3D cad design sequence

[Zeb]

What the heck are you talking about!?

Well, you could make the main bearing axis the starting point, or a point in space somewhere. The problem comes when assembly constraints start pushing things around. Assembly constraints are among the most glitchy in CAD programs. The tooling plate is not a bad place to start, as the engine has a lot of closed mechanical loops.

Another reason is the individual parts can become overwhelming. Having assemblies blocked out helps visualize design intent for the smaller pieces. It also validates the hand-drawn dimensions. He probably got cursed a lot before writing a disclaimer that the project is for experienced builders. There's a likelyhood that if all the parts were sent out to a machine shop as drawn, the engine would not assemble and run.

Along with software issues, I also borrow some of my ideas from ASME Y14.5, where:

Datum features are selected to meet design requirements. When selecting datum features, the designer should consider the following characteristics:

Functional surfaces
Mating surfaces
Readily accessible surfaces
Surfaces of sufficient size to allow repeatable measurements

I mean, I wasn't totally being altruistic. I made the instructions for meselfish too.

 

[johnmcc69]

Agreed. & a great example as well.

"Having assemblies blocked out helps visualize design intent for the smaller pieces."

I think this may be overlooked, (this is why creating sub-assemblies is beneficial), where you can hide other assemblies. In my case, I can simply pick an assembly, right click & "Hide" it. This simply removes it from the screen, not to be confused with the "Suppress" command or whatever your program may call it. (I think this was covered earlier.)

Apologies for my derailing the thread, but it is relevant.

I would like to hear how you other experienced users do this in any other software, but, I don't want to kill Gordons thread. Sometimes these things blow up & fall off target. It all started with "3D design sequence" about where to first start designing an engine in 3D.

I'm sorry if I got off track, I'm a CAD junkie.

I hope everyone has a great Thanksgiving, day off, whutever...
Enjoy your time with family & friends.

John

 

[Zeb]

I think you're on topic. It's about different approaches to structuring the work.

The cylinder/ assembly in the test case we're doing would be a good candidate for a subassembly. It has to be pre-assembled on the bench before final install and there's a few dozen pieces hooked up to it. It can also be in place before final rigging of the engine.

Thankfully most engines here have less than a couple hundred parts.

 

[Jasonb]

There are errors in those Middleton drawings as with most of his, I did do the corrections for when it was in ME Magazine but not sure if I kept them or not. It's also not a beam engine but a side lever engine

Couple of images from an old e-mail and the revised base drawing, not sure if these were to correct errors on teh ME drawing , Middletons errors or a bit of both

https://www.dropbox.com/s/5m2042cwgw2cod8/Middleton Doubkle Beam Baseplate.pdf?dl=0

 

[Zeb]

Nice! I caught my error last night. It looks like my sketch was identical to your revised, save for the 3.875 dimension.

Unless I made an error somewhere else, it looks like the original is correct. It just looked more like 3 3/8" not 3 7/8". It's obvious from the image that the feature is farther than a 1/4", but I was cruising along.


Another note about CAD process is a single dimensional change updated the entire pattern for me on both sides without affecting other downstream features.

Something really nice about Alibre (Design) and other packages is that for those awful (ha) BA thread drill sizes, I could type in a description after making the first hole, then click on it to reuse it in other places that called for them. No hands on keyboard is nice.