneath them. So I'll have to work that one out. Easy to make but have to get the adjustment right, much harder now the wheels are on. Sometime that joint will fail, as Murphy's Law is always true.

[Philip Hall]

Hi Julian,

I don't think that Araldite 24hr would be unduly affected by heat, at least with a fine bit and the small amount of solder you would use to attach a pickup wire.

However, I have been having a think about this and I now realise I went about the 72XX in a slightly different way. Hopefully I can describe this adequately as I no longer have the engine... I think now that I used a slightly thicker PCB strip to form the brake cross shaft by soldering, to the upper side of the strip, a length of wire (I guess about 0.7mm) which fitted into the holes on the lower edge of the brake hangers. It looks as though I have cranked the ends of the wires slightly downwards so the bottom of the cross shaft is approximately the right distance away from the rails. The longitudinal brake rodding was notched slightly on its upper edge where it crossed the cross shafts and soldered to the underside, which thus appeared to be passing through the rodding. The copper under surface of the PCB was cut through either side of of the longitudinal rods so they could become the pickup bus bars to which the pickup wires were soldered. One wire from each side of the motor went to each of the bus bars. If you enlarge the attached picture (which I have posted at the largest resolution I have) you will just be able to see this between the first and second driving wheels, and a pickup wire is just visible there as well.

Sorry to have slightly led you up the garden path! Hope this helps. More importantly, reads as to make sense! I might add that seasoned Great Western men might notice that the engine is not completely accurate but I had modified the kit so many times during construction that I called a halt as it was taking so long. But the owner likes it, and this was well before Hornby decided to do a turn on it...

Philip

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[jim s-w]

Bill - I make kits I don't design them. Surely it's the designer who needs to understand the drawing?

That is only true if you built the kit as intended. As soon as you start making 'improvements' then you need the information the drawings provide to understand where the kit is deficient.

Isn’t that Julian’s point. If you have the drawings the kit designer had and you can make ‘improvements’ from them them by definition the kit designer hasn’t understood the drawings.

[Noel]

If you have the drawings the kit designer had and you can make ‘improvements’ from them them by definition the kit designer hasn’t understood the drawings

Or maybe he's understood them better than the person making the 'improvements', or he's made compromises to make production easier, or to keep the costs down or to make it easier for other people to build...

[Julian Roberts]

Wonder if a separate thread should be created Bachmann Crab Comet Conversion Pony Truck in case this runs and runs ?

[Julian Roberts]

Thanks Phil for advice and superb pic of the superb 72xx. My Nucast kit awaits the unlikely day of its construction, nowadays I am focusing on types seen on ScR metals.

The Crab compensation beams were beefed up via a set up to prevent them altering

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I found I had just the right pcb already. The rear brakes were wrapped in the stuff. One side gapped all the way along. The chassis has a busbar arrangement that will be modified to get power from suitably bent phosphor bronze strip. (Somewhere along the line I had changed the square tube chassis strengthener into a bit of doublesided pcb.) So the idea is that the pick ups can transfer power to the chassis without any soldering via the detachable brakeset.

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I'll edit out the duplications here later

[Julian Roberts]

Sitting the loco on its rear footsteps and pony truck the gap between chassis and wheel looked about right compared with photos.

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I hooked up the rear 4 wheels and it turned out a little high and slightly tilting,so I took 1mm off the high side beam and half a mm off both. I was foxed a little by having put wire on the motor with insufficient flexibility so I replaced it with Society very flexible wire. A little thought resulted in concluding the pony needed to be a little more distant so a 0.25mm packing piece was added to the front of the chassis - the roller attachment is not easily adjusted this minute amount. After these mods the loco sat just right with its tender and a wagon. The tender is still 00 gauge but the track is dual gauged so the flange difference is no issue. Obviously I'm assuming the tender is correct and the wagon too. I tried various wagons etc so that is a reasonable datum.

So all that remains is to get the front sprung wheelset to join the party without spoiling the fun, i.e. not disturb the height, and behave appropriately... to follow...

[Le Corbusier]

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[Julian Roberts]

Magic Tim. Thanks!!!

[Julian Roberts]

Well getting appropriate behaviour has taken me a day with a helpful dose of manflu to enable total dedication of time available.

The front wheelset inc springs was inserted, model placed on mirror, and the ride height looked OK. Close examination of the pony showed the loco front was by a whisker not actually resting on its roller.

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There was insufficient upward travel available for the hornblocks. So I filed them deeper by 1mm, plus associated recesses for the springs. This was getting problematic as the spacer onto which the whole paraphernalia for the motion is bolted, was there. The line I had originally drawn for the default ride height is beginning to look a long way off...

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With front wheelset duly reinstated and the pony department looking healthy it was time for a push test on my test track.

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A glory term for a metre of Exactoscale Fast Track not gauge widened, with a pair of reverse curves 4ft radius. No problem so I screwed in the grubscrew and tested under power. The rear wheels slipped helplessly even with pickups solely on that wheelset but traversed the track perfectly in each direction both ways round. I got the rods on and all was perfect for roadholding. The movement left a lot to be desired but that was not the immediate issue. So time to put it on my running in plank which is a 6ft straight with auto reverse at each end.

All was fine until I upped the speed to about 20mph scale. You may laugh. The pony derailed. Haha. Very funny.

Now that test track has never seen a derailment and all my locos (7 so far) have run in on it. So I didn't start looking for a track fault. It was easy to see. A little jiggle at a rail join and uneven height. But none of my 0-4-4/
4-4-0s had derailed there so it was nothing to do with small wheels or carrying wheels.

After our BB discussion on another thread I checked the BB. 17.55 - 63ish! [ Edit. 2 days later I found the measurement was 17.63-71ish] How could I have got that so wrong? I'd used my usual gauge which is 17.67 in the middle and 17.75 at the ends. I tried pulling the wheels outward but the Loctite had done it's job well. As I'm pretty sure my other loco carrying wheels have similar delinquently narrow tendencies, and I hate loose wheels, I decided to leave it alone for now.

Anyway I reduced the speed and let it run for an hour which was fine. Then I realised the pony wheel was skidding along, not turning. I oiled it. No difference. So that said to me: not enough weight on the pony. And that might explain the derailment.

So I checked the upward travel of the front wheelset with loco on mirror and pushed down the ensure it was sitting on the pony. Sure enough there was almost no upward movement possible. All the effective weight was on the front wheels.

So it was out with a drill to make a pair of holes in the spacer to enable the hornways give another half mm of travel. That little task took all morning.

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Now set all up again. Derails on test track. Front wheel off the line at first whiff of a curve. Haha. Thinks.

Earlier I found that stretching one of these springs increased its force. So I found that one which is stretched about 2mm, and stretched another one. Insert them. Back with the wheels. By now I wasn't bothering with the front pair of rods! The action felt promising. Test drive - perfect! Back onto the plank: perfect! No derailment whacking it up to its full 30mph maximum.

Haha. Happy New Year! Early days yet but hopefully this is a goer now. So what I intended to achieve and hopefully have done is enough spring force on the front wheels to have them firmly on the rails and able to give directional force, but still with enough weight on the pony to keep it firmly on the rails and controlling the ride height. A balance. More weight may come from the cast cylinders and perhaps some nooks and crannies may take some lead. All will help. As it is the weight is 311g.

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