Model Rail - Gem Kit

Very true, the most important aspect is to get all the main axle holes in line, this is something that I'm not sure if the Comet jig (LS16 or LS17) does this correctly, the best and easiest method is to use special axle jigs [1] and the locos coupling rods. Alan Gibson (Workshop) used to sell such a set of gigs but I can't see them listed now...

The most important aspect to any chassis is that the axles are in line and the same distance apart, an out of line buffer beam will look crap but it will not prevent the loco running, axles that don't match the coupling rods (and each set of rods match the other side) will mean that there is no way that the chassis will run correctly. Whilst the Comet jig seems to make a good job of aligning the frames and axle boxes it fails to make sure that the assembly matches the coupling rods IYSWIM.

[1] Hope the ASCII art displays OK (fixed font?)

|X|| ||X| ||X| = frames/bearings ] ------------------- [ === FAILS AXLE === < crank-pin dia. ] ------------------- [ |X|| ||X| [ = actual coupling rods

Suspect this is out of my experience, cos if the axles are correctly aligned at right angles to the frames and correctly positioned am not sure how coupling rods can be incorrect. Is cos am assuming all from same kit that is of siutable quality.

With my latest attempt at GEM chassis had only to open out holes in coupling rods sufficiently to fit comfortably over the crankpins and that was it. CHassis with wheels and coupling rods ran as freely and smoothly as a good RTR wagon.

Cheers, Simon

: >

: Suspect this is out of my experience, cos if the axles are correctly aligned : at right angles to the frames and correctly positioned am not sure how : coupling rods can be incorrect. : Is cos am assuming all from same kit that is of siutable quality.

You're assuming that all chassies match the coupling rod...

I'm not saying that the Comet (or any other) system won't/can't work, just that dummy axles that use the coupling rods to align the driving wheel axles/bearings is fool proof - the only way for it to fail is if each set of coupling rods doesn't match it's opposite, if that is the case then *any* alignment jig/method will fail at the driving wheel quartering stage!

Seems to me youre looking for a way to correct an error in one component by introducing an error in another. If rods that bad then as Iain Rice would say - send em back. However do admit to inserting crankpins not quite verticle into Alan Gibson wheels to give problem as you describe - if anyone has a foolproof method of doing these would be very interested as I have a second hand kit that has these wheels. I prefer Romfords as they cannot be slightly out of quartering and the crankpins are more likely to be correctly inserted.

Cheers, Simon

: >

: Seems to me youre looking for a way to correct an error in one component by : introducing an error in another. If rods that bad then as Iain Rice would : say - send em back.

Whhoooossssshhhhh.... :~((

How do you ensure the frames themselves are accurately aligned? To what degree of accuracy?

How do you ensure the bearings are aligned and positioned correctly when you solder them into the chassis?

Some form of alignment jig is essential if you are fitting sliding hornblocks that require you to cut slots in the chassis.

MBQ

On 11/02/2009 21:18, simon said,

That is a very rash assumption, especially if the coupling rods are not on the same fret as the frames (not the case in the Mainly Trains chassis in the OP). It's a fundamental principle of chassis building that bearings/hornblocks are aligned using the coupling rods. I winced when I saw Chris Leigh's article because he hasn't done that, and to compound it he's had to remove the etching cusp (what he calls a burr) in the frames to get the bearings to fit. At some point, he'll need to open out the holes in the coupling rods to suit the crankpin bushes, so that's two sets of holes that he's worked on independently, whilst hoping the centres will remain identical. I doubt they will.

This is all why when building a chassis, the coupling rods are always prepared first then the axle bearings jigged to suit. In the March issue, look carefully to see how much slop there'll be on the coupling rod/crankpin. Fortunately Chris has chosen a good quality chassis to demonstrate with, but there are many chassis around where the approach shown will result in disaster.

Must admit was afraid that would be the case. But how about giving a poor misguided beginner a bit of assistence in understanding so progress can continue. One day I might reach the exalted levels....

CHeers, Simon

How do you ensure the frames themselves are accurately aligned? To what degree of accuracy?

How do you ensure the bearings are aligned and positioned correctly when you solder them into the chassis?

Some form of alignment jig is essential if you are fitting sliding hornblocks that require you to cut slots in the chassis.

MBQ

Suspect you are over my knowledge/experience too. Havent reached the level of sliding hornblocks.

Cheers, Simon

Thanks for the warning, as said had no problem with alignment of coupling rods after minimum reaming with the GEM chassis. Will be aware of possible one with other kits though.

Cheers, Simon

: : > Some form of alignment jig is essential if you are : > fitting sliding hornblocks that require you to cut slots : > in the chassis. : : : Suspect you are over my knowledge/experience too. : Havent reached the level of sliding hornblocks. :

But it's as relevant to 1/8" 'top-hat' bearings and all types of chassis (even the old style castings found with some kits) as it is to sliding horns or the Flexichas system, what this way of aligning the axles does is ensure that they ARE in alignment - by using the one item that can't easily be adjusted, even a few thousandths of an inch discrepancy can cause binding of the coupling rods or more slop that the you know what up a shirt sleeve, but has to be correct - it's all to easy to build a chassis that requires so much metal to be reamed from the coupling rods that not only are they grossly sloppy on the crank pins but their structural integrity is compromised. As I pointed out before, the most critical stage in any build is making sure that both sets of coupling rods have *exactly* the same pin centres [1] and that the main axle bearings match those centres, the rest of the chassis can be quite some way out of true and the loco will still run down the track looking like a mechanical master rather than a duck...

It's actually easier to adjust the fit of the chassis bearings before they are fixed (soldered) to the chassis frames/block that try to adjust the 'fit' of the coupling rods at the wheel quartering stage - many 'wheel quarting problems' are nothing what so ever to do with the wheels but the fact that the axle centres do not match the coupling rod centres!

[1] o=====o-------o CORRECT o=====o-------o

Compared to;

o=====O-------0 WRONG! O=====o-------o

OK, understand better. When reach a kit that requires such considerations then will re-read the relevant sections of those wonderful books by Iain Rice, consider the purchase of a £200 jig and muddle on. But thus far GEM kits have not required any more than a simple comet jig and blind faith.

However could you explain one thing to me. I see talk of slop in the coupling rods of thousandths of an inch, yet when look at my new Bachmann Jubilee which runs like a dream it seems to have slop in the hundredths of an inch. It is - I must add - a serious question.

Cheers, Simon

.

No need to spend =A3200. If you have the tools, you can make your own by turning down a length of 1/8" rod, or buy the same type for a tenner or so.

How many axles did your GEM kits have? Are they etched chassis?

RTR locos are mass produced and they cannot individually fettle each loco like you would with a kit. To get down to thousandths of an inch, they would need to tighten the tolerances on the whole production process, and that would push up costs. Some RTR locos are geared to drive every axle and the conn rods just come along for the ride.

You can run a kit built loco with considerable slop but if you take pride in it, it will not be as satisfactory, even if it's only you who knows.

MBQ

: OK, understand better. When reach a kit that requires such considerations : then will re-read the relevant sections of those wonderful books by Iain : Rice, consider the purchase of a £200 jig and muddle on.

More like a £2 jig (and that is with a hansom mark-up)!... Quite literally, these jigs are just 1/8" bar, longer than a normal axle with the ends turned down to match the crank pin bearing dia.

But thus far GEM : kits have not required any more than a simple comet jig and blind faith.

So far you have been in luck!

: : However could you explain one thing to me. I see talk of slop in the : coupling rods of thousandths of an inch, yet when look at my new Bachmann : Jubilee which runs like a dream it seems to have slop in the hundredths of : an inch. : It is - I must add - a serious question. :

Mass production tolerances I guess, there really is no need for that sort of slop - from engineering point of view.

Interesting, do you have info on where can get one ?

Thanks, Simon

Not set up for turning, so interested in where can buy one if you have info ?

This is a 4-4-0 George Vth. But have done a 0-6-0 Cauliflower (but not as well as soldering technique very poor at that time).

Am having problems with weight distribution as its front heavy, and trying to understand how to get the bogie arm right so bogie chassis doesnt foul main chassis - theres isnt much clearence on the prototype.

I want a loco thats not available in RTR (or a variation) so will kit build. If its a good runner thats a bit sloppy from an engineering point of view then will live with that. Its all about compromise with attempting to improve skills whilst having a life with other interests within modelling.

Do think some people are put off trying as they arent experts with great experience right from the start. I'm a hacker in a rush with poor dexterity and struggle with understanding engineering. Yet can assemble, paint and line a GEM kit to give a model that am happy see running on my layout.

On sunday we're going to GCR at loughborogh, his nibs will see Thomas and I will see a hundred year old hunslett (unless its ex Coventry pit loco).

Cheers, Simon

: : Interesting, do you have info on where can get one ? :

As I said, 'Alan Gibson (workshop)' used to do them and might well still do, the problem is that the business changed hands a year or so back meaning that not everything is back in production yet or that everything that is back in production is listed yet - email them (or even phone them, at a reasonable time...) and ask, if they don't still do them they will probably know who does.

Picking up on what you said in your reply to MBQ, if you are serious about improving your skills have you though about joining the EM Gauge Society - many people who have no intention of adopting EM gauge join just for the technical tips and the like, and AIUI they are not elitist like the P4 lot can sometimes be!

To be honest I can't remember and I'm not at home at the moment. It probably came from Mainly Trains, but I moved to N gauge now and rarely look there.

Looking at the Comet ones, there are actually two different issues. They sell frame assembly jigs. With most etched kits these days, its relatively easy to build a square chassis.

The type of jigs I'm thinking of are to ensure accuracy and alignment of the axles within the chassis. They are 1/8" dia and come with springs that go between the chassis to hold the hornbock guides and top hat bushes in place. The ends are turned down to fit the holes in the con rods. Thus, the holes in the con rods determine the axle spacing (and using the jig this way ensures they are the same). It does depend on the con rods being a matching pair either side!

There are ways to transfer the weight around but I would suggest you read Iain Rice's books or Mike Sharman's Flexichas rather rthan try to explain it here. It's not too difficult.

Indeed and that's where magazine articles can do a lot of good. But using no jig or using one of the very expensive ones doesn't really help the average modeller. Perhaps there's scope for an article about how the different types of jigs work and can be used.

MBQ

Thanks, used brain and made sure ends of bogie arm remained flat and couldnt tilt. Then set height correctly. Obvious now !

Yep, thats why started this thread. With GEM chassis and comet jig a beginner can put together a good chassis from the included instructions, from Model Rail article would struggle. Although I acknowledge the type of jig you and Jerry suggestion will be useful in a wider context.

Thanks, Simon