Comet 3F Jinty Chassis Issue

[ianLMS]

Good afternoon,

I am in the process of building a Western Precision Cotswold 3F Jinty and have replaced the solid brass chassis with a Comet one.

 

I have assembled it (using Comet jigs/spacers), but when I fitted the Romford wheels I noticed that the centre axle position was 1-2mm higher than the outer two. The wheels sat off the ground and came nowhere near to the track . Thinking there must be a good reason for this (to accomodate older wheels with rubber tyres perhaps), I cut out the two axles points, allowed them to rest in position and soldered them back in.

 

When I tested the chassis with motor/gearbox/wheels and coupling rods attached, the whole thing binded up, the coupling rod bent and the middle axles snapped out of position. It didnt help that when I opened up the holes on the coupling rod, the rod twisted and weakened mind. Even so, bottom line, the repair I made didnt work.

 

So, in true problem solving fashion, I  purchased some Alan Gibson Sprung Hornblocks and some MJT Hornblocks from Warley.

 

So, the questions are;

 

1. Is this a duff chassis which I now need to bin and buy another one (too late to return as its assembled), or use the solid chassis

2. Is it worth messing with the sprung hornblocks on the centre axle or should I do all 3?

3. If so, which are better - MJT or Alan Gibson?

4. I guess I need to buy new coupling rods - any suggestions as the ones in the Cotswold kit have different crankpin hole positions to the Comet chassis (assume designed for the axle spacing in their solid chassis)

 

Thank you

Ian

[Worsdell forever]

If you need a new kit I'd go for High Level and/or their hornblocks. It might be rescuable using hornblocks but re-assembled in a jig.

[hayfield]

Ian

 

I am surprised that the Comet chassis is that much out, it is common that the centre axle is about 0.5 mm higher than the other two.

 

I gave a friend an etched brass Wills Finecast chassis to build, he bought it round as the chassis was binding. We rebuilt it with less slop and it works a treat

 

If you want a rigid chassis I would suggest that you take the chassis apart and rebuild it, initially solder up the chassis with loose brass axle bearings, then using hornblock alignment jigs and the coupling rods solder the brass bearings in place

[k22009]

Gents

 

The late Geoff Brewin explained it all some time back about why the Comet chassis were designed that way, see link.

 

http://www.rmweb.co.uk/community/index.php?/topic/84556-rigid-comet-0-6-0-chassis-middle-bearing-not-in-line/&do=findComment&comment=1417873

 

Dave

[ianLMS]

Gents

 

The late Geoff Brewin explained it all some time back about why the Comet chassis were designed that way, see link.

 

http://www.rmweb.co.uk/community/index.php?/topic/84556-rigid-comet-0-6-0-chassis-middle-bearing-not-in-line/&do=findComment&comment=1417873

 

Dave

Thanks for that - that explains it all. 

 

We will see how I go with the sprung hornblock solution.

[Barry Ten]

It's only ever a small amount of difference as Hayfield says. The only time I've come across a Comet chassis with a 1- 2 mm difference was when the chassis had been fixed to a body

which had then distorted due to the lead shot expanding, causing the chassis to go somewhat banana-shaped in the process. As a disclaimer, I've not built the Jinty chassis but

with all the others (King, Castle, Prairie, Dean Goods, 57xx, Britannia and Patriot) the difference is very slight, and not really evident once you've finished the model.

Edited November 26, 2018 by Barry Ten

[ianLMS]

So, as usual, instead of consulting the experts on here, i went straight for the Xuron's and probably ruined a perfectly good, working chassis. I think I will use this chassis to practice fitting hornblocks, but in the meantime order a replacement chassis/coupling rods and do it properly.

 

The gap appeared quite large when fitted with the Romford wheels, but as I didnt measure it, its tought to say exactly how much it was. I should have photographed it as well.

 

£20 spent on a very valuable lesson - bargain in my book!

Edited November 26, 2018 by ianLMS

[Barry Ten]

So, as usual, instead of consulting the experts on here, i went straight for the Xuron's and probably ruined a perfectly good, working chassis. 

 

I'm sure we've all been there - I certainly have.

[Steamport Southport]

So, as usual, instead of consulting the experts on here, i went straight for the Xuron's and probably ruined a perfectly good, working chassis. I think I will use this chassis to practice fitting hornblocks, but in the meantime order a replacement chassis/coupling rods and do it properly.

 

The gap appeared quite large when fitted with the Romford wheels, but as I didnt measure it, its tought to say exactly how much it was. I should have photographed it as well.

 

£20 spent on a very valuable lesson - bargain in my book!

 

If it's just the coupling rods that are unusable then there are replacements available.

 

Try Alan Gibson.

 

 

Jason

[ianLMS]

The coupling rods are definately toast, and the chassis centre axles cant be used again either.

 

I have just ordered a new chassis complete with coupling rods. As I said earlier, I will use the older chassis to see how each type of hornblocks fit. I have ordered a set from Comet as well so now I have three types to play with to see which appear easier, work better etc. Having a chassis just to mess with will pay off I am sure, especially when I come to build my Jubilee, Black 5 and Patriot kits.

Edited November 27, 2018 by ianLMS

[ianLMS]

I had a spare couple of hours last night so I fiddled a bit more with the chassis. I fitted the Alan Gibson sprung hornblocks for the centre axle only and was really quite easy to do. I then used the Cotswold coupling rod as a backing plate and soldered the two halves of the broken Comet coupling rod onto it. Reamed out the holes and fitted this to one side of the chassis. The other side I fitted the working Comet coupling rod. After a little fettling getting openeing the holes up etc, it worked. The wheels turned with only very light tight-spots.

 

I will wait for the replacement coupling rods from Comet before progressing any further as my mock up one is way too thick and was purely for experimental purposes. I will still use this chassis to play with - the new chassis will be used on the loco.

 

Question: How do the sprung hornblocks work with a gearbox arrangement if I want to spring all 3 axles? The gap between the two hornblocks doesnt look large enough for a gearbox so slide in between? So, do you mount the hornblocks sitting proud of the face on the chassis, or file them down flat on the inside, use a different gearbox option or dont spring the geared axle at all?

 

Thank you

Ian

[hayfield]

Ian

 

I could be very wrong and thinking about it whilst writing with CSB method all axles are sprung so  there must be slim gearboxes available. I would aim to fix one axle and spring the other(s). 

 

Or move to EM gauge as the chassis are wider 

 

Good luck, perhaps have a word with Chris at High Level for his thoughts

 

Edit

 

I am always amazed at how some chassis work whilst others fail to. I have a (second hand) really loosley made chassis with jointed coupling rods, runs like a dream !!  On the other hand have built some chassis to what I believed was a very high standard and they binded when put under power. Jointed rods help plus there is this majical point of how much slop to build in for optimum performance

Edited November 27, 2018 by hayfield

[uax6]

Branchlines do slim gearboxes... 8mm wide iirc.

 

Andy G

[John_Hughes]

Branchlines do slim gearboxes... 8mm wide iirc.

 

Andy G

 

So do High Level, in various gear / motor / axle combinations. (Usual disclaimer!)

[Barclay]

If you don't already have some I would recommend some horn block alignment jigs from London Road Models. They are just basically extra long axles with very pointy ends onto which you fit the rods, ensuring that when you install the horn blocks the loco wheelbase exactly matches the rods. It makes the process very simple and when it's done you know it will work. Good luck with it.

[ianLMS]

If you don't already have some I would recommend some horn block alignment jigs from London Road Models. They are just basically extra long axles with very pointy ends onto which you fit the rods, ensuring that when you install the horn blocks the loco wheelbase exactly matches the rods. It makes the process very simple and when it's done you know it will work. Good luck with it.

I have a chassis jig from Poppy's which will ensure I correctly align the hornblocks, and I have the Comet chassis assembly jigs to help me ensure I have the correct width - I have all the tools, just need the skills!

 

Andy /  John - thank you for those tips - I will look at Branchlines and High Level ready for when I tackle the Jubilee/Patriot kits.

[PenrithBeacon]

http://www.cometmodels.co.uk/

 

The pdf file for the Jinty chassis pack shows that it is arranged to use sprung hornblocks

[hayfield]

Ian

 

Surprised your jig did not align the bearings, the London Road Models alignment jigs are cheap and very effective. I have found the Comet frame jigs are fine for assembling the frames, but the alignment jogs set the bearings to the coupling rods. Not too certain how the Poppy chassis jig works

[ianLMS]

The chassis was actually fien when assembled. The issue was caused by me cutting out the centre axle guide thinking that it should be set at the same height as the outer two. However, as previosuly answered above, the Comet centre axle is always set higher. It was completely my own fault why the chassis was messed up.

 

I have now fitted the sprung hornblocks in the centre axle to replace the ones I cut out, and rigged up a coupling rod to test. All works fine. 

 

The Poppy's jig is MDF and has 1/8" pins which run across the jig, through the bearings in the chassis and has pins on the end for the coupling rods to ensure alignment. Link below;

 

http://www.poppyswoodtech.co.uk/

Edited November 27, 2018 by ianLMS

[farren]

OK I can't offer any advice on this but to clarify if building a Comet chassis on my Avonside jig the centre pin would be best to left out?

[ianLMS]

The Avonside jig looks a serious pro piece of kit and way more engineered than my MDF laser cut one so I definately would not know the answer. 

[hayfield]

Ian

 

I think the principle may well be the same, which is where the London Road Models jigs come into their own, having said this if jointed rods are used it must be difficult to set the bearings !! 

[Buhar]

OK I can't offer any advice on this but to clarify if building a Comet chassis on my Avonside jig the centre pin would be best to left out?

 

You'd have to omit it, but then the Avonside jig is really designed around using hornblocks on all axles rather than a rigid chassis. 

 

CSB is reckoned to be pretty near impossible in 16.5mm as there aren't the clearances within the frames for the wires and their guides plus a gearbox.  If you use an 8mm gearbox as mentioned above you would only have 1.75mm either side within 11.5mm spaced frames.  Similar applies to pivoted beam arrangements bearing on the axles or bushes leaving the gearbox free to move, but is a bit more do-able as the pivot point (where you need a tad more width) can be worked with the gearbox alignment so as not to interfere.

[farren]

Thank you for your answer will remember this in the future.

[mark axlecounter]

I've built 3 comet 3f chassis plus around 17 more chassis and I have found the poppy jig easy and the best I have used. Non of the chassis have had any problems in running or setting them up.

Here is a pic of the jig with the comet caprotti black 5 on it,

 

 

 

Mark