Help! Alibre: Changing Part Size?

[Nick WheelerParticipant @nickwheeler]

Nigel, you’re still working far too hard.

You know the cylinder spacing, so draw a horizontal construction line that long starting at the origin. That immediately constrains it and the cylinders that are centred on its ends. You can also make the rectangle that will become the block symmetrical around it. The only other dimensions needed are how long the block will be, its width and how far one of the edges are from one of your fixed points(the origin is the obvious choice!) This is all editable without breaking the geometry. Extrude the rectangle minus the cylinder circles.

I would probably sort the bolt holes next, as they will determine how much bigger than the cylinders the cover needs to be. It makes sense to me to place the first hole half way between the cylinder edges. You could calculate that distance is, but it’s a faff and is only applicable to that spacing and bore sizes. Instead, you draw a line between both circles, constrain it to be horizontal with both their centres, and put a point on the mid point of the line – no dimensions required for any of this, and if you do change any of the bores or spacing it stays where you want it. Use the hole command centred on that point to create your first, threaded hole. That feature is then used with the circular pattern tool, using a bore axis as the centre of rotation and however many holes you want – six is a good number if you’re going to be doing manual marking out. Repeat the same process for the other cylinder.

Now you can visually juggle with the size of the bolthead you’re going to use to give a sensible size for the head cover diameters. And you build that using projected bore centres for the end arcs, with lines tangent to them for the joining sides. Project the bolt-hole point as well so you can repeat the process used above for all the holes, knowing they’ll align with the ones in the block. Again, no working out required and everything will still fit even if dimensions are edited.

 

I would contend that assembly of your components is ‘impossible’ because of your preliminary elevations, not despite them. I would have started this with the crankshaft in its required position in the chassis, built the rods and pistons to suit, then added the eccentrics and their linkages so everything worked correctly. Finally I’d have built the block and end covers around all these as both parts are effectively only fancy brackets to hold everything in place.

[Nigel Graham 2Participant @nigelgraham2]

Thank you Nick.

Looks as if this site is having IT problems too. This afternoon it was off completely by a server failure, it said. It’s still flakey now!

 

Oh Dear. I thought I’d asked a simple question – a small dimension-change. I had no idea it is so complicated, nor that my drawing was so bad. It took Jason and David quite some work to analyse and correct it. Now I’m seeing there are umpteen right ways to the same drawing, but I go and pick only the wrong ones; so I’m becoming ever more confused and disheartened.

 

You describe how you’d use projections but, sorry, you’ve lost me. That is an area of Alibre I know nothing about, but I did not have to project anything. I knew the seed hole’s location from centres that cannot be moved, and I knew the pitch-circles operate on those. I then Assembled the two covers alone, to verify the dimensions all match. I think in practice I’ll pilot-drill the covers on a rotary-table then spot through them to the block, rather than rely on co-ordinates I obviously cannot plot correctly.

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I have to design a new cylinder block to match the original and other parts already made: I have to design and often re-design the entire project as I go along, often retrospectively; hoping that Part B just made does not need a new edition of Part A made twenty years ago….. (The chassis rails already have plenty of spare holes.)

 

All the main dimensions are fixed. They can not be changed; but I needed to draw cylinder covers to fit everything properly.

 

I could see no alternative to geometrical construction, in 2D; manually or as I used, TurboCAD’s direct orthogonal mode. This gave the dimensions to feed into Alibre so I did not need “juggle” anything there. It was my only feasible approach!

 

My mistake was simply forgetting a diameter  already designed, and I made the block slightly too narrow. My original question was how to correct only that – I had no idea the whole drawing was complete rubbish.

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I can’t draw my engine as you suggest, even with simplifying shapes of parts too hard in CAD. It’s an enclosed, inverted-vertical compound with Stephenson’s Link Motion (I’m cribbing the valve-gear from published designs). So two of everything. The side on dead-centre is slightly simpler but everything on the mid-stroke side is all at very odd angles. So I can’t see at all how to assemble all those parts in an Alibre 3D model, but I can’t draw the crankshaft and engine-case anyway.

So I could create a CAD G.A. for the engine only orthographically, in TurboCAD, whose extremely powerful 3D mode is impossible for me – but, the huge fly in the steam-oil is that printing a TurboCAD drawing is almost impossible too!

 

[Nigel Graham 2Participant @nigelgraham2]

Just tried drawing the block again, using what I’ve managed to glean from everyone’s advice above.

I can’t get it to work, just drawing the basic rectangle to start the thing, symmetrically about the origin.

Using the Rectangle tool didn’t work – it drew the figure the right size but placed randomly –  so I tried constructing it from four lines.

I managed to place them by dimensions from the origin, as I could not make the Constraints do anything, but they all ended up as Magnitude and Position Undefined.

I can’t even draw a b****y rectangle anchored centrally to the grid…..

Hopeless.

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