A Case for Assembly Level Features.

“Greatness is a by-product of usefulness”

Greatness quotes

Before going into the 'gory details' of the subject, what is an assembly level feature?

An assembly level feature is a cut that's made into components at the assembly level, after they've been put together. At the part level, no cut appears. The cut only appears at the assembly, and only at that level.

A question I'm sometimes asked, is 'why would I want to place a cut only in the assembly, and not down at the part level?'

The cheeky answer is you may not. It's a tool, that much like a pair of safety wire pliers, may not be useful to anyone but those who truly need it.

Safety Wire Pliers
(click to enlarge)

However, here's the example that taught me the place where an assembly level feature can be very useful.

I was building a night stand for a woodshop class I was taking, and naturally, since I had access to Inventor, I used it. But part of the design encountered a challenge, although not a difficult problem to overcome.

The side of the table was 24 inches wide, and about 30 inches tall. Now if you're using lumber (not plywood), it's nearly impossible to find a board that size, and of you can find it, it's going to be expensive.

A more realistic solution is to use a 'glue up' take smaller boards, and glue them up to create the board you need.

Boards to be 'Glued up'
(click to enlarge)

But there are slots that need to be cut in the side that will hold the rails for the drawers, floor, top, etc.

These slots will all run through three of the four boards. This is where we run into the beginnings of our challenge.

1) The boards are the same, with the exception of the slots, they would come out of sizing (jointer and planer) the exact same shape.

2) Why wouldn't I cut these slots separately and try to join them later? It's nearly impossible to get them to line up, and even if I could, it would take so much work, it wouldn't be worth it.

(click to enlarge)

So the result is to cut the feature after the boards have been glued up, using a simple jig to guide to tool. But how to I represent it correctly in Inventor?

I could 'fake it' (and in the old days we did), by cutting the features at the part level, but one of the parts has a blind slot. That would mean creating two distinct parts (three with the through slot, and one with the blind slot).

That translates into an inaccurate Bill of Materials, since the parts have the slots cut after their assembled. In effect, the blanks are the same.

So that's where the assembly level features can make a lot of sense. You can cut the slots at the assembly level, where they should be, and still have a nice, accurate bill of materials that reflect the materials you need to buy and prepare.

(click to enlarge)

So you may never need them, like that pair of safety wire pliers. But just because a given user doesn't find a tool useful, it doesn't mean it doesn't have a purpose If you do need them, they can be a critical tool that is indispensable it its usefulness. Just like the mechanic that needs those pliers.

And if you're wondering what those pliers are used for? It's used to twist wire through bolts and prevent them from vibrating and falling out in racing and aviation applications. It also ensures that the bolt has been checked for proper torque (the bolt should be checked before it's wired).

So even though we don't personally use it, the mechanics fixing the commercial aireliner we may fly on are!

An example of safety wire on an older airplane.
But it's still used today.
(click to enlarge)

Happy inventing!