Comet Gresley 8'3" bogies

Porcy Mane
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Re: Comet Gresley 8'3" bogies

Postby Porcy Mane » Fri Nov 11, 2022 8:10 pm

Crepello wrote:
Porcy Mane wrote:Hello boys,
Tight crop of a BR official that shows the roof profile quite well.

Those trees look familiar. Hatfield south curve by any chance?


A tad north. Brookmans Park.

davebradwell
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Re: Comet Gresley 8'3" bogies

Postby davebradwell » Fri Nov 11, 2022 9:57 pm

I reckon you could well be right about the Comet bogie, Bill, especially after they produced a roof that is wrong for everyone. At least Mike Trice rescued the LNER folk.

It seems Harris introduced the idea that later builds of full brakes had the longer bogie but no-one's produced any photographic evidence of this here. Certainly the heavy/light is given away by the 8 or 10 ton load capacity.

This might be useful as the numbers are legible. Much easier to enter these than mess about scaling a drg.

Scan10036.JPG


Comparing my 8ft light and 8-6 heavy you're not going to be far out just shortening the 8ft 6in. Worth confirming that 8ft is same depth in the centre, but it is very likely. Perhaps you'll be minded to improve the top profile while you're making changes.

DaveB
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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Sat Nov 12, 2022 4:50 pm

I modeled up an 8' version and tried to make the bolster more like the photograph.
David Bradwell - could you suggest how the top profile needs changing?

8foot-bogie.jpg


thanks
David
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davebradwell
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Re: Comet Gresley 8'3" bogies

Postby davebradwell » Sun Nov 13, 2022 8:01 am

Just compare your radii with the ones given on the drg. If they're the same it's an illusion but to me the bumps look too high and too short with much sharper corners - it might just be the viewing angle, of course. Conversely, the bottom radii look too large but I'm wary of going by the look of the thing which is why I like numbers. Don't you just enter prototype inches into the cad - the computer sorts out all the scaling?

It's your model, of course!

DaveB

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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Sun Nov 13, 2022 2:11 pm

I had a go at modifying the profile, I'll admit not to finding the 'sketching' in 3D CAD systems anywhere as easy to use as the drawing in 2D systems. All the additional constraints based stuff just seems to get in the way and I find myself accidentally moving one point and the whole drawing goes mad! I've now made the arcs for the curves match the draws so hopefully it looks better.

8foot-bogie-v2.jpg


David
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davebradwell
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Re: Comet Gresley 8'3" bogies

Postby davebradwell » Tue Nov 15, 2022 9:26 am

It looks a whole lot better than the Comet casting you started with, David, which was the point of the exercise. Hope you didn't mind being encouraged to finish the job otherwise you were just swapping one set of errors for another. And you know it's right, now.

DaveB

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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Tue Nov 15, 2022 11:56 am

davebradwell wrote:It looks a whole lot better than the Comet casting you started with, David, which was the point of the exercise. Hope you didn't mind being encouraged to finish the job otherwise you were just swapping one set of errors for another. And you know it's right, now.

DaveB


Thanks DaveB
No problem at all with the encouragement.
My version of Scalefour - 'Trying to get it a bit more right one step at a time'
David

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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Wed Nov 30, 2022 10:20 pm

So Eileen's Emporium supplied a set of Bill Bedford's 8' sprung bogies which I assembled per the instructions, apart from leaving out the brake gear. These always run nicely and glide along.

aIMG_9923.jpg


I 3D printed the cosmetic sides and attached these to the brass bogies.

aIMG_9927.jpg


aIMG_9925.jpg


I've always found the brakes the trickiest bit of Bill's bogies so I had in the past tried to 3D print them. I modified my previous version to fit the 8' bogies and also tried adding the safety loop this time.

aIMG_9932.jpg


These fit nicely onto the internal bracing of the bogie and clear the wheels, though I did need to use a little grit paper to just shape them to clear the wheels as they spring up and down.

aIMG_9935.jpg


aIMG_9930.jpg


I don't think they look too bad but I wait with baited breath for comments!

Now for a roof related question: Can anyone point me at a diagram showing the correct profile? I would need to print it in two halves to get the length onto my printer but wonder if this might be a better route than trying to fettle the Comet one to shape?

David
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billbedford

Re: Comet Gresley 8'3" bogies

Postby billbedford » Wed Nov 30, 2022 10:45 pm

From Michael Harris' first? book:

The Doncaster roof was a true semi-ellipse with the axis - at the level of the bottom of the cant-rail - 2ft 5 1/2in below the highest point of the roof curve.

If you need more help, Mike Trice posted a whole lot of Gresley coach drawings, including roofs, on Western Thunderer during August last year.

davebradwell
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Re: Comet Gresley 8'3" bogies

Postby davebradwell » Wed Nov 30, 2022 10:47 pm

That all looks very fine and a simple construction. I may well have been able to find drgs of the end cross members but you have to call a halt somewhere.

I'm sure I quoted the roof ellipse earlier in the thread - Nov 7th. Your cad system should get it right in one hit. I've tried tracing the curve from drgs and it's hopeless - they just aren't drawn properly as they're defined on other drgs.

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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Wed Nov 30, 2022 11:13 pm

Yes, reading back over the thread I saw them again, my brain was probably too concerned about bogies at the time to note them down. An ellipse makes things very easy to draw.

roof-profile.jpg
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davebradwell
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Re: Comet Gresley 8'3" bogies

Postby davebradwell » Thu Dec 01, 2022 9:14 am

Don't forget to pinch Mike Trice's idea and print a shallow groove down the back so you get the vents on the centreline.

DaveB

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Re: Comet Gresley 8'3" bogies

Postby davebradwell » Thu Dec 01, 2022 9:17 am

Did you notice that height of roof is less than half height of ellipse? You might make it even nearer to Mike's by extending edges to include rebate for sides and the cornice.

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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Thu Dec 01, 2022 7:08 pm

I've just registered on Western Thunder so I'll try and find the drawings.

What I think you are saying is that the major axis of the ellipse was 8'8 1/2" but the roof was only 8'8" wide. So rather than having the half of the 4'6 3/4" of the minor axis you actually cut the bottom once you get the the 8'8 width. The result makes the roof in 4mm .9mm lower than if it had just been half of the ellipse.

roof-profile1.jpg


Does that make any sense?
David
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davebradwell
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Re: Comet Gresley 8'3" bogies

Postby davebradwell » Thu Dec 01, 2022 10:28 pm

Try this: I have, amongst others, a drg of a Thompson 61ft 6in Corridor 3rd Brake. Like other similar coaches, the body is 8ft 6 1/4in high to top of roof. Suggest you subtract the height of your sides from this to give roof depth required. Drawing also gives height from top of roof to top of cornices as 2ft 0 1/2in and these are 2 1/2in X 1/2in section so you can work out if this can be part of your print.

This may give a figure close to your 0.9 dimension but it will be less susceptible to minor rounding errors in LNER drawing office.

You may need to produce some new coach ends as well to meet your new roof. If so then straight portion of ends is 4ft 1in wide. Are you able to print your roof with thinner ends? Don't forget central groove under.

I've signed up with Western Thunder, too. I think he shows the Pullman gangway.

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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Thu Dec 01, 2022 11:44 pm

OK, so now we are reaching 'why do I bother' territory.

The overall height of the coach should be 8'6 1/4" or 34.83mm.
If I use the dimensions of the ellipse as given then my roof part ends up at 8.225mm tall.
Measuring the etched sides these are 24.5mm tall

so I end up with a total height of 32.725mm or almost 2mm too short.

ARRRRRRRRRRRRRRRRRRRRRRR!

The etched side has the bead above the top window as .66mm(ish I think the etched artwork was probably draw with crayons)
Looking at the picture posted by Porce Mane there seems to be quite a lot of material between the top of the windows and the noticeable curve of the roof so I think I can add a bit in there but if I hold the etched sides against the drawing of the coach end printed on the instructions which measures damn on 34.83mm then it looks like the etches are about 1mm too short.

David

DougN
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Re: Comet Gresley 8'3" bogies

Postby DougN » Fri Dec 02, 2022 12:35 am

David, I can see your frustrations... but I can see you are achieving a better model than taking the kit as given and just building it... all these trials and tribulations will produce a better model :thumb Keep going. I am enjoying the read and amazed at the capabilities of 3d printing. I just wish I had time to have a go at it. A mate has just run up some things for me in about 2 hours... He has even said if I want to print anything he'll do it for me.
Doug
Still not doing enough modelling

billbedford

Re: Comet Gresley 8'3" bogies

Postby billbedford » Fri Dec 02, 2022 9:45 am

Here is part of a roof section drawing which shows the arrangement at the cant-rail:

Roof section.jpg


The roof ellipse is drawn from the bottom face of the cant-rail.
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billbedford

Re: Comet Gresley 8'3" bogies

Postby billbedford » Fri Dec 02, 2022 9:54 am

... and here's a drawing of the arrangement of the destination boards, which shows a complete section at the cant-rail.


dest.jpg


Most etched sides will include the cornice (the raised weather strip outside the cant-rail) in the etch.
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davebradwell
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Re: Comet Gresley 8'3" bogies

Postby davebradwell » Fri Dec 02, 2022 10:09 am

Might this help:

Thompson brake end.jpeg


This is the steel brake 3rd again.

The dimensions on the right are the wood framing and confirm Bill's statement pretty closely.

Those on the left which aren't quite readable are the steel panelling. Might this help you discover where Comet may have drifted?

This is why I usually start with a real drawing and a blank computer screen. I have very few unmade kits - they were sold off years ago after a few bad experiences. I suspect usual source of error is not starting with reliable information but someone's interpretation of it.

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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Fri Dec 02, 2022 2:06 pm

thanks for those drawings.

Pumping those figures into my CAD drawing confirms my fears. If I make the roof and cant rail area to prototype and use the existing Comet sides then I end up with a coach which is 1.5mm too short.

Interesting I just compared the Comet etch sides to the Comet etched ends and found that the ends, from the cut-out for the cant rail to the bottom is almost 1mm taller than the height of the etches sides!

aIMG_9938.jpg


So, given where I am with the kit I think I have a few possibilities to try to fudge things, though the best option would probably be to forget the etched sides completely, start with the drawing and etch my own. Unfortunately I think this option is too expensive to bother with, I'd probably just forget about the whole thing.

Option 1. Build the kit with a correct profile roof and live with the overall height ending up too short.

Option 2. Try to add some material to the bottom of the side etch. The etch has a half etch line along the bottom of each door where I think the door actually just went down to the bottom of the side. There would probably be a load of filling with solder and filing to get it vaguely smooth (maybe I model a prototypical rusted out bottom of side). This probably means that all the half etch dots for door stops/ handles and even the door windows end up too high.

Option 3. slightly exaggerate the roof profile and flat portion of the cant rail to try to get vehicle nearer to scale height.

Option 4. some combination of all the above!

If I solder a piece of scrap etch to the bottom of the side and then it file back to match the front of the etch I can increase the depth of the sides by .45mm. I can slightly increase the profile of the roof by .5mm and live with the coach being .5mm too low.

Any better ideas?
David
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billbedford

Re: Comet Gresley 8'3" bogies

Postby billbedford » Fri Dec 02, 2022 2:15 pm

The etching is drawn to fit underneath the cornice, ie at more or less the bottom of the cant-rail.

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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Fri Dec 02, 2022 3:34 pm

Yes, and if I put the top of the side against where the bottom of the cant rail would be the bottom of the side is 1mm too short.
aIMG_9939.jpg


I filed the ends of the Comet roof extrusion so that the etched end would fit in the correct place. Loose fitting the sides and ends shows the problem.

aIMG_9940.jpg


The etched end fits the extrusion quite well, apart from the profile being wrong and having a non-existent ridge on the edge. However the bottom corner shows just how badly out the side is.

aIMG_9941.jpg


David
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billbedford

Re: Comet Gresley 8'3" bogies

Postby billbedford » Fri Dec 02, 2022 4:10 pm

Check which is wrong. The height of the side from the underside of the floor to the underside of the cantrail is 6'2".

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barhamd
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Re: Comet Gresley 8'3" bogies

Postby barhamd » Fri Dec 02, 2022 6:34 pm

I think I've been staring at this for too long and getting confused.

The etched coach side is approximately 24.5mm which doesn't sound far off 6'2".

I tried to redraw the roof profile from scratch using the drawings and numbers gathered so far.

Starting with a clear sketch I drawn an ellipse, major axis of 8'8 1/2",17.415mm and minor of 4'6 3/4", 9.125mm,
This, we are told is centred level with the bottom of the cant rail.
The cant rail is 2 1/4" high, .75mm so I draw a line .75mm up from the major axis and then down from where this meets the ellipse, squaring off the ends of the sides.

The overall width of the roof has been given as 8'8" or 34.66mm, my drawing gives that same dimension as 34.712mm which I think is close enough for P4.

Then we look at the height of the roof, The annotated drawing posted by Dave Bradwell appears to show a height from top of cant rail to top of roof of 1'11 7/8" (if my maths is right) which would be 7.875mm however my drawing now has this dimension as 8.375mm so the roof is profile appears taller than quoted.

We are told the height from the bottom of the body to the top of the roof is 8' 6 1/4", 34.08mm
We now have a coach side of 24.5mm, a roof height from bottom of cant rail to top of roof of 8.375 giving us an overall height of 33.625mm

roof-profile2.jpg


Measuring the Comet extrusion the height from bottom gutter / cant rail to the top of the roof comes in at 8.4mm, call it 2'3 1/2"

So total height if built as intended, reducing the height of the ends to match the sides, would be 8'5 1/2", again not far off the prototype 8'6 1/4 figure given by Dave Bradwell.

Rather than print a new roof I wondered about printing a tool which matches the profile and using it to sand the extrusion to a better shape.
David
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