micro-BOM management

I’ve now built 3 or 4 complete quad A1 style robots depending upon how you look at it. Each was somewhat of a one-off, incrementally modified over time as I discovered failure modes and improved the design. Before starting to serially build quad A1 style robots, I wanted to get a better understanding of how much actually goes into making one. The quad A1 has a fair number of sub-assemblies, custom PCBs, harnesses, and assembly steps that go into its production. During previous builds, I kept running into problems where I would run out of some component, fastener, or raw material unexpectedly, then have to wait for its lead time before I could continue.

I’m currently using a simplified Kanban inventory tracking system where I re-order parts and components when their inventory level reaches a critical threshold. However, given the discontinuous nature of my production, setting those levels is really hard. I didn’t know, for instance, just how many M3x8 bolts were necessary between the chassis, legs, and all the other sub-assemblies, since I had never really built multiple of all of them at one time before.

Incremental improvement

To make my life a little easier, I’ve started on a “micro-BOM” management solution. It currently is just a simple C++ application that reads a tree of JSON5 (mostly so I can use the obvious comment style) files on disk. Each JSON5 file describes a single stock item, what sub-components it consists of, and what it takes to acquire it in terms of cash, assembly time, 3d printer time, ordering information, and assembly procedures.

{
  "name" : "quada1_leg_lower",
  "version" : "20200701.0",
  "uuid" : "ad3f6ce2-aa13-42a2-8a4c-2d4b4e799ab0",
  "components" : [
    { "name" : "quada1_leg_foot", "qty" : 1 },  // the squash ball
  ],
  "resources" : [
    { "mk3s" : 7.5, "petg_black" : 71.54, "hours_tech1" : 0.1 },
  ],
  "procure" : {
    "print" : "20200625-lower_leg_and_bracket.3mf",
  },
}

That tool can then follow the hierarchy to count up how many stock units of each type are necessary for any top level item, and how many “resources” are necessary to make that happen.

Thus after entering in ~85 separate “stock units”, I now know that the current version of a full quad A1 requires approximately 300 hours of MK3s printer time between all of its assemblies and sub-assemblies, 132 heat set inserts, and 310 fasteners of various types (assuming the qdd100s are pre-assembled)!

Next steps

Given that I have relatively deep sub-assembly trees, and that I expect to be keeping some stock of all of them, I want to make this tool inventory-aware. That way it knows I already have N leg sub-assemblies in stock or Y qdd100s already built, and thus can tell me how many more bolts and such are necessary to build a full robot.

I’d also like to be able to export the results from this to replace the human-readable bom.txt entries sprinkled throughout git. That will make them both more accurate, and easier to maintain.

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