This thread is about how 3D CAD helps designers spot errors as a design develops. It's founded on JasonB's build thread, which is inspired by the Stuart Turner Real Engine, based on a full-size type.

Learning Solid Edge, I was looking for an advanced Assembly exercise in which I develop an entire engine from a photograph only.  The goal to create engineering drawings from an idea of the layout without the dimensions needed to build a working model. Jason often does this, and I can work from his latest .

In CAD Jason's Real Engine looks like this::

And my unfinished blinged-up clone in SolidEdge:

Not bad, but CAD helps spot important errors:

A – eccentric can't connect to the slide-valve (when it's added). Easy fix
B – Flywheel is on the wrong way round and hits the frame. Easy fix.
C – Base is foreshortened so the piston can't connect to the crank via the conrod. Fixing it is a good test of the CAD package: can I just slide the front of the base and the cylinder without disturbing the pillars and complicated headstock? Fingers crossed, I think it's easy.

A serious flaw in the cylinder:

Here, one of the 8 PCD stud-holes, circled D, is too close to the steam port. All 8 holes need to be rotated to provide clearance.

I need a dimensioned Third Angle Plans to fabricate parts in my manual workshop, no castings. CAD generates the plans, and the operator adds dimensions and other annotations as necessary by clicking.

The cylinder example reveals another glitch – the dimensions highlight the chance to use standard stock sizes has been missed! Reducing the amount of unnecessary machining and swarf.is always worthwhile.

Makes me appreciate Jason's efforts even more!

Dave

Edited By SillyOldDuffer on 24/05/2022 15:42:23

After dinner and beer, I found the duff constraint and fixed it allowing the flywheel, crank and piston do move together. Problem: the piston hits the bottom of the cylinder just before bottom dead centre, crunch:

I can turn off the cylinder to show the piston has about 4mm clearance at top dead centre:

Suggests shortening the conrod by 4mm will allow the piston to run full travel inside the cylinder without hitting the ends. Hope so because the other options are more complicated – changing the cylinder length, crank throw etc.

Dave

Posted by Nicholas Wheeler 1 on 24/05/2022 22:05:53:

Posted by SillyOldDuffer on 24/05/2022 21:19:30uggests shortening the conrod by 4mm will allow the piston to run full travel inside the cylinder without hitting the ends. Hope so because the other options are more complicated – changing the cylinder length, crank throw etc.

That's why you design the mechanism first, and add the pretty pillars and base once you know it's all going to work. That way, all you'd need to move would be a well defined plane….

As an example here is my doodle for a small, fabricated, powered shaper that would work, but shows where the material needs to be rather than where I thought it should be(the blue top plate supporting the ram needs to be shorter in the front and longer in the back)

That would be my usual approach, essential in 2D whenever parts have to fit and move together in complicated ways. But 3D CAD offers new opportunities, and Solid Edge has a feature I've not come across before: it can create new parts by building on the existing assembly. The cylinder top was done that way. In the assembly display I selected the top of the cylinder and used it as a template, with no need to remember any dimensions or alignments. Quick and natural.

In this case Jason, Stuart Turner, and 19th century engineers have already done most of the hard work for me! They created the mechanism and a layout that works, and I can imitate their build-style as well. All I have to do is approximate the layout and tweak dimensions until the model 'runs'. Previously I'd have started as described by PatJ, roughed out a blocky layout like your shaper example, and not worried about fanciful curves until everything else was tickety-boo. But as it's trivial to model a Jason style base and pillars I did so. I had to change both, but not a big problem. Otherwise, most of my parts are block- unfinished: the cylinder has no steam ways, the eccentric is impossible to make, and there are no fixings yet. Although Jason's version is far more complete, mine could be brought closer to his standard by filling in lots 'mere details'.

This is the latest version: I had to alter the piston rod and cylinder lengths to get it to work. Looks much the same, except the flywheel spins, turns the crank, which lifts and drops the piston inside the cylinder via the conrod without hitting the ends. The eccentric works too, though it's not connected to anything yet.

Much delay caused by finding and fixing broken relationships, likely caused by a stupid learner driving a stupid computer! Whilst I appreciate Solid Edge more and more, I'm not finding it easy to learn. Behind the scenes, what I'm doing is very like the 'Goes Wrong Show'. Making reasonable progress though, the cat hasn't been kicked yet.

Dave

I'm impressed – that fixed the problem! I spent ages looking for an incorrect constraint, when it was missing entirely. A mistake!

Choosing constraints calls for a level of judgement. For example, I've discovered leaving pins out of joints works well and is less likely to go wrong!

Also rotating the crank shows the conrod comes perilously close to the guide rails, but I got away with it. However, if I upgrade to a proper tapered conrod like Jason's, there's a high risk it will collide with the stanchion. I have a choice: fix it properly, or pretend a straight conrod was deliberately chosen to be easier to make…

Have to go shopping now and resent it. As Pat says 3D modelling is addictive – another kind of Joint!

Dave

Thanks, I wondered about that. Blame my unofficial mentor – that part of the eccentric and it's rod aren't visible in the image I pinched off you!

Today I added the belt-pulley and defined all the part materials before moving on to 'Explode Render Animate'. Animation was a little fiddly, but I can now generate a video of the engine turning.

Latest version:

The purple dot is the Centre of Mass.

Explode is quite fun and potentially useful when trying to work out how a pile of incautiously removed parts go back together. (Mum's kitchen clock never worked again!) Below is auto-explode, I think I could do better with the manual tool.

Identifying the materials and looking at the final dimensions shows this to be big by my standards, Would stand about 300mm tall, weigh 3.75kg, and contain 34 parts of which 26 are unique (excluding nuts and bolts). Mostly structural steel and brass, except I assumed the cylinder is cast-iron,

The Centre of Mass is slightly higher than the cylinder top, about a third of the total height. Better than I expected, and the COM doesn't move much when the engine rotates! Another credit to the original designer is the excellence of the static balance: X is only out by 0.35mm and Y by -0.94mm. Likely the Dynamic balance is less satisfactory because the whirling crank and power take-off are perched on top of 4 tall unbraced pillars, but even so may not have been a problem at steam speed.

My reading of the CAD model is that this engine is a good design.

As to making one, I have to confess I started filing a small slot in an Aluminium tube last Sunday and still haven't finished… This would keep me out of mischief for months.

Other excuses, there's ore to do. I haven't tackled the steam passages or the valve yet.

Dave

No problem, my engine is now bigger than yours!

Back in the Real world, I've started work on the cylinder and am unhappy about exhausting at the front through a belt; I can't think of a way of fabricating it with a lathe and mill.

Complicated around the back too:

A section view like one you did with Alibre would be handy:

Today's mission is finding out if Solid Edge can do the same. And then swap armchair for bench and catch up on some actual metalwork. (Tedious filing, which I can't keep straight.)

Dave

Edited By SillyOldDuffer on 27/05/2022 09:19:41

Wow, that locomotive cylinder is impressive. I see I'm trailing behind giants! Duncan's comment has set my mind at rest about the need to worry about this stuff, but I'm tempted to add extra drilled passages to my model because CAD makes it easy! But if I made a physical model, I'd probably leave them out because drilling several straight holes is the sort of thing I mess up.

Not even thought of spigots let alone forgotten them. They need adding.

I've fitted the Mark 2 Cylinder to the engine with no bother except a mistake has crept in, not sure how – I swear I was careful!. In this version the piston is a little high at TDC, and markedly so at BDC. A happy accident because it leaves room for spigots!

Doing this exercise has made me appreciate real models even more. Not only impressive construction skills, but also a myriad of design problems to be solved. Hats off to anyone who did complicated stuff with a drawing board and a slide rule! I'm finding 3D CAD makes it much easier to spot and correct mistakes, it's fast once you know the ropes, and there's a high confidence the real thing will work because all the joints and movements can be tested realistically as well. Doesn't eliminate the need for prototypes, but sure reduces it.

Dave