[this log entry copied from #Minamil where it was updated 26 Oct 2021]

Started tracking part prices because they've been a bit volatile and small changes to small prices can be significant. Still tracking because still changing -- some changes not small.

Example BoM cost

Sep 2021

• $30.10 - 3x motors (Aug: up again. >2x from -1yr)
• $5.83 - 12x bearings (3 extra)
• $11.06 - 2x 600mm rods (100mm-kerfs & 200mm-kerfs extra)
• $1.79 - 100x screws
• <$1.00 - ~9"x30" 1/8" hardboard (cut from panel; keep remainder for more laser cutting stock)

Total <$49.78 (+tax) 

+21% from March 2021; now up to ~$17/axis; +51% from Jan 2020 start with this configuration at $33 :-/

Getting parts faster in the continental US

A lot faster for a little more $:

• motors from Bangood buy 3 (cost from AliExpress sellers up --> penalty for buying from US stock down)
  • (26 Oct 21: Bg currently shipping from China, tho promised a little faster than via AliExpress; Bg has dropped & restored US stock before so maybe it will be back...) 
• bearings from Amazon 1x10pcs (assuming Prime or total order over $25 for free shipping)

A lot faster for  ̶a̶ ̶l̶i̶t̶t̶l̶e̶  more $:

• rods from Amazon buy 3x6mmx401mm (longer stock to cut yourself; this & bearings will break $25 for free shipping if not Prime)

Screws from Grainger or Ace Hardware if you live near a store -- maybe not in stock but will ship free to a store.

Hardboard I've bought in 4x8 sheets (or 2x4) from Home Depot -- or check other building supply/lumber sources that have a panel saw for cutting big sheets down on request. Such places usually post a low limit on free cuts, like 1, but the person running the saw will often (IME always) do more if you ask respectfully and have your measurements ready. Check the max material size for the laser cutter you will use and think about how to divide the large sheet into pieces of that size with the fewest cuts. For example: the laser cutter I use most often takes material up to 32" x 18". Two cuts at 32" divide a 48"x96" sheet into three 48"x32" pieces. A third cut at 18" across the three together gives me three 32"x18" pieces ready to use and three 32"x30" pieces I can transport easily and cut down later. That's a lot of material for nearly free compared to acrylic, Baltic ply, etc. that you can use to iterate a design before cutting the expensive stuff. Or find that hardboard works well enough for a project -- like a little CNC mill or something.

Part cost details

For orders from US

• CH-SM1545 -- motor + leadscrew
  • No cost changes for listed motor sources from Aug to 26 Oct 2021 -- stabilizing?
  • 26 Oct 21: no Aliexpress sellers remain other than ChiHai Motor outlets; and Banggood now ships from China (tho promised somewhat faster than via Aliexpress) -- which looks like consolidation to a single source at 2.1-2.3x cost when found this part :-/ )
  • Aliexpress "Shenzhen Chihai Motor Store" 6 Aug 21
    • link
    • spend: 3x $7.05 + $8.95 = $30.10
    • unit cost: $30.10 / 3 = $10.03
  • Aliexpress "CHIHAI Factory Store" 6 Aug 21
    • spend: 3x $7.05 + $9.35 = $30.50
    • unit cost: $30.50 / 3 = $10.17
  • Aliexpress "CHIHAIMOTOR Store" 6 Aug 21
    • spend: 3x $7.05 + $11.61 = $32.76
    • unit cost: $32.76 / 3 = $10.92
  • Banggood 10 Aug 21
    • link
    • spend: 3x $10.99 = $32.97
    • unit cost: $10.99
  • Aliexpress "TO-HO store" 21 Sep 21 (26 Oct 21: ceased selling some time since 21 Sep)
    • was $4.78 shipped Jan-Aug '20 and some time before that, which made a stronger argument for designing around this part!
    • spend: 3x $11.53 + $4.00 = $38.59
    • unit cost: $38.59 / 3 = $12.86 (+169% up from prior low)

• LM6UU -- 6mm linear bearing
  • about buying bearings:
    • IME the common generic bearings all seem to come from the same or indistinguishable sources with stochastic quality
    • buying a dozen allows for selecting the best 9 with up to three rejects, which will probably be ok
    • while "real" bearings for grownups are expensive, like $19 each from McMaster-Carr, the MSM branded bearings below are a definite step up from generic for cost that's not so radically out of scope for this project
  • cheap: Aliexpress "VENSTPOW Official Store" 26 Oct 21
    • link
    • spend: 1x 12pcs $5.83
    • unit cost: $5.83 / 9 = $0.65
    • extra: 3 (larger pool for selecting best 9, or enough for another single slide if all good)
    • unit cost: $5.83 / 12 = $0.49 if use extra 3 for another slide
  • fast: Amazon "DollaTek US" 26 Oct 2021
    • link
    • relatively instantaneous delivery in area served, for free with Prime or order over $25.
    • spend: 1x 10pcs = $6.99
    • unit cost: $6.99/9 = $0.78
    • extra: 1 (only one reject, but quick supply if need more to select from)
  • fast+extra: Amazon "JS Electric" 16 Oct 2021
    • link (eh? 26 Oct: links to different bearing, but this still comes up as 12xLM6UU in search - and links to the different product. maybe transient bogosity?)
    • spend: 1x 12pcs = $10.99
    • unit cost: $10.99/9 = $1.22 ($0.92 if $/12)
    • extra: 3 (larger pool for selecting best 9, or enough for another single slide if all good)
    • unit cost: $10.99 / 12 = $0.92 if use extra 3 for another slide
  • better: Aliexpress "MSM Industrial Store" 26 Oct 21
    • superior branded parts not generics 
    • link
    • spend: 1x 10pcs $13.47
    • unit cost: $13.47 / 9 = $1.50
    • extra: 1
• 6x100mm rod
  • Aliexpress "VENSTPOW Official Store" 26 Oct 21
    • cut your own 100mm rods from 2x600mm
    • link
    • spend: 2pcs  6x600mm $11.06
    • unit cost: $11.06 / 9 = $1.23
    • extra: 100mm-5*kerf + 200mm-4*kerf
  • Aliexpress "VENSTPOW Official Store" 26 Oct 21
    • pre-cut to 100mm
    • link
    • spend: 5x 2pcs $2.64 = $13.20
    • unit cost: $13.20 / 9 = $1.47
    • extra: 1
  • Aliexpress "Hardwar &Yool Store"[sic] 21 Sep 21
    • pre-cut to 100mm
    • link (26 Oct 21 out of stock)
    • stainless 304
      • con: not recommended for use with ball bearings because softer than bearing balls
      • but: might work ok in this light-duty this application?
      • pro: smooth & uniform diameter among my sample; 5.98mm = not perfect but pretty good 
    • spend: 10pcs $7.35 + $2.71 = $10.06
    • unit cost: $10.06/9 = $1.12
    • extra: 1
  • fast: Amazon "uxcell" 26 Oct 2021
    • cut your own 100mm rods from 3x401mm
    • link
    • relatively instantaneous delivery in area served, for free with Prime or order over $25.
    • spend: 3x 6x401mm $7.99 = $23.97
    • unit cost: $6.99/9 = $2.66
    • extra: 3x 101mm-3*kerf
  • Scavenge
    • pretty common in printers, etc.
    • look for bright shiny smooth rods/shafts that other metals don't scratch
    • "6mm" ≠ 6mm
      • rods/shafts will be made to some specified tolerance depending on application -- which oversimplifies to "a little under" or "a little over"
      • the bearings fit rods "a little under" 6mm e.g. 5.99 but not 6.01
      • measure if you have means - can help decide what to disassemble or not
      • or test with a bearing - if the rod crunches into the bearing balls, stop; it won't work even you succeed in forcing in through
    • hardened rod is better but harder to cut
      • big bad angle grinder is great if you have one
      • maybe use rotary tool + abrasive disk
      • maybe cut bulk with a hack saw, and maybe some oil, after breaking hardened surface
      • cut for clean surface over measured length because rods are clamped by their ends - chamfers are nice but need extra length - especially for motor + thicker (>3.0mm) material
• Screws #6-32x1/4in -- general assembly; no shipping if near one of these retail locations
  • Grainger 26 Oct 2021
    • spend: 1x 1MU12 100pcs pan head Phillips $1.79
    • = Fabory U24522.013.0025
  • Ace Hardware 26 Oct 2021
    • spend: 1x 56297 100pcs round head combo $3.79
    • = Hillman 90114
  • Non-Imperial: look for #6-32x6mm because PC cases
  • Scavenge
    • Common PC case screw
      • but need a pile, so less likely by casual scavenging
      • unless building less, like just one axis

• Screws #6-32xSmaller -- mounting motor/leadscrew units
  • roll your own
    • spend: $0.00 if you drop no more than 2 from the 100pc bag (as of some recent version)
    • unit cost: $0.00 + file
    • 12x file heads to <2mm thickness
    • 4x file length to material thickness
  • Supermicro 3.5in hdd caddy screws (via Amazon seller) 26 Oct 2021
    • link
    • spend: 1x MCP-410-00005-0N $13.27 (very volatile over crazy wide range)
    • unit cost: $13.27 / 12 = $1.11
    • extra: 88x screws, 24x caddy labels
    • but probably not if you don't already have some (& do have a file)
  • other
    • not really a standard screw (that I know of - comment plz?)
    • #6-32x4mm screws exist
      • some of which have thin heads
    • thin head
    • thin/shallow countersunk flathead ideal for constraining position
      • it looks like the magic feature is 100deg countersink; bonus 100deg+undercut
      • McMaster-Carr 91771A977 might work. $9.35/100 (1/4inch)
      • or McMaster 91771A976 $7.22/50 (3/16inch)

• Material
  • 1/8 in hardboard
    • finish one side
    • current design could tile inside 9" x 30"
  • Home Depot 26 Oct 2021 (up again since Sep; these prices rising too...)
    • link
    • spend: 1pcs 2'x4' $5.26 (+28% from Oct 2020)
    • fits 3x cut layout at ~9"x30"
    • unit cost by potential cut count: $1.75
    • fits >= 4x area (if globally efficient tiling)
    •  unit cost by area: $1.31
      • or
    • link
    • spend: 1pcs 4'x8' $10.53 (+56% from Oct 2020 :-/ )
    • fits 15x cut layout at ~9"x30"
    • unit cost by potential cut count: $0.70
    • fits >= 17x area (if globally efficient tiling)
    • unit cost by area: $0.62