Had Another Go

[JasonBModerator @jasonb]

This is the video I did previously. Looks like rather than model the crank all as one part I used the other option of assembling it from constituent parts which is another way to do it. You can see that I offset each pair of webs by 90, 180 and 270deg from the first. You would only need do it once for your two throw crank.

Also found the ones for your crosshead

[Nigel Graham 2Participant @nigelgraham2]

Thankyou for those.

David Jupp has also contacted me separately, and this is from my reply, and augmented:

 

Light-bulb moment, while out for a walk this morning:

…. I don’t need draw this engine to anything like its full complexity.

 

Most of a simple twin-cylinder steam-engine’s components are in pairs. They need drawing properly, but if I can’t draw the entire assembly I might be able to create sufficient of the layout, though may have to resort to a 2D manual (or TurboCAD) drawing.

 

The engine is prominent, nearly-vertical, has to fit a defined volume of fresh air in the vehicle chassis, and fairly represent the prototype, as by the different top covers’ diameters to suggest the original compound. The crankshaft protrudes to carry the flywheel on one end, the coupling to the transmission-gears on the other. It is also largely enclosed, allowing some liberties with internal details whose originals are unknown anyway.

 

I identified key regions constraining the internal design by working clearances, relationships to surrounding parts, servicing access and influence on the external appearance.

So trying to draw the complete engine in full, glorious detail is not necessary. For example:

 

Represent the big-ends and their sweep by a circumscribing cylinder orbiting the crankshaft axis, and avoiding the casing walls. So it does not matter if I can’t draw the crankshaft in true form with one crank 90º round from the other. Drawn with the cranks pointing sideways and the big-end represented by a cylinder, it will show the room needed across the casing. The “sump” depth is simply two radii plus a bit. This simplifying also avoids trying to represent a connecting-rod leaning over at an angle, in mid-stroke, as it does not need drawing fully in the GA.

The other important considerations are the lengths of the moving parts, the piston-rod glands and their servicing access, and the positions of the crossheads.

The cylinder block is rectangular, and needs fit the casing with sufficient room for the bolts and spanners without the fastenings fouling the other parts. The original casing had “windows” for reaching those and the glands, but the scale versions won’t accommodate even my delicate little mitts. Below those the case had a big cover, described as “quickly detachable” – held by a lot of wing-nuts. I will follow suit. Whatever the original “sump”, I will use a removeable cover there, too.

The GA needs only one of the two sets of identical components, and not in detail. My futile attempt to stick the cylinder covers together is therefore a pure Alibre Atom exercise to see what I can’t do. I need design the cylinder block as fully as I can, but add only one crank-end cover, and not fully.

Similarly with the valve-gear and valves (adapted from K.N. Harris’ modified gear for LBSC’s Maid Of Kent locomotive, with further guidance from Martin Evans’ Manual of Model Steam Locomotive Construction).

 

If I can’t create a 3D GA model in Alibre, a 2D version should suffice. It is to help me build a real thing, not a pretty picture. I tried a 3D impression of the whole vehicle, but even with a lot of simplifying, found it too hard.

[Julie AnnParticipant @julieann]

Here are a couple of parts recently 3D printed in stainless steel by PCBway:

Surface finish is fairly good; I prefer to exchange CAD data as STEP files where possible as they are more accurate than STL. The parts are surprisingly accurate dimensionally, within a thou or two. There are internal support structures, so not quite prototypical, but the structures are not really visible.

Total cost was about $100; $60 for the two parts and $40 shipping via DHL. I paid with a credit card and got a lousy exchange rate. I have Euro and US$ accounts, so if I use PCBway in the future I’ll try to do a direct bank transfer. Time from order to delivery was less than two weeks.

PCBWay look to be very competitive on PCBs so if I start dabbling in electronics again at home, as I am no longer involved in electronics professionally, I’ll use PCBway for the bare PCBs.

Julie

[Nick WheelerParticipant @nickwheeler]

Your 2D simplifying isn’t that much simpler than creating a basic, unanimated 3D model. It will require more thought to both create and use, so you’ll be getting the worst of both with few of the advantages of either.

 

There are several ways you could join your pair of covers to each other as an exercise, but none of them are particularly relevant to the way they would actually be used – bolted to an end of the cylinder block, and aligned with the cylinder bore. Which is also how they would be designed if using CAD efficiently.

[JasonBModerator @jasonb]

Julie Ann, They look to have come out well, it will be interesting to compare the finish and cost of the lost wax ones when they arrive.

There is another thread over on MECH that may be of interest as it shows work by a couple of other similar companies. And also this one on HMEM where you can see the better quality from the more expensive prints that Barry had done. But for a part that got slung in the nearest pond or ditch what you have should be fine for your engines.

[David JuppParticipant @davidjupp51506]

In Nigel’s assembly file, the 2 coincident constraints duplicated each other functionally – leading to the over-constrained errors.  Suppressing or deleting one of them resolved that.

His coaxial constraint would have worked if the selected edges hadn’t been on the top of one cover and bottom of the other.  Editing the selected edges and using the ‘Flip’ button resolved that.

That left the covers free to slide past each other but otherwise correct.  Adding another constraint got everything into the correct arrangement.  I used another coincident on flat faces of the stuffing boxes, a coaxial on the 2 ‘half holes’ would also have worked.

There are many constraint combinations that could have been used.

 

The key learning here, if an assembly constraint fails – don’t leave it, either remove it or edit it to resolve the error.  A pending error can prevent other things working correctly.

[David Jupp]

On David Jupp Said:

 

The key learning here, if an assembly constraint fails – don’t leave it, either remove it or edit it to resolve the error.

I have a supplementary question about constraints in Alibre; background I’ve been using Alibre Design Expert professionally for 18 years. I recently wanted to delete a constraint on a line. I couldn’t find any way of selecting the constraint and removing it. The constraint symbols are impossibly small, and I couldn’t see one on the line in question anyway. In the end I had to delete the line and redraw it. What am I missing?

Julie

[David JuppParticipant @davidjupp51506]

Julie,

Sounds like you are referring to sketch constraints, rather than assembly constraints.

You can make sketch constraint symbols larger – System Options -> Parts/Assemblies -> Sketching.  There are 3 size options for constraint symbols.

There is a similar setting in System Options -> Drawings -> Sketching.

If you are using a 4K monitor you’ll probably have to set Windows scaling to a higher value or experiment with the High DPI settings of Compatibility for the program, in order to get symbols at a good size.

You can delete sketch constraints by either

• right click on symbol, choose Delete from context menu
• left click on symbol, press Delete key on keyboard

The coincident sketch constraint doesn’t have a symbol, but it can be overridden by holding down Ctrl key whilst dragging the figure away.

 

[Julie AnnParticipant @julieann]

Oooops, yes I meant sketch constraints. Sometimes they need removing as I need to change the way I create a part. Generally I’ve never had a problem with assembly constraints. I’ll go and have a look at increasing contraint font size now.

Julie

[Julie AnnParticipant @julieann]

Wow, that’s much better, thanks David – Julie

[JasonBModerator @jasonb]

Even if they are small you should see the cursor change when you hover over the constraint, it shows bottom right of the + so you know you are over it then the right click brings up the options.

[Nigel Graham 2Participant @nigelgraham2]

Dave (SOD) –

Those acerbic comments were not what I’d expect from you. I was poking gentle fun at two things:

–  NINETEENTH-CENTURY attitudes when universities like Oxford still insisted on Latin as an entry qualification, despite their luminaries pushing areas like mathematics well beyond what was used in a practical sense at the time, and

– myself. If you don’t like my tongue-in-cheek humour you might at least credit me with the courage to admit my weaknesses. It is not a crime to find Mathematics, or CAD, far harder than you do; nor is it a crime to confess that.

Yes that was the era of Boole, Dodgson, Venn etc. – and the history of Matrices extends back far beyond them – but they were investigating pure-mathematical concepts, not engineering mathematics although the great 19C engineers were beginning to use maths more and more in their work.

 

Of course I know engineering relies on maths. I knew that when a lad, as my Dad, with BSc after his name, was a Chartered Electrical Engineer, employed in that field as an MoD scientist.

 

I’ve also said I worked, at laboratory-floor level, in a field of engineering that is extremely mathematical indeed – and by which I finally twigged Logarithms decades after pre-calculator arithmetic at school.

That was because the stock-in-trade scale for underwater acoustics uses the deciBel referred to one microPascal, for sound pressure levels. Airborne sound’s own 0dB equates to 20 µPa: their difference in dB is within mental arithmetic grasp if you remember the formula and the common-logarithm of 2 (it’s 0.3, near enough).

It was also the place that introduced me to computers, back in MS DOS days, and taught me basic ‘Word’ and ‘Excel’.

 

I did attempt A-Level Maths at school, yes, including 3D Graphs; but struggled and on the teacher’s advice stepped back to re-visiting O-Level Maths and “Additional Maths” (roughly half the A-level syllabus.) I left school with an A-level in Technical Drawing, and I gained an A-Level in Geology as an evening-class student in my 50s.

So please don’t accuse me of not wanting to learn anything. I do want to learn, but cannot help life-long, low learning ability. We can’t all be brilliant, and I have to work around what I can do. It is a matter of Nature, not choice, so please don’t deride people for not having your IQ and education.

…….

Meanwhile, somehow I need develop understanding Alibre Atom to a practical level within my reach. If I still need use some sort of dual existence to design things, so be it. Not ideal but better than nothing.

[Nigel Graham 2Participant @nigelgraham2]

Sorry David.

I tried exactly as you’d sent me, with your example and a new Assembly both open so I could switch back to read your list of directions, but still could not make it work. I could not see exactly what to select for each constraint, nor how to choose the correct constraint type.

The only blessing was that the large boss on both copies of the cover was facing the same way, though of course the flats were also facing the same way. For I have never been able to use that Flip tool either.

Even if I become vaguely good at drawing machine Parts, I don’t think I’ll ever be able to Assemble them.

[David JuppParticipant @davidjupp51506]

Much of the time Atom3D automatically selects the constraint type, based on the selected entities.  It also indicates which other constraint types are potentially valid, in case you wish to choose one of those.  See my earlier post for current & potential constraint indication – then click the icon if you want to change type (that will rarely be necessary).

I suggest selecting planes or flat faces when possible for constraints – there tends to be less confusion.   Worry about edges, vertices, points, curved surfaces, only if you can’t get a result with planes/faces.

 

[David JuppParticipant @davidjupp51506]

Nigel,

The Alibre/Atom Help file is quite good on assembly constraints – the types overviews is good but can be slow to load…

https://help.alibre.com/articles/#!alibre-help-v27/assemblies-assembly-constraints

Note the ‘mechanical’ constraints are not supported by Atom3D.

 

If you select any existing constraint, the entities it involves are highlighted in the workspace.

You can right click any assembly constraint in the Design Explorer, then choose Edit – this shows more detail and allows you to alter the selected entities or the type of constraint to apply.

[Julie AnnParticipant @julieann]

I could see other constraints in the Alibre sketch, but the one I wanted to delete was so small I couldn’t see it. Easier to delete the line than move the cursor around until something happens.

There is one lost wax casting of the part on order from Stanier Engineering in New Zealand. It should be on it’s way now. It was ready before Christmas but we decided it would be wise to avoid the Christmas delivery chaos and ship in the New Year. One casting is significantly more expensive than the PCBway parts. I won’t be using it on my engine as it will be cast in silicon bronze.  But I am curious to see what standard the professionals can achieve.

After a mess up in December when I ordered a 3D resin printer, had second thoughts and cancelled the order, then third thoughts and tried to uncancel but couldn’t, I’ve created a new order for the printer and matching washer/drier/curer today.

I’ve been using as filament 3D printer for 12 years so it will be interesting to see how the resin printer compares. Certainly the slicing process is more involved, particularly with respect to supports.

Julie

 

[Julie AnnParticipant @julieann]

One quick post on mathematics and then I promise I’ll be quiet, like a good girl.

Same as SoD I didn’t see the point of matrices at school, although I had no issues manipulating them. It would have been a big help if the teachers had explained why they were useful. Either they were blindly following the ‘New Math’ syllabus or they didn’t actually know.

Matices have been used in anger since the mid 19th century in the field of linear alegbra. I’ve used them extensively when computing the Fourier transform. They are key to understanding how the original Cooley and Tukey fast Fourier transform works.

I have used the Laplace transform, and the resulting complex s-plane, for the design and analysis of control systems and especially analogue filters.

Centrifugal governors can be analysed using the s-plane and it helps one to understand how they work and what the stability criteria are, at least in a handwaving fashion. Maxwell was the first person to develop an analysis of centrifugal governors in an 1868 Royal Society paper. They can be viewed as lowpass filters.

Julie

[Nigel Graham 2]

On Nigel Graham 2 Said:

Dave (SOD) –

Those acerbic comments were not what I’d expect from you….

Nigel, sorry about my outburst.  Far more caustic than it should have been. I’m in constant pain and it makes me short-tempered and easily frustrated.   Also trashes my concentration and judgement.  Was building to a peak when I typed the post and I was back in bed an hour later.  Shouldn’t have sent it.

No excuses though, I shouldn’t have indulged my irritation on you.  I apologise.

Dave

 

[Nigel Graham 2Participant @nigelgraham2]

Thankyou David.

I examined that link, though I found the rather excitable animations a bit confusing in places and the text reminiscent of business Terms & Conditions. The simple planar one is fairly obvious (a slide and its base), so is the concentric (a pin in a hole); others so unlikely I could not see what they do.

I’m obviously missing something somewhere, but it was always normal for me to learn anything very patchily, with gaps in my knowledge that are difficult or impossible to fill.

……

My generation was never taught matrices at school, in the 1960s for me. I first heard of them only when I took the standard GCSE Mathematics course as an adult student in the 1990s, as refresher for work reasons. The text-book I still have, introduces them by an imaginary shop sales report and saying the shop-keeper finds he does not need its row and column headings… before moving abruptly onto adding boxes of numbers with unexplained names. The book and lecturer taught us only the basic moves: no definitions, links to other topics, purposes, etc. Not even what a matrix really is!  We were not taught to understand them. Later I discovered their ancestors stretch back many centuries, though not what those were used for.

When at school, c.1965-6, one Maths lesson a week was devoted to the ‘School Mathematics Project’ syllabus, complete with a BBC TV Schools programme. This snobbishly called normal maths “traditional” and taught us making cardboard dodecahedra (calling the developments, “nets”), octal arithmetic and Venn diagrams. Again, no reasons given but I later found the octal and binary arithmetic was in the era when just using a computer needed such skills. While John Venn never meant his circles to be arithmetical, just symbolic; and dodecahedra are useful to radar dome designers.  It also had a geometrical section on “translations” and “rotations”; explained, as I recall, non-numerically, no co-ordinates, no arithmetic.

 

Oh, and I passed the GCSE Maths exam, taken in a hall full of students young enough to be my progeny, with high marks – though I forget the grade!

Incidentally I have examined the county’s college adult-education prospectii over the years since, but the curriculae shrink depressingly. No maths to speak of, no science; the only IT-related subjects are basic introductions to e-mails and digital photography. Some basic book-keeping but no meatier uses like spreadsheets and databases, let alone CAD, though the latter is probably within full-time, further-education, engineering courses.

[Andy StopfordParticipant @andystopford50521]

have you seen this, Nigel:

https://www.alibre.com/AlibreDownloads/Manuals/AlibreDesign_ExerciseManual_v24.pdf

Personally, I find video tutorials for software extremely irritating and would far rather have a written text like the above. I can’t answer as to whether it’s any good since i don’t use Alibre, but it looks like a good introduction to modelling and assemblies.

I was taught maths from the School Mathematics Project in the early 70s – I quite liked the bit at the beginning of each chapter which introduced the concept of whatever it was, and used to read that rather than listening to the master droning on. Of course, I then didn’t actually know what to do when it came to doing the exercises, and generally did dismally.

We didn’t make any cardboard dodechahedra though – I feel cheated.

 

[Nigel Graham 2Participant @nigelgraham2]

Julie –

I like those strainers!

 

Andy –

Ah, well, you lost there in no CAD (Cardboard Aided Dodecahedra)! In later years I made a hollow sheet-brass one, as a pomander, for a girlfriend’s Christmas present. It was a shell of two halves held together with a central, internal column and a fancy “acorn” nut. Though the pattern of holes in each pentagonal facet of the upper half was intended to let the fragrance out, Nina actually used it as a stand for ornamental grasses instead. Sometimes I wonder if she still has it, some 40-odd years after we split up….

…..

Been busy:

I have already made this crankshaft but need still make the bearings, transmission coupling, etc.

[JasonBModerator @jasonb]

Not much wrong with that crankshaft. Keep it up.

[Nigel Graham 2Participant @nigelgraham2]

Dave –

I am sorry to read of your illness.

Thankyou! I do have a rather satirical turn at times – but sometimes aimed at me!

I disliked maths at school because I struggled with it, it seemed (as did French) merely to pass an exam without understanding or purpose, and two particular teachers were very good at helping you be bad at the subject. Unfortunately it meant I could not become as I had once hoped, a professional scientist or engineer, though I spent my working life in “shop-floor” level work related to those. Working for a sonar manufacturer was my nearest to that lost early dream.

What helped later were fresh angles on things. Logarithms fell to working with deciBels, and I use mensuration, pi, and simple trigonometry in model-engineering. Most unexpectedly, I finally learnt what Differentiation does from a geology-club lecture on analysing stream gradients to find possible geological boundaries concealed below their valley floors. It uses a graph of the results from iterating just one, very simple formula. Something made me replace the altitude change term [H-h] with dH; and I twigged we were sort of differentiating each measured length of river.

As for French… I was not to know that I would go on my first foreign holiday only a few years after leaving school !

….

Jason –

Thankyou!

I went the simple way: using Mirror to form the second half of each crank based on the face of a half-length pin, but plotting the second crank itself rather than trying to rotate a copy of the first. I also found a shorter way to plot the 2D figure to start the webs.  The ends are were all just extruded cylinders, then the chamfers and fillets for appearance.

….

 

Andy –

I’ll sample that link – thankyou.

I find that problem with videos in any subject, not just CAD, but am fine with .pdf material.

I have two generic text-book primers on using CAD, one 2D-only and probably AutoCAD based; the other more modern and very much 3D-model based. These give the basic principles but obviously cannot be specific to make or programme.

I don’t think you’ll find many specific, printed IT text-books now, even in the “Xxxxx for Dummies” series, or similar, because as a second-hand book dealer told me, they go out of date so rapidly they are not viable for publishers. I am not sure if the print-on-demand system would ease that difficulty, but it makes commercial sense for the software publishers to issue directly e-posted material.

That dealer won’t buy computer text-books because they are so hard to sell on even second-hand, yet he does stock engineering text-books written many decades ago. I bought two or three for model-engineering references, for when modelling Edwardian machinery from sparse information such books explain its contemporary practices and design principles, often using illustrations probably quoted from works drawings.

[Andy Stopford]

On Andy Stopford Said:

have you seen this, Nigel:

https://www.alibre.com/AlibreDownloads/Manuals/AlibreDesign_ExerciseManual_v24.pdf

Personally, I find video tutorials for software extremely irritating and would far rather have a written text like the above. I can’t answer as to whether it’s any good since i don’t use Alibre, but it looks like a good introduction to modelling and assemblies.

[…]

 

Thanks for sharing that link, Andy

I don’t have Alibre, but after a few minutes looking at the early pages, I have already decided to read the lot.

MichaelG.

[David JuppParticipant @davidjupp51506]

Andy, Nigel, and others

The link Andy shared is for Alibre Design.  There is an equivalent link for the Atom3D version of that document   (might lead to less frustrations for Nigel if the images are closer to his owned software).

https://www.alibre.com/AlibreDownloads/Manuals/Atom3D_ExerciseManual_v24.pdf

The modelling task in the two versions of the document is the same, but the interface shown is different.  Atom3D has a deliberately simplified user interface.

I’ll warn people now – you do have to follow the instructions closely, or you will end up with parts that won’t assemble.

[Author]

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