Member postings for Roger Froud

I used Smartsketch 2D CAD also known as Imagineer for years, and it's probably the most intuitive and easy one to use.

Personally, I wouldn't advise anyone to buy 2D CAD because the benefits are nowhere near as great as going to 3D CAD. I use Alibre design and have a more expensive version of that package for my business. There's a cheaper version that does pretty much all that mine does but I need the CAM module too.

The trouble with 2D CAD is that you still have to draw any views you want and it's not much use for visualising assemblies. 3D CAD on the other hand creates the 2D views for you from the 3D model. Initially you find yourself wanting more control over that process, after all, you're used to doing it all the hard way. When you overcome that resistance and use the tools in the way they were intended, life becomes very easy. Ask for whatever 2D views you want, create section views.... it's all easy.

Making 3D assemblies is easy too, just tell parts to mate etc and they do. It takes a bit of getting used to , but it's so powerful. Once you've got to grips with all this, you'd never go back.

That's fair comment, and I think the ten percent difference between our two figures is indicative of the scale of the problem. Anyone who's studied the science of flow in pipes will know that the diameter and the length of the pipe has a huge bearing on the flow rate through it, never mind what happens when you partially obstruct it.

If you gave any of these designs to an F1 test team, I bet they could model the gas flow for a variety of pressure drops across the flues and do the same for the water turning into steam in the boiler. They have the Finite Element Analysis tools to model what happens over aerodynamic surfaces cooling passages in the water jackets in the engines. There's no mystery as to how to solve these problems but sadly I don't have access to these tools, and wouldn't know how to apply them even if I did.

Julian has sent me some pictures of a boiler that has very few tubes indeed, only 12 in a boiler for the same size locomotive as Speedy. Instinct tells me that it's not enough, knowing what everyone else seems to think is necessary, yet is works very well indeed. I think any attempt to use arbitrary rules of thumb is futile because the governing factors in the design are just not represented by those figures. It's a fascinating spreadsheet though, and I think its main value is in debunking these rules of thumb rather than reinforcing them.

I'm not sure why the superheater tubes are treated as if they have the same cross sectional area as a single small flue though, The figure I get for the free gas flow using the actual area of the 5 superheater tubes is 29%, a long way from the 19% if you work it out the other way. I'm not sure what any of this reveals to be honest, but then I don't know anything about the subject. There are such huge variations in the figures and of course I don't know which ones steamed well and which ones had issues. It looks like you can get away with murder and it will still work, that's the only conclusion I can come to as a total outsider.

Thanks, I'd only glanced down the list without realising that there are many sheets! I've got it now, It's interesting that it's got one of the highest ratios of tube cross section to firebox grate area of all the locomotives. It backs up what Julian is saying that there's far more tube area than is really necessary and losing tubes of changing the layout entirely might be a good option.

I'd like to take 1/4" off the outside diameter for choice so there's room for some lagging without making an oversize smokebox. Changing the tube layout would make that a lot easier to achieve.

Thanks John, that explains a lot, and yes, you're right, I've calculated the total surface area of the outside of the tubes. I'll have to put Speedy into that and see what happens.

Roger

I'd love to see that spreadsheet John, I've created my own and the figures are nothing like yours. I've getting Speedy's grate area as 2.9% of the total flue area using the outside diameters of the tubes for the area calculations. Just looking at the cross sectional view of the firebox tube plate, one circumference of the small flues would cover half the width of the grate, and the tubes are about twice as long as the firebox. In other words, one small tube is about the same as the firebox grate area and there are 26 of them.

I don't think your spreadsheet can be measuring the area but some other parameter. Can you shed any light on that? Your parameter, whatever it is, presumably results in successful boilers, I'm curious to know how that works.

Roger

I think you're mistaken, I've created an accurate model of the complete boiler and the flanges are close enough to interfere. I don't think you'd want to stop the tube short of the 1/16" that protrudes through, you surely want to braze right round the tube showing through?

I agree entirely that it's no big deal in as much as it can all be made to work, it's just not as satisfactory as it could be. The point I was making was that we really shouldn't need to have this discussion, it ought to be right on the plans.

Thanks Bob, I had finally concluded just that. Whatever the plate thickness, the flanged plates are better left at the size drawn. I'll swap the two outer bottom tubes for 5/16" though and move then in to the same clearance, it will solve all of those clearance issues. I've also drawn up a double flanged throatplate as suggested by Julian, I think that's worth doing.

Did you have to file clearances for the tubes in the flange of the firebox tubeplate? My guess is that they don't make that to the drawing because there isn't enough clearance on those either.

The flues have a run of about 1/6" upwards towards the smokebox, but I doubt if anyone would ever notice that.

I agree, it doesn't make sense to be unnecessarily precise about much of this. Now I can see where the errors are on the drawing I'm happy that it's not a complete can of worms. It's amazing how much confusion can be created. Most of that would have been avoided if they simply put 1/8" as the thickness of the boiler wrapper. Oh well.

Well, I have that as a material store and there's not enough room for much else. I like your thinking though,

The key for me is to get the most from the machines I've got. I already do CNC turning jobs on the milling machine by putting the tooling on the bed and the stock in a collet or chuck in the quill. The benefit of doing that is that it's effectively an 'auto toolchange' lathe because you simply move from tool to tool.

I've also made a mount for a belt driven 24,000RPM high speed spindle that can be used for fine work or engraving.

So the milling machine is also a CNC Lathe and Engraving machine too. There's no reason why it can't have a 3D printing head attached too so it can be used for that. The sky's the limit when you have an accurate machine with a computer attached to it.

You're lucky to have enough space for a beast like that. I only have a single garage, and every machine I have has to do many jobs. I already have a CNC Mill, a large Warco lathe and a Jones and Shipman tool and cutter grinder so it's bursting at the seams.

Hi John,

That certainly explains the difference in the difference in the smokebox tubeplate. It's tempting to follow suit and make it out of 1/8" copper, after all, the outer firebox wrapper is 1/8".

I think your calculations may be wrong on the reduction. I estimated that the equivalent are of the 5 large tubes to be 8.5 small ones from the ratio of their circumferences. That gives a reductions of 2 / 34.5 x 100 = 5.8% approx.

Maybe I'm missing something. Obviously your spreadsheet is more sophisticated because you must be using the surface area.

Anyway, fitting two 5/16% tubes is such a neat way to get round the mechanical difficulties that I think it's the way ahead. Everything clears nicely and it isn't a struggle any more.

Is that a shaping machine I see in your gallery? I haven't seen or used one of those since my apprenticeship at General Motors, 40 years ago!

Ok, I've just fitted two 5/16" tubes there, keeping the same clearance as the others to the inboard tube. This works really well from a clearance point of view and means a loss of around 3% in surface area, This is what I'm probably going to go with unless there's a compelling reason not to.

The more I pore over the 3D computer model, the more I think this is the best solution. It's those two tubes that are causing all of the clearance issues. The smokebox tubeplate needs large scollups to be taken out of the flange and the "piston ring" joint has a similar problem. I'm not keen on doing that and the tubes are already close enough to the top of the firebox tubeplate flange. I really don't want to raise the top of the firebox crown either. I could slightly squeeze the tubes closer together or probably the least offensive would be to fit two smaller tubes there.

Removing those two tubes reduced the surface area by 5.9% which doesn't seem much. Does anyone else have a view on this that they would like to share?

So how much tweaking did you have to do to make it all fit? It looks to me like the tubes can't be parallel to the bottom of the boiler but slope slightly upwards.... no bad thing. Several of the tubes in the firebox tubeplate also graze the edge of the flange if it's drawn precisely to the dimensions.

What diameter did you make the smokebox tubeplate? 4-3/4" isn't consistent with the 5" outside diameter at that end.

Thanks Bob. I presume the diameter of the smoke box needs to be slightly increased if it's going to look right.

I'm having an absolute nightmare trying to create a 3D computer model to verify the drawings. Whichever way I look at it, things simply don't add up. The drawing for the smokebox tubeplate states 4-3/4" diameter which is clearly wrong. I can see that he was probably thinking that the boiler at that point was 1/8" thick when in fact it's 3/32" or 2.5mm (13 gauge)

It also looks like he's callculated the heignt of the crown stays or the height of the firebox from the sectional view omitting the inner flange of the firebox tubeplate. Either way, it's never going to fit if it's made to the drawing unless I've made an error.

How far have you got? What did you have to change to make it all fit if at all?

That's an interesting thought, I'll chew that one over John.

You've lost me there Bob, I convert everything to within a micron so it's the same dimension. It really is about time we finally changed to metric, I was a teenager when we went over to it, and I'm sure it puts a lot of young people off the hobby. Most of them don't know what an inch is, and why should they?

I see where you've had to file away the flange on the bottom outside flues, just like my computer model shows. I don't see that as a big problem, it's just not ideal. I'll take a look at the dimension I've ended up with and compare that with the figure you've found. Quite why that's not on the drawing is a mystery. I notice that the Smokebox Tubeplate drawing is incorrect in as much as it doesn't show the forward facing flange in that view. It give the impression that there's more room than there is.

Anyway, it can clearly all be made to fit one way or another, and if moving the tubes very slightly makes it easier to braze up then I'll probably do that. I'm not looking to change things unnecessarily, but I have the opportunity to do these things so I may as well if it helps. I see that some builders have added more stays to the backhead and I'll do that too.

Are you planning to lag your boiler? I'm curious to know what needs to be done about that.

I hear what you say and take it all on board. I'm lucky in that I can build the computer model and look at it from every angle, tweaking things where they look a bit close. It's tempting to move the outer two of the bottom row of small flues slightly. in fact, it might make sense to very slightly bunch up the diagonal row of small flues towards the centre of the crown. I'm sure an extra millimeter could clearance between that tube and the barrel diameter could be achieved without compromising anything else.

I think you're mistaken about the motivation for Don Ashton's reworking of the valve gear. It's true that the Union link isn't correct on LBSCs design, but it's the fact that it radically changes the performance that prompted it being moved back to where it should be. Don has hopefully come up with the best geometry based on modern simulations and years of experience, and that's the pattern I'll be following.